Comparative morphometrics of echinoderm larvae. II. Larval size, shape, growth, and the scaling of feeding and metabolism in echinoplutei

Abstract

The relationships among egg size, larval growth and form, and the functional consequences of larval form, (e.g. the changing relationship between ciliated band length and metabolic activity) were compared among four co-occurring echinoids, Strongylocentrotus droebachiensis (O. F. Müller), S. franciscanus (A. Agassiz), S. purpuratus (Stimpson), and Dendraster excentricus (Eschscholtz). Development time from fertilization to the formation of the echinus rudiment, just prior to metamorphic competence, was inversely related to egg volume. Egg sizes spanned a 6-fold range in volume, corresponding to a doubling of developmental duration. Species with smaller eggs should have greater fecundity per unit reproductive effort. However, the larval stages should suffer greater planktonic predation as a result of slower development. In general, the initial differences in larval size and shape that are related to egg size do not persist throughout development. The quantitative importance of shape change in increasing the size of feeding structures relative to support structures is uniformly high (80–85% of total change is allometric) in these species. During the period of arm formation (2-armed stage to 8-armed stage), ciliated band length scaled with strong positive allometry with respect to body length. The allometric exponents ranged from 2.7 to 3.4 and were positively related to egg size. Species with larger eggs have greater inherent capability for increasing the size of feeding structures relative to support structures during growth. Likewise, the sealing of feeding capability with respect to metabolic activity was positively related to egg size, with the allometric exponents ranging from 0.8 to 1.5. An important consequence of increased egg size is the increased ability to seale feeding structures positively with respect to the geometric and energetic size of the larva.