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.