Sphenopus exilis, Fujii, Takuma & Reimer, James Davis, 2016

Fujii, Takuma & Reimer, James Davis, 2016, A new solitary free-living species of the genus Sphenopus (Cnidaria, Anthozoa, Zoantharia, Sphenopidae) from Okinawa-jima Island, Japan, ZooKeys 606, pp. 11-24 : 13-19

Taxon classification Animalia Zoantharia Sphenopidae

Sphenopus exilis sp. n. Figures 2, 3

Holotype.

Specimen number NSMT-Co1576 (MISE-TF-107): Kin Bay, Uruma, Okinawa-jima Island, Japan (26°22'25"N, 127°53'30"E), 15 m depth, collected by Takuma Fujii, 29 October 2011, fixed in 5-10% SW formalin, deposited in National Museum of Nature and Science, Tokyo, Japan (NSMT). Polyp length 2.4 cm, maximum width 0.8 cm, minimum width 0.3 cm. Figure 2B.

Paratypes.

Specimen number NSMT-Co1577 (MISE-TF-107), a lot of total 11 polyps collected on the same dive, collection data same as holotype, five polyps fixed in 5-10% formalin, six polyps fixed in 99% EtOH, polyp length 1.3 to 2.2 cm (average 1.7 ± 0.3 cm), maximum width 0.4 to 1.0 cm (average 0.5 ± 0.2 cm), minimum width 0.2 cm, deposited in NSMT. GenBank accession numbers: COI, KX400760-KX400768; mt 16S rDNA, KX400756-KX400759; ITS-rDNA, KX400769-KX400772. Figure 2A and B; Specimen number RMNH Coel. 42121 (MISE-TF-144): a lot of total 16 polyps collected on the same dive, Kin Bay, Uruma, Okinawa-jima Island, Japan (26°22'25"N, 127°53'30"E), 15 m depth, collected by Takuma Fujii, 24 May 2012, 11 polyps fixed in 5-10% formalin, five polyps fixed in 99% EtOH, polyp length 1.1 to 2.2 cm (average 1.7 ± 0.4 cm), maximum width 0.4 to 0.5 cm (average 0.5 ± 0.1 cm), minimum width 0.1 to 0.3 cm (average 0.2 ± 0.1), deposited in Naturalis Biodiversity Center, Leiden, Netherlands (RMNH); Specimen number NSMT-Co1578 (MISE-TF-151), a lot of total six polyps collected on the same dive, Oura Bay, Nago, Okinawa-jima Island, Japan (26°32'29"N, 128°3'16"E), 17 m depth, collected by Takuma Fujii, 13 November 2012, five polyps fixed in 5-10% formalin, 1 polyp fixed in 99% EtOH, polyp length 1.0 to 1.9 cm (average 1.5 ± 0.4 cm), maximum width 0.3 to 1.1 cm (average 0.7 ± 0.3 cm), minimum width 0.1 to 0.3 cm (average 0.2 ± 0.1 cm), deposited in NSMT. Figure 2C and D.

Diagnosis: external morphology.

Solitary, cylindrical polyp. Length of polyps 1.0 to 2.4 cm (average 1.7 ± 0.3 cm), maximum width 0.3 to 1.1 cm (average 0.6 ± 0.2 cm), minimum width 0.1 to 0.3 cm (average 0.2 ± 0.1 cm) (n=34). Tentacles longer than half diameter of the expanded oral disc (Figure 2A). Oral disc gently hollowing into mouth, with stellate grooves as many as tentacles (Figure 2A, C). Capitular ridges present but not strongly pronounced when polyps closed (Figure 1). The upper part of the polyp between capitulum and the column slightly constricted (the width of the most constricted region approximately 0.1 cm to 0.4 cm thinner than the width of contracted capitulum) when polyp contracted (Figures 1, 2B, D). Upper part of the column generally thick and oval (Figures 1, 2B, D). Aboral narrow bottom portion of column extended (=foot), thinner than upper portion of column, like a cone (Figures 1, 2B, D), with the distal portion round and thicker than the extended foot (=physa) (Figures 1, 2B, D). Column smooth, with encrusted fine dense sand particles. Occasionally broken piece(s) of bivalve shells attached to the aboral end (Figure 2B).

Diagnosis: internal morphology.

Fine sand particles heavily encrusted into ectoderm and mesoglea. Mesenteries in brachycnemic arrangement. Mesentery number 36, complete 18, incomplete 18 (Figure 3A; n=6 polyps). Single siphonoglyph apparent. Mesogleal sphincter muscle well developed, visible under dissecting microscope (Figure 3B). Endosymbiotic Symbiodinium spp. (zooxanthellae) absent (=azooxanthellate).

Diagnosis: cnidae.

Basitrichs and spirocysts in tentacles and actinopharynx. Basitrichs, holotrichs, microbasic p-mastigophores and basitrichs in mesenterial filaments. Holotrichs in column (Table 1).

Habitat.

Specimens were found at approximately 10 to 20 m depths on the slopes of silty seafloors in enclosed bays. Most polyps semi-burrowed in silt, with only the open oral disc visible and protruding out from the seafloor.

Colour.

Tentacles and oral disc whitish and translucent in life. Faint black narrow horizontal bands appear on tentacles, and similar faint patterns on the oral disc of a few polyps (Figure 2C). Column colour of encrusted sand particles, a few polyps with 2 to 6 faint black vertical stripes approximately 15 mm wide on the upper part of the column, reaching from oral end to aboral end (Figure 2C, D).

Etymology.

Named from latin ' exilis ' meaning ‘slender’ or ‘small’, as polyps have an elongate and narrow foot more slender than other known species in this genus to the exception of Sphenopus pedunculatus . Polyps of this species are also much smaller than those of all three other species in the genus.

Common name.

Hime-daruma-sunaginchaku (new Japanese name)

Molecular phylogeny.

The results of the phylogenetic analyses of both mitochondrial cytochrome oxidase subunit I(COI) and 16S rDNA showed very few differences between sequences of our specimens and those of Sphenopus marsupialis , as well as compared with those of various Palythoa species. These results are not incongruous with previous studies on the molecular phylogeny of family Sphenopidae , where intra-family variation levels of mitochondrial DNA sequences were relatively low ( Reimer et al. 2006, 2012).

The results of the phylogenetic analyses of nuclear internal transcribed spacer rDNA region showed Sphenopus exilis sp. n. forming a well-supported clade in the maximum likelihood and Bayesian analyses (Figure 4; ML=94%, Bayes=0.91). As well, together with sequences of Sphenopus marsupialis , Sphenopus exilis sp. n. formed a strongly supported Sphenopus clade (Figure 4; ML=99%, Bayes=1.00). In comparing the ITS-rDNA sequences between Sphenopus exilis sp. n. and Sphenopus marsupialis , there were 12 to 27 b.p. differences over a total 470 b.p. (=2.5~5.7% difference).

Remarks.

Until now three species have been considered valid within Sphenopus ; Sphenopus marsupialis (Gmelin, 1791), Sphenopus arenaceus Hertwig, 1882, and Sphenopus pedunculatus Hertwig, 1888. Sphenopus exilis sp. n. is easily distinguished from these other species by its small polyp size (length of Sphenopus exilis sp. n. <2.5 cm and width <1 cm), and by the shape of its elongated foot and physa. Polyps of both Sphenopus marsupialis and Sphenopus arenaceus are round on the aboral end, and not elongated as in Sphenopus exilis sp. n. (Figure 3C). Soong et al. (1999) examined various sized Sphenopus marsupialis collected from around Taiwan including small polyps without any narrow elongated foot (length <2 cm). Additionally, Reimer et al. (2016) recently reported on a Sphenopus marsupialis specimen of the typical rounded shape and large size (~9 cm in height) from Okinawa-jima Island. No polyps with intermediate morphology between Sphenopus marsupialis and Sphenopus exilis sp. n. have ever been found. Thus, the specimens collected in this study cannot be considered to be immature polyps of Sphenopus marsupialis . The morphologically most similar species to Sphenopus exilis sp. n. is Sphenopus pedunculatus as it also has a narrow foot, but Sphenopus pedunculatus is much larger than Sphenopus exilis sp. n., with polyp lengths of 2.4 to 3.2 cm and widths of 2 to 2.4 cm, and with approximately 60 mesenteries. As well, the aboral end of Sphenopus pedunculatus is shaped like a clasping disc, different from that of Sphenopus exilis sp. n. with a narrow rounded shape ( Hertwig 1888, Reimer et al. 2014).

In contrast to the morphological differentiation from other Sphenopus species, only a few differences were found in molecular analyses. The COI sequences of Sphenopus exilis sp. n. were identical to those of Sphenopus marsupialis , Palythoa tuberculosa (Esper, 1805), and Palythoa umbrosa Irei, Singer & Reimer, 2015. However, it is known that the evolutionary rate of mitochondrial DNA markers is quite slow in most Anthozoa (Shearer et al. 2004; Huang et al. 2008; Stampar et al. 2014), and the nuclear ITS-rDNA region is currently the fastest evolving DNA marker that has been utilized for species-level analyses of suborder Brachycnemina ( Reimer et al. 2007). Although there are only relatively few differences between the ITS-rDNA sequences of Sphenopus exilis sp. n. and Sphenopus marsupialis (2.5~5.7% sequence divergence), the formation of a supported monophyletic clade confirms the results of our morphological analyses that the specimens collected in this study belong to a species different from Sphenopus marsupialis (Figure 4). Moreover, these results suggest the possibility of the presence of multiple, cryptic species within Sphenopus marsupialis as previously mentioned by Soong et al. (1999).

Currently, very little is known about the ecology and species diversity of the genus Sphenopus , demonstrated by the fact that there have been no or few records of both Sphenopus arenaceus and Sphenopus pedunculatus within the last 100 years. Thus, morphological and molecular analyses of newly obtained specimens from type localities followed by reviewing each species’ description carefully are required to clearly understand the species distinction of Sphenopus species. As mentioned in previous studies, the phylogenetic results of this study indicate a need to re-examine the validity of the genus Sphenopus as it is positioned within the genus Palythoa , and by extension the definition of genera within the family Sphenopidae should be reconsidered ( Reimer et al. 2012, Irei et al. 2015).

In the ITS-rDNA molecular phylogeny, it is notable that two recently described azooxanthellate Palythoa species from caves, Palythoa umbrosa and Palythoa mizigama , form a well-supported subclade with Sphenopus exilis sp. n. and Sphenopus marsupialis . As the phylogenetic relationship between Sphenopus and Palythoa is not yet clear, and likely does not reflect the traditional taxonomy ( Reimer et al. 2012), construction of a large ITS-rDNA phylogeny with additional sequences from other Palythoa and Sphenopus species is needed. At the same time, investigation with additional DNA markers asides from the mt DNA and ITS-rDNA currently utilized in zoantharian phylogeny may be helpful.