On the advantages and disadvantages of larval stages in benthic marine invertebrate life cycles

Abstract

Many benthic marine invertebrates develop by means of free-living, dispersive larval stages. The presumed advantages of such larvae include the avoidance of competition for resources with adults, temporary reduction of benthic mortality while in the plankton, decreased likelihood of inbreeding in the next generation, and increased ability to withstand local extinction. However, the direction of evolutionary change appears generally biased toward the loss of larvae in many clades, implying that larvae are somehow disadvantageous. Possible disadvantages include dispersal away from favorable habitat, mismatches between larval and juvenile physiological tolerances, greater susceptibility to environmental stresses, greater susceptibility to predation, and various costs that may be associated with metamorphosing in response to specific chemical cues and postponing metamorphosis in the absence of those cues. Understanding the forces responsible for the present distribution of larval and non-larval (aplanktonic) development among benthic marine invertebrates, and the potential influence of human activities on the direction of future evolutionary change in reproductive patterns, will require a better understanding of the following issues: the role of macro-evolutionary forces in selecting for or against dispersive larvae; the relative tolerances of encapsulated embryos and free-living larvae to salinity, pollutant, and other environmental stresses; the degree to which egg masses, egg capsules, and brood chambers protect developing embryos from environmental stresses; the relative magnitude of predation by planktonic and benthic predators on both larvae and early juveniles; the way in which larval and juvenile size affect vulnerability to predators; the extent to which encapsulation and brooding protect against predators; the amount of genetic change associated with loss of larvae from invertebrate life cycles and the time required to accomplish that change; and the degree to which continued input of larvae from other populations deters selection against dispersive larvae. The prominence of larval development in modern life cycles may reflect difficulties in losing larvae from life cycles more than selection for their retention.