Inbreeding shapes the evolution of marine invertebrates.

Inbreeding is a potent evolutionary force shaping the distribution of genetic variation within and among populations of plants and animals. Yet, our understanding of the forces shaping the expression and evolution of nonrandom mating in general, and inbreeding in particular, remains remarkably incomplete. Most research on plant mating systems focuses on self-fertilization and its consequences for automatic selection, inbreeding depression, purging, and reproductive assurance, whereas studies of animal mating systems have often assumed that inbreeding is rare, and that natural selection favors traits that promote outbreeding. Given that many sessile and sedentary marine invertebrates and marine macroalgae share key life history features with seed plants (e.g., low mobility, modular construction, and the release of gametes into the environment), their mating systems may be similar. Here, we show that published estimates of inbreeding coefficients (FIS ) for sessile and sedentary marine organisms are similar and at least as high as noted in terrestrial seed plants. We also found that variation in FIS within invertebrates is related to the potential to self-fertilize, disperse, and choose mates. The similarity of FIS for these organismal groups suggests that inbreeding could play a larger role in the evolution of sessile and sedentary marine organisms than is currently recognized. Specifically, associations between traits of marine invertebrates and FIS suggest that inbreeding could drive evolutionary transitions between hermaphroditism and separate sexes, direct development and multiphasic life cycles, and external and internal fertilization.

Chemoattractant-Mediated Preference of Non-Self Eggs in Ciona robusta Sperm.

Self-fertilization in hermaphroditic species might or might not be advantageous based on the level of inbreeding or outbreeding depression and the opportunity to outcross. This study examined whether chemoattractants can influence selfing rates through changes in sperm swimming behavior in the hermaphroditic tunicate Ciona robusta. The first set of experiments tested sperm preference in a dichotomous choice chamber by allowing the sperm to choose between wells with no eggs and wells with eggs, while the second experiment gave sperm a choice between self eggs and non-self eggs from another C. robusta individual. We found that sperm were about 5 times more likely to be captured in wells with eggs than in empty wells (P < 0.001) and that they were about 1.6 times more likely to be captured in wells with non-self eggs than in those with self eggs (P = 0.002). Additionally, we found that although sperm were activated by water pretreated with eggs, there was no difference in sperm swimming speed and motility in water treated with pooled-egg water compared to self-egg-treated water (P = 0.636 and P = 0.854, respectively). Our results indicate that while chemoattractant identity does not affect the basic mechanics of sperm activation and thus fertilization ability, it can cause sperm to aggregate near non-self eggs in greater numbers. This may allow for sperm to fertilize non-self eggs in greater numbers when available while still retaining the ability to fertilize self eggs.

Diel variation in the sizes of larvae of Bugula neritina in field populations.

The scale of planktonic larval dispersal affects a variety of ecological and evolutionary processes. Recent work suggests that the dispersal ability of obligately lecithotrophic larvae is influenced by the amount of energy supplied to each larva: larger larvae may stay in the plankton longer and thus travel greater distances than smaller larvae. We examined a prediction of this hypothesis in the bryozoan Bugula neritina, which each morning releases brooded larvae that settle within a few hours. If larger larvae stay in the plankton longer than smaller larvae, than larger larvae should increase in frequency in the planktonic population as the day progresses. However, field surveys revealed a negative relationship between time of day and the sizes of planktonic larvae. Because these results may have been complicated by prolonged larval release, we sequestered groups of brooding colonies in field mesocosms to examine release patterns. Larvae were released over a period of 8-9 h, with smaller larvae increasing in frequency as the day progressed. We conclude that populations of larvae of B. neritina may not be homogenous in energetic content throughout the day; this must be taken into consideration when designing studies of many aspects of larval biology.