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Achieving long-term retention of pop-up satellite archival tags (PSATs) has proven difficult for all fishes but is particularly challenging for small migrant species due to the relatively large size of tags. In this study, the authors tested the latest and smallest PSAT model on the market, the mark-report satellite tag (mrPAT), and developed a simple, cost-effective method of tag attachment on sheepshead Archosargus probatocephalus (Walbaum 1792), a small marine fish. During laboratory trials, the method of tag attachment used in this study outperformed the existing methods with two c. 40 cm fish retaining their tags for 3 months (the duration of the laboratory study). During field deployments, data were successfully obtained for 17 of the 25 tagged fish [37-50 cm fork length (FL)]. Of these, 14 tags (82%) remained on the fish until the pre-programmed release date resulting in tag retention times of up to 172 days (mean: 140 days). The investigation represents the first extensive study into the feasibility of PSATs for monitoring fishes in this size range. The authors demonstrate that their method of attachment and this latest PSAT model are feasible for c. 5-month deployments on fishes that are relatively small (c. 45 cm FL). These results with A. probatocephalus represent a potentially significant advance in PSAT methodology for fishes of this size. Future investigations are needed to determine if this method is transferrable to other species in the same size range., (© 2023 Fisheries Society of the British Isles.)

As the largest and most diverse vertebrate group on the planet, fishes have evolved an impressive array of sensory abilities to overcome the challenges associated with navigating the aquatic realm. Among these, the ability to detect Earth's magnetic field, or magnetoreception, is phylogenetically widespread and used by fish to guide movements over a wide range of spatial scales ranging from local movements to transoceanic migrations. A proliferation of recent studies, particularly in salmonids, has revealed that fish can exploit Earth's magnetic field not only as a source of directional information for maintaining consistent headings, but also as a kind of map for determining location at sea and for returning to natal areas. Despite significant advances, much about magnetoreception in fishes remains enigmatic. How fish detect magnetic fields remains unknown and our understanding of the evolutionary origins of vertebrate magnetoreception would benefit greatly from studies that include a wider array of fish taxa. The rich diversity of life-history characteristics that fishes exhibit, the wide variety of environments they inhabit, and their suitability for manipulative studies, make fishes promising subjects for magnetoreception studies., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Magnetotactic bacteria (MTB) are a diverse group of highly motile Gram-negative microorganisms with the common ability to orient along magnetic field lines, a behavior known as magnetotaxis. Ubiquitous in aquatic sediment environments, MTB are often microaerophilic and abundant at the oxic/anoxic interface. Magnetic field sensing is accomplished using intracellular, membrane-encased, iron-containing minerals known as magnetosomes. The chemistry, morphology and arrangement of magnetosomes differs substantially among different MTB. Although magnetic field sensing mechanisms, genetic bases and protein functions have been elucidated in select model organisms such as the Magnetospirillum strains and Desulfovibrio RS-1, not all findings are applicable to diverse clades of MTB. As the number of identified species has increased, it has become evident that many of the characteristics and mechanisms once presumed to be prototypical of MTB are in fact not universal. Here we present a general overview of the current state of MTB research for readers outside of the realm of prokaryotic research, focusing on recent discoveries, knowledge gaps and future directions. In addition, we report new insights acquired using holographic technology to observe and quantify microbial responses in magnetic fields that are earth-strength or weaker, providing a new ecophysiological approach to in situ MTB research., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

In addition to providing animals with a source of directional or 'compass' information, Earth's magnetic field also provides a potential source of positional or 'map' information that animals might exploit to assess location. In less than a generation, the idea that animals use Earth's magnetic field as a kind of map has gone from a contentious hypothesis to a well-established tenet of animal navigation. Diverse animals ranging from lobsters to birds are now known to use magnetic positional information for a variety of purposes, including staying on track along migratory pathways, adjusting food intake at appropriate points in a migration, remaining within a suitable oceanic region, and navigating toward specific goals. Recent findings also indicate that sea turtles, salmon, and at least some birds imprint on the magnetic field of their natal area when young and use this information to facilitate return as adults, a process that may underlie long-distance natal homing (a.k.a. natal philopatry) in many species. Despite recent progress, much remains to be learned about the organization of magnetic maps, how they develop, and how animals use them in navigation., (© 2021. The Author(s).)

Radio frequency electromagnetic noise (RF) of anthropogenic origin has been shown to disrupt magnetic orientation behavior in some animals. Two sources of natural RF might also have the potential to disturb magnetic orientation behavior under some conditions: solar RF and atmospheric RF. In this review, we outline the frequency ranges and electric/magnetic field magnitudes of RF that have been shown to disturb magnetoreceptive behavior in laboratory studies and compare these to the ranges of solar and atmospheric RF. Frequencies shown to be disruptive in laboratory studies range from 0.1 to 10 MHz, with magnetic magnitudes as low as 1 nT reported to have effects. Based on these values, it appears unlikely that solar RF alone routinely disrupts magnetic orientation. In contrast, atmospheric RF does sometimes exceed the levels known to disrupt magnetic orientation in laboratory studies. We provide a reference for when and where atmospheric RF can be expected to reach these levels, as well as a guide for quantifying RF measurements., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Sea turtles are often restrained manually for brief periods during veterinary evaluation and care in rescue, rehabilitation, research, and aquarium settings. Blood gas values and lactate are routinely evaluated during triage of sea turtles, and lactate clearance is of prognostic significance in cold-stunned individuals. Although increases in blood lactate have been associated with muscle exertion, experimental forced submergence, trawl and pound net capture, and general anesthesia, changes in blood lactate associated with short periods of manual restraint have not been evaluated. Venous blood gas and lactate values were tested in 16 juvenile loggerhead sea turtles ( Caretta caretta ) before and after manual restraint for a 15-min routine veterinary examination. The agreement of blood lactate values between two point-of care analyzers (i-STAT and Lactate Plus) was also compared. Blood pH and bicarbonate (HCO 3 - ) decreased significantly ( P < 0.001), and partial pressure of carbon dioxide (pCO 2 ) increased significantly ( P < 0.0001) after 15 min. Lactate increased significantly between time points for both analyzers ( P < 0.0001). Linear regression analysis showed excellent correlation for lactate measurements obtained on both analyzers ( r = 0.998). The mean difference in lactate concentrations between the analyzers was statistically significant, indicating that the methods cannot be used interchangeably ( P < 0.0001). Deming regression and Bland-Altman plots identified a slight negative proportional bias for lactate measurement by the Lactate Plus compared with the i-STAT. These results suggest that clinicians should evaluate blood gas values and lactate at the beginning of health evaluations and interpret serial lactate values in sea turtles with caution, because even short periods of manual restraint can induce lactic acidosis and considerably influence these values.

Diverse taxa use Earth's magnetic field in combination with other sensory modalities to accomplish navigation tasks ranging from local homing to long-distance migration across continents and ocean basins. Several animals have the ability to use the inclination or tilt of magnetic field lines as a component of a magnetic compass sense that can be used to maintain migratory headings. In addition, a few animals are able to distinguish among different inclination angles and, in effect, exploit inclination as a surrogate for latitude. Little is known, however, about the role that magnetic inclination plays in guiding long-distance migrations. In this paper, we use an agent-based modelling approach to investigate whether an artificial agent can successfully execute a series of transequatorial migrations by using sequential measurements of magnetic inclination. The agent was tested with multiple navigation strategies in both present-day and reversed magnetic fields. The findings (i) demonstrate that sequential inclination measurements can enable migrations between the northern and southern hemispheres, and (ii) demonstrate that an inclination-based strategy can tolerate a reversed magnetic field, which could be useful in the development of autonomous engineered systems that must be robust to magnetic field changes. The findings also appear to be consistent with the results of some animal navigation experiments, although whether any animal exploits a strategy of using sequential measurements of inclination remains unknown.

Diverse organisms use Earth's magnetic field as a cue in orientation and navigation. Nevertheless, eliciting magnetic orientation responses reliably, either in laboratory or natural settings, is often difficult. Many species appear to preferentially exploit non-magnetic cues if they are available, suggesting that the magnetic sense often serves as a redundant or 'backup' source of information. This raises an interesting paradox: Earth's magnetic field appears to be more pervasive and reliable than almost any other navigational cue. Why then do animals not rely almost exclusively on the geomagnetic field, while ignoring or downplaying other cues? Here, we explore a possible explanation: that the magnetic sense of animals is 'noisy', in that the magnetic signal is small relative to thermal and receptor noise. Magnetic receptors are thus unable to instantaneously acquire magnetic information that is highly precise or accurate. We speculate that extensive time-averaging and/or other higher-order neural processing of magnetic information is required, rendering the magnetic sense inefficient relative to alternative cues that can be detected faster and with less effort. This interpretation is consistent with experimental results suggesting a long time course for magnetic compass and map responses in some animals. Despite possible limitations, magnetoreception may be maintained by natural selection because the geomagnetic field is sometimes the only source of directional and/or positional information available., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2020. Published by The Company of Biologists Ltd.)

Diverse animals use Earth's magnetic field to guide their movements, but the neural and molecular mechanisms underlying the magnetic sense remain enigmatic. One hypothesis is that particles of the mineral magnetite (Fe 3 O 4 ) provide the basis of magnetoreception. Here we examined gene expression in the central nervous system of a magnetically sensitive invertebrate, the Caribbean spiny lobster (Panulirus argus), after applying a magnetic pulse known to alter magnetic orientation behavior. Numerous genes were differentially expressed in response to the pulse, including 647 in the brain, 1256 in the subesophageal ganglion, and 712 in the thoracic ganglia. Many such genes encode proteins linked to iron regulation, oxidative stress, and immune response, consistent with possible impacts of a magnetic pulse on magnetite-based magnetoreceptors. Additionally, however, altered expression also occurred for numerous genes with no apparent link to magnetoreception, including genes encoding proteins linked to photoreception, carbohydrate and hormone metabolism, and other physiological processes. Overall, the results are consistent with the magnetite hypothesis of magnetoreception, yet also reveal that in spiny lobsters, a strong pulse altered expression of > 10% of all expressed genes, including many seemingly unrelated to sensory processes. Thus, caution is required when interpreting the effects of magnetic pulses on animal behavior.

A variety of animals sense Earth's magnetic field and use it to guide movements over a wide range of spatial scales. Little is known, however, about the mechanisms that underlie magnetic field detection. Among teleost fish, growing evidence suggests that crystals of the mineral magnetite provide the physical basis of the magnetic sense. In this study, juvenile Chinook salmon ( Oncorhynchus tshawytscha ) were exposed to a brief but strong magnetic pulse capable of altering the magnetic dipole moment of biogenic magnetite. Orientation behaviour of pulsed fish and untreated control fish was then compared in a magnetic coil system under two conditions: (1) the local magnetic field and (2) a magnetic field that exists near the southern boundary of the natural oceanic range of Chinook salmon. In the local field, no significant difference existed between the orientation of the control and pulsed groups. By contrast, orientation of the two groups was significantly different in the magnetic field from the distant site. These results demonstrate that a magnetic pulse can alter the magnetic orientation behaviour of a fish and are consistent with the hypothesis that salmon have magnetite-based magnetoreception., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2020. Published by The Company of Biologists Ltd.)

Certain long-distance migratory animals, such as salmon and sea turtles, are thought to imprint on the magnetic field of their natal area and to use this information to help them return as adults. Despite a growing body of indirect support for such imprinting, direct experimental evidence thereof remains elusive. Here, using the fruit fly as a magnetoreceptive model organism, we demonstrate that exposure to a specific geographic magnetic field during a critical period of early development affected responses to a matching magnetic field gradient later in life. Specifically, hungry flies that had imprinted on a specific magnetic field from 1 of 3 widely separated geographic locations responded to the imprinted field, but not other magnetic fields, by moving downward, a geotactic behavior associated with foraging. This same behavior occurred spontaneously in the progeny of the next generation: female progeny moved downward in response to the field on which their parents had imprinted, whereas male progeny did so only in the presence of these females. These results represent experimental evidence that organisms can learn and remember a magnetic field to which they were exposed during a critical period of development. Although the function of the behavior is not known, one possibility is that imprinting on the magnetic field of a natal area assists flies and their offspring in recognizing locations likely to be favorable for foraging and reproduction., Competing Interests: The authors declare no competing interest.

Objective: To determine the combined mydriatic effects of topical rocuronium bromide and phenylephrine in juvenile loggerhead turtles and identify any adverse effects associated with treatment., Animals Studied: Eleven juvenile loggerhead turtles (Caretta caretta)., Procedures: Four 20 μL drops of rocuronium bromide and four 20 μL drops of 10% phenylephrine were placed into the right eye at 2-minute intervals of 5 turtles, while the same volume of saline was administered to six control turtles. A pupilometer recorded pupil measurements at rest and following a light stimulus at 2, 15, 30, 60, 120, 150, 180, 210, 240, 300, and 360 minutes following delivery of the final drop to the ocular surface. Intraocular pressure (IOP) was also measured at similar time points., Results: The nonilluminated and light-stimulated pupillary diameter of the right eye of treated turtles was significantly greater than baseline starting at 120 and 15 minutes, respectively. Light-stimulated pupillary diameter of treated eyes was greater than that of control eyes from time 15 minutes until the end of the treatment period. No systemic side effects were noted over a 24 hours period following treatment and all turtles showed normal behavior and appetite. No mydriasis was noted in either eye at 24 hours and the anterior segment was normal., Conclusions: A combination of topical ophthalmic rocuronium bromide and 10% phenylephrine is safe and effective for mydriasis in juvenile loggerhead turtles., (© 2019 American College of Veterinary Ophthalmologists.)

Diverse marine animals migrate across vast expanses of seemingly featureless ocean before returning as adults to reproduce in the area where they originated. How animals accomplish such feats of natal homing is an enduring mystery. Growing evidence suggests, however, that sea turtles and salmon imprint on the magnetic field of their home area when young and then use this information to return as adults. Both turtles and salmon have the sensory abilities needed to detect the unique 'magnetic signature' of a coastal area. Analyses have revealed that, for both groups of animals, subtle changes in the geomagnetic field of the home region are correlated with changes in natal homing behavior. In turtles, a relationship between population genetic structure and the magnetic fields that exist at nesting beaches has also been detected, consistent with the hypothesis that turtles recognize their natal areas on the basis of magnetic cues. Salmon likely use a biphasic navigational strategy in which magnetic cues guide fish through the open sea and into the proximity of the home river where chemical cues allow completion of the spawning migration. Similarly, turtles may also exploit local cues to help pinpoint nesting areas once they have arrived in the vicinity. Throughout most of the natal homing migration, however, magnetic navigation appears to be the primary mode of long-distance guidance in both sea turtles and salmon., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2019. Published by The Company of Biologists Ltd.)

In the beginning there was great confusion about animal migration. Aristotle, noting that the types of birds around him changed with the seasons, concluded that summer redstarts turned into robins at the onset of winter, and that garden warblers became blackcaps [1]. Others thought that birds disappear in winter because they hibernate submerged in mud. In a case of art decidedly not imitating life, a 16th century illustration accompanying the writings of Swedish Archbishop Olaus Magnus showed a fishing net filled with hibernating swallows being pulled from a lake [1]. Gradually, over centuries, these fanciful early explanations gave way to an understanding that migration is a widespread phenomenon and that Earth is alive with itinerant animals traversing continents, seas, and skies (Figure 1)., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The Galapagos sea lion ( Zalophus wollebaeki) is an otariid species endemic to the Galapagos archipelago and is currently listed as endangered. The ocular trematode Philophthalmus zalophi was recently reported to affect the survival of juvenile Galapagos sea lions on Santa Cruz Island, resulting in marked ophthalmic changes. This study evaluated the ophthalmic disease and histopathologic effects of P. zalophi on juvenile Galapagos sea lions in the largest rookery located on San Cristóbal Island. Twenty juvenile Galapagos sea lions (10 male and 10 female) were evaluated among five sites in the rookery El Malecón. Ophthalmic examination, including fluorescein staining and evaluation of the adnexa, cornea, and sclera, were performed on each eye. The presence, number, and location of ocular parasites were determined, and parasites were collected for identification. Conjunctival biopsy was performed on 11 animals: 2 that lacked parasites and gross lesions and 9 with both parasites and gross lesions. All parasites collected were confirmed as P. zalophi and identified in 80% (16/20) of the study animals and 70% (28/40) of the examined eyes. Philophthalmus zalophi was most frequently found attached to the nictitating membrane but also located on the palpebral conjunctiva or cornea. The most common clinical signs were varying degrees of conjunctival hyperemia (28/40 eyes), most frequently of the nictitating membrane and mucoid ocular discharge (12/40 eyes). The number of parasites was significantly associated with the degree of conjunctival hyperemia ( P < 0.001). Histopathology of conjunctival biopsies revealed organized lymphoid follicles and lymphoplasmacytic infiltrates. The histopathologic changes and gross lesions were likely due to the parasite's attachment to the conjunctiva. This study provides additional details of P. zalophi infection in juvenile Galapagos sea lions. Further research is warranted to detail the life cycle of this parasite, transmission to sea lions, and potential treatment protocols.

The San Cristóbal lava lizard, Microlophus bivittatus , is one of nine species of lava lizards endemic to the Galápagos Islands of Ecuador. No information presently exists about baseline health parameters for any of these species. We analysed blood samples drawn from 47 lizards (25 males and 22 females) captured at two locations on San Cristóbal Island. A portable blood analyser (iSTAT) was used to obtain near-immediate field results for total CO 2 , lactate, sodium, potassium, ionized calcium, glucose and haemoglobin. Standard laboratory haematology techniques were employed for differential white blood cell counts and haematocrit determination. Body temperature, heart rate and body measurements were also recorded. We found significant differences in haematocrit values between males and females. The values reported in this study provide baseline data that may be useful in detecting changes in health status among lava lizards affected by natural disturbances or anthropogenic threats. Our findings might also be helpful in future efforts to demonstrate associations between specific biochemical or haematological parameters and disease. Because there are several related species on different islands in the Galápagos archipelago, comparisons between populations and species will be of interest. Lay Summary:Haematology and biochemistry values of the San Cristóbal lava lizard Microlophus bivittatus , along with several other health parameters (morphometrics and temperature), are reported for the first time.

The ability to perceive the Earth's magnetic field, or magnetoreception, exists in numerous animals. Although the mechanism underlying magnetoreception has not been clearly established in any species, in salmonid fish, it is hypothesized to occur by means of crystals of magnetite associated with nervous tissue such as the brain, olfactory organ or retina. In this study, rainbow trout ( Oncorhynchus mykiss ) were exposed to a brief magnetic pulse known to disrupt magnetic orientation behaviour in several animals. Changes in gene expression induced by the pulse were then examined in the retina. Analyses indicated that the pulse elicited differential expression of only a single gene, gamma-crystallin M3-like ( crygm3 ). The near absence of an effect of the magnetic pulse on gene expression in the retina stands in sharp contrast to a recent study in which 181 genes were differentially expressed in brain tissue of O. mykiss after exposure to the same pulse. Overall, our results suggest either that magnetite-based magnetoreceptors in trout are not located in the retina, or else that they are unaffected by magnetic pulses that can disrupt magnetic orientation behaviour in animals., (© 2018 The Author(s).)

The canonical drivers of population genetic structure, or spatial genetic variation, are isolation by distance and isolation by environment. Isolation by distance predicts that neighboring populations will be genetically similar and geographically distant populations will be genetically distinct [1]. Numerous examples also exist of isolation by environment, a phenomenon in which populations that inhabit similar environments (e.g., same elevation, temperature, or vegetation) are genetically similar even if they are distant, whereas populations that inhabit different environments are genetically distinct even when geographically close [2-4]. These dual models provide a widely accepted conceptual framework for understanding population structure [5-8]. Here, we present evidence for an additional, novel process that we call isolation by navigation, in which the navigational mechanism used by a long-distance migrant influences population structure independently of isolation by either distance or environment. Specifically, we investigated the population structure of loggerhead sea turtles (Caretta caretta) [9], which return to nest on their natal beaches by seeking out unique magnetic signatures along the coast-a behavior known as geomagnetic imprinting [10-12]. Results reveal that spatial variation in Earth's magnetic field strongly predicts genetic differentiation between nesting beaches, even when environmental similarities and geographic proximity are taken into account. The findings provide genetic corroboration of geomagnetic imprinting [10, 13]. Moreover, they provide strong evidence that geomagnetic imprinting and magnetic navigation help shape the population structure of sea turtles and perhaps numerous other long-distance migrants that return to their natal areas to reproduce [13-17]., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

On a global scale, the geomagnetic field varies predictably across the Earth's surface, providing animals that migrate long distances with a reliable source of directional and positional information that can be used to guide their movements. In some locations, however, magnetic minerals in the Earth's crust generate an additional field that enhances or diminishes the overall field, resulting in unusually steep gradients of field intensity within a limited area. How animals respond to such magnetic anomalies is unclear. The Caribbean spiny lobster, Panulirus argus , is a benthic marine invertebrate that possesses a magnetic sense and is likely to encounter magnetic anomalies during migratory movements and homing. As a first step toward investigating whether such anomalies affect the behavior of lobsters, a two-choice preference experiment was conducted in which lobsters were allowed to select one of two artificial dens, one beneath a neodymium magnet and the other beneath a non-magnetic weight of similar size and mass (control). Significantly more lobsters selected the control den, demonstrating avoidance of the magnetic anomaly. In addition, lobster size was found to be a significant predictor of den choice: lobsters that selected the anomaly den were significantly smaller as a group than those that chose the control den. Taken together, these findings provide additional evidence for magnetoreception in spiny lobsters, raise the possibility of an ontogenetic shift in how lobsters respond to magnetic fields, and suggest that magnetic anomalies might influence lobster movement in the natural environment., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2018. Published by The Company of Biologists Ltd.)

Organisms use a variety of environmental cues to orient their movements in three-dimensional space. Here, we show that the upward movement of young Chinook salmon ( Oncorhynchus tshawytscha ) emerging from gravel nests is influenced by the geomagnetic field. Fish in the ambient geomagnetic field travelled farther upwards through substrate than did fish tested in a field with the vertical component inverted. This suggests that the magnetic field is one of several factors that influences emergence from the gravel, possibly by serving as an orientation cue that helps fish determine which way is up. Moreover, our work indicates that the Oncorhynchus species are sensitive to the magnetic field throughout their life cycles, and that it guides their movements across a range of spatial scales and habitats., (© 2018 The Author(s).)

Although sea turtles face significant pressure from human activities, some populations are recovering due to conservation programs, bans on the trade of turtle products, and reductions in bycatch. While these trends are encouraging, the status of many populations remains unknown and scientific monitoring is needed to inform conservation and management decisions. To address these gaps, this study presents methods for using unmanned aerial systems (UAS) to conduct population assessments. Using a fixed-wing UAS and a modified strip-transect method, we conducted aerial surveys along a three-kilometer track line at Ostional, Costa Rica during a mass-nesting event of olive ridley turtles (Lepidochelys olivacea). We visually assessed images collected during six transects for sea turtle presence, resulting in 682 certain detections. A cumulative total of 1091 certain and probable turtles were detected in the collected imagery. Using these data, we calculate estimates of sea turtle density (km -2 ) in nearshore waters. After adjusting for both availability and perception biases, we developed a low-end estimate of 1299 ± 458 and a high-end estimate of 2086 ± 803 turtles per km -2 . This pilot study illustrates how UAS can be used to conduct robust, safe, and cost-effective population assessments of sea turtle populations in coastal marine ecosystems.

Diverse taxa use Earth's magnetic field to aid both short- and long-distance navigation. Study of these behaviors has led to a variety of postulated sensory and processing mechanisms that remain unconfirmed. Although several models have been proposed to explain and understand these mechanisms' underpinnings, they have not necessarily connected a putative sensory signal to the nervous system. Using mathematical software simulation, hardware testing and the computational neuroscience tool of dynamic neural fields, the present work implements a previously developed conceptual model for processing magnetite-based magnetosensory data. Results show that the conceptual model, originally constructed to stimulate thought and generate insights into future physiological experiments, may provide a valid approach to encoding magnetic field information. Specifically, magnetoreceptors that are each individually capable of sensing directional information can, as a population, encode magnetic intensity and direction. The findings hold promise both as a biological magnetoreception concept and for generating engineering innovations in sensing and processing.

The hawksbill turtle, Eretmochelys imbricata , is a marine chelonian with a circum-global distribution, but the species is critically endangered and has nearly vanished from the eastern Pacific. Although reference blood parameter intervals have been published for many chelonian species and populations, including nesting Atlantic hawksbills, no such baseline biochemical and blood gas values have been reported for wild Pacific hawksbill turtles. Blood samples were drawn from eight hawksbill turtles captured in near shore foraging locations within the Galápagos archipelago over a period of four sequential years; three of these turtles were recaptured and sampled on multiple occasions. Of the eight sea turtles sampled, five were immature and of unknown sex, and the other three were females. A portable blood analyzer was used to obtain near immediate field results for a suite of blood gas and chemistry parameters. Values affected by temperature were corrected in two ways: (i) with standard formulas and (ii) with auto-corrections made by the portable analyzer. A bench top blood chemistry analyzer was used to measure a series of biochemistry parameters from plasma. Standard laboratory haematology techniques were employed for red and white blood cell counts and to determine haematocrit manually, which was compared to the haematocrit values generated by the portable analyzer. The values reported in this study provide reference data that may be useful in comparisons among populations and in detecting changes in health status among Galápagos sea turtles. The findings might also be helpful in future efforts to demonstrate associations between specific biochemical parameters and disease or environmental disasters.

Diverse animals use Earth's magnetic field in orientation and navigation, but little is known about the molecular mechanisms that underlie magnetoreception. Recent studies have focused on two possibilities: (i) magnetite-based receptors; and (ii) biochemical reactions involving radical pairs. We used RNA sequencing to examine gene expression in the brain of rainbow trout ( Oncorhynchus mykiss ) after exposure to a magnetic pulse known to disrupt magnetic orientation behaviour. We identified 181 differentially expressed genes, including increased expression of six copies of the frim gene, which encodes a subunit of the universal iron-binding and trafficking protein ferritin. Functions linked to the oxidative effects of free iron (e.g. oxidoreductase activity, transition metal ion binding, mitochondrial oxidative phosphorylation) were also affected. These results are consistent with the hypothesis that a magnetic pulse alters or damages magnetite-based receptors and/or other iron-containing structures, which are subsequently repaired or replaced through processes involving ferritin. Additionally, some genes that function in the development and repair of photoreceptive structures (e.g. crggm3 , purp , prl , gcip , crabp1 and pax6 ) were also differentially expressed, raising the possibility that a magnetic pulse might affect structures and processes unrelated to magnetite-based magnetoreceptors., (© 2017 The Author(s).)

The Caribbean spiny lobster, Panulirus argus, is a migratory crustacean that uses Earth's magnetic field as a navigational cue, but how these lobsters detect magnetic fields is not known. Magnetic material thought to be magnetite has previously been detected in spiny lobsters, but its role in magnetoreception, if any, remains unclear. As a first step toward investigating whether lobsters might have magnetite-based magnetoreceptors, we subjected lobsters to strong, pulsed magnetic fields capable of reversing the magnetic dipole moment of biogenic magnetite crystals. Lobsters were subjected to a single pulse directed from posterior to anterior and either: (1) parallel to the horizontal component of the geomagnetic field (i.e. toward magnetic north); or (2) antiparallel to the horizontal field (i.e. toward magnetic south). An additional control group was handled but not subjected to a magnetic pulse. After treatment, each lobster was tethered in a water-filled arena located within 200 m of the capture location and allowed to walk in any direction. Control lobsters walked in seemingly random directions and were not significantly oriented as a group. In contrast, the two groups exposed to pulsed fields were significantly oriented in approximately opposite directions. Lobsters subjected to a magnetic pulse applied parallel to the geomagnetic horizontal component walked westward; those subjected to a pulse directed antiparallel to the geomagnetic horizontal component oriented approximately northeast. The finding that a magnetic pulse alters subsequent orientation behavior is consistent with the hypothesis that magnetoreception in spiny lobsters is based at least partly on magnetite-based magnetoreceptors., (© 2016. Published by The Company of Biologists Ltd.)

Sea turtles are capable of navigating across large expanses of ocean to arrive at remote islands for nesting, but how they do so has remained enigmatic. An interesting example involves green turtles (Chelonia mydas) that nest on Ascension Island, a tiny land mass located approximately 2000 km from the turtles' foraging grounds along the coast of Brazil. Sensory cues that turtles are known to detect, and which might hypothetically be used to help locate Ascension Island, include the geomagnetic field, airborne odorants, and waterborne odorants. One possibility is that turtles use magnetic cues to arrive in the vicinity of the island, then use chemical cues to pinpoint its location. As a first step toward investigating this hypothesis, we used oceanic, atmospheric, and geomagnetic models to assess whether magnetic and chemical cues might plausibly be used by turtles to locate Ascension Island. Results suggest that waterborne and airborne odorants alone are insufficient to guide turtles from Brazil to Ascension, but might permit localization of the island once turtles arrive in its vicinity. By contrast, magnetic cues might lead turtles into the vicinity of the island, but would not typically permit its localization because the field shifts gradually over time. Simulations reveal, however, that the sequential use of magnetic and chemical cues can potentially provide a robust navigational strategy for locating Ascension Island. Specifically, one strategy that appears viable is following a magnetic isoline into the vicinity of Ascension Island until an odor plume emanating from the island is encountered, after which turtles might either: (1) initiate a search strategy; or (2) follow the plume to its island source. These findings are consistent with the hypothesis that sea turtles, and perhaps other marine animals, use a multi-modal navigational strategy for locating remote islands.

The marine iguana, Amblyrhynchus cristatus, is an iconic lizard endemic to the Galápagos Islands of Ecuador, but surprisingly little information exists on baseline health parameters for this species. We analysed blood samples drawn from 35 marine iguanas captured at three locations on San Cristóbal Island. A portable blood analyser (iSTAT) was used to obtain near-immediate field results for pH, lactate, partial pressure of O2, partial pressure of CO2, bicarbonate (HCO3 (-)), percentage O2 saturation, haematocrit, haemoglobin, sodium, potassium, ionized calcium and glucose. Parameter values affected by temperature were auto-corrected by the iSTAT. Standard laboratory haematology techniques were employed for differential white blood cell counts and haematocrit determination; resulting values were also compared with the haematocrit values generated by the iSTAT. Body temperature, heart rate, respiratory rate and body measurements were also recorded. Body length was positively correlated with several blood chemistry values (HCO3 (-) and glucose) and two haematology parameters (haemoglobin and manually determined haematocrit). A notable finding was the unusually high blood sodium level; the mean value of 178 mg/dl is among the highest known for any reptile. This value is likely to be a conservative estimate because some samples exceeded the maximal value the iSTAT can detect. For haematocrit determination, the iSTAT blood analyser yielded results significantly lower than those obtained with high-speed centrifugation. The values reported in this study provide baseline data that may be useful in comparisons among populations and in detecting changes in health status among marine iguanas affected by natural disturbances or anthropogenic threats. The findings might also be helpful in future efforts to demonstrate associations between specific biochemical parameters and disease.

During long-distance migrations, animals navigate using a variety of sensory cues, mechanisms and strategies. Although guidance mechanisms are usually studied under controlled laboratory conditions, such methods seldom allow for navigation behavior to be examined in an environmental context. Similarly, although realistic environmental models are often used to investigate the ecological implications of animal movement, explicit consideration of navigation mechanisms in such models is rare. Here, we used an interdisciplinary approach in which we first conducted lab-based experiments to determine how hatchling loggerhead sea turtles (Caretta caretta) respond to magnetic fields that exist at five widely separated locations along their migratory route, and then studied the consequences of the observed behavior by simulating it within an ocean circulation model. Magnetic fields associated with two geographic regions that pose risks to young turtles (due to cold wintertime temperatures or potential displacement from the migratory route) elicited oriented swimming, whereas fields from three locations where surface currents and temperature pose no such risk did not. Additionally, at locations with fields that elicited oriented swimming, simulations indicate that the observed behavior greatly increases the likelihood of turtles advancing along the migratory pathway. Our findings suggest that the magnetic navigation behavior of sea turtles is intimately tied to their oceanic ecology and is shaped by a complex interplay between ocean circulation and geomagnetic dynamics., (© 2015. Published by The Company of Biologists Ltd.)

Natal homing is a pattern of behavior in which animals migrate away from their geographic area of origin and then return to reproduce in the same location where they began life [1-3]. Although diverse long-distance migrants accomplish natal homing [1-8], little is known about how they do so. The enigma is epitomized by loggerhead sea turtles (Caretta caretta), which leave their home beaches as hatchlings and migrate across entire ocean basins before returning to nest in the same coastal area where they originated [9, 10]. One hypothesis is that turtles imprint on the unique geomagnetic signature of their natal area and use this information to return [1]. Because Earth's field changes over time, geomagnetic imprinting should cause turtles to change their nesting locations as magnetic signatures drift slightly along coastlines. To investigate, we analyzed a 19-year database of loggerhead nesting sites in the largest sea turtle rookery in North America. Here we report a strong association between the spatial distribution of turtle nests and subtle changes in Earth's magnetic field. Nesting density increased significantly in coastal areas where magnetic signatures of adjacent beach locations converged over time, whereas nesting density decreased in places where magnetic signatures diverged. These findings confirm central predictions of the geomagnetic imprinting hypothesis and provide strong evidence that such imprinting plays an important role in natal homing in sea turtles. The results give credence to initial reports of geomagnetic imprinting in salmon [11, 12] and suggest that similar mechanisms might underlie long-distance natal homing in diverse animals., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Loggerhead sea turtle hatchlings (Caretta caretta) use regional magnetic fields as open-ocean navigational markers during trans-oceanic migrations. Little is known, however, about the ontogeny of this behaviour. As a first step towards investigating whether the magnetic environment in which hatchlings develop affects subsequent magnetic orientation behaviour, eggs deposited by nesting female loggerheads were permitted to develop in situ either in the natural ambient magnetic field or in a magnetic field distorted by magnets placed around the nest. In orientation experiments, hatchlings that developed in the normal ambient field oriented approximately south when exposed to a field that exists near the northern coast of Portugal, a direction consistent with their migratory route in the northeastern Atlantic. By contrast, hatchlings that developed in a distorted magnetic field had orientation indistinguishable from random when tested in the same north Portugal field. No differences existed between the two groups in orientation assays involving responses to orbital movements of waves or sea-finding, neither of which involves magnetic field perception. These findings, to our knowledge, demonstrate for the first time that the magnetic environment present during early development can influence the magnetic orientation behaviour of a neonatal migratory animal., (© 2014 The Author(s) Published by the Royal Society. All rights reserved.)

The green turtle, Chelonia mydas, is an endangered marine chelonian with a circum-global distribution. Reference blood parameter intervals have been published for some chelonian species, but baseline hematology, biochemical, and blood gas values are lacking from the Galapagos sea turtles. Analyses were done on blood samples drawn from 28 green turtles captured in two foraging locations on San Cristóbal Island (14 from each site). Of these turtles, 20 were immature and of unknown sex; the other eight were males (five mature, three immature). A portable blood analyzer (iSTAT) was used to obtain near immediate field results for pH, lactate, pO2, pCO2, HCO3-, Hct, Hb, Na, K, iCa, and Glu. Parameter values affected by temperature were corrected in two ways: (1) with standard formulas; and (2) with auto-corrections made by the iSTAT. The two methods yielded clinically equivalent results. Standard laboratory hematology techniques were employed for the red and white blood cell counts and the hematocrit determination, which was also compared to the hematocrit values generated by the iSTAT. Of all blood analytes, only lactate concentrations were positively correlated with body size. All other values showed no significant difference between the two sample locations nor were they correlated with body size or internal temperature. For hematocrit count, the iSTAT blood analyzer yielded results indistinguishable from those obtained with high-speed centrifugation. The values reported in this study provide baseline data that may be useful in comparisons among populations and in detecting changes in health status among Galapagos sea turtles. The findings might also be helpful in future efforts to demonstrate associations between specific biochemical parameters and disease.

Migratory marine animals exploit resources in different oceanic regions at different life stages, but how they navigate to specific oceanic areas is poorly understood. A particular challenge is explaining how juvenile animals with no prior migratory experience are able to locate specific oceanic feeding habitats that are hundreds or thousands of kilometers from their natal sites. Although adults reproducing in the vicinity of favorable ocean currents can facilitate transport of their offspring to these habitats, variation in ocean circulation makes passive transport unreliable, and young animals probably take an active role in controlling their migratory trajectories. Here we experimentally demonstrate that juvenile Chinook salmon (Oncorhynchus tshawytscha) respond to magnetic fields like those at the latitudinal extremes of their ocean range by orienting in directions that would, in each case, lead toward their marine feeding grounds. We further show that fish use the combination of magnetic intensity and inclination angle to assess their geographic location. The "magnetic map" of salmon appears to be inherited, as the fish had no prior migratory experience. These results, paired with findings in sea turtles, imply that magnetic maps are phylogenetically widespread and likely explain the extraordinary navigational abilities evident in many long-distance underwater migrants., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

In the final phase of their spawning migration, Pacific salmon use chemical cues to identify their home river, but how they navigate from the open ocean to the correct coastal area has remained enigmatic. To test the hypothesis that salmon imprint on the magnetic field that exists where they first enter the sea and later seek the same field upon return, we analyzed a 56-year fisheries data set on Fraser River sockeye salmon, which must detour around Vancouver Island to approach the river through either a northern or southern passageway. We found that the proportion of salmon using each route was predicted by geomagnetic field drift: the more the field at a passage entrance diverged from the field at the river mouth, the fewer fish used the passage. We also found that more fish used the northern passage in years with warmer sea surface temperature (presumably because fish were constrained to more northern latitudes). Field drift accounted for 16% of the variation in migratory route used, temperature 22%, and the interaction between these variables 28%. These results provide the first empirical evidence of geomagnetic imprinting in any species and imply that forecasting salmon movements is possible using geomagnetic models., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

During their long-distance migrations, sea turtles of several species feed on jellyfish and other invertebrates that are particularly abundant in ocean regions characterized by high productivity. An ability to distinguish productive oceanic regions from other areas, and to concentrate foraging activities in locations where prey density is highest, might therefore be adaptive. The volatile compound dimethyl sulfide (DMS) accumulates in the air above productive ocean areas such as upwelling and frontal zones. In principle, DMS might therefore serve as an indicator of high prey density for turtles. To determine whether turtles perceive DMS, juvenile loggerhead sea turtles (Caretta caretta) were placed into a water-filled arena in which DMS and other odorants could be introduced to the air above the water surface. Turtles exposed to air that had passed over a cup containing 10 nmol l(-1) DMS spent more time at the surface with their noses out of the water than control turtles, which were exposed to air that had passed over a cup containing distilled water. Odors that do not occur in the sea (cinnamon, jasmine and lemon) did not elicit increased surface time, implying that the response to DMS is unlikely to reflect a generalized response to any novel odor. The results demonstrate for the first time that sea turtles can detect DMS, an ability that might enable the identification of favorable foraging areas.

Young loggerhead sea turtles (Caretta caretta) from eastern Florida, USA, undertake a transoceanic migration in which they gradually circle the Sargasso Sea before returning to the North American coast. Loggerheads possess a 'magnetic map' in which regional magnetic fields elicit changes in swimming direction along the migratory pathway. In some geographic areas, however, ocean currents move more rapidly than young turtles can swim. Thus, the degree to which turtles can control their migratory movements has remained unclear. In this study, the movements of young turtles were simulated within a high-resolution ocean circulation model using several different behavioral scenarios, including one in which turtles drifted passively and others in which turtles swam briefly in accordance with experimentally derived data on magnetic navigation. Results revealed that small amounts of oriented swimming in response to regional magnetic fields profoundly affected migratory routes and endpoints. Turtles that engaged in directed swimming for as little as 1-3 h per day were 43-187% more likely than passive drifters to reach the Azores, a productive foraging area frequented by Florida loggerheads. They were also more likely to remain within warm-water currents favorable for growth and survival, avoid areas on the perimeter of the migratory route where predation risk and thermal conditions pose threats, and successfully return to the open-sea migratory route if carried into coastal areas. These findings imply that even weakly swimming marine animals may be able to exert strong effects on their migratory trajectories and open-sea distributions through simple navigation responses and minimal swimming.

Young loggerhead sea turtles (Caretta caretta) from eastern Florida, U.S.A., undertake a transoceanic migration in which they gradually circle the North Atlantic Ocean before returning to the North American coast. Hatchlings in the open sea are guided at least partly by a 'magnetic map' in which regional magnetic fields function as navigational markers and elicit changes in swimming direction at crucial locations along the migratory route. The magnetic map exists in turtles that have never migrated and thus appears to be inherited. Turtles derive both longitudinal and latitudinal information from the Earth's field, most likely by exploiting unique combinations of field inclination and intensity that occur in different geographic areas. Similar mechanisms may function in the migrations of diverse animals., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Young loggerhead sea turtles (Caretta caretta) from the east coast of Florida, USA, undertake a transoceanic migration around the North Atlantic Gyre, the circular current system that flows around the Sargasso Sea. Previous experiments indicated that loggerhead hatchlings, when exposed to magnetic fields replicating those that exist at five widely separated locations along the migratory pathway, responded by swimming in directions that would, in each case, help turtles remain in the gyre and advance along the migratory route. In this study, hatchlings were exposed to several additional magnetic fields that exist along or outside of the gyre's northern boundary. Hatchlings responded to fields that exist within the gyre currents by swimming in directions consistent with their migratory route at each location, whereas turtles exposed to a field that exists north of the gyre had an orientation that was statistically indistinguishable from random. These results are consistent with the hypothesis that loggerhead turtles entering the sea for the first time possess a navigational system in which a series of regional magnetic fields sequentially trigger orientation responses that help steer turtles along the migratory route. By contrast, hatchlings may fail to respond to fields that exist in locations beyond the turtles' normal geographic range.

Long-distance animal migrants often navigate in ways that imply an awareness of both latitude and longitude. Although several species are known to use magnetic cues as a surrogate for latitude, it is not known how any animal perceives longitude. Magnetic parameters appear to be unpromising as longitudinal markers because they typically vary more in a north-south rather than an east-west direction. Here we report, however, that hatchling loggerhead sea turtles (Caretta caretta) from Florida, USA, when exposed to magnetic fields that exist at two locations with the same latitude but on opposite sides of the Atlantic Ocean, responded by swimming in different directions that would, in each case, help them advance along their circular migratory route. The results demonstrate for the first time that longitude can be encoded into the magnetic positioning system of a migratory animal. Because turtles also assess north-south position magnetically, the findings imply that loggerheads have a navigational system that exploits the Earth's magnetic field as a kind of bicoordinate magnetic map from which both longitudinal and latitudinal information can be extracted., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Sea turtles undertake long migrations in the open ocean, during which they rely at least partly on magnetic cues for navigation. In principle, sensitivity to polarized light might be an additional sensory capability that aids navigation. Furthermore, polarization sensitivity has been linked to ultraviolet (UV) light perception which is present in sea turtles. Here, we tested the ability of hatchling loggerheads (Caretta caretta) to maintain a swimming direction in the presence of broad-spectrum polarized light. At the start of each trial, hatchling turtles, with their magnetic sense temporarily impaired by magnets, successfully established a steady course towards a light-emitting diode (LED) light source while the polarized light field was present. When the LED was removed, however, hatchlings failed to maintain a steady swimming direction, even though the polarized light field remained. Our results have failed to provide evidence for polarized light perception in young sea turtles and suggest that alternative cues guide the initial migration offshore.

Patterns of abundance across a species's reproductive range are influenced by ecological and environmental factors that affect the survival of offspring. For marine animals whose offspring must migrate long distances, natural selection may favour reproduction in areas near ocean currents that facilitate migratory movements. Similarly, selection may act against the use of potential reproductive areas from which offspring have difficulty emigrating. As a first step towards investigating this conceptual framework, we analysed loggerhead sea turtle (Caretta caretta) nest abundance along the southeastern US coast as a function of distance to the Gulf Stream System (GSS), the ocean current to which hatchlings in this region migrate. Results indicate that nest density increases as distance to the GSS decreases. Distance to the GSS can account for at least 90 per cent of spatial variation in regional nest density. Even at smaller spatial scales, where local beach conditions presumably exert strong effects, at least 38 per cent of the variance is explained by distance from the GSS. These findings suggest that proximity to favourable ocean currents strongly influences sea turtle nesting distributions. Similar factors may influence patterns of abundance across the reproductive ranges of diverse marine animals, such as penguins, eels, salmon and seals.

Across the geographic area that a species uses for reproduction, the density of breeding individuals is typically highest in locations where ecological factors promote reproductive success. For migratory animals, fitness depends, in part, on producing offspring that migrate successfully to habitats suitable for the next life-history stage. Thus, natural selection might favor reproduction in locations with conditions that facilitate the migration of offspring. To investigate this concept, we studied the Kemp's ridley sea turtle (Lepidochelys kempii) to determine whether coastal areas with the highest levels of nesting have particularly favorable conditions for hatchling migration. We modeled the passive drift of young Kemp's ridley turtles from seven nesting regions within the Gulf of Mexico to foraging grounds using the particle-tracking program ICHTHYOP and surface-current output from HYCOM (HYbrid Coordinate Ocean Model). Results revealed that geographic regions with conditions that facilitate successful migration to foraging grounds typically have higher abundance of nests than do regions where oceanographic conditions are less favorable and successful migration is difficult for hatchlings. Thus, our findings are consistent with the hypothesis that, for the Kemp's ridley turtle and perhaps for other migrants, patterns of abundance across the breeding range are shaped in part by conditions that promote or impede the successful migration of offspring., (© The Author 2010. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved.)

Sea turtles are known to detect chemical cues, but in contrast to most marine animals, turtles surface to breathe and thus potentially have access to olfactory cues both in air and in water. To determine whether sea turtles can detect airborne chemical cues, captive loggerhead turtles (Caretta caretta) were placed into a circular, water-filled arena in which odorants could be introduced to the air above the water surface. Air that had passed across the surface of a cup containing food elicited increased activity, diving and other behavior normally associated with feeding. By contrast, air that had passed across the surface of an identical cup containing distilled water elicited no response. Increases in activity during food odor trials occurred only after turtles surfaced to breathe and peaked in the first post-breath minute, implying that the chemical cues eliciting the responses were unlikely to have been detected while the turtles were under water. These results provide the first direct evidence that sea turtles can detect airborne odors. Under natural conditions, this sensory ability might function in foraging, navigation or both.