Resource allocation between somatic growth and reproductive output in the pelagic chordate Oikopleura dioica allows opportunistic response to nutritional variation

Abstract

Pelagic urochordate appendicularians are a vital component of marine zooplankton communities, second in abundance only to copepods. Found in all major ocean systems, they are capable of rapid blooms, attaining densities exceeding 53 000 ind. m -3 . We maintained a widely dis- tributed species, Oikopleura dioica, under controlled laboratory conditions, and examined some key life history parameters, including growth rates, fecundity, and generation time, in response to vary- ing temperature and food regimes. The results allowed us to divide the life cycle into 4 distinct phases, during which somatic growth of the animal appeared highly programmed as a function of temperature, but non-responsive to food concentrations exceeding a minimum level necessary for survival. All resources above this level were directed to the reproductive organ, yielding clear differ- ences in fecundity as a function of food regime. Generation times and spawning windows were also independent of food concentration, but dependent on temperature. The generation time, which is extremely short for a complex metazoan, combined with high fecundity, yielded a relationship between animal size and maximal intrinsic rate of natural increase that considerably exceeded values recorded for other poikilothermic metazoans. Intrinsic rates of natural increase were similar to those determined for some algal species reproducing by binary fission, explaining the capacity of O. dioica to respond quickly and opportunistically to algal blooms.