Motility of copepod nauplii and implications for food encounter

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 acquisition 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, Eurytemora affinis and Euterpina acutifrons). Behaviors of individual nauplii were divided into sequences of sinking, swimming and jumping events. Motility behavior is both stageand 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 behavior-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. Infrequent jumping as observed in many cyclopoids is sufficient to prevent prey diffusion from reaching steady state rates, which are too low to account for observed clearance rates reported in the literature. Frequent jumpers (e.g. Acartia tonsa) jump at a frequency close to the optimum required to maximize the use of prey motility and to avoid local food depletion.