Size, larvae, and videotape: Causes and consequences of variation in suspension-feeding rates of echinoderm larvae

I studied mechanisms and rates of suspension feeding by larvae of echinoderms. The larvae swim and feed by the coordinated beat of cilia in a band of epithelial cells (the ciliated band). Observations of free-swimming larvae and manipulations of the ionic composition of seawater support the hypothesis that food particles are concentrated from suspension in seawater by the brief, localized reversal of the direction of beat of cilia in the region of close approach of a particle to the ciliated band. Concentration of suspended particles at the band leads to predictions about feeding rates of larvae. I tested these predictions in subsequent experiments. First, I tested a hypothesis about the evolution of egg size in free-spawning marine invertebrates with feeding larvae. The test uses experimental manipulation of egg size in a sea urchin. Small eggs developed into smaller larvae with reduced feeding rates. As a result, larvae from smaller eggs required more time to complete larval development and metamorphosed into smaller juvenile sea urchins. The result indicates that the benefits of increased fecundity are approximately balanced by the costs of slower larval feeding and growth. Second, I compared feeding rates for different larval forms that are characteristic of echinoderm classes. The pluteus larvae of sea urchins and brittle stars had lower feeding rates per length of ciliated band that the larvae of sea stars and sea cucumbers. However, growth rates of plutei were high. Though plutei are less effective suspension feeders, they may be energetically less expensive feeding devices. The general result indicates that all echinoderm larvae are not equivalent solutions to the problem of building a large juvenile from a small egg. Third, I measured feeding rates and observed the development and larval form of an unusual sea urchin, Brisaster latifrons. Brisaster spawn relatively large, yolky eggs, and the larvae are able to feed but do not require food to complete development. These rare, facultatively feeding larvae may represent evolutionary intermediate forms between the more common feeding and nonfeeding larvae. Feeding rates of Brisaster larvae are low for their size in comparison with other, obligate-feeding sea urchin larvae. The observations suggest that large eggs, independence from food, and reduced feeding capacity evolve before other kinds of developmental changes in the evolutionary transformation from feeding to nonfeeding larval development.