Larger larvae from the colonial bryozoan species Bugula neritina had higher survival and growth relative to smaller larvae, but when amongst siblings, smaller larvae were positively advantaged and grew as large (or even larger) than their bigger counterparts.

There may be an adaptive explanation for these findings. Larger mothers may produce larger offspring to facilitate their dispersal to habitats where they perform best (that is, in isolation from siblings) and smaller mothers may produce smaller offspring that disperse less and therefore will end up in habitats with siblings where they perform best.

The results from this study by Hayley Cameron and colleagues from the Centre for Geometric Biology and the National Oceanic and Atmospheric Administration in the USA, contrast with classical theories that predict that larger offspring are produced by larger, more fecund, mothers to offset the competitive effects of more siblings.

Identifying the mechanisms for these underlying correlations between maternal size and offspring size has important ecological implications. Average body size of individuals has been reduced in many systems, and this research suggests – for some species at least – the smaller offspring that will result from smaller mothers may still be able to perform well in the maternal habitats, but that dispersal to new habitats may be constrained.

This is one of the few field studies testing these theories. Hayley and her colleagues were able to experimentally manipulate both sibling density and offspring size of the arborescent bryozoan, Bugula neritina, and monitor the survival and growth of different-sized individual larvae following deployment in the field. Further studies will be needed to increase our understanding of why offspring size co-varies with maternal size across a range of taxa.

This research was published in the journal Ecology.