A synthesis of growth rates in marine epipelagic invertebrate zooplankton

Abstract

We present the most extensive study to date of globally compiled and analysed weight-specific growth rates in marine epi-pelagic invertebrate metazoan zooplankton. Using specified selection criteria, we analyse growth rates from a variety of zooplanktonic taxa, including both holo- and mero-planktonic forms, from over 110 published studies. Nine principal taxonomic groups are considered, the copepods (number of individual data points (n) = 2,528); crustaceans other than copepods (n = 253); cnidarians (n = 77); ctenophores (n = 27); chaetognaths (n = 87); pteropods (n = 8); polychaetes (n = 12); thaliaceans (n = 88); and larvaceans (n = 91). The copepods are further examined by subdividing them into broadcasters or sac-spawning species, and as nauplii (N1-N6), copepodites (C1-C5) and adults (C6). For each taxonomic group relationships between growth, temperature and body weight are examined using a variety of methods. Weight-specific growth tends to increase with increasing temperature and with decreasing body weight in the crustacean group. Growth does not relate to body weight in the case of chaetognaths and larvaceans, but does increase with temperature. In the cnidarian and ctenophore groups growth does not relate to temperature, but is negatively related to body size. For the thaliceans growth increases with both increasing body weight and temperature. In the entire broadcasting copepod data set, weight-specific growth increases with increasing temperature and decreasing body weight. In sac-spawners, growth increases with increasing temperature, and increases with decreasing body weight at temperatures below 20 degrees C, but decreases with body weight at temperatures above this. Comparison between the different taxa shows important differences and similarities. Our extensive synthesis of data generally confirms that larvaceans, pteropods, cnidarians and ctenophores have rates of weight-specific growth that are typically greater than the copepods, chaetognaths and other crustaceans of similar carbon weight. For the cnidarians, ctenophores and larvaceans growth rates are almost always greater than the general relationship describing copepod growth, and are also at the upper limits or beyond the maximum rates for copepods of a similar weight. For the pteropods, growth rates are generally greater than those of copepods, although the data set was limited to a single carnivorous species in a single study (i.e. Clione limacina). The thaliaceans have the highest growth rates for animals with body weights greater than around 1 mg C ind-1, with rates of up to 2.1 d-1 for Pegea bicaudata. Whilst the larvaceans can achieve rates of 2 d-1 in warm tropical waters (28 degrees C), and as high as > 3 d-1 for < 0.2 mg C individual-1 animals of Oikopleura diocia. These are possibly the highest rates every recorded in epi-pelagic metazoans. Reasons for the differences between taxonomic groups are discussed in relation to intrinsic and extrinsic factors and limitations. The importance of this investigation not only lies in it being the most comprehensive overview of patterns of growth to date, but because the data set highlight the gaps in measurements and current knowledge. We examine the inadequacies in the current data sets, and in the methods being used to measure growth and production. Most of the data are for animals collected from coastal and estuarine waters, and it is clear that for a fuller understanding there is an urgent need for work in the open ocean, and for investigations outside temperate regions. There is also a need to explore the role of food availability, and how food concentrations in incubations, and under food saturation, relate to those experienced in the natural environment.