Delayed metamorphosis by larvae of benthic marine invertebrates: Does it occur? Is there a price to pay?

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Abstract
In the laboratory, the swimming larvae of benthic marine invertebrates are generally capable of prolonging their larval period beyond the time that they first become physiologically competent to metamorphose. Larvae seem to differ markedly in their ability to postpone metamorphosis, both interspecifically and intraspecifically. The proportion of this variability that is genetically controlled has not yet been determined; if under genetic control, both pre-competent and competent periods would be subject to selection, although the selective pressures and the physiological or developmental mechanisms through which such pressures might operate remain purely speculative. Limited data strongly suggest that at least a few species delay metamorphosis in the field. The frequency with which they do so, and under what conditions they do so, and for how long, are unexplored questions. Some of these issues can now be explored, with the finding that various inorganic and organic substances can trigger the metamorphosis of competent larvae in a number of species; competence, and the extent to which larvae delay their metamorphosis, may now be assessed directly for field populations and in laboratory cultures, rather than by means of unreliable morphological or behavioral criteria. Careful laboratory studies may enable greater prediction of minimum and maximum dispersal periods in the field by revealing the extent to which environmental factors, such as temperature and salinity, influence pre-competent and competent periods in different species. The extent to which postponing metamorphosis alters post-metamorphic fitness is also now subject to investigation through laboratory studies and possible field transplants.