The phylogenetic position of the solitary zoanthid genus Sphenopus (Cnidaria: Hexacorallia)

Abstract

The zoanthid genus Sphenopus (Cnidaria: Anthozoa: Zoantharia), like many other brachycnemic zoanthids, is found in shallow subtropical and tropical waters, but is uniquely unitary (solitary, monostomatous), azooxanthellate, and free-living. With sparse knowledge of its phylogenetic position, this study examines the phylogenetic position of Sphenopus within the family Sphenopidae utilizing specimens from southern Taiwan and Brunei collected in 1999-2011, and furthermore analyzes the evolution of its unique character set via ancestral state reconstruction analyses. Phylogenetic analyses surprisingly show Sphenopus to be phylogenetically positioned within the genus Palythoa, which is colonial (polystomatous), zooxanthellate, and attached to solid substrate. Ancestral state reconstruction strongly indicates that the unique characters of Sphenopus have evolved recently within Palythoa and only in the Sphenopuslineage. These results indicate that zoanthid body plans can evolve with rapidity, as in some other marine invertebrates, and that the traditional definitions of zoanthid genera may need reexamination.

Shallow-water zoantharians (Cnidaria, Hexacorallia) from the Central Indo-Pacific

“…This species has been reported from many locations in the Indo-West Pacific, including Taiwan (Soong et al 1999) and Brunei Darussalam (Reimer et al 2012b). …”

“…This azooxanthellate species was originally described from the Philippines, and has not been reported in the literature for over 80 years, excepting two brief mentions in Reimer et al (2012b). Easily discernable from other Sphenopus species by the presence of a ‘foot’ (=peduncle) that is attached to substrate (e.g.…”

“…The type species of the azooxanthellate genus Sphenopus , this species has an Indo-West Pacific distribution (Reimer et al 2012b). Uniquely for the order, species in this genus are unitary (not colonial), and usually free-living, as they are not attached to substrate, and instead embedded in sand or loose gravel/substrate (Figures 10A, B).…”

“…Individuals can often grow to large sizes (for zoantharians); up to several cm in both length and polyp diameter. Taxonomic examination of this genus is quite limited, with only two recent studies (Soong et al 1999, Reimer et al 2012b), both of which clearly state that further research is needed to more clearly understand this group.…”

“…The first molecular works of Burnett and co-workers (Burnett et al 1994, 1995, 1997) combined with the ecological and descriptive works of Ryland (Ryland and Lancaster 2003, 2004) have led to more recent papers dealing with the molecular phylogeny of zoantharians (Reimer et al 2004, 2012b, Sinniger et al 2005, Swain 2010), resulting in a reassessment of zoantharian taxonomy (Fujii and Reimer 2011, 2013, Sinniger et al 2013). Consequently, zoantharians are now perhaps the hexacorallian order for which the taxonomy most accurately reflects molecular phylogenetic understanding.…”