Abstract
A new species of free-living solitary zoantharian is described from Okinawa, Japan. Sphenopus exilis sp. n. occurs on silty seafloors in Kin Bay and Oura Bay on the east coast of Okinawa-jima Island. Sphenopus exilis sp. n. is easily distinguished from other Sphenopus species by its small polyp size and slender shape, although there were relatively few differences between Sphenopus exilis sp. n. and Sphenopus marsupialis in the molecular phylogenetic analyses. Currently, very little is known about the ecology and diversity of Sphenopus species. Thus, reviewing each species carefully via combined morphological and molecular analyses by using newly obtained specimens from type localities is required to clearly understand and distinguish the species within the genus Sphenopus.
Highlights
The suborder Brachycnemina (Cnidaria: Anthozoa: Hexacorallia: Zoantharia) consists of zoantharians commonly found in shallow warm waters, as almost all species within this group contain endosymbiotic photosynthetic Symbiodinium spp. (e.g. Swain 2010)
Three species are currently considered valid within the genus Sphenopus; S. marsupialis (Gmelin, 1791), S. arenaceus Hertwig, 1882, and S. pedunculatus Hertwig, 1888
In contrast to S. marsupialis with a wide distribution in the Indo-Pacific (Soong et al 1999, Reimer et al 2012, 2014), there have been no further records of S. arenaceus and only one additional record of S. pedunculatus after their original descriptions
Summary
The suborder Brachycnemina (Cnidaria: Anthozoa: Hexacorallia: Zoantharia) consists of zoantharians commonly found in shallow warm waters, as almost all species within this group contain endosymbiotic photosynthetic Symbiodinium spp. (e.g. Swain 2010). The genus Sphenopus belongs to the family Sphenopidae within Brachycnemina based on its brachycnemic mesenterial arrangement, mesogleal sphincter muscles, and heavy encrustation of granules into the body column. This genus is unique as Sphenopus individuals consist of free-living solitary polyps, in contrast to other Sphenopidae and Brachycnemina species and to all other known zoantharians. This unique feature is considered to be an adaptation to Sphenopus’ muddy/sandy sea floor habitats that are difficult for most sessile benthos to inhabit Comparatively very little is known about the species diversity of the genus Sphenopus
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