Larger offspring associated with lower temperatures across species of Microporella, a widespread colonial invertebrate

Abstract

Among life history traits, offspring size has one of the most direct impacts on fitness, influencing growth, recruitment and survival of the individual, therefore affecting population, and ultimately macroevolutionary outcomes. Despite its ecological and evolutionary importance, little is known about how offspring size varies in lineages over macroevolutionary timescales, especially for colonial organisms. Here, we use the cheilostome bryozoan genus Microporella to investigate variation in offspring size over the history of the lineage. The genus Microporella is species-rich, cosmopolitan, has a calcified skeleton (and is hence readily preserved in the fossil record), and is a brooder. The brood structure (ovicell) reliably reflects the space occupied by a larva and hence ovicell size is a good proxy for offspring size. Using a suite of biotic and abiotic factors, we ask what best explains offspring/larval size variation in contemporary and fossil species of Microporella, and how offspring/larval size changes through the millions of years of the history of the lineage. We find that offspring size is affected by a combination of module size and water temperature (or latitude when fossil species are included), while fecundity and levels of nutrients have a weak to no effect on this life history trait. Among Microporella species, descendant species are statistically more likely to have larger offspring than their putative ancestors, with the size difference between species pairs little explained by temporal latitudinal shifts. Our results suggest that both contemporary ecological controls and historical considerations are important in understanding life history trait evolution.