Constraints on Performance and Pelagic Periods of Larvae

University Of Washington, Seattle WA

Investigators

Abstract

Constraints on Performance and Pelagic Periods of Larvae Richard R. Strathmann Many marine animals live on the sea bed as sedentary adults but swim and are carried by ocean currents as planktonic larvae. Many of these larvae feed 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 at metamorphosis. There are costs to this means of supporting early growth and development, however. The longer the period of larval growth, the more larvae are eaten by predators or carried by currents away from areas of good habitat on the sea bed. Many larvae feed by concentrating algal cells and other small particles of food This food can be scarce, and food-limited growth has been demonstrated. Maximum rates of concentrating scarce food from suspension depend on traits of larvae that differ among major groups (phyla) of marine animals. Because food is often in limiting supply, one might expect larvae in the same waters, whatever their other differences, to increase their capacity to concentrate scarce planktonic food as they increase in weight and energy requirements and in order to achieve greater growth rates as they develop. Surprisingly, some animals' larvae appear to be poorly adapted to low concentrations of food. They have a very limited capacity to concentrate scarce planktonic food, even though they grow from tiny eggs to much larger juveniles at metamorphosis. Moreover, these larval forms have remained unchanged during much of animal evolution. The proposed research tests the hypothesis that food-limited growth rates depend on highly conserved traits of larvae. The prediction is that some forms are particularly constrained to slower growth when food is scarce. Some of these larvae feed with short ciliary bands throughout their development. These include the larval forms of bryozoans, which are colonial animals that live on rocks and seaweeds, and nemerteans (ribbon worms), which are common predators on other small marine animals. The bryozoan and nemertean larvae do not seem to fit predictions based on the more extensively studied larvae of molluscs and sea urchins and thus challenge biologists' understanding of the ecology and evolution of planktonic larvae. The growth of bryozoan and nemertean larvae at limiting and satiating concentrations of food will be compared to growth of sea urchin larvae, whose maximum rates of clearing food from suspension are greater and increase more nearly in proportion to body mass. If the hypothesis is correct, then major groups of animals differ in food-limitation for their larvae, with consequences for death rates and dispersal of their larvae. Another case of an apparently ill-equipped feeding larva is the planula larva of sea anemones. These larvae lack ciliary bands entirely and are anatomically the simplest feeding larval forms, yet some of them grow substantially before metamorphosis. In other groups of animals, larvae that are this simple do not catch particulate food. Analyses of feeding capacity and growth rates of larval sea anemones will indicate the capabilities of the simplest feeding larvae. The results will indicate constraints on the origination of ciliated feeding larvae in other groups of animals.