Macromedaeus adelus Mendoza, 2021

Macromedaeus adelus Mendoza, 2021 View in CoL # ( Fig. 25A–D View Fig )

Xantho distinguendus View in CoL — Gordon 1931: 528, 543, text-figs. 21, 22C; Forest and Guinot 1961: 57, pl. 1(3).

Macromedaeus adelus View in CoL — present record.

Taxonomy: Mendoza (2021).

Type: Holotype ò ZRC 2017.0465 View Materials and paratypes in ZRC.

Type locality: Punggol End, Singapore.

Distribution: South China Sea: so far from Singapore ( Mendoza 2021) and South China (present study).

Habitat: Intertidal; middle to low tidal levels, under rock fragments.

Remarks: In describing Macromedaeus adelus from Singapore and providing an overview of the genus, Mendoza (2021) revisited Buitendijk’s (1960) doubts about the identities of Xantho distinguendus reported by Gordon (1931), in comparison with then syntypes of C. (X.) distinguendus . Gordon (1931) commented on the morphological similarities in the specimens of Medaeops granulosus (as Medaeus ), Leptodius affinis (as Xantho exaratus ) and Macromedaeus distinguendus (as Xantho ) collected from Hong Kong. The material of the latter species was described as “(g)ranulation of chelipeds almost obsolete”, “(u)pper margin of merus of walking-legs not crested” and “(carpus and propodus of walking-legs) hardly any trace of the granulation” (p. 544). The morphologies of the chelae and ambulatory legs described as such differ substantially with that of M. distinguendus , which are common along intertidal mudflats of Hong Kong ( Fig. 25E–H View Fig ).

The second author recently reexamined material deposited at the NHM previously examined by Gordon, with the catalogue number NHM 1930.12.2.150, collected by Barney from Hong Kong. The lot comprises three males (two adults, one juvenile) and nine females (three ovigerous). A large male (10.0 × 6.2 mm) is illustrated herein. In comparison with M. distinguendus , which is common in Japan, Korea and North China ( Fig. 25E; e.g View Fig ., Shen 1932b; Sakai 1939 1976; Kim 1973; Yuan et al. 2022), both adult males of Gordon’s material have ambulatory legs with carpi and propodi not crested ( Fig. 25D View Fig ; vs. Fig. 25H View Fig ), and frontal lobes medially separated by a deep V-shaped cleft ( Fig. 25A View Fig ; vs. Fig. 25E View Fig ). The surface of the major chela in these specimens is externally granular on upper half and nearly smooth on lower portion ( Fig. 25B View Fig ), in contrast with M. distinguendus being entirely coarsely granular and dorsally rugose ( Fig. 25F View Fig ). The features observed in Gordon’s (1931) material confirm the presence of M. adelus in Hong Kong. Macromedaeus distinguendus , as Serène (1984) noted (p. 178), appears to be distributed in the Sino-Japanese area in temperate-subtropical East Asia.

Concerning the two recently described, and apparently tropical M. adelus and M. hainanensis Yuan, Jiang & Sha, 2022 , both showing overlapping geographical reported thus far (from northern South China Sea towards the equatorial Southeast Asia), recent studies by the second author (PKLN) and J.C.E. Mendoza (ZRC) has concluded both names being synonyms, with M. adelus enjoying priority, that M. adelus published on 5 November 2021 while M. hainanensis was released on 18 January 2022. Our present comparisons agree with this. Nevertheless, the presence of both sub-tropical to temperate ( M. distinguendus ) and tropical ( M. adelus ) lineages in Hong Kong is not surprising.