Zoological Studies

Investigations into the Reproductive Patterns, Ecology, and Morphology in the Zoanthid Genus Palythoa (Cnidaria: Anthozoa: Hexacorallia) in Okinawa, Japan

Eriko Shiroma and James Davis Reimer (2010) The zoanthid genus Palythoa is found in shallow subtropical and tropical waters worldwide; yet many questions remain regarding the diversity of species and their evolution.  Recent progress using molecular techniques has advanced species identifications but also raised new questions.  In previous studies, it was hypothesized that P. sp. yoron may be the result of interspecific hybridization between the closely related species P. tuberculosa and P. mutuki.  Here, in order to further assess the relationships among these 3 species, their sexual reproductive patterns, distribution, and morphology (tentacle number, colony shape and size, polyp shape, etc.) were investigated in 2008 at Odo Beach, Okinawa, Japan.  Results show clear differences in morphology and distribution among all 3 species, with P. sp. yoron apparently intermediate between P. tuberculosa and P. mutuki.  From observations of oocyte/egg production, sizes, and frequency, P. tuberculosa is believed to spawn within 5 d after the full moon in July.  No gamete production was observed in either P. mutuki or P. sp. yoron, although P. mutuki was found to prolifically reproduce asexually.  How P. sp. yoron propagates remains unknown, but at least it appears that the 3 species do not spawn synchronously.  From morphological data collected in this study in conjunction with previous DNA phylogenetic results, it is proposed that P. sp. yoron may be the product of a hybridization event in the past between a P. tuberculosa egg and P. mutuki sperm, followed by occasional introgression with P. tuberculosa, although no evidence of sexual reproduction was seen in either P. mutuki or P. sp. yoron.  Some Palythoa spp. diversity may be due to reticulate evolution, and this may also contribute to Palythoa’s potential for adaptation.