Amphiura (Amphiura) digitula (H. L. Clark, 1911)

Shin, Taekjun Lee and Sook, 2020, A newly recorded brittle star, Amphiura (Amphiura) digitula (H. L. Clark, 1911) (Ophiuroidea: Amphilepidida: Amphiuridae), from Geoje Island, Korea, Journal of Species Research 9 (3), pp. 273-279 : 274-278

Amphiura (Amphiura) digitula

(H.L. Clark, 1911) ( Fig. 1 )

dzẅŋụẹHṘḇṻIJƞ (신ḡ)

Amphiodia digitula H.L. Clark, 1911: 162 .

Amphiura leptopholida H.L. Clark, 1915: 226 .

Amphiura digitula Matsumoto, 1917: 199 ; D’yakonov, 1954: 71; A.M. Clark, 1965: 49.

Diamphiodia digitula Fell, 1962: 14 .

Amphiura (Amphiura) digitula : Stöhr et al., 2020: 243028.

Material examined. Four specimens, Geoje Island (34° 44′41″N, 128°40′01″E), 27 August 2017, Lee T., 47 m depth by grab ( NIBRIV0000863921 , NIBRIV0000863922 , NIBRIV0000863926 , NIBRIV0000863927 ) .

Description. Disk small (diameter: 5.2 - 7.3 mm), slightly concave at interradii and covered by numerous fine scales ( Fig. 1A ). Arms long (length: 25.0 - 35.1 mm), approximately 4.8 times longer than disk diameter. Disk scales circular form, overlapping, with indistinguishable primary scales ( Fig. 1A ). Radial shield small ( Fig. 1A ), longer than wide ( Fig. 1B ), separated by disk scales proximally but connected distally ( Fig. 1B ). Small projection on distal end of radial shield, stumpy hook situat- ed at outermost of projection, hook tip divided into three or four ( Fig. 1B ). Oral shield ovoid form, slightly longer than wide, more angular distally than proximally ( Fig. 1C, D ). Madreporite shield more rounded than others ( Fig. 1C ). Tooth papilla one and two adoral shield spines on an adoral shield ( Fig. 1D ). The 2nd adoral shield spine longer than other, tapered dramatically toward dull tip ( Fig. 1D ). Dorsal arm plate rounded triangular, proximal end more angular than distal end ( Fig. 1E ). Lateral arm plate wider than long ( Fig. 1E ). Arm spines four. Ventral arm plate longer than wide, pentagonal with dull angles, and proximal end relatively more angular ( Fig. 1F ). Two tentacle shields in single tentacle pore, not fully covered ( Fig. 1F ).

Distribution. Korea ( Korea Strait), Japan (Enoshima, Sadoga Island, Sagami Bay).

Habitat. Clay or sandy clay substrates.

Color. In live specimens, disk is gray and arms are pale brown.

Remarks. This species has several distinguishing morphological characteristics, including a long, tapered oral papilla and small, hooked stumpy spine on the distal part of the radial shield. As such, the species is easily distinguished from other Amphiura species in Korea. Matsumoto (1917) reported A. (A.) digitula specimens with four oral papillae, but A.M. Clark (1965) described that A. leptopholida , with three oral papillae, had almost morphological characteristics nearly identical to A. digitula . So, A. leptopholida synonymized A. digitula , even though the number of the oral papilla differs. All of the Korean specimens of A. digitula were identical to the description of A. digitula forma leptopholida in Clark, 1965, so we classified these samples as A. digitula .

We also obtained a partial sequence of COI from one A. (A.) digitula specimen in this study. The amplified sequence was 657 bp and matched the general DNA barcoding region. The NJ and ML phylogenetic analyses revealed that species of the Amphiura group was monophyletic, but supported by a low bootstrap value ( Fig. 2 ). But, the group of Amphiura shown high interspecific pairwise distance (p-distance) values between each species in this study, and which was higher than the mean of interspecific p-distance in previous echinoderms DNA barcoding studies ( Layton et al., 2016; Boissin et al., 2017). The interspecific p-distance between A. (A.) digitula and the other 20 Amphiura species ranged from 20.4% (A. (A.) elandiformis ) to 26.6% (A. (A.) angularis ), with a mean of 24.5% ( Table 2). This is slightly higher than the mean in ophiuroids (18.9%) reported by Boissin et al. (2017) but corresponds to the mean and average value of interspecific Amphiura : mean = 24.1% and range = 17.0 - 30.0% ( Table 2). The pairwise distance of COI presented the identification clearly between each species of the Amphiura , even though not shown the clear phylogenetic relationships within in NJ and ML trees. This study used morphological and molecular analyses to identify the recently collected specimens as A. (A.) digitula , a newly recorded species in the Korea marine fauna.

Figures

Fig. 1. Morphological characteristics of Amphiura (Amphiura) digitula (H.L. Clark). (A) dorsal side of disk; (B) radial shields; (C) ventral side of disk; (D) oral frame; (E) dorsal side of proximal arm; (F) ventral side of proximal arm. AS, arm spines; AdSh, adoral shield; Ad- ShSp, adoral shield spine; D, dorsal arm plate; d, distal; L, lateral arm plate; M, madreporite; OP, oral papilla; OSh, oral shield; p, proximal; RS, radial shield; TPa, tooth papilla; TS, tentacle scale; V, ventral arm plate.

Fig. 2. Neighbor joining (NJ) and maximum likelihood (ML) trees based on 657 bp of the mitochondrial COI gene from 21 Amphiura species, including A. (A.) digitula. Bootstrap support values indicated on each node. (A) Phylogenetic tree of neighbor joining based on the kimura 2-parameter distance model, with 1000 non-parametric bootstrap replicates; (B) phylogenetic tree of maximum likelihood based on the GTR+I+G model, with 1000 bootstrap replicates.