Meeting Abstract
P3.13 Saturday, Jan. 5 Multiple origins of respiratory structures in sea urchins MOOI, Rich; BRESLIN, Cara Marie*; California Acad. of Sciences, San Francisco; Univ. of California, Santa Cruz rmooi@calacademy.org
The clade Irregularia consists of more or less bilaterally symmetric sea urchins such as heart urchins (Spatangoida), lamp urchins (�cassiduloids�), and sand dollars (Clypeasteroida). Many of these possess respiratory structures called petaloids on the top (aboral) surface. In the more familiar, crownward Irregularia, petaloids are generally made up of closely packed, gill-like tube feet arranged in curved columns along the ambulacra. These columns are paired so that a petal-like shape is demarcated in each aboral ambulacrum. Each tube foot in a petaloid sits on a pair of pores that pierce the calcite plates making up the test. Among extant Irregularia petaloids are easily recognized, but variation among basal forms makes recognition of petaloids more problematic. Therefore, it was necessary to develop criteria by which to map out the evolution of respiratory structures within these echinoid groups. By combining trees from various analyses, we produced a phylogeny for the Irregularia that links all the major clades, both extant and fossil. Analysis of more than 60 taxa demonstrated that petaloids evolved more than once. Critical to this discovery is the placement of extinct taxa, many of which lack petaloids, at the bases of each of the major branches in the phylogeny. Closer examination revealed anatomical differences in the respiratory pores between heart urchins and other irregulars, supporting the fact that the petaloids evolved independently. Therefore, in spite of superficial similarities, petaloids in the clade incorporating the heart urchins are convergent with those in the clade containing oligopygoids, sand dollars, and lamp urchins. This has major implications for our understanding of the evolution of respiratory structures in echinoids, as well as for the development of criteria that lead to the recognition of petaloids.
