Morphological Evolution of Newly Metamorphosed Sea Urchins—A Phylogenetic and Functional Analysis

Newly metamorphosed juvenile sea urchins are highly variable across taxa. This contribution documents and illustrates structural, functional, and phylogenetic variation among newly metamorphosed juvenile sea urchins for 31 species from 12 ordinal or familial lineages. The classic juvenile with five primary podia, 20 interambulacral spines, and variable numbers of juvenile spines is found commonly among new metamorphs across lineages, but there are many examples, which depart from this pattern and most likely reflect adaptation to settlement habitats. At metamorphosis juveniles can have 5-25 functional podia. They can have 0-65 spines, 0 or 5 sphaeridia (balance organs). They may have zero or up to eight pedicellariae. While competent larvae that delay metamorphosis may continue to develop juvenile structures, variation across species is much greater than within species and there are strong phylogenetic and functional differences among juveniles. Heterochronic changes in expression of these structures can account for differences among taxa. Based on this sample, juvenile characters such as spines, podia, and larval pedicellariae are expressed in ways that suggest they are developmental modules whose expression can be readily changed relative to one another and to the time of metamorphosis.