More than meets the eye: Species limits and diversity in sea cucumber (Echinodermata: holothuroidea).

摘要

Identifying accurately species is critical for our understanding of patterns of diversity and speciation. However, for many organisms with simple and variable morphological traits, the characters traditionally used by taxonomists to identify species might lead to a considerable under appreciation of their diversity. Recent advances in molecular-data based computational methods have considerably improved our ability to identify and test species limits.;In Chapter 2, we redefine the genus based on combined morphological and molecular analyses. Phyrella unlike most dendrochirotids eviscerate posteriorly. The number of tentacles is variable (14--20), as is the degree of fragmentation of the calcareous ring, calling into question the separation of Phyllophorinae and Semperiellinae, and suggest that Semperiella and Thyonidiella are synonymous with Phyrella. We recognize five species in Phyrella. Molecular phylogenetic analysis recovers a well-supported Phyrella, but suggests that some genera and subfamilies of Phyllophoridae are not monophyletic.;In Chapter 3, we use an integrative approach to delineate species in the complex "Holothuria impatiens", a common, shallow-water species, occurring across the Indo-Pacific, the Western Atlantic and the Mediterranean Sea, and show that it is a complex of at least 13 species. Estimates of the timing of diversification showed that some of these species diverged less than 2 Ma, which is the fastest case of closely related species occurring in sympatry for a marine metazoan. This study demonstrates how clarifying species limits contribute to refining our understanding of speciation.;In Chapter 4, we present the results of a large scale barcoding effort for holothuroids. The number of cryptic species is much higher than currently recognized. The vast majority of sister species have allopatric distributions, with species showing genetic differentiation between ocean basins, and some are even differentiated among archipelagos. However, many closely related and sympatric forms, that exhibit distinct color patterns and/or ecology, show little differentiation in, and cannot be separated by, COI sequence data. This pattern suggests that echinoderms acquire reproductive isolation at a much faster pace than other marine phyla. Understanding the causes behind such patterns will refine our understanding of diversification and biodiversity in the sea.