Zooplankton is a morphologically and taxonomically diverse group and includes organisms that vary in size by many orders of magnitude, but they are all faced with the common problem of collecting food from a very dilute suspension. In order to maintain a viable population in the face of mortality, zooplankton in the ocean have to clear daily a volume of ambient water for prey particles that is equivalent to about 10(6) times their own body volume. While most size-specific vital rates and mortality rates decline with size, the clearance requirement is largely size-independent because food availability also declines with size. There is a limited number of solutions to the problem of concentrating dilute prey from a sticky medium: passive and active ambush feeding; feeding-current feeding, where the prey is either intercepted directly, retained on a filter, or individually perceived and extracted from the feeding current; cruise feeding; and colonization of large particles and marine snow aggregates. The basic mechanics of these food-collection mechanisms are described, and it is shown that their efficiencies are inherently different and that each of these mechanisms becomes less efficient with increasing size. Mechanisms that compensate for this decline in efficiency are described, including inflation of feeding structures and development of vision. Each feeding mode has implications beyond feeding in terms of risk of encountering predators and chance of meeting mates, and they partly target different types of prey. The main dichotomy is between (inefficient) ambush feeding on motile prey and the more efficient active feeding modes; a secondary dichotomy is between (efficient) hovering and (less efficient) cruising feeding modes. The efficiencies of the various feeding modes are traded off against feeding-mode-dependent metabolic expenses, predation risks, and mating chances. The optimality of feeding strategies, evaluated as the ratio of gain over risk, varies with the environment, and may explain both size-dependent and spatio-temporal differences in distributions of various feeding types as well as other aspects of the biology of zooplankton (mating behaviour, predator defence strategies).

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Rates of clearance, ingestion, ammonia excretion, respiration and egg production were measured in food-acclimated (0 to 1700 W C 1-l) planktonic copepods Acartia tonsa in relation to food concentration. Carbon and nitrogen budgets were constructed. Clearance peaked at a food concentration of 150 W C 1-l, and decreased at both higher and lower concentrations. Ingestion and egg production rates increased sigmoidally with food concentration approaching plateaus equivalent to 180 and 64 % body C d-l, respectively. Rates of ammonia excretion and respiration increased with algal concentration in a decelerating manner. Respiration and excretion rates of copepods fed at saturation food concentration were more than 4 times higher than those for starved individuals. The causality of the increased respiration rate in association with feeding (specific dynamic action, SDA) is discussed by considering the physiology and biochemistry of the processes that potentially contribute to SDA. The theoretical biochemical minimum costs of biosynthesis accounted for between 50 and 116 % of observed SDA, while assimilation costs equalled 18 to 28 %. Costs of feeding, digestion and excretion (-1 % of SDA), and the mechanical work required to transport food down the gut, contributed insignificantly to SDA. It is concluded that the increment in metabolic rate of feeding A. tonsa largely relates to biosynthesis ('growth') and transport, and that the efficiency of egg production in this species is near its theoretical maximum.

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Activity and autonomous motion are fundamental in living and engineering systems. This has stimulated the new field of "active matter" in recent years, which focuses on the physical aspects of propulsion mechanisms, and on motility-induced emergent collective behavior of a larger number of identical agents. The scale of agents ranges from nanomotors and microswimmers, to cells, fish, birds, and people. Inspired by biological microswimmers, various designs of autonomous synthetic nano-and micromachines have been proposed. Such machines provide the basis for multifunctional, highly responsive, intelligent (artificial) active materials, which exhibit emergent behavior and the ability to perform tasks in response to external stimuli. A major challenge for understanding and designing active matter is their inherent nonequilibrium nature due to persistent energy consumption, which invalidates equilibrium concepts such as free energy, detailed balance, and time-reversal symmetry. Unraveling, predicting, and controlling the behavior of active matter is a truly interdisciplinary endeavor at the interface of biology, chemistry, ecology, engineering, mathematics, and physics.

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, N 2 -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.

Leaking organic solutes form an elongated plume in the wake of a sinking aggregate. These solutes may both be assimilated by suspended bacteria and guide bacteria with chemokinetic swimming behavior toward the aggregate. We used modifications of previously published models of the flow and concentration fields around sinking aggregates and of chemokinetic behavior of bacteria to identify the behavior that optimizes aggregate colonization and plume utilization. The optimal solution is governed by physical constraints and is a trade off between a high sensitivity to chemical signals and a long signal integration time. For a run-and-tumble swimming behavior, the predicted tumbling frequency is between 1 and 10 s Ϫ1 , similar to that reported for marine bacteria. The predicted optimal sensitivity to chemical signals is similar to or greater than that known for Escherichia coli. The optimal behavior was used to examine the potential contribution of aggregate-generated solute plumes for water column bacterial production. Despite occupying only a small volume fraction, the plumes may provide important growth habitats for free bacteria and account for a significant proportion of water column bacterial production at typical concentrations of marine snow aggregates.