Hyastenus inermis ( Rathbun, 1911 )

Yan, Bee, Lee & Ng, Peter K. L., 2019, On the identity of Hyastenus inermis (Rathbun, 1911), and description of a new species from Sulawesi, Indonesia (Crustacea: Decapoda: Majoidea: Epialtidae), Raffles Bulletin of Zoology 67, pp. 490-497 : 491-495

Hyastenus inermis ( Rathbun, 1911)

( Figs. 1 , 2 A–F)

Halimus inermis Rathbun, 1911: 194 (list), 250, pl. 20 fig. 6 (type locality: Amirante Islands , outer islands of Seychelles).

Hyastenus inermis – Balss, 1935: 123 (list). – Guinot, 1967: 294 (list). – Griffin, 1974: 15. – Griffin & Tranter, 1974: 171. – Griffin & Tranter, 1986: 124 (list), 125 (key), 148. – Ng et al., 2008: 103 (list).

Hyastenus convexus var. hendersoni – Monod, 1938: fig. 4H, I [not Hyastenus convexus var. hendersoni Laurie, 1906 ].

Material examined. Holotype: male (15.5 × 10.6 mm) ( USNM 41402 ), stn E6, Amirante islands , Indian Ocean, 51 m, coll. 9 October 1905 . Paratypes: 3 males (12.5 × 7.8 mm, 11.6 × 7.6 mm, 10.3 × 6.4 mm), 3 females (13.9 × 9.1 mm, 10.3 × 6.2 mm, 7.6 × 4.8 mm), 1 ovigerous female (11.6 × 7.0 mm), 1 juvenile, same locality data as holotype ; 1 male (9.5 × 5.5 mm) ( USNM 41403 ), stn C12, Saya de Malha , 86 m, coll. 6 September 1905 ; 1 male (11.9 × 7.1 mm), 1 female (10.2 × 6.1 mm) ( USNM 41404 ), stn B15, Cargados Carajos , 55 m, coll. 30 August 1905 ; 3 males (15.0 × 9.3 mm, 11.6 × 7.5 mm, 11.1 × 6.8 mm) ( USNM 41401 ), stn F5, Seychelles, 80 m, coll. 20 October 1905 . Others : 1 juvenile male (3.0 mm × 5.0 mm), 2 females (9.8 × 6.0 mm, 11.6 × 7.6 mm) (NHM 1912.2.10.85-87), Seychelles, “Sealark” expedition, coll. JS Gardiner.

Diagnosis. Carapace pyriform; covered with setae, carapace surface smooth when denuded. Carapace region defined; gastric region swollen with 2 granules on edge of distinctly swollen gastric region; cardiac region slightly swollen, intestinal region slightly swollen ( Fig. 1A, C ). Antennal flagellum slightly shorter than or nearly as long as pseudorostral spines. Basal antennal article longer than broad; sharp distolateral angle, outer margin concave, slightly constricted, base of distal lateral margin of article rounded. Pterygostomial region with 2 granules on outer margin ( Fig. 1B ). Male thoracic sternum slightly concave anteriorly; sternites 3, 4 constricted proximally, widest basally, with lateral margin slightly constricted. Male pleon slender, triangular, telson triangular ( Fig. 1B ). G1 straight, with ear-like lobe at tip, distal half slightly twisted ( Fig. 2 A–D).

Description. Carapace pyriform; covered with setae, carapace surface smooth when denuded. Pseudorostral spines straight, diverging; approximately 0.2–0.4 of carapace length. Supraorbital eave forms rectangular lobe; preorbital angle distinct, antorbital angle short, blunt; orbital hiatus between supraorbital and postorbital lobes form narrow U-shape; postorbital lobe cup-shape. Carapace regions defined; gastric region prominently swollen with 2 granules on edge of swollen gastric; cardiac region slightly swollen, intestinal region slightly swollen ( Fig. 1A, C ).

Antennal flagellum slightly shorter than or nearly as long as pseudorostral spines. Antennules completely retractable in deep fossae. Basal antennal article longer than broad; sharp distal angle of article, outer margin concave, slightly constricted, base of distal lateral margin of article rounded with a granule. Buccal frame covered by third maxilliped. Pterygostomial region with 2 granules on outer margin ( Fig. 1B ).

Male cheliped with propodus slightly swollen, 3 granules on dorsal surface, 1 granule on posterior margin, 1 granule on outer surface; carpus with 4 granules on outer surface; merus with spine on distal angle. Female cheliped slender and smooth. Ambulatory legs slender, surface smooth; P2 longest ( Fig. 1A ).

Male thoracic sternum slightly concave anteriorly; sternites 3, 4 constricted proximally, widest basally, lateral margin slightly constricted. Male pleon slender, triangular, telson triangular, somites 1–6 and telson free ( Fig. 1B ). G1 straight, with ear-like lobe at tip, distal half slightly twisted; G2 slightly curved, with rounded distal tip ( Fig. 2 A–F).

Remarks. Hyastenus inermis was described by Rathbun (1911) from 16 males, 14 females and 13 juvenile specimens collected from Cargados carajos, Saya de Malha, the Amirante Islands, and Seychelles, in the Indian Ocean. In the material listed provided by Rathbun (1911: 250), one male specimen collected from the Amirante Islands, station E6, was designated as the type for the species, and in the captions of her plates ( Rathbun, 1911: 261), she also identified a male as the “type”. This male specimen (USNM 41402) is regarded as the holotype, while the remaining specimens should be treated as paratypes. Out of the series of material listed by Rathbun (1911), a total of seven males, two females and one juvenile specimen, inclusive of the holotype, were examined in this study.

Rathbun (1911) compared H. inermis to Thusaenys irami ( Laurie, 1906) , due to both species lacking a lateral branchial spine on the carapace. Thusaenys irami , however, has a granule on the outer margin of the carapace between the preorbital and postorbital lobes, and long pseudorostral spines (approximately 0.5–0.6 of carapace length) ( Laurie, 1906: 379, pl. 1 fig. 4), characters which are absent in H. inermis . Rathbun (1911) also suggested that a specimen from the Philippines identified as H. diacanthus (De Haan, 1837) by Miers (1886) is more likely H. inermis instead. Rathbun (1911), however, also commented that even though the orbital, antennal and pterygostomial regions of H. inermis are similar to H. diacanthus , the latter has a carapace surface covered with granules and lateral branchial spines, subparallel pseudorostral spines, and attains a larger adult size. In addition, Miers (1886) commented that young specimens of H. diacanthus are more similar to H. planasius ( Adams & White, 1848) but differ in the degree of setation on the carapace, the divergent pseudorostral spines, and the structure of the distal angle of the basal antennal article. As such, it seems unlikely that the Philippine material is H. inermis .

Comparison of the holotype G1 morphology of H inermis s. str. to that of the Iranian specimen as figured by Naderloo (2017: fig. 16.4e) shows that the latter lacks the characteristic ear-like lobe at the tip observed in H. inermis ( Fig. 2 A–D). It is likely that the Iranian specimens (see Naderloo & Sari, 2007a; 2007b; Naderloo et al., 2015) are a separate species, but as we do not have access to this material, we cannot be certain.

Hyastenus inermis is distinct among congeners in that it lacks the typical lateral branchial spines on the carapace, which is observed on other species of Hyastenus either as weak or strong spine; and has a very distinct swollen gastric region ( Fig. 1A ). It is most similar to H. tabolongi , new species, but the two species can easily be separated by several distinct differences (see next species).

The biology of H. inermis is not known, and we do not know if it has the same habits as H. tabolongi , new species. All the type specimens have been cleaned and no epibionts were visible.

Distribution. Hyastenus inermis is known from its type locality, Amirante, Indian Ocean, and between Mauritius and Cape Guardafui ( Rathbun, 1911; Griffin, 1974).

RAFFLES BULLETIN OF ZOOLOGY 2019

Figures

Fig. 1. Hyastenus inermis (Rathbun, 1911), holotype male (15.5 × 10.6 mm) (USNM 41402), Indian Ocean. A, overall dorsal view of carapace; B, overall ventral view; C, lateral view of carapace.

Fig. 2. Drawing of left G1 and G2. A–F, Hyastenus inermis (Rathbun, 1911), holotype male (15.5 × 10.6 mm) (USNM 41402), Indian Ocean; G, H, H. tabolongi, new species, holotype male (10.7 × 6.6 mm) (MZB Cru 5010), north Sulawesi. A, G, ventral view; B, ventral view of distal portion; C, H, dorsal view; D, dorsal view of distal portion; E, ventral view of G2; F, dorsal view of G2. Scale bars = 1 mm.