Most marine animals have complex life cycles, with two stages, the familiar adult stage, and tiny dispersive stages (usually larvae), which disperse through the plankton. The role of these larval stages has been the subject of considerable attention from marine ecologists in the 1980s and 1990s, because populations of many species, including many commercially important ones, are limited by the recruitment of these larval stages. Recruitment of most species is also highly variable in space and time, and explaining this variability is central to explaining population fluctuations, and for resource managers to identify ways of maintaining or enhancing commercially exploited or vulnerable species. Several components of the larval period are likely to influence recruitment success, especially their dispersal during the planktonic phase, their ability to make an effective transition from larval to adult stage, and their successful establishment after this transition.
Do larval experiences in the plankton influence their success later in life?
After settling, animals must survive and grow to enter the adult population as recruits. Because the animals are still tiny, even after they settle, they suffer high mortality, so variation in their early survival might be a major contributor to variation in recruitment.
One intriguing suggestion that has emerged in the marine ecological literature in the past few years has been the suggestion that experiences of larvae in the plankton “carry over” to affect their fate later in life. This view is in contrast to widely held earlier views that because marine animals undergo such massive tissue reorganization during metamorphosis, they start life afresh after metamorphosis.
We have shown a wide range of sources of carry over effects:
- We have provided the first demonstration that exposure to pollutants during the larval stage can have serious effects that show up months later in the adults.
- We have shown that, for a range of species, the energy resources invested in each larva by its maternal parent affects its ability to swim for sustained period, its responses to settlement cues, and its growth and survival after metamorphosis.
- In collaboration with one of the originators of the carry over hypothesis, Prof. Jan Pechenik (Tufts University), we have shown that levels of larval activity also affect juvenile performance – highly active larvae burn up energy reserves, and have little left for subsequent growth.
These results are critical elements in the emerging view that settling larvae are not a pool of very similar organisms, but a heterogenous collection reflecting parental investment, exposure to stresses, and time spent in the plankton. The work leads to the interesting prediction that variation in larval “quality” may explain significant amounts of variation in recruitment of marine animals.