Search

BackgroundMahi-mahi (Coryphaena hippurus) is a commercially and ecologically important fish species that is widely distributed in tropical and subtropical waters. Biological attributes and reproductive capacities of mahi-mahi make it a tractable model for experimental studies. In this study, life development of cultured mahi-mahi from the zygote stage to adult has been described.ResultsA comprehensive developmental table has been created reporting development as primarily detailed observations of morphology. Additionally, physiological, behavioral, and molecular landmarks have been described to significantly contribute in the understanding of mahi life development.ConclusionRemarkably, despite the vast difference in adult size, many developmental landmarks of mahi map quite closely onto the development and growth of Zebrafish and other warm-water, active Teleost fishes.

Cardiovascular performance is altered by temperature in larval fishes, but how acute versus chronic temperature exposures independently affect cardiac morphology and physiology in the growing larva is poorly understood. Consequently, we investigated the influence of water temperature on cardiac plasticity in developing mahi-mahi. Morphological (e.g. standard length, heart angle) and physiological cardiac variables (e.g. heart rate fH, stroke volume, cardiac output) were recorded under two conditions by imaging: (i) under acute temperature exposure where embryos were reared at 25°C up to 128 h post-fertilization (hpf) and then acutely exposed to 25 (rearing temperature), 27 and 30°C; and (ii) at two rearing (chronic) temperatures of 26 and 30°C and performed at 32 and 56 hpf. Chronic elevated temperature improved developmental time in mahi-mahi. Heart rates were 1.2-1.4-fold higher under exposure of elevated acute temperatures across development (Q10≥2.0). Q10 for heart rate in acute exposure was 1.8-fold higher compared to chronic exposure at 56 hpf. At same stage, stroke volume was temperature independent (Q10∼1.0). However, larvae displayed higher stroke volume later in stage. Cardiac output in developing mahi-mahi is mainly dictated by chronotropic rather than inotropic modulation, is differentially affected by temperature during development and is not linked to metabolic changes.

Mahi-mahi (Coryphaena hippurus) are photopic predators that rely on vision for predator avoidance and foraging behavior. Research conducted on Mahi-mahi vision has been primarily focused on adults, with limited knowledge of the visual function and eye development in early life-stages. In this study, larval Mahi-mahi were obtained from spawning wild-caught broodstock and exposed to an optomotor response assay to assess visual function. Following behavioral assessment, histological examination was conducted on the retinal layers to determine changes in eye morphology and visual acuity early in development. There was an age-dependent increase in the ability of Mahi-mahi to exhibit an optomotor response at increasing speeds. Furthermore, this corresponded to an increase in retinal area and lens thickness between 7 and 10 day post-hatch (dph) larvae, where anatomical visual acuity (measured as the minimum separable angle) also improved. These findings help further the knowledge of visual development in early life-stage pelagic fishes. [ABSTRACT FROM AUTHOR]

Mahi-mahi Coryphaena hippurus is a promising species for aquaculture development and has been used as a model species for oil toxicology and physiology studies. This species has one of the fastest growth rates of any marine teleost and a unique reproductive biology due to its high spawning frequency and reproductive energy allocation. These characteristics lend the species to being an excellent model for understanding broodstock nutrition for other high energetic pelagic species. In this study, egg morphometrics and larval survival were tracked over a 10-week period from the initial capture of wild mahi-mahi broodstock. Larval quality from subsequent spawns collected over time was quantified using larval survival activity indices ( SAIs) as a metric to assess egg quality. Larval SAIs were maintained and did not significantly decrease ( p < .05) over the time course of this study. A multiple linear regression based on the elapsed time in captivity of the broodstock, egg diameter and larval SAI at 1 dph provided the most accurate prediction of larval SAI at 3 dph ( R2 = 0.996 p < .05). There were strong positive correlations with larval SAIs at 1 and 3 dph and the key nutrients: eicosapentaenoic acid (C20: 5n-3, EPA) and related fatty acid ratios, vitamin E and nearly all amino acids under investigation. This study demonstrated that larval survival was maintained over time due to the supply of these key nutrients in the broodstock diet. [ABSTRACT FROM AUTHOR]

Windows of exposure to a weathered Deepwater Horizon oil sample (slick A) were examined for early life stage mahi-mahi ( Coryphaena hippurus) to determine whether there are developmental periods of enhanced sensitivity during the course of a standard 96-h bioassay. Survival was assessed at 96 h following oil exposures ranging from 2 h to 96 h and targeting 3 general periods of development, namely the prehatch phase, the period surrounding hatch, and the posthatch phase. In addition, 3 different oil preparations were used: high- and low-energy water accommodated fractions of oil and very thin surface slicks of oil (∼1 μm). The latter 2 were used to distinguish between effects due to direct contact with the slick itself and the water underlying the slick. Considering the data from all 3 exposure regimes, it was determined that the period near or including hatch was likely the most sensitive. Furthermore, toxicity was not enhanced by direct contact with slick oil. These findings are environmentally relevant given that the concentrations of polycyclic aromatic hydrocarbons eliciting mortality from exposures during the sensitive periods of development were below or near concentrations measured during the active spill phase. Environ Toxicol Chem 2017;36:1887-1895. © 2016 SETAC [ABSTRACT FROM AUTHOR]

During the spring and summer of 2010, the Deepwater Horizon (DWH) oil well released over three million barrels of crude oil into the Gulf of Mexico. As the oil dispersed it contaminated ecosystems that support numerous Gulf species including mahi-mahi (Coryphaena hippurus). The timing of the spill, and location of the surface slick, coincided with the spawning of many species in the region, raising concerns over embryonic and larval exposure. Numerous abnormalities due to crude oil exposure have been documented in fish early life stages, including cardiotoxicity; however, knowledge of the molecular mechanisms that cause these phenotypes is still limited. Several transcriptomic studies have presented cholesterol biosynthesis as one of the top enriched pathways following PAH exposure. In this study we exposed mahi-mahi embryos to DWH oil collected from the surface slick. At exposures ranging from ∑PAH 1.69 μg/L to ∑PAH 5.99 μg/L, the resulting larvae demonstrated significant increases in farnesyl-diphosphate farnesyltransferase 1 (fdft1) and an upward trend in 3-Hydroxy-3-Methylglutaryl-CoA Reductase (hmgcr) expression, genes that encode key enzymes in the cholesterol biosynthetic pathway. In addition to the increased expression of genes in cholesterol biosynthetic pathway, a significant decrease in total cholesterol was observed in larval homogenates, at ∑PAH 8.3 μg/L. These data confirm earlier transcriptomic studies and show that oil may diminish cholesterol and adversely impact numerous cellular functions due to altered membrane stability. Unlabelled Image • Cholesterol levels are reduced with oil exposure in larval mahi-mahi. • Hmgcr and fdft1 expression in larval mahi-mahi are increased with oil exposure. • Findings corroborate transcriptomic studies which highlight the cholesterol pathway [ABSTRACT FROM AUTHOR]

Abstract Pelagic fish have long been presumed to produce buoyant embryos which float and hatch at or near surface waters. Due to their transparency and rapid development, mahi embryos are thought to be especially vulnerable to stressors occurring in surface waters, such as ultraviolet radiation (UVR) and increased temperatures. In the present study, we suggest a possibly critical mechanism of UVR avoidance by pelagic fish embryos. Specifically, we provide evidence that mahi embryos are able to sense UVR and may alter buoyancy as a means of reducing exposure to the most harmful stressors occurring in the upper layers of the water column. Further, once the UVR exposure was terminated, embryos displayed fast recovery of positive buoyancy indicating this response is rapidly dynamic and not pathological. The mechanism behind buoyancy control is not known, but evidence from the current study suggests that ammonia sequestration, as seen in multiple other fish species, is not the primary control mechanism employed by embryonic mahi. Finally, expression of antioxidant and UV repair enzymes were investigated to elucidate possible involvement in observed buoyancy changes and to explore alternative methods of repairing UVR damage. Graphical abstract Unlabelled Image Highlights • Mahi embryos display early onset of negative buoyancy when exposed to UVR. • Recovery of positive buoyancy observed once UVR exposure was terminated. • Rapid upregulation of photolyase suggests efficient UV-repair mechanism in mahi. • Ammonia sequestration/excretion unlikely mechanism of buoyancy control. [ABSTRACT FROM AUTHOR]

Key differences in the developmental process of pelagic fish embryos, in comparison to embryos of standard test fish species, present challenges to obtaining sufficient control survival needed to successfully perform traditional toxicity testing bioassays. Many of these challenges relate to the change in buoyancy, from positive to negative, of pelagic fish embryos that occurs just prior to hatch. A novel exposure system, the pelagic embryo-larval exposure chamber (PELEC), has been developed to conduct successful bioassays on the early life stages (ELSs; embryos/larvae) of pelagic fish. Using this unique recirculating upwelling system, it was possible to significantly improve control survival in pelagic fish ELS bioassays compared to commonly used static exposure methods. Results demonstrate that control performance of mahi-mahi ( Coryphaena hippurus ) embryos in the PELEC system, measured as percent survival after 96-hrs, significantly outperformed agitated static exposure and static exposure systems. Similar significant improvements in 72-hr control survival were obtained with yellowfin tuna ( Thunnus albacares ). The PELEC system was subsequently used to test the effects of photo-induced toxicity of crude oil to mahi-mahi ELSs over the course of 96-hrs. Results indicate a greater than 9-fold increase in toxicity of Deepwater Horizon ( DWH ) crude oil during co-exposure to ambient sunlight compared to filtered ambient sunlight, revealing the importance of including natural sunlight in 96-hr DWH crude oil bioassays as well as the PELEC system's potential application in ecotoxicological assessments. [ABSTRACT FROM AUTHOR]

Teleost fishes are diverse and successful, comprising almost half of all extant vertebrate species. It has been suggested that their success as a group is related, in part, to their unique O 2 transport system, which includes pH-sensitive hemoglobin, a red blood cell β-adrenergic Na+/H+ exchanger (RBC β-NHE) that protects red blood cell pH, and plasma accessible carbonic anhydrase which is absent at the gills but present in some tissues, that short-circuits the β-NHE to enhance O 2 unloading during periods of stress. However, direct support for this has only been examined in a few species of salmonids. Here, we expand the knowledge of this system to two warm-water, highly active marine percomorph fish, cobia (Rachycentron canadum) and mahi-mahi (Coryphaena hippurus). We show evidence for RBC β-NHE activity in both species, and characterize the Hb-O 2 transport system in one of those species, cobia. We found significant RBC swelling following β-adrenergic stimulation in both species, providing evidence for the presence of a rapid, active RBC β-NHE in both cobia and mahi-mahi, with a time-course similar to that of salmonids. We generated oxygen equilibrium curves (OECs) for cobia blood and determined the P 50 , Hill, and Bohr coefficients, and used these data to model the potential for enhanced O 2 unloading. We determined that there was potential for up to a 61% increase in O 2 unloading associated with RBC β-NHE short-circuiting, assuming a − 0.2 ∆pH a-v in the blood. Thus, despite phylogenetic and life history differences between cobia and the salmonids, we found few differences between their Hb-O 2 transport systems, suggesting conservation of this physiological trait across diverse teleost taxa. At the gills, under stress, catecholamines (1) stimulate β-NHE activity, extruding excess H+ from the red blood cell (2) to maintain intracellular pH (3) and hemoglobin (Hb)-O 2 binding (4). Cell increases in volume (5) due to osmosis from β-NHE activity. At the tissues, plasma-accessible carbonic anhydrase (CA) catalyzes dehydration of extruded H+ into CO 2 (6), which rapidly diffuses back into the red blood cell (7), decreasing intracellular pH (8) and increasing O 2 unloading to the tissues (9). [Display omitted] • Enhanced O 2 unloading due to RBC β-NHE short-circuiting has previously been shown in three salmonids • Cobia and mahi-mahi both possess rapidly activated red blood cell β-NHE, which is abolished by propranolol • A decrease in blood pH by 0.2 units may enhance O 2 unloading in cobia by over 60% in cobia • These data provide further support for the hypothesis that enhanced O 2 unloading may be a general trait among teleosts [ABSTRACT FROM AUTHOR]