Zoological Studies

Vol. 49 No. 2, 2010

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

Eriko Shiroma1 and James Davis Reimer2,3,*

1Department of Marine Science, Biology and Chemistry, Faculty of Science, University of the Ryukyus, Senbaru 1, Nishihara, Okinawa 901-0213, Japan
2Molecular Invertebrate Systematics and Ecology, Rising Star Program, Transdisciplinary Research Organization for Subtropical Island Studies, University of the Ryukyus, Senbaru 1, Nishihara, Okinawa 901-0213, Japan
3Marine Biodiversity Research Program, Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, 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.

Key words: Spawning timing, Zooxanthellae, Zoantharia, Reticulate evolution, Hybridization.

*Correspondence: Tel: 81-98-8958542.  Fax: 81-98-8958576.  E-mail:jreimer@sci.u-ryukyu.ac.jp