Morphology and Anatomy of Corallimorpharian Metarhodactis aff. boninensis from Thailand
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Abstract
Corallimorpharians are sessile cnidarians that are widely distributed from tropical to the polar region. Corallimorpharians exhibit a high level of intraspecific morphological variation and this causes problems in identification and uncertainty of the species boundary. Metarhodactis is a monotypic genus of the family Discosomidae and widely distributed in Thai waters. However there were a few studies on the detailed morpho-anatomical structure of Metarhodactis species in Thailand. This study aimed to examine the morphological and anatomical characteristic of Metarhodactis aff. boninensis, and to confirm the taxonomic status of this species. Our specimens collected from Prachuap Khiri Khan Province, Thailand, exhibited an oval-shaped oral disc with curled edge and produced only unbranched papilliform tentacles. Zooxanthellae densely inhabit in the tentacle gastrodermal tissue. M. aff. boninensis bears only one nematocyst in which six cnidae types were found. Holotrichs III and microbasic p-mastigophores were the most abundant cnidae, followed by holotrichs I. Male and female reproductive organs were observed in separate individuals. It was obvious that the M. aff. boninensis in this study was clearly distinct from the original description of M. boninensis from Japan on the basis of tentacle shape, cnidae type, and its locality. We suggest that further study to clarify the taxonomic position of M. aff. boninensis is required considering a combined data of morpho-anatomical characteristics and molecular markers.
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