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Coevolution between predator and prey plays a central role in shaping the pelagic realm and may have significant implications for marine ecosystems and nutrient cycling dynamics. The siliceous diatom frustule is often assumed to have coevolved with the silica-lined teeth of copepods, but empirical evidence of how this relationship drives natural selection and evolution is still lacking. Here, we show that feeding on diatoms causes significant wear and tear on copepod teeth and that this leads to copepods becoming selective feeders. Teeth from copepods feeding on thick-shelled diatoms were more likely to be broken or cracked than those feeding on a dinoflagellate. When fed a large diatom, all analyzed teeth had visible wear. Our results underscore the importance of the predator-prey arms race as a driving force in planktonic evolution and diversity., Competing Interests: Competing interests statement:The authors declare no competing interest.

Unicellular flagellated protists are a key element in aquatic microbial food webs. They all use flagella to swim and to generate feeding currents to encounter prey and enhance nutrient uptake. At the same time, the beating flagella create flow disturbances that attract flow-sensing predators. Protists have highly diverse flagellar arrangements in terms of number of flagella and their position, beat pattern, and kinematics, but it is unclear how the various arrangements optimize the fundamental trade-off between resource acquisition and predation risk. Here we describe the near-cell flow fields produced by 15 species and demonstrate consistent relationships between flagellar arrangement and swimming speed and between flagellar arrangement and flow architecture, and a trade-off between resource acquisition and predation risk. The flow fields fall in categories that are qualitatively described by simple point force models that include the drag force of the moving cell body and the propulsive forces of the flagella. The trade-off between resource acquisition and predation risk varies characteristically between flow architectures: Flagellates with multiple flagella have higher predation risk relative to their clearance rate compared to species with only one active flagellum, with the exception of the highly successful dinoflagellates that have simultaneously achieved high clearance rates and stealth behavior due to a unique flagellar arrangement. Microbial communities are shaped by trade-offs and environmental constraints, and a mechanistic explanation of foraging trade-offs is a vital part of understanding the eukaryotic communities that form the basis of pelagic food webs., Competing Interests: The authors declare no competing interest.

Many planktonic suspension feeders are attached to particles or tethered by gravity when feeding. It is commonly accepted that the feeding flows of tethered suspension feeders are stronger than those of their freely swimming counterparts. However, recent flow simulations indicate the opposite, and the cause of the opposing conclusions is not clear. To explore the effect of tethering on suspension feeding, we use a low-Reynolds-number flow model. We find that it is favorable to be freely swimming instead of tethered since the resulting feeding flow past the cell body is stronger, leading to a higher clearance rate. Our result underscores the significance of the near-field flow in shaping planktonic feeding modes, and it suggests that organisms tether for reasons that are not directly fluid dynamical (e.g., to stay near surfaces where the concentration of bacterial prey is high)., Competing Interests: The authors declare no competing interest., (Copyright © 2020 the Author(s). Published by PNAS.)

Our traditional view of the interactions between marine organisms is conceptualized as food webs where species interact with one another mainly via direct consumption. However, recent research suggests that understudied non-consumptive interactions, such as behaviourally mediated indirect interactions (BMIIs), can influence marine ecosystems as much as consumptive effects. Here, we show, to our knowledge, the first experimental evidence and quantification of bottom-up BMIIs in plankton food webs. We used observational, modelling and experimental approaches to investigate how behavioural responses to resource availability influence predation mortality on grazers with different foraging strategies (ambushing versus active foraging). A three-level food chain was used: phytoplankton as resource, copepod nauplii as grazers of phytoplankton and a large copepod as a predator. Ambushers showed little change in foraging activity with resource availability, whereas active foragers decreased their foraging activity with increasing resources, which led to a decrease (24-50%) in predation mortality. Therefore, an increase in resources ('initiator') causes behavioural changes in active grazers ('transmitter'), which ultimately negatively affects predator ('receiver') consumption rates. Consequently, increase in resource abundance may result in decreasing energy transfer to higher trophic levels. These results indicate that behaviourally mediated interactions drive marine food web dynamics differently from that predicted by only density-mediated or consumptive interactions.

Statistical species distribution models (SDMs) are increasingly used to project spatial relocations of marine taxa under future climate change scenarios. However, tests of their predictive skill in the real-world are rare. Here, we use data from the Continuous Plankton Recorder program, one of the longest running and most extensive marine biological monitoring programs, to investigate the reliability of predicted plankton distributions. We apply three commonly used SDMs to 20 representative plankton species, including copepods, diatoms, and dinoflagellates, all found in the North Atlantic and adjacent seas. We fit the models to decadal subsets of the full (1958-2012) dataset, and then use them to predict both forward and backward in time, comparing the model predictions against the corresponding observations. The probability of correctly predicting presence was low, peaking at 0.5 for copepods, and model skill typically did not outperform a null model assuming distributions to be constant in time. The predicted prevalence increasingly differed from the observed prevalence for predictions with more distance in time from their training dataset. More detailed investigations based on four focal species revealed that strong spatial variations in skill exist, with the least skill at the edges of the distributions, where prevalence is lowest. Furthermore, the scores of traditional single-value model performance metrics were contrasting and some implied overoptimistic conclusions about model skill. Plankton may be particularly challenging to model, due to its short life span and the dispersive effects of constant water movements on all spatial scales, however there are few other studies against which to compare these results. We conclude that rigorous model validation, including comparison against null models, is essential to assess the robustness of projections of marine planktonic species under climate change., (© 2016 John Wiley & Sons Ltd.)

Changes in marine plankton communities driven by environmental variability impact the marine food web and global biogeochemical cycles of carbon and other elements. To predict and assess these community shifts and their consequences, ecologists are increasingly investigating how the functional traits of plankton determine their relative fitness along environmental and biological gradients. Laboratory, field and modelling studies are adopting this trait-based approach to map the biogeography of plankton traits that underlies variations in plankton communities. Here, we review progress towards understanding the regulatory roles of several key plankton functional traits, including cell size, N2 -fixation and mixotrophy among phytoplankton, and body size, ontogeny and feeding behaviour for zooplankton. The trait biogeographical approach sheds light on what structures plankton communities in the current ocean, as well as under climate change scenarios, and also allows for finer resolution of community function because community trait composition determines the rates of significant processes, including carbon export. Although understanding of trait biogeography is growing, uncertainties remain that stem, in part, from the paucity of observations describing plankton functional traits. Thus, in addition to recommending widespread adoption of the trait-based approach, we advocate for enhanced collection, standardisation and dissemination of plankton functional trait data., (© 2013 Blackwell Publishing Ltd/CNRS.)

In this paper we consider the advective/diffusive transport of a solute near a hovering zooplankter. We approximate the fluid flow with that of a Stokeslet, corresponding to the plankter exerting a point force on the water, and assume that the plankter acts as a point source for the transported solute, located at the same point as the force. We find an analytical expression in closed form for the steady-state concentration of the solute. We also discuss the situation where the plankter performs Brownian motion. Finally we apply the results to the courtship of the marine copepod Pseudocalanus elongatus, where the male performs a mating dance below the hovering female. For this situation, our model supports the hypothesis that the mating dance is guided by the plume of a signalling pheromone.

Many planktonic organisms have motility patterns with correlation run lengths (distances traversed before direction changes) of the same order as their reaction distances regarding prey, mates and predators (distances at which these organisms are remotely detected). At these scales, the relative measure of run length to reaction distance determines whether the underlying encounter is ballistic or diffusive. Since ballistic interactions are intrinsically more efficient than diffusive, we predict that organisms will display motility with long correlation run lengths compared to their reaction distances to their prey, but short compared to the reaction distances of their predators. We show motility data for planktonic organisms ranging from bacteria to copepods that support this prediction. We also present simple ballistic and diffusive motility models for estimating encounter rates, which lead to radically different predictions, and we present a simple criterion to determine which model is the more appropriate in a given case.

Bivalves are important grazers on phytoplankton in shallow waters. However, very little is known about their ability to capture actively moving zooplankton. We investigated the escape response and success of early and late nauplii of three copepod species (Acartia tonsa, Temora longicornis and Eurytemora affinis) in the flow field of a blue mussel, Mytilus edulis, using both video observations and incubation experiments. An empirical model was created to describe the spatial distribution of the fluid deformation rate. Nauplii responded with escape jumps at mean fluid deformation rates of 0.6–1.9 s-1. Escape success differed between taxa. T. longicornis was the poorest escaper, while A. tonsa and E. affinis were more efficient and similar to one another. Deformation rates differed in different parts of the flow field, which resulted in differences in escape success between the sectors. Nauplii were caught most often in the sector furthest away from the exhalent siphon, where the deformation rate was the weakest. There the nauplii were unable to detect an escape signal in time to react and flee. [ABSTRACT FROM AUTHOR]

Flocculation of phytoplankters into large, rapidly sinking aggregates has been implicated as a mechanism of vertical transport of phytoplankton to the sea floor which could have global significance. The formation rate of phytoplankton aggregates depends on the rate at which single cells collide, which is mainly physically controlled, and on the probability of adhesion upon collision (=coagulation efficiency, stickiness), which depends on physico-chemical and biological properties of the cells. We describe here an experimental method to quantify the stickiness of phytoplankton cells and demonstrate that three species of diatoms grown in the laboratory ( Phaeodactylum tricornutum, Thalassiosira pseudonana, Skeletonema costatum) are indeed significantly sticky and form aggregates upon collision. The dependency of stickiness on nutrient limitation and growth was studied in the two latter species by investigating variation in stickiness as batch cultures aged. In nutrient replete T. pseudonana cells stickiness is very low (< 5 × 10), but increases by more than two orders of magnitude as cell growth ceases and the cells become nutrient limited. Stickiness of S. costatum cells is much less variable, and even nutrient replete cells are significantly sticky. Stickiness is highest (> 10) for S. costatum cells in the transition between the exponential and the stationary growth phase. The implications for phytoplankton aggregate formation and subsequent sedimentation in the sea of these two different types of stickiness patterns are discussed. [ABSTRACT FROM AUTHOR]

We describe different modes of prey rejection in a filter-feeding appendicularian, Oikopleura dioica. Nonselective prey rejection occurs by intermittent rejection through reversal of the feeding current of all particles when the pharyngeal filter is overloaded, and by accidental loss through the spiracles of all particles that have entered the house when the pharyngeal filter occasionally breaks. In addition, selective prey rejection of individual particles may occur in the mouth by reversal of the ciliary current in the spiracle: the unwanted particle is expelled with a small amount of water. Up to four rejections of individual particles may occur per second. This active rejection is based on both the size and the chemical characteristics of the individual particle. A significantly higher rejection rate was found for toxic dinoflagelate (68%) compared to similar sized nontoxic ones (34%) & semi; exponentially growing algae were less rejected than senescent ones & semi; and nutrient-depleted algae were more rejected than nutrient-replete cells. For nontoxic particles, prey size relative to the size of the appendicularian was the main prey selection criterion and the optimal prey size (> 80% acceptance) for O. dioica is between 0.4% and 2% of their own size, whereas the minimum and maximum particle sizes that they can ingest range from 0.04% to 20% of their own size. This prey size spectrum is much broader than that found in most other planktivorous organisms and demonstrates that appendicularians are simultaneous microphageous and macrophageous planktonic grazers. [ABSTRACT FROM AUTHOR]