Meeting Abstract
Pedicellariae are numerous, microscopic, jawed structures scattered over the test (calcite body wall skeleton) among spines of many taxa in the Echinoidea (sea urchins and relatives). A pedicellaria consists of a rod-like stem extending upwards from and articulating with a small tubercle on the test’s surface. The stem terminates in a set of 2 to 4 intricately hinged, calcite jaw pieces, or valves. There are at least 5 different types of pedicellariae, with varying functions that remain poorly understood. The order Clypeasteroida (sand dollars and sea biscuits) shows high diversity within one type of pedicellariae, the ophicephalous. Ophicephalous pedicellariae consist of 3 toothed valves connected by sophisticated hinges. Proximal to the hinges are calcite loops (“handles”) that nest one below the other and connect to the stem through a “strap” made of connective tissue. The strap/handle complex increases the gripping strength of the valves without muscular input. Ophicephalous pedicellariae can provide phylogenetic characters, but little is known about their growth, evolution, and inter-specific variation. Detailed analyses of three species of sea biscuit reveal allometric relationships among components of the pedicellariae themselves as well as the fact that pedicellariae exhibit strong negative allometry with respect to overall test size. SEM and light microscopy from more than 25 species of clypeasteroids, and sister taxa such as the echinolampadoids, reveal important size and shape differences throughout pedicellarial evolution, raising major questions concerning convergence and the conservation of genetic signal to make such complex structures.
