Zooplankton body composition

I compiled literature on zooplankton body composition, from protozoans to gelatinous plankton, and report allometric relations and average body composition. Zooplankton segregate into gelatinous and non-gelatinous forms, with few intermediate taxa (chaetognaths, polychaetes, and pteropods). In most groups body composition is size independent. Exceptions are protozoans, chaetognaths, and pteropods, where larger individuals become increasingly watery. I speculate about the dichotomy in body composition and argue that differences in feeding mechanisms and predator avoidance strategies favor either a watery or a condensed body form, and that in the intermediate taxa the moderately elevated water content is related to buoyancy control and ambush feeding. The chemical body composition is a significant property of an organism’s body plan and hence an important organismal trait. It varies dramatically between different groups of zooplankton, mainly segregating zooplankton into gelatinous and non-gelatinous forms. The gelatinous plankton have an inflated volume and a low dry mass to body volume ratio compared to non-gelatinous forms (Vinogradov 1953) and this difference has significant implications for the functional ecology of the respective groups (Hamner et al. 1975; Acuña et al. 2011). The body composition may also vary within taxonomic groups, as documented for protozoans (Menden-Deuer and Lessard 2000) as well as for some phytoplankton (Strathmann 1967), and such size-dependent variation likewise has implications for nutrient acquisition (Thingstad et al. 2005) and other aspects of the organism’s ecology (Kiørboe 2008). Vital rates—metabolism, growth, feeding—constitute other important characteristics of an organism, and they are all related to body size. Comparisons of vital rates between and within taxa must be made using common and relevant body mass units. Carbon (C) is the primary structural component of zooplankton and hence is often chosen as the common unit, and C and nitrogen (N) are the most commonly used currencies of biogeochemical ocean models and in assessments of zooplankton biomass. Despite the need for a common unit, body masses of zooplankton are reported in a variety of units: volume, wet mass, dry mass, ash-free dry mass, C, or N. There are many reports on the body composition of zooplankton in the primary literature, including several compilations focusing on specific groups, such as jellyfish (Lucas et al. 2011), copepods (Båmstedt 1986), and protists (Menden-Deuer and Lessard 2000), but to my knowledge there are no extensive compilations that examine and compare patterns in body composition across a broader range of taxa. Also, most previous papers report bulk composition and—implicitly—assume that the relative body composition is size independent, which we know in some cases is not the case (e.g., protozoans; Menden-Deuer and Lessard 2000). Here, I compile data on zooplankton body composition, from flagellates to jellyfish and euphausiids, with two purposes. The first is to examine how the body composition trait varies as a function of size and between taxonomic groups, and particularly to see how groups segregate. Although it is well known that gelatinous plankton have a high water content compared to most other zooplankton, the questions are whether the water content is similar or different among the taxonomically and functionally very different gelatinous groups (Cnidarians, Ctenophores, Tunicates), and whether other groups segregate with respect to content of water or other constituents. Such information will allow speculation about the functionality of body composition. The other purpose of the compilation is to facilitate the comparison of vital rates between groups by providing body mass conversion factors, although such comparisons themselves are beyond the scope of this paper.