Araeosoma owstoni Mortensen, 1904

Lee, Taekjun & Shin, Sook, 2022, A newly recorded sea urchin, Araeosoma owstoni Mortensen, 1904 (Echinoidea; Echinothurioida; Echinothuriidae), from the Korea Strait, Journal of Species Research 11 (3), pp. 188-193 : 191-192

Araeosoma owstoni Mortensen, 1904 ( Fig. 2 )

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Araeosoma owstoni Mortensen, 1904: 82 ; Clark, 1925: 63; Shigei, 1981: 198; 1986: 35; Rowe and Gates, 1995: 217; Liao and Clark, 1995: 326; Chao and Lee, 2001: 19; Liu, 2008: 864; Kroh and Mooi, 2022: 513119.

Material examined. One specimen, Korea Strait (33° 45 ʹ 36.0 ʺ N, 127°10 ʹ 58.0 ʺ E) ( Fig. 1B ), 24 April 2017, T . Lee and S. Bae, 90 m depth by trawling, deposited vouch- er of NIBR: NIBRIV0000895385 .

Description. Test large, flexible, low form, rounded edge and oral side flat ( Fig. 2A ). Apical area rather small ( Fig. 2A ). Genital and ocular plates widely separated. Genital pores large, and closely situated middle of genital plate. Madreporite plate distinctly larger than other genital plates. Interambulacral areas twice as broad as ambulacral ( Fig. 2A ). Interambulacra 38 and ambulacra 60 in number. Ambulacral plate with small and these tubercles not forming regular series. Interambulacral plate with two primary tubercles and numerous military tubercles. Length of the primary spines on the aboral side sig- nificantly varying. Secondary spines have poison gland near tip of spine. Only one kind of tridentate pedicellariae present and variable in size ( Fig. 2C ). Triphyllous pedicellariae elongate, narrow form ( Fig. 2D ). Spicules of tube feet rather large, irregular, thorny, fenestrated plates.

Size. test diameter = 102.3 mm; test height = 28.4 mm; apical system = 17.9 mm; peristome = 25.8 mm; length of longest primary spine = 15.3 mm.

Color. Aboral side was dark brown with reddish brown color on the margin of ambulacral area. Oral side was brown when alive ( Fig. 2A, B ).

Distribution. Korea ( Korea Strait: off eastern Jeju), Japan (Okinoshima, Sagami Bay to Kagoshima Bay, and Okina- wa), East China Sea, Taiwan, Philippines, Australia (Cape Lambert, and Brisbane) ( Fig. 1A ).

Remarks. Despite being peeled off on the aboral side and lacking tetradactyle pedicellarie, the collected specimen provided enough evidence for morphological identification. This specimen conserves its original test shape and has two types of pedicellaria: tridentate and triphyllous. The morphological characteristics of the specimen were consistent with the following major morphological characteristics of Araeosoma owstoni in previous studies ( Mortensen, 1904; Shigei, 1986): 1) including patterns and numbers of amburacra and interambucra, 2) ratio of diameter with apical systems and test, and 3) pedicellariae forms. Araeosoma owstoni has three subspecies so far ( Kroh and Mooi, 2022). Among them, morphological characteristics of A. owstoni bicolor (A. Agassiz and H. L. Clark, 1907) and A. owstoni nudum Mortensen, 1934 were slightly different from A. owstoni . For instance, A. owstoni bicolor had higher numbers of interambulacral and ambulacral plates than A. owstoni and A. owstoni nudum had more naked aboral side ( Mortensen, 1934; 1948). Araeosoma owstoni bicolor distributed from Kagoshima, Japan ( Agassiz and Clark, 1907; 1909), and Araeosoma owstoni nudum from Philippine to Hong Kong ( Mortensen, 1934; 1948), however, their distributions are relatively similar to A. owstoni ( Fig. 1A ). Mortensen (1934) was the first to suggest Araeosoma owstoni nudum as a variant of A. owstoni . Furthermore, Shigei (1981) synonymised Asthenosoma bicolor as A. owstoni and did not mention about the presence of all subspecies when he redescribed A. owstoni ( Shigei, 1986) . Accordingly, we present our newly collected echinoid as A. owstoni .

DNA barcoding analysis. In terms of morphological identification, the genus Araeosoma is one of the most difficult echinoids. Thus, a partial sequence of mitochondrial COI (1,212 bp) was obtained and deposited in the Gen- Bank (accession number: OK094487) for further studies such as DNA barcoding and molecular phylogenetics. The pairwise distances were calculated based on 522 bp sequences of COI and this dataset consisted of seven species of Araeosoma including A. owstoni , and seven other echinothuriids ( Table 1). Numerous data of Araeosoma in the GenBank were not identified to the species level. Fortunately, A. thetidis , a species closely related to A. owstoni , was correctly identified and registered. In the result of pairwise distance analysis, divergence between genus Araeosoma and other echinothuriids was 14.5% and the mean of interspecific divergence of genus Araeosoma is 6.5% ( Table 1). The average interspecific divergence between A. owstoni and other Araeosoma species was 6.8%, ranging from 3.2% with A. thetidis to 10.8% with Araeosoma sp. 6 and 7 ( Table 1). For most cases, the interspecific divergence in echinoderm ranged from 2.5 to 24.2% (Arndt et al., 1996; Hart et al., 1997; Ward et al., 2008) and the mean was around 12.0% ( Layton et al., 2016). Interestingly, closely related echinoid species have a low interspecific divergence as 2-3% ( Palumbi et al., 1997). As a result of the DNA barcoding in this study, A. owstoni from Korea can be distinguished from other Araeosoma species and has a close relationship with A. thetidis .

Figures

Fig. 2. Araeosoma owstoni Mortensen, 1904. A, aboral side; B, oral side. C, tridentate pedicellaria and its valves; D, valves of triphyllous pedicellaria.

Fig. 1. Distribution of Araeosoma owstoni Mortensen, 1904. A, collection localities of A. owstoni, A. owstoni bicolor (A. Agassiz and H.L. Clark, 1907) and A. owstoni nudum Mortensen, 1934 from previous studies; B, collecting site of A. owstoni of this study and marked as ‘★’ on the map.