Constraints on Performance and Pelagic Periods of Larvae

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Constraints on Performance and Pelagic Periods of LarvaeRichard R. Strathmann Many marine animals live on the sea bed as sedentary adults but swim andare carried by ocean currents as planktonic larvae. Many of these larvaefeed and grow for weeks or months before they can settle on the bottom.Larval feeding supports growth from a small egg to a larger size atmetamorphosis. There are costs to this means of supporting early growth anddevelopment, however. The longer the period of larval growth, the morelarvae are eaten by predators or carried by currents away from areas of goodhabitat on the sea bed. Many larvae feed by concentrating algal cells and other small particlesof food This food can be scarce, and food-limited growth has beendemonstrated. Maximum rates of concentrating scarce food from suspensiondepend on traits of larvae that differ among major groups (phyla) of marineanimals. Because food is often in limiting supply, one might expect larvaein the same waters, whatever their other differences, to increase theircapacity to concentrate scarce planktonic food as they increase in weightand energy requirements and in order to achieve greater growth rates as theydevelop. Surprisingly, some animals' larvae appear to be poorly adapted tolow concentrations of food. They have a very limited capacity toconcentrate scarce planktonic food, even though they grow from tiny eggs tomuch larger juveniles at metamorphosis. Moreover, these larval forms haveremained unchanged during much of animal evolution. The proposed research tests the hypothesis that food-limited growthrates depend on highly conserved traits of larvae. The prediction is thatsome forms are particularly constrained to slower growth when food isscarce. Some of these larvae feed with short ciliary bands throughout theirdevelopment. These include the larval forms of bryozoans, which arecolonial animals that live on rocks and seaweeds, and nemerteans (ribbonworms), which are common predators on other small marine animals. Thebryozoan and nemertean larvae do not seem to fit predictions based on themore extensively studied larvae of molluscs and sea urchins and thuschallenge biologists' understanding of the ecology and evolution ofplanktonic larvae. The growth of bryozoan and nemertean larvae at limitingand satiating concentrations of food will be compared to growth of seaurchin larvae, whose maximum rates of clearing food from suspension aregreater and increase more nearly in proportion to body mass. If thehypothesis is correct, then major groups of animals differ infood-limitation for their larvae, with consequences for death rates anddispersal of their larvae. Another case of an apparently ill-equipped feeding larva is the planulalarva of sea anemones. These larvae lack ciliary bands entirely and areanatomically the simplest feeding larval forms, yet some of them growsubstantially before metamorphosis. In other groups of animals, larvae thatare this simple do not catch particulate food. Analyses of feeding capacityand growth rates of larval sea anemones will indicate the capabilities ofthe simplest feeding larvae. The results will indicate constraints on theorigination of ciliated feeding larvae in other groups of animals.