Research Article |
Corresponding author: Takuma Fujii ( takumagnum0213@hotmail.co.jp ) Academic editor: Bert W. Hoeksema
© 2016 Takuma Fujii, James Davis Reimer.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Fujii T, Reimer JD (2016) A new solitary free-living species of the genus Sphenopus (Cnidaria, Anthozoa, Zoantharia, Sphenopidae) from Okinawa-jima Island, Japan. ZooKeys 606: 11-24. https://doi.org/10.3897/zookeys.606.9310
|
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 S. 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.
Zoantharia , Sphenopus , new species, free-living, enclosed bay, identification key
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.
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 (
Recently, we discovered comparatively small Sphenopus specimens (polyp lengths <2.5 cm) from the shallow silty seafloors of enclosed bays on the east coast of Okinawa-jima Island, Japan. Combined morphological and molecular phylogenetic analyses lead us to conclude that the specimens belong to a previously unknown species. Thus, in this paper, a new Sphenopus species is formally described and a dichotomous key to identify all known Sphenopus species is provided. This report represents only the second formal record of this genus in Japan after
Sample collection. Specimens from Okinawa were collected by SCUBA. Prior to collecting, in situ images of expanded polyps were taken to assist in morphological analyses (colour, tentacle counts and size, polyp form). Half of the specimens collected were preserved in 99% EtOH for DNA analyses, and the other specimens were fixed for morphological analyses in 5 to 10% formalin sea water after anesthesia using MgCl2, and subsequently transferred to 70% EtOH some days later.
Morphological analyses. The lengths, maximum widths (largest diameter of column) and minimum widths (width at the top of physa where the aboral ampullaccous ends) of the column of preserved polyps were measured using calipers to the nearest 0.1 mm (Figure
Cnidae. Undischarged cnidae were measured from small pieces of tissue from the tentacles, column (external portion), actinopharynx, and mesenterial filaments of specimen
DNA processing and amplification. DNA was extracted from ethanol preserved specimens by following a guanidine extraction protocol (
Phylogenetic analyses. New sequences obtained in this study were deposited in GenBank (accession numbers: COI, KX400760–KX400768; mt 16S rDNA, KX400756–KX400759; ITS-rDNA, KX400769–KX400772). Obtained DNA sequences were manually aligned using Bioedit ver. 7.1.3.0 (
For phylogenetic analyses of mt 16S rDNA, COI, and ITS-rDNA the same methods were independently applied. The maximum-likelihood (ML) method was performed using MEGA5 (
Specimen number
Polyps of Sphenopus exilis sp. n. A In situ image of S. exilis sp. n., polyp with no black patterns, from the type locality in Kin Bay, Okinawa, Japan on 29 October 2011 B Polyps of
Specimen number
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
Fine sand particles heavily encrusted into ectoderm and mesoglea. Mesenteries in brachycnemic arrangement. Mesentery number 36, complete 18, incomplete 18 (Figure
Morphological features of Sphenopus exilis sp. n. A Cross section of holotype
Basitrichs and spirocysts in tentacles and actinopharynx. Basitrichs, holotrichs, microbasic p-mastigophores and basitrichs in mesenterial filaments. Holotrichs in column (Table
Cnidae types and sizes in different tissue sections of the holotype of Sphenopus exilis sp. n.
Image (Scale bars: 50 μm) | Length* | Width* | Frequency** | ||
---|---|---|---|---|---|
Tentacle | Basitrich |
|
10.4 (21.4–19.1) |
3.0 (2.6–3.3) |
Occasional (n=8) |
Spirocyst |
|
13.1 (12.0–14.5) |
2.6 (2.4–2.8) |
Numerous (n=20) | |
Column | Holotrich |
|
26.7 (16.3–34.5) |
14.7 (12.5–16.2) |
Rare (n=3) |
Actinopharynx | Holotrich |
|
28.0 (21.0–34.9) |
10.5 (5.5–15.5) |
Rare (n=2) |
Basitrich |
|
26.9 (26.4–27.4) |
4.4 (3.9–5.0) |
Numerous (n=20) | |
Spirocyst |
|
13.4 (10.8–15.2) |
2.9 (2.8–2.9) |
Common (n=13) | |
Filaments | Holotrich |
|
21.4 (17.0–25.6) |
10.4 (5.9–15.25) |
Occasional (n=4) |
Basitrich |
|
31.1 (30.6–31.6) |
3.2 (2.8–3.5) |
Numerous (n=20) | |
p-mastigophore |
|
15.4 (14.6–16.2) |
4.4 (3.7–4.9) |
Occasional (n=5) |
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.
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
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 S. pedunculatus. Polyps of this species are also much smaller than those of all three other species in the genus.
Hime-daruma-sunaginchaku (new Japanese name)
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 S. 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 (
The results of the phylogenetic analyses of nuclear internal transcribed spacer rDNA region showed S. exilis sp. n. forming a well-supported clade in the maximum likelihood and Bayesian analyses (Figure
Maximum likelihood tree of nuclear internal transcribed spacer of ribosomal DNA (ITS-rDNA) region for newly obtained sequences from Sphenopus exilis sp. n. in this study along with previously published GenBank sequences of family Sphenopidae. Bootstrap values of ML >60% are shown at respective nodes. Nodes supported by Bayesian posterior probabilities >0.90 are marked with asterisks. Species names’ of sequences obtained from GenBank follow with accession numbers. The subtree shown in b) shows only the clade formed by genus Sphenopus, Palythoa mizigama and P. umbrosa, delineated by the gray square in a).
Until now three species have been considered valid within Sphenopus; S. marsupialis (Gmelin, 1791), S. arenaceus Hertwig, 1882, and S. pedunculatus Hertwig, 1888. S. exilis sp. n. is easily distinguished from these other species by its small polyp size (length of S. exilis sp. n. <2.5 cm and width <1 cm), and by the shape of its elongated foot and physa. Polyps of both S. marsupialis and S. arenaceus are round on the aboral end, and not elongated as in S. exilis sp. n. (Figure
In contrast to the morphological differentiation from other Sphenopus species, only a few differences were found in molecular analyses. The COI sequences of S. exilis sp. n. were identical to those of S. marsupialis, Palythoa tuberculosa (Esper, 1805), and P. 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;
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 S. arenaceus and S. 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 (
In the ITS-rDNA molecular phylogeny, it is notable that two recently described azooxanthellate Palythoa species from caves, P. umbrosa and P. mizigama, form a well-supported subclade with S. exilis sp. n. and S. marsupialis. As the phylogenetic relationship between Sphenopus and Palythoa is not yet clear, and likely does not reflect the traditional taxonomy (
1 | Aboral end rounded, column shape oval, never having a very narrow stalk | 2 |
– | Aboral end elongated and narrow, forming a foot | 3 |
2 | Polyp colored earthy gray | S. marsupialis |
– | Polyp colored rusty red | S. arenaceus |
3 | Aboral end forms clasping disc, or the narrow stalk part of younger polyps very short compared to the oval part of the column. Comparatively large polyps (=polyp lengths > 2.4 cm, width > 2 cm) with approximately 60 mesenteries | S. pedunculatus |
– | Aboral end forms a rounded anchor. Comparatively small polyps (length < 2.4 cm, width < 2 cm), and approximately 36 mesenteries | S. exilis sp. n. |
Unlike some other recently described zoantharian taxa such as Nanozoanthidae and Microzoanthidae, S. exilis sp. n. is not very small in size and does not inhabit a cryptic habitat. However, silty, sandy and rubble habitats are often overlooked in biodiversity surveys in favor of coral reef habitats, and this sampling bias has resulted in a relative lack of understanding of the diversity of these habitats and the evolutionary position of their inhabitants (
Thus, not only S. exilis sp. n., but also the diverse and various organisms that exclusively inhabit soft substrates in coral reef regions in the world face issues of decreasing habitat despite our lack of knowledge of their biodiversity (e.g. Cnidaria: Ceriantharia:
We thank Tadashi Yamada (Yamato Co., Ltd.), Shin Nishihira (Diving Team Snack Snufkin), and Dr. Masami Obuchi (University of the Ryukyus) for supporting collection of specimens, and we also thank Fumihito Iwase (Shikoku Marine Life Laboratory) for providing valuable information and comments. We are also grateful to the reviewer and the editor for improving this manuscript. The first author was supported by a research fellowship from the Japan Society for the Promotion of Science for young scientists, JSPS KAKENHI (24-3048), and the Special Budget of MEXT in 2015 (Establishment of Research and Education Network on Biodiversity and its Conservation in the Satsunan Islands). The second author was funded by a Japan Society for the Promotion of Science (JSPS) ‘Zuno-Junkan’ grant entitled ‘Studies on origin and maintenance of marine biodiversity and systematic conservation planning’, and a JSPS ‘Kiban B’ grant entitled ‘Global evolution of Brachycnemina and their Symbiodinium’.