Semelparity and Iteroparity

Abstract

Diversity in reproduction schedules is a central component of life history variability, with life span and age at maturity as key traits. Closely linked is the number of reproductive attempts and if organisms reproduce only once followed by death (semelparity) or spread reproduction over multiple and separated episodes during the reproductive lifespan (iteroparity). Amphipoda and Isopoda are two crustacean groups with many semelparous species, but semelparity is also part of other groups such as Decapoda, Copepoda, and Lepostraca. We briefly review theories posited for the evolution of semelparity and iteroparity, covering models on demography in both deterministic and fluctuating environments, and examine models on optimal resource allocation. We provide predictions of these theories, a guide on how to test them in crustaceans, and illustrate how theory can help us understand the diversity within this major taxon. We also point out a few shortcomings of these theories. One is that immediate recruitment is usually assumed in studies of semelparity, which is a poor assumption for the many crustaceans that form egg banks with prolonged recruitment. Another is the lack of models where iteroparity versus semelparity emerge as a consequence of life history trade-offs, rather than the more common approach that assumes demographic parameters. Furthermore, we argue that treating semelparity and iteroparity as a dichotomy is sometimes problematic and that viewing these strategies as a continuum can be useful. We discuss life history correlates and the particularly relevant links between the semelparity-iteroparity axis and capital breeding and seasonality, parental care, and terminal molts. We also discuss some of the indirect methods used to conclude if a crustacean is semelparous or not, such as a rapid drop in adult abundance after reproduction or signs of growth or storage after reproduction. A central message in the chapter is the high value of life history theory as a guide when formulating explanations and projecting evolutionary changes in reproductive lifespan of crustaceans.