Body form and skeletal morphometrics during larval development of the sea urchin Lytechinus variegatus Lamarck

Development of the echinopluteus of the subtropical/tropical sea urchin Lyt echinus variegatus Lamarck, was studied using quantitative measures of size , shape, and the growth of the larval body and skeleton. Larvae of L. varie gatus developed rapidly, attaining metamorphic competence in 9-10 days (dep ending on diet) at a culture temperature of 26 degrees C. Larvae grew subst antially from the initial 2-arm pluteus stage to the fully developed eight- arm pluteus with a juvenile rudiment. Larval length increased 2.8-fold, tot al arm length (summed over all larval arm pairs) increased 7.3-fold. Ciliat ed band length (an index of feeding capability) increased 6.8-fold, 76% of the increase was allometric, i.e., due to shape change. An index of shape, the ratio of ciliated band length: body length increased 2.8-fold during de velopment to a maximum value of 13.87. Allometric growth occurred both duri ng the period of arm formation and later during rudiment formation by dispr oportionate growth of the larval arms. The echinopluteus skeleton of Lytech inus variegatus consists of six separate elements: 2 sets of paired (left a nd right) elements, the body-postoral-anterolateral rod complex and the pos terodorsal rods; and 2 unpaired elements, the dorsal arch and the posterior transverse rod. The skeleton of the main body region (body rods) did not g row after the formation of the initial 2-arm larval stage. Body length and arm length increases were due entirely to elongation of the arm skeleton. I ncreases in larval circumference and development of various lobes were supp orted by elongation of the dorsal and ventral transverse rods. Pedicellaria e (1-3) developed around day 7 at the posterior of the larva. Juvenile skel etal plates developed in association with the proximal tips of several larv al skeletal rods, such as the base of the dorsal arch and the bases of the posterodorsal rods. There are several common features of pluteus larval gro wth that are shared among species, with different egg sizes; from different orders, and from different geographic regions. The most important similari ty is the scaling of feeding structures (ciliated band length) relative to larval body size. Elaboration of the ciliated band is by growth of the larv al arms with 60-70% of the band located on the arms. However, there are dif ferences in which arm pairs contribute most to form change and feeding capa bility. These descriptions and comparisons suggest both similarities and di fferences in larval growth and form that are functionally important and des erve greater attention from larval ecologists. (C) 1999 Elsevier Science B. V. All rights reserved.