Effect of body size and temperature on generation time in zooplankton

The relationships of generation time to both adult body mass and temperature are critical to our understanding of numerous aspects of zooplankton behavior, ecology and evolution, yet these relationships remain poorly understood. Here, I use a meta-analysis to define the relationship of generation time to body mass and temperature for both marine and freshwater zooplankton species (rotifers, copepods and cladocera), for temperatures from 5–20°C. I also define a relationship for the constituent parts of generation time (embryonic and postembryonic development time) as a function of egg and body mass for zooplankton across this temperature range. These relationships provide powerful tools for evaluating differences in generation time between individual zooplankton species. Introduction Generation time is the single most important determinant of the rate of population growth in species (Cole, 1954) and thus, differences in generation time affect numerous ecological processes, including intraand interspecific interactions (e.g. competition, predation). Differences in generation time (time from egg to maturity) among species are sometimes attributed to differences in body mass, as generation time has been shown to scale positively to body mass for diverse assortments of plants and animals (Yarwood, 1956; Blueweiss et al., 1978), and within taxonomic groups of endotherms [e.g. (Taylor, 1968) for birds and mammals]. However, for ectotherms, the value of body mass scaling relationships is limited because they do not account for the influence of temperature on generation time. Generation times of ectotherms are generally thought to increase with body size, but also decrease with increasing temperature for a given body size. Thus, for a scaling relationship to be useful to the study of ectotherms, it must account for the effects of both body size and temperature. The question of how generation time varies with temperature and/or body mass remains poorly understood in ectotherms. Zooplankton are an excellent group of organisms for evaluating the combined effects of body mass and temperature on generation time because they have relatively short generation times and are conducive to laboratory studies. Moreover, the manner in which generation time relates to body mass and temperature is critical to understanding the ecology, behavior and evolution of zooplankton. For example, knowledge of the relationship of generation time to body size and temperature could contribute to the understanding of zooplankton seasonal dynamics (e.g. species abundance or species interactions) because generation times of many zooplankton occur in a seasonal context. Yet, as with most ectotherms, knowledge of the effects of body mass and Journal of Plankton Research Vol.22 no.2 pp.241–251, 2000 241 © Oxford University Press 2000 temperature on generation time, or its constituent parts [embryonic development time (egg to hatch) and postembryonic development time (hatch to maturity)], remains sparse and disparate. Research has typically focused on a single group (i.e. rotifers, cladocerans or copepods), partially because based on the classic work of Allan, these traits have long been considered specific to each group (Allan, 1976). The lack of understanding of the relationship between generation time, body mass and temperature in zooplankton is reflected in the long-standing debate regarding the nature of the relationship, and in the numerous and diverse models formulated to predict generation time (Huntley and Lopez, 1992; Hirst and Sheader, 1997). Some investigators have concluded that generation time scales positively to body mass among certain groups [(Hirst and Sheader, 1997 ) for copepods], while others contend there is no such relationship (Kiorboe and Sabatini, 1995), and that generation time is uniquely a function a temperature (McLaren, 1978; Allan and Goulden, 1980). Yet, even the relationship of generation time with temperature is unclear as zooplankton growth rates have been found to be both dependent and independent of temperature for different groups of species (Hirst and Lampitt, 1998). In this study, I address the combined effects of body mass and temperature on generation time for all major zooplankton groups (rotifers, copepods and cladocera), both marine and freshwater species. I begin by establishing the relationship of generation time to body mass and temperature across a wide range of body sizes at each of four temperatures (5, 10, 15 and 20oC) from data obtained from the published literature. Next, I investigate the relationship between body mass and temperature (5, 10, 15 and 20oC) for a portion of generation time, postembryonic development time. Last, I describe the proportion of time spent in the embryonic versus postembryonic development phases of generation time as a function of relative egg size (egg mass/adult mass). Method Zooplankton generation times and postembryonic development times were obtained from published studies of individual species cultured at one or more of four constant temperatures (5, 10, 15 and 20°C) (Appendix I). In a small number of cases, development times at one or more of these four temperatures were estimated from fitted lines describing these relationships across a broader range of temperatures. Adult body mass values were obtained from synthesis papers unless these sizes were specifically noted in the studies of generation times (Appendix I). Species included in our analyses comprise a broad range of body sizes from all major zooplankton groups (cladocera: 14 spp.; cyclopoid copepods: five spp.; calanoid copepods: 17 spp.; rotifers: four spp.). In general, I consider all zooplankton species together because small sample sizes and/or a narrow range of body sizes within groups prohibit robust comparisons of generation time within or between groups. Embryonic development times and egg size data used to calculate the proportion of time spent in the embryonic versus postembryonic phase of generation time were obtained from Gillooly and Dodson (Gillooly and Dodson, 1999a). J.F.Gillooly