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

Abstract

Development of the echinopluteus of the subtropical/tropical sea urchin Lytechinus variegatus Lamarck, was studied using quantitative measures of size, shape, and the growth of the larval body and skeleton. Larvae of L. variegatus developed rapidly, attaining metamorphic competence in 9–10 days (depending on diet) at a culture temperature of 26°C. Larvae grew substantially from the initial 2-arm pluteus stage to the fully developed eight-arm pluteus with a juvenile rudiment. Larval length increased 2.8-fold, total arm length (summed over all larval arm pairs) increased 7.3-fold. Ciliated 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 development to a maximum value of 13.87. Allometric growth occurred both during the period of arm formation and later during rudiment formation by disproportionate growth of the larval arms. The echinopluteus skeleton of Lytechinus variegatus consists of six separate elements: 2 sets of paired (left and right) elements, the body-postoral-anterolateral rod complex and the posterodorsal rods; and 2 unpaired elements, the dorsal arch and the posterior transverse rod. The skeleton of the main body region (body rods) did not grow 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. Increases in larval circumference and development of various lobes were supported by elongation of the dorsal and ventral transverse rods. Pedicellariae (1–3) developed around day 7 at the posterior of the larva. Juvenile skeletal plates developed in association with the proximal tips of several larval 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 growth that are shared among species, with different egg sizes, from different orders, and from different geographic regions. The most important similarity is the scaling of feeding structures (ciliated band length) relative to larval body size. Elaboration of the ciliated band is by growth of the larval arms with 60–70% of the band located on the arms. However, there are differences in which arm pairs contribute most to form change and feeding capability. These descriptions and comparisons suggest both similarities and differences in larval growth and form that are functionally important and deserve greater attention from larval ecologists.