The Spread of Sibling Larvae of Sedentary Marine Invertebrates

Abstract

1. It is unlikely that individual selection simply for increasing distance between juveniles or between juveniles and parents would produce the patterns of reproduction and large-scale dispersal exhibited by many benthic invertebrates with pelagic larvae. However, these patterns are compatible with the hypothesis that pelagic larval phases of several weeks are the result of individual selection for spreading sibling larvae. 2. Selection for spread of siblings as a fixed feature of the life cycle depends on survival and reproduction in the benthic phase of life and in early and late larval stages varying independently from time to time and place to place. Data on benthic populations is apparently insufficient to determine whether or not such variation occurs. However, information on horizontal diffusion rates gained from dye diffusion experiments indicates the geographic scale of independent variation required by the hypothesis. 3. Rate of diffusion in the sea increases with time (or size of diffusing patch), but even with an accelerating rate of diffusion, the advantage in adding an extra day to pelagic life probably decreases as the duration of the pelagic phase increases because the added increment of spread is smaller relative to the spread already achieved. 4. Selection for spread of sibling larvae could result in multiple spawning, greater variability in vertical swimming, or variability in length of pelagic period as well as increased length of pelagic period. Selection for multiple spawning would be more intense in invertebrates with a short pelagic larval phase and should be more common in these species if selection commonly favors spread of offspring. Unfortunately, the disadvantages of increasing dispersal by any of these supplementary means cannot be assessed. Observations on large numbers of species may be required to detect predicted trends. Specialists on some groups of invertebrates may have such data but be unaware of its relation to dispersal and evolution of complex life cycles. 5. Alternative hypotheses can explain the evolution of most patterns of development and behavior discussed here. Observations on variability in survival and reproduction in the benthic phase over areas of increasing size would provide the strongest test of the hypothesis that spread of siblings is a function of long pelagic larval stages. In such studies it is particularly important to distinguish whether variability among areas results from variation in early mortality or variations in numbers of settling larvae. A minimum estimate of the short-term advantage of spreading offspring could be gained from data on the fraction of settlers surviving and age-specific fecundity, gathered over a large area for 5 or more years.