Morphometric and metabolic analysis of the growth and form of an echinopluteus

Abstract

The pluteus larva of the echinoid, Dendraster excentricus (Eschscholtz, 1829), increases in size and changes shape during development. Larval length increases from 311 to 699 μm between the two-armed stage and the formation of the echinus rudiment. Protein content increases from 130 to 600 ng, while metabolic activity increases 28-fold during this period. Larval volume increases from 1867 × l0 3μm 3 to 5506 × 10 3μxm 3 during growth from the two- to the eight-armed stage. Initially, shape changes through the formation of four pairs of larval arms. Later, body proportions are altered because of positive allometric growth of the posterodorsal and postoral arms. The changes in body form from the two- to the eight-armed stage result in positive scaling of larval surface area with respect to body volume ( b = 1.6, size independent) ( b is the allometric coefficient, Y= aX b ). The larval feeding structure (ciliated band) grows with positive allometry with respect to midline body length ( b = 2.8), overall larval length ( b = 2.1), surface area ( b = 1.3), and volume ( b = 1.4). Shape change, rather than size increase, accounts for most (83%) of the increase in feeding capability. Larval body wall tissue volume scales isometrically with metabolic demand ( b = 0.9) during the period of arm formation. Throughout development, most of the growth of the larva occurs in the arms. By the eight-armed stage the arms account for 74% of the ciliated band, 48% of the surface area, 27% of the total volume, and 79% of the body wall tissue volume. The contribution of the arms to these measures of area and volume increases continuously during larval development. In contrast, the percentage of the ciliated band on the arms increases from 41% in the two-armed pluteus to 64% in the four-armed stage and-then remains nearly constant throughout larval growth, averaging 68%. The important functional consequence of the changes in body form that characterize the pluteus growth pattern is the maintenance of feeding capability relative to larval body size (body wall tissue volume, b = 1.0) and metabolic demand ( b = 1.1). Because later larval stages can eat a wider size range of food organisms, the isometry of clearance capability and metabolism indicates that later stages should be less food limited.