Adult performance: driven by environmental effects or offspring size?

When organisms grow up to reproductive maturity, they are shaped by the environment that surrounds them and the way they interact with it. Intrinsic effects shape this interaction, and are a product of an organisms’ phenotype. In early development, size is postulated to be one of the most influential intrinsic-effects to determine later performance as an adult. This tends to work in a ‘bigger is better’ manner, even though reverse patterns have been documented.

Our field site in Blairgowrie, VIC, Australia (on top of the post), where we grow sessile marine invertebrates on PVC panels suspended from pontoons.

Our goal was to determine the influence of offspring size-effects on adult performance, relative to the environment they grow up in. To do this we had two approaches: one was experimental, the other was a re-analysis of old data. For both approaches, we looked at the sessile marine invertebrate species Watersipora subtorquata (a colonial bryozoan), which is one of the critters you can see growing on the underside of boats. It is invasive, grows quickly, and is a considered pest. Some of these attributes, however, make it an ideal study organism as it is hardy and, usually, highly fecund. For our experimental study we manipulated environmental variation by lab-spawning mature colonies and transplanting their settled larvae (which have a range of different sizes) into two different environments: shallow and deeper waters.

Left hand side: One of our PVC panels on which we grow our study organism, Watersipora subtorquata, which can be seen in the right hand picture, as the circular orange colonies growing on a plate.

For our revisiting of old data, we re-analysed datasets which had recorded offspring size and various environmental parameters in the same species. Our lab has been studying offspring size effects for the last decade, from which we have a wealth of data from different experiments that were aimed to study how offspring size affects adulthood and included various environmental parameters. Using this data and the experimental results we partitioned variance in adult performance into environmental components (temporal, spatial, and treatment effects), offspring size, and residual variance. We analysed two components of adult performance: colony size and survival. Surprisingly, survival was hardly affected by either offspring size or environmental effects. However, when analysing effects on adult colony size, environmental components dwarfed offspring size-effects in importance, in both our experimental approach and re-analysis of old data.

Rolanda Lange

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