Sea urchins of the genus Gracilechinus Fell & Pawson, 1966 from the Pacific Ocean: Morphology and evolutionary history.

Gracilechinusis a widely distributed genus of regular echinoids with a centre of species diversity in the North Atlantic (five of 10 species). Only three species of this genus are known from the Pacific Ocean:Gracilechinus multidentatus,G. euryporusandG. lucidus. The first two species were previously assigned to the genusEchinus, which differs fromGracilechinusin ambulacral tuberculation. According to our data, all three Pacific species possess ambulacral tuberculation characteristic of the genusGracilechinusand differ from each other in the structure of the valves of tridentate pedicellariae and primary spines. Assignment of these three species to the genusGracilechinusis also confirmed by phylogenetic analysis of the 657 bp fragment of the COI gene. The COI tree provides evidence for the validity of the generaGracilechinusandEchinusand agrees in general with morphological data. Among species ofGracilechinus, two lineages are distinguished on the COI tree, differing in the shape of the tridentate pedicellariae valves. One lineage comprises only the North Atlantic speciesGracilechinus alexandri, the other one includes the North Atlantic, South Atlantic and Pacific species. The genusGracilechinusapparently originated in the North Atlantic and only representatives of the second lineage have penetrated the South Atlantic and the Pacific. [ABSTRACT FROM PUBLISHER]

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