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

Gracilechinus is 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. euryporus and G. lucidus. The first two species were previously assigned to the genus Echinus, which differs from Gracilechinus in ambulacral tuberculation. According to our data, all three Pacific species possess ambulacral tuberculation characteristic of the genus Gracilechinus and differ from each other in the structure of the valves of tridentate pedicellariae and primary spines. Assignment of these three species to the genus Gracilechinus is also confirmed by phylogenetic analysis of the 657 bp fragment of the COI gene. The COI tree provides evidence for the validity of the genera Gracilechinus and Echinus and agrees in general with morphological data. Among species of Gracilechinus, two lineages are distinguished on the COI tree, differing in the shape of the tridentate pedicellariae valves. One lineage comprises only the North Atlantic species Gracilechinus alexandri, the other one includes the North Atlantic, South Atlantic and Pacific species. The genus Gracilechinus apparently originated in the North Atlantic and only representatives of the second lineage have penetrated the South Atlantic and the Pacific.