Small planktonic marine copepods (＜ 1 mm in length) are the most abundant metazoans on Earth. Included are adults and copepodites of calanoid genera such as Paracalanus, Clausocalanus, and Acartia; cyclopoid genera such as Oithona, Oncaea, and Corycaeus; planktonic harpacticoids of the genus Microsetella; and nauplii of almost all copepod species. Despite the abundance of small copepods, they have historically been undersampled due to the use of nets with meshes ＞ 200- 333 μm. Recent studies have shown, however, that when appropriate net meshes of 100 μm or less are used, small copepods vastly exceed the abundance and sometimes the biomass of larger ones. Failure to adequately account for small copepods may cause serious underestimations of zooplankton abundance and biomass, the copepod grazing impact on phytoplankton primary production, zooplankton-mediated fluxes of chemicals and materials, and trophic interactions in the sea. The feeding ecology of small copepods is less well-known than that of adults of larger copepod species, such as members of the genus Calanus. Further, most feeding information for small copepods is for coastal genera such as Acartia, rather than for offshore taxa. Although it is generally assumed that small copepods, including nauplii, feed primarily upon small-sized phytoplankton cells, most such information comes from rearing or feeding studies on limited laboratory diets. There have been few examinations of actual copepod feeding on mixed diets of natural phytoplankton and microzooplankton found in the sea, but some of those have produced surprises. For instance, some species of Oithona and Paracalanus and even nauplii of Arctic Calanus spp. may feed primarily as predators upon heterotrophic protists, rather than as grazers of phytoplankton. Also, nauplii of various tropical copepod species have been shown to feed upon bacterioplankton. Thus, numerous basic questions remain as to the feeding ecology and grazing/predation impact of small copepods in the sea. Despite limited knowledge of what small copepods eat, it is clear that many higher-trophic-level consumers eat them. Numerous studies have shown that copepod nauplii, Oithona spp., and other small copepods are important prey of fish larvae and other planktivores. Small copepods exhibit a variety of reproductive strategies to compensate for losses to their populations due to predation. These include having high fecundity and growth rates, when not limited by insufficient food; having high reproduction and growth rates at warmer temperatures; having limited motion and low respiration rates, allowing the investment of more energy in reproduction; and having extended longevity to maximize lifetime reproductive output. Thus, small copepods are important links in marine food webs, serving as major grazers of phytoplankton, as components of the microbial loop, and as prey for ichthyoplankton and other larger pelagic carnivores. Our present inadequate understanding of the true abundance, biomass, trophic ecology, and role of small copepods in biogenic fluxes precludes proper understanding of the ecology of the sea.

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The Importance of Small Planktonic Copepods and Their Roles in Pelagic Marine Food Webs

Models of particle encounter efficiency borrowed from aerosol fil- tration theory have revolutionised ideas about mechanisms of suspension-feeding (a form of hydrosol filtration), although they continue to be under-utilised in generating predictions concerning feeding ecology and morphological evolution and in guiding quantitative experimentation . As a complementary tool we model encounter rate, demonstrated with parameterisations ofthe same particle-encounter mechanisms . The rate models include specific encounter geometry, absent from the prevalent efficiency indices, which is used to refine estimates of the absolute and relative effectiveness of the various encounter mechanisms . Using rate (as opposed to effi- ciency and in accord with optimal foraging theory) as the dependent variable yields very different predictions for optimisation schemes . For example, a larger collector radius reduces encounter efficiency for most mechanisms, but it either increases or leaves unaffected encounter rate . Similarly, an increase in advection velocity often reduces encounter efficiency, but it increases encounter rate for most mechanisms

Physical mechanisms and rates of particle capture by suspension-feeders

The copepod community observed during an 18-month period at the mouth of eutrophic Kingston Harbour, Jamaica, was dominated by small species of Parvocalanus, Temora, Oithona, and Corycaeus. Mean copepod biomass was 22.1 mg AFDW m−3 (331 mg m−2). Annual production was 1679 kJ m−2, partitioned as 174 kJ m−2 naupliar, 936 kJ m−2 copepodite, 475 kJ m−2 egg and 93 kJ m−2 exuvial production. All nauplii, most copepodites and many adults, equivalent to half of the biomass and production, were missed by a standard 200-μm plankton net, emphasizing the importance of nauplii and small species in secondary production estimates. The evidence suggests that growth rates and production are generally not food limited, and we speculate that size-selective predation shapes the structure of the harbour community. Biomass and production are higher than previous estimates for tropical coastal waters, but comparable to other eutrophic tropical embayments and many productive temperate ecosystems. Far from being regions of low productivity, tropical zooplankton communities may have significant production and deserve greater research attention than they currently receive.

Production of tropical copepods in Kingston Harbour, Jamaica : the importance of small species

Three features that are unique or almost unique to pelagic copepods among zoo-plankton may account for their numerical dominance in the pelagic realm of the oceans: (i) the torpedo-shaped body, the sensory armed antennules and the 'gearing' of the muscle motor make pelagic copepods very efficient in detecting and escaping predators. The specific force production and velocities during escape jumps are both about an order of magnitude higher than for other similarly sized organisms. (ii) The capability to remotely detect prey and to either capture these as they arrive in a scanning current, or as they swim through the perceptive sphere of the copepod, releases the copepods from the penalties associated filtering sticky water for prey particles. Scanning currents and active attack jumps are found in only few other zooplankters. (iii) Efficient mate finding in a mate-dilute environment allows for sexual reproduction with actual mating in each generation, otherwise uncommon in the plankton, and yields advantages in terms of deleting bad mutations and promoting 'good genes' through sexual selection.

https://hal.archives-ouvertes.fr/hal-00653068/document

What makes pelagic copepods so successful

Velocity differences drive all encounter processes Therefore, knowledge of both prey and predator motility are essential in order to understand feeding behavior and predict food acquisi- tion rates Here, we describe and quantify the motility behavior of young and old naupliar stages of 6 copepods (Centropages typicus, Calanus helgolandicus, Temora longicornis, Acartia tonsa, Eury- temora affinis and Euterpina acutifrons) Behaviors of individual nauplii were divided into sequences of sinking, swimming and jumping events Motility behavior is both stage- and species-specific in terms of appearance of tracks, speeds, durations and frequencies of events as well as time budgets Motility mode often changes drastically during naupliar ontogeny Crudely, nauplii can be divided into those moving with a jump-sink type of motility of various frequencies (1 min -1 to 3 s -1 ) and those swimming with a smoother glide of varying continuity We apply observed time budgets and behav- ior-specific speeds in simple models to examine mechanisms of food encounter The motility of all nauplii may account for clearance rates reported in the literature, but through different mechanisms Smoothly swimming nauplii encounter food by scavenging, which also allows for some food capture inefficiency Jump-sink types rely on motile food We demonstrate how diffusivity of motile prey may account for observed clearance rates for jump-sink types, which seem to employ 2 strategies In- frequent jumping as observed in many cyclopoids is sufficient to prevent prey diffusion from reach- ing steady state rates, which are too low to account for observed clearance rates reported in the liter- ature Frequent jumpers (eg Acartia tonsa) jump at a frequency close to the optimum required to maximize the use of prey motility and to avoid local food depletion

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Motility of copepod nauplii and implications for food encounter

The goals of this and following studies were to describe how nauplii of related calanoid copepods gather and ingest phytoplankton cells, and to compare their feeding behavior with that of copepodids and adult females of the same species. Nauplii of the calanoids Eucalanus pileatus and E. crassus draw particles towards themselves creating a feeding current. They actively capture diatoms > 10 pm width with oriented movements of their second antennae and mandibles. The cells are displaced toward the median posterior of the mouth and then are moved anteriorly for ingestion. The nauplii gather, actively capture, and ingest particles using 2 pairs of appendages, whereas copepodids and females use at least 4 of their 5 pairs of appendages (second antennae, maxillipeds, first and second maxillae) to accomplish the same task. These nauplii are not able to passively capture small cells efficiently like copepodids and females because they lack a fixture similar to the second maxillae. Gathering and ingestion by late nauplii of E. crassus and E. pileatus require together an average of 183 ms for a cell of Thalassiosira weissflogii ( l 2 pm width) and 1.17 S fox Rhizosolenia alata (150 to 500 pm length). Although na.uplii of related species show little difference in appendage morphology, they differ markedly in feeding and swimming behavior. Their behavior is partly reflected in the behavior of copepodids and adult females.

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G. A. Paffenhöfer

Papers

Feeding behavior of nauplii of the genus Eucalanus (Copepoda, Calanoida)