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

E Di Martino,Lh Liow

doi:10.3354/meps13656

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.

Highlights

Life history traits have direct impact on both the fitness of the individuals involved and the growth and decline of the populations to which they belong (Stearns 1992, Marshall & Keough 2008)
We focus on a lineage, using multiple species of the same genus, Microporella, to ask the following questions: (1) Does zooid size explain most of the variation in larval size in contemporary species, or do trade-offs with density of ovicells and/or environmental variation increase our ability to explain interspecific offspring/larval size variation? (2) We ask if the inferences we make for contemporary species hold for fossil species, using paleolatitude as a proxy for temperature
Models that include fecundity and/or chl a are much less preferable based on AICc (Table 1)

Summary

Introduction

Life history traits have direct impact on both the fitness of the individuals involved and the growth and decline of the populations to which they belong (Stearns 1992, Marshall & Keough 2008). Our understanding of the historical covariance of marine offspring and adult size is largely informed by work on solitary organisms, including mollusks Colonial metazoans, which represent a major component of marine global biodiversity (Jackson 1977), often do not fit well into the ecological and evolutionary models developed for unitary organisms Jackson 1985, Hiebert et al 2020), the necessity of performing quantitative studies and/or testing model predictions directly on colonial organisms. The paucity of studies on offspring size of colonial invertebrates (e.g. cnidarians, bryozoans, ascidians, and sponges) over macroevolutionary timescales is partly due to the extremely low preservation potential of marine larvae in the fossil record (Raff et al 2006), i.e. we have to rely on inferences made from extant organisms

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