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Zoological Science
Abstract
Among family Zoanthidae (Anthozoa: Hexacorallia: Zoantharia), Zoanthus and Isaurus are widespread benthos on coral reefs, but the reproductive biology of both genera is generally unknown. Accordingly, sexual reproduction of two Zoanthidae species in Japan, Zoanthus kuroshio in Okinawa, and Isaurus tuberculatus in Okinawa and Kochi, was investigated by annual sampling. Between 2012 and 2013, polyps of tagged colonies of Z. kuroshio and I. tuberculatus were sampled monthly, and gonads in mesenteries were examined under a stereomicroscope. In Z. kuroshio, sizes of oocytes were estimated and in situ spawning observations were conducted at night at Okinawa sites. Monthly sampling indicated that gametogenesis periods of both species in Okinawa were from May to August, and oogenesis set in earlier than spermatogenesis, and thus both species can be considered protogynous functional hermaphrodites. On the other hand, I. tuberculatus in Kochi had hermaphroditic polyps in July and August. Notably, fertile colonies of Z. kuroshio were not only hermaphrodites but also included exclusively male and female colonies. Overall, four sexual types were identified in Z. kuroshio: male, female, functional protogynous hermaphrodites, and unclassified sterile colonies. In situ gamete release of Z. kuroshio was observed in August 2012 and July to August 2013. First spawning of the year occurred 1 day after the new moon; colonies shed their bundles 3.5–4 h after sunset. We speculate that lunar phase and time after sunset influence spawning timing in Z. kuroshio. Basic knowledge on sexual reproduction of Zoanthidae is critically needed to better understand the basic biology of these benthic cnidarians.
We thank Profs. Kazuhiko Sakai and Michio Hidaka (U. Ryukyus) for acting as vice-referees during the first author's master's degree, upon which this manuscript is based. For sampling in Kochi, we thank Dr. Masami Obuchi and Shu Nakachi (Biological Institute on Kuroshio). The following people and institutions are thanked for their support in sampling and logistics: researchers at the Biological Institute on Kuroshio, Dr. Shusuke Ono (Miyakonojo High School), and all Molecular Invertebrate Systematics and Ecology laboratory members at U. Ryukyus. We thank Dr. Yoko Nozawa (Academia Sinica, Taiwan) for help with statistics. We thank Prof. Euichi Hirose, Prof. Hideyuki Yamashiro (both U. Ryukyus), and Dr. Mamiko Hirose (Tokai U.) for histological advice, and we thank Dr. Danwei Huang (National U. of Singapore), Dr. Andrew Baird (James Cook U.), and Dr. Shashank Keshavmurthy (Academia Sinica) for advice on cryptic species.
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