Redescription of Vellumnus penicillatus (Gordon, 1930) from Hong Kong and description of a new species from Macclesfield Bank, South China Sea (Crustacea: Brachyura: Pilumnidae)

Abstract The poorly known pilumnid, Vellumnus penicillatus (Gordon, 1930), is redescribed by examining the Hong Kong types. A new species, Vellumnus tki n. sp., is described from Macclesfield Bank, South China Sea, and it is distinguished from its closest congener, Vellumnus minabensis (Sakai, 1969) from Japan, by the setal pattern on the carapace and pereiopods, carapace proportions, structure of the front and anterolateral teeth, armature on the chelipeds and relatively shorter ambulatory legs.

Vellumnus penicillatus was described from Hong Kong; it is poorly known and has only been reported once since, from Singapore (Balss, 1938).During a recent visit to the Natural History Museum (NHM), London, the opportunity was taken to re-examine the V. penicillatus type material.In this paper, the types of Gordon's species are redescribed and a new species is recognised from the Macclesfield Bank, South China Sea.
Material examined is deposited in the NHM and the Zoological Reference Collection (ZRC), Lee Kong Chian Natural History Museum, National University of Singapore.Measurements provided are in millimetres and are of the maximum carapace width and length, respectively.The following abbreviations are used: coll., collected by; G1, male first gonopods and G2, male second gonopod.The terminology used follows Davie et al. (2015).
Remarks.Of the five Vellumnus species originally recognised, Ng (2010) expressed doubt about the assignment of V. vermiculatus (A. Milne-Edwards, 1873) to this genus stating that it may require transferring to Lophoplax Tesch, 1918.Examination of type specimens as well as material from the West Pacific were examined and the carapace characters show it is closer to Lophoplax, notably in the hepatic, gastric, and intestinal regions being swollen and setae covering the surfaces around them (see also Maenosono, 2019;Trivedi et al., 2022).It has since been transferred to this genus (Ng and Rahayu, 2023).
Remarks.Vellumnus penicillatus is here described in detail with additional figures.The dorsal surface of the carapace comprises dense setae arranged in approximate patterns (Figs.2C, 3A) but is without underlying ridges, the surface is generally smooth with only scattered granules on the lateral parts (Fig. 1A, B).Takeda (1977a: 131) listed a number of characters that distinguished V. penicillatus from V. pygmaeus (type locality: Ogasawara Islands): "the median gastric region is indicated by a longitudinal strand ending in a diamond, a tomentum on each protogastric region forms a circle or ocellus, the areolation beneath the tomentum is only faintly indicated, the carapace is apparently broader with the nearly truncated second and third anterolateral teeth, the latter of which forms the lateral angle of the carapace, and the male first pleopod bears a strong subdistal spine."His carapace setal patterns, however, do not appear to be a good character because the original figure by Gordon (1931) is somewhat schematic (present Fig. 3A).The two species, however, are different; in V. penicillatus, the supraorbital and anterolateral margins are relatively longer; Fig. 3A-C (Gordon, 1930).A, B, Holotype female (4.2 × 2.9 mm) (NHM 1930.12.2.123), Hong Kong; C-I, paratype male (3.6 × 2.9 mm) (NHM 1930.12.prominent swelling posterior to the first anterolateral tooth in V. pygmaeus; Takeda, 1977a: fig.5A); and the G1 is more slender and sinuous with the distal part proportionately longer and more distinctly tapering; Fig. 3H, I (vs.stouter and less sinuous with the distal part shorter in V. pygmaeus; Takeda, 1977a: fig.5B, C).
The specimen from Singapore reported as "Planopilumnus penicillatus" by Balss (1938: 60) and "Vellumnus penicillatus" by Ng (2010: 51) is not this species.This specimen was re-examined.It appears to be a male in poor condition, being damaged and extremely soft with all the appendages detached.
New Vellumnus species from Macclesfield Bank, South China Sea Nauplius, 31: e2023014 It seems to be a young V. labyrinthicus.As such, V. penicillatus s. str.remains restricted to Hong Kong.
Etymology.This species is named for the High Commissioner of the Republic of Singapore, London, in gratitude for all his support of the natural history digital repatriation project between the Lee Kong Chian Natural History Museum, Singapore, and the Natural History Museum, London.His Excellency Lim Thuan Kuan is known as "TK" to his friends (pronounced "tee-kay"), and is the origin of "tki".Remarks.The holotype was examined by Heinrich Balss in June 1932 and identified as "?Lophoplax (Pilumnoplax) sculpta", but it is not this pilumnid species (see Takeda, 1977b;Maenosono, 2019;Ng and Rahayu, 2023).Instead, it is considered here to be an undescribed Vellumnus species.
In the relatively larger size, (ca.carapace width > 4-5 mm), a more quadrate shaped carapace, distinct carapace areolation, e.g., regions well demarcated, and relatively wider male anterior thoracic sternum, Vellumnus tki n. sp. is most similar to V. minabensis, but differs in that the carapace is more quadrate in shape, e.g., width to length ratio 1.30; Figs.4A, 5A (vs.carapace proportionately wider, e.g., width to length ratio 1.4 in V. minabensis); the frontal lobe is convex; Figs.5A, 6A (vs.frontal lobe strongly convex in V. minabensis); the regions of carapace have scattered granules only on the anterior margins; Figs.5A, 6A (vs.regions of carapace covered with numerous granules in V. minabensis); the anterolateral teeth are proportionately smaller; Figs.5A, 6A (vs.anterolateral teeth large and prominent in V. minabensis); the granules on the carpus and chela are distinctly more dispersed; Fig. 5B, C (vs. granules on the carpus and chela densely packed in V. minabensis); and the ambulatory merus appears to be relatively shorter; Figs.4A, 5D (vs.ambulatory merus relatively longer in V. minabensis; cf.Sakai, 1969: text-fig. 13;Sakai, 1976: text fig. 263, pl. 176, fig. 1).The G1 structures of the two species are similar, with the distal part slightly shorter with the tip more truncate in V. tki n. sp.; Fig. 6G-I (vs. longer and more tapering in V. minabensis;Sakai, 1969: text-fig. 12a) but this difference is not significant at the species level.The arrangement of setae on the carapace of V. tki n. sp. is also different from that of V. minabensis.Sakai (1969) described the setae on the carapace of V. minabensis as arranged in a "vermiform" manner (Sakai, 1969: 266) but his original figure (Sakai, 1969: text-fig. 13) does not illustrate this character.In a later color figure of the species, however, he showed the vermiform setal patten (Sakai, 1976: pl. 176 fig.1).This carapace setation pattern resembles that of V. labyrinthicus except that in V. minabensis, the carapace is proportionately wider and the anterolateral teeth are relatively weaker and directed obliquely anteriorly (Sakai, 1969: fig. 13;Sakai, 1976