Tropiometra macrodiscus ( Hara, 1895 )

Kim, Philjae & Shin, Taekjun Lee and Sook, 2022, New record of the unstalked crinoid Tropiometra macrodiscus (Crinoidea: Comatulida: Tropiometridae) from Korea, Journal of Species Research 11 (1), pp. 22-28 : 24-27

Tropiometra macrodiscus ( Hara, 1895)

Antedon macrodiscus Hara, 1895: 115 ; Clark, 1908: 481; Hara and Okada, 1921: 33.

Tropiometra macrodiscus Clark, 1907: 349 ; Clark, 1908: 316.

Tropiometra afra Clark, 1908: 315 ; 1912: 176; Gislén, 1934: 6.

Tropiometra afra var. macrodiscus Gislén, 1922: 90 ; 1922: 44, 51, 53, 78, 90.

Tropiometra afra macrodiscus Clark, 1947: 272 .

Material examined. Korea: One specimen, waters adjacent to Busan (34°53′25.0″N, 128°57′22.4″E), Aug 3, 2010, with SCUBA ; One specimen, waters adjacent to Tongyeong (34°39′12.9″N, 128°34′42.5″E), Oct 20, 2011, with SCUBA .

Description. Specimen from Busan; a total of 10 arms, 310 mm long. Centrodorsal width 12 mm, height 4 mm. Dorsal pole 6 mm in diameter. Centrodorsal, very thick disk with slightly sloping side, middle of dorsal pole clearly concave form. Cirrus sockets arranged in three irregular rows. Cirrus ⅩL Ⅴ, 32-42 segments, 35-50 mm long, stout and smoothly curved, non-spine development. Terminal claw blunt and short. P 1 31 mm length, 39 segments, slender, non-genital pinnule. P 2 slightly stouter than P 1, 34 mm in length, 40 segments. P 3, similar to P 2, 36 mm in length 44 segments. The entire body parts solid and non-flexible, even the pinnules or cirrus. The axillaries nearest to arms very stout, solid, and strong; the terminal parts of arms becoming slender. IBr 1 longish rectangular shape. IBr 2 (axillaries) broadly pentagonal, low height, and almost triangular. Syzygies occurring between the brachial 3 + 4, 8 + 9, and 13 + 14. The specimen was collected in Tongyeong; 10 arms, robust, and longish. Large stout body. All arms were broken and some regenerated, with a length of 210 mm. Centrodorsal width 13 mm height 6 mm. Dorsal pole 5 mm in diameter. Cirrus sockets arranged in three irregular rows. Cirrus Ⅹ LIII, 33-41 segments, 35-53 mm long and stout, smoothly curved. In cirrus segments, no opposing spines or dorsal spines. Terminal claws short and curved, and blunt never sharp. P 1 33 mm in length, 39 segments, slender, and non-genital part. P 2, 36 mm in length, 35 segments, slightly stouter than P 1. P 3, 38 mm in length, 37 segments, similar to P 2. IBr 1 longish oblong shape. IBr 2 (axillaries), similar with triangle-like pentagons with edges against adjacent axillaries, very short. Syzygies shown in 3 + 4 and usually 8 + 9.

Size. Arms = 310 mm (Busan) and 210 mm (Tongyeong). Centrodorsal = 12 mm (Busan) and 13 mm (Tongyeong). Cirrus = 50 mm (Busan) and 53 mm (Tongyeong).

Color. Body yellowish dark brown in 95% ethanol, rgenerated parts bright yellow.

Distribution. Korea ( Korea Strait), Japan (Sagami Bay, Ogasawara Is), China, Hong Kong.

Remarks. The large crinoids, T. macrodiscus and T. afra , are confused in species-level classification because of their similar morphological characteristics. When Antedon macrodiscus was first recorded ( Hara, 1895), it was regarded as a unique species separated from A. afra at the species level. Hara (1895) only confused the specimens with Amphimetra miberti accepted as Amphimetra tessellate because of their morphological similarities, such as pinnules, brachials, and intersyzygial interval. The specimens are distinguished by the number of cirri and the presence/absence of spines on cirrus segments. After the establishment of Tropiometra , A. macrodiscus was synonymized into T. macrodiscus . Tropimetra macrodiscus specimens from the Sagami Bay in Japan were reported in April 1908, and these were identified as T. afra depending on the ambiguous morphological differences with the specimens collected in Australia ( Clark, 1908). Clark placed T. macrodiscus under the synonymy of T. afra , and reported the specimen collected in the Korea Strait as T. afra ( Clark, 1947) . Gislén (1922) recognized the difference between the two species, but suggested that the several different features could be considered as variations that may be due to the intervening territory. At present, T. macrodiscus and T. afra are understood to be different species with extremely different distribution ranges. However, T. macrodiscus specimens from the Sagami Bay reported in Hara (1895) have stouter and larger forms than the specimens recorded as T. afra , and have longer and more numerous cirrus segments. These morphological differences according to the distribution range were emphasized as the basis for the species-level classification, and these differences were identified as the main classification keys for the current Tropiometra classification.

We compared our specimen with previous descriptions of T. macrodiscus and T. afra . Our specimens clearly showed differences from T. afra described by Clark (1947) in characteristics that are the key to the morphological classification of crinoids: 1) long cirrus and arms, 2) more numerous cirrus segments and cirrus quantity, 3) pentagon-shaped axillaries, and 4) no opposing spines on cirrus. In addition, our specimen differs slightly from T. macrodiscus described by Clark (1947), whereas it was in accordance with the description of T. macrodiscus Gislén (1922) . Furthermore, the collection sites of the specimens we have were included in the Korean Strait known as the distribution range of T. macrodiscus (from Hong Kong northward to the Korean Strait and eastward to the Sagami Bay, Japan, and the Bonin Islands), distinct from the distribution range of T. afra ( Philippines and southward to Bowen, Queensland, and to Fremantle and Geraldton, Western Australia). In the description of T. macrodiscus Clark (1947) , the specimens from the Korean Strait showed that the length of the arms was not longer and the number of cirri was less than that of the other specimens. Our specimens were similar to specimens from China and Japan. Therefore, we presumed that our specimens were mature in accordance with obviously long arms and numerous cirri, and we concluded that the T. macrodiscus specimens of Clark (1947) are possibly immature specimens, i.e., the length of the arm is shorter than that of our specimens and the slightly fewer number of cirri.

In the phylogenetic analysis, 15 COI sequences from nine crinoids belonging to the family Tropiometridae were used for species molecular identification. Pairwise distances ranged from 0.0% to 26.1% within the nine Tropiometridae species. The intraspecific variation for T. macrodiscus ( MT 086599, MT 086600 from Korea; JX236097 from Japan) and T. afra (GU327867 from Japan) ranged from 0.0% to 0.9% ( Table 1 ). According to our molecular data, T. afra reported in Japan should be considered as same species, T. macrodiscus . The COI sequence of T. afra (GU327867 from Japan) was reported by Rouse et al. (2013). Rouse et al. (2013) only determined the molecular phylogeny status of extant crinoids, and we could not find any information on the morphological description of the source of the sequence. The only additional information available in the paper is the collection site of the specimen. However, the paper was accepted on September 2012, and the corresponding author noticed descriptive notes by personal communication for T. afra macrodiscus in World Register of Marine Species (Worms, https://www.marinespecies.org/) that the two subspecies, T. afra afra and T. afra macrodiscus , are distinct enough to separate species based on molecular data on 6 June 2013 (Unpublished data, accessed on 4 November 2021). Moreover, Clark (1947) recorded that the significant classification key for T. afra distributed in Philippines and Australia, and T. macrodiscus distributed in Hong Kong, China, and Japan is the distribution range. Therefore, some questions remain regarding the specimens used in Rouse et al. (2013). The specimen collection site location belongs to the distribution range of T. macrodiscus , and it makes us to consider that their specimen might be T. macrodiscus , not T. afra .

The COI sequences of 657 bp obtained in this study were registered in the NCBI GenBank database (https:// www.ncbi.nlm.nih.gov/) under accession numbers MT 086599 (Busan) and MT 086600 (Tongyeong). Our results will be useful for examining the molecular phylogenetic relationships of crinoids and establishing genetic differences between T. macrodiscus and T. afra in the future.