Speciation in four closely related species of sea urchins (genusEchinometra) with special reference to the acrosome reaction

Abstract

Sea urchins of the genus Echinometra are abundant on Okinawa reef flats in southern Japan. The Okinawan Echinometra is subdivided into four sympatric and closely related species (Ea, Eb, Ec, and Ed). To elucidate the relationships between them, we have examined the effect of egg jelly on the acrosome reaction, a process sea urchin spermatozoa undergo before fertilization. When Echinometra spermatozoa were treated with eggjelly, the acrosome reaction in each species was, as expected, effectively induced by jelly from the same species. The acrosome reaction in Ed spermatozoa was induced by egg jelly regardless of species, whereas the acrosome reaction of Eb spermatozoa took place only when it was treated with Eb egg jelly. The acrosome reaction of Ea spermatozoa was induced by jelly from species Ea or Eb, while Ec spermatozoa responded to egg jelly from Ea, Ec, or Ed. These results suggest that the separation of these four species of Okinawan Echinometra has taken place quite recently. An additional finding of our study is that the rate of sperm motility differs from species to species. In order from slowest to fastest, the species are Ed, Ec, Eb, and Ea. Motility may be related to reproductive isolation among the four species of Echinometra.

Comparative ultrastructure of spermatozoa from two regular and two irregular New Zealand echinoids

“…In contrast, in E. chloroticus lipid reserves may be located between the flagellum and the surrounding mitochondria (although their presence requires further investigation), as described for the closely related Echinometra species (Mita et al. ) and Hemicentrotus pulcherrimus (A. Agassiz 1864) (Mita & Nakamura ). Like the deep water echinothuroid Phormosoma placenta ( Thomson 1872) (Eckelbarger et al.…”

The complete mitochondrial genome of the sea urchin, Echinometra sp. EZ

“…Echinometra species present in the Indo-Pacific region are best understood as a complex consisting of four distinct species. Echinometra mathaei and E. oblonga correspond to types 'B' and 'D,' respectively (Arakaki and Uehara 1999;Mita et al 2004). The other two species are referred to as E. sp.…”

“…Echinometra mathaei and E. oblonga correspond to types ‘B’ and ‘D,’ respectively (Arakaki and Uehara 1999 ; Mita et al. 2004 ). The other two species are referred to as E .…”

Effect of temperature on interactions between eggs and spermatozoa in four closely related species of sea urchins of the genusEchinometra

“…As confirming here, sperm head shapes vary among the four species of Echinometra (Arakaki et al, 1998;Mita et al, 2002Mita et al, , 2004. The spermatozoon of Ea, Eb, and Ec each has a head that is longer and more slender than the preceeding species (Fig.…”

“…reported that these two types are incapable of cross-fertilization and that there M. Mita et al / IRD 50 (2007) 181-185 182 are differences in larval morphology and karyotype. More recently, Uehara and co-workers (Uehara and Shingaki, 1985;Uehara et al, 1986Uehara et al, , 1990Uehara and Taira, 1987;Arakaki and Uehara, 1991;Mita et al, 2004) designated two additional types, C and D, based on differences in chromosome and spicule characteristics and gamete compatibility. In view of the substantial differences among the four types, Okinawan types of E. mathaei seem to be four distinct species.…”