Dorhynchus Wyville Thomson, 1873

Ng, Peter K. L., Forges, Bertrand Richer De & Lee, Bee Yan, 2023, New and rare deep-sea majoid crabs (Crustacea: Brachyura) from Papua New Guinea, Zootaxa 5318 (1), pp. 1-44 : 2-3

Genus Dorhynchus Wyville Thomson, 1873

Type species. Dorhynchus thomsoni Wyville Thomson, 1873 , by monotypy.

Remarks. Manning & Holthuis (1981: 280–281) commented that the species in Achaeopsis Stimpson, 1857 , can be split into two groups: one with shorter pseudorostral spines, relatively shorter and stouter P2–P5, the P2 merus is shorter than the carapace length, the P2 and P3 dactyli are gently curved and the P4 and P5 dactyli falciform, and occurring in waters less than 200 m depth ( A. spinulosa Stimpson, 1857 , type species); and the other with longer pseudorostral spines, relatively longer and more slender P2–P5, the P2 merus is longer than the carapace length, the P2–P5 dactyli all gently curved and occurring in waters greater than 200 m depth ( Dorhynchus thomsoni Wyville Thomson, 1873 ; Lispognathus furcatus A. Milne-Edwards, 1880 ; Stenorhynchus ramusculus Baker, 1906 , Achaeopsis rostrata Sakai, 1932 ). They suggested that the second group of species should be referred to Dorhynchus Wyville Thomson, 1873 , instead. This was followed by Davie (2002) and has been adopted ever since. We are not fully convinced that this is a natural dichotomy, and some of the characters of the present new species challenge this separation. The type species of Dorhynchus ( Dorhynchus thomsoni ) is unusual in that the pseudorostral spines, frontal carapace regions, epistome and antennules are transversely much narrower (cf. Barnard 1950: fig. 4d) compared to most of the other species placed by Manning & Holthuis (1981) in the genus. Only the more recently described D. basi MacPherson, 1983 , from Namibia shares similar characters ( MacPherson 1983: figs. 21A, B, 22). The carapace of D. furcatus , however, is actually very similar to that of A. spinulosa , with both possessing relatively short pseudorostral spines, and notably, the lateral branchial spine is directed dorsally and not laterally (A. Milne-Edwards, 1880b: pl. 31A fig. 4). In other species of Dorhynchus (including D. thomsoni and the present new species, D. profundus ), the lateral branchial spine is directed laterally. In addition, the male anterior thoracic sternum (sternites 1–4) of D. thomsoni is relatively wide (cf. Barnard 1950: fig. 4c) while that of D. profundus n. sp. is transversely narrower, with sternites 3 and 4 laterally constricted ( Fig. 4E ). The condition of the male anterior thoracic sternums of the other species is not known. The ambulatory dactyli of D. furcatus , nevertheless, are like the other described species of Dorhynchus (A. Milne-Edwards, 1880b: pl. 31A fig. 4), with the exception of the present new species (see below). A reappraisal of Achaeopsis and Dorhynchus is clearly necessary, with all the constituent species re-examined to clarify generic delineations. As for the lengths of the P2 and P3 merus, we find that they are sometimes shorter than the pcl; one male (ZRC 2022.0067) has the P2 merus 1.15 times the pcl, while its P3 merus is 0.94 times the pcl; while a female (MNHN-IU-2015-888) has the P2 and P3 meri 0.89 and 0.86 times the pcl, respectively. In the specimen of D. rostratus examined, the P2 and P3 meri are 1.18 and 1.04 times the pcl. These proportions therefore need to be used very carefully. A large series of specimens of a few species will be needed to ascertain its reliability and degree of variation in members of Dorhynchus .

A note on the nomenclature of D. furcatus is necessary. The two names Lispognathus furcatus A. Milne-Edwards, 1880a , and Lispognathus furcillatus A. Milne-Edwards, 1880b , certainly refer to the same species as their accompanying data is the same. The genus Lispognathus was first established in A. Milne-Edwards (1880a) and the only described species was “ Lispognathus furcatus A. Milne-Edwards, 1880a ”. The genus was again described in a follow-up paper by A. Milne-Edwards (1880b) but this time, the only species described was “ Lispognathus furcillatus A. Milne-Edwards, 1880b ” (same spelling used in the plates). Why A. Milne-Edwards changed the spelling is not known but both are clearly objective synonyms as they share the same type series.

Figures

FIGURE 4. Dorhynchus profundus n. sp., holotype male (pcl 9.2 mm) (MNHN-IU-2011-2139), Woodlark Island.A, lateral view of cephalothorax; B, dorsal view of anterior part of carapace showing pseudorostrum and orbits; C, ventral view of epistome, antennae, antenules, orbit and pseudorostrum; D, right third maxilliped; E, anterior thoracic sternum and sternopleonal cavity; F, anterior thoracic sternum showing spinate ridge demarcating sternopleonal cavity on sternite 4; G, pleon.