Brachynotus, de Haan, 1835

McLay, C. L. & Schubart, C. D., 2004, Short Communication On the correct name for Hemigrapsus edwardsii (Hilgendorf, 1882) (Brachyura: Varunidae) from New Zealand, Journal of Natural History 38 (6), pp. 695-704 : 697-698

Comparison of Brachynotus and Hemigrapsus

Morphological differences between adult Hemigrapsus and Brachynotus (based on the types H. crassimanus and B. sexdentatus (as B. lucasi )) have been outlined by Rathbun (1931) (see Introduction). Examination of the New Zealand material showed that H. sexdentatus and H. crenulatus share the morphological characters defined as typical for Hemigrapsus by Rathbun (1931).

The genus Brachynotus de Haan, 1835 is nowadays restricted to the Atlantic Ocean. Currently, there are four valid species, namely B. atlanticus Forest, 1957 , B. foresti Zariquiey Alvarez, 1968 , B. gemmellari (Rizza, 1839) and B. sexdentatus (type species) (Risso, 1827). Cuesta et al. (2000) and Schubart et al. (2001) point out that B. gemmellari might be the same species as B. sexdentatus , representing an eco-phenotypic variation. However, additional results will be needed to confirm this possible synonymy. Ingle (1980: 124) gives the following definition of Brachynotus de Haan, 1835 : ‘Carapace broader than long, convex longitudinally or somewhat flattened; lateral margins relatively straight. Carpus of third maxilliped articulating at middle of dorsal margin of merus; a very narrow rhomboid-shaped gap between inner margins of third pair of maxillipeds. Chelipeds relatively smooth or carinate’.

The genus Hemigrapsus Dana, 1851 is only found in the Indo-Pacific Oceans, with the exception of recent introductions of two species into the Atlantic Ocean (Breton et al., 2002). Dana (1851: 348) defined Hemigrapsus as follows: ‘Carapace nearly smooth, not marked by transverse lines, front nearly straight, sides arcuate. Third maxillipeds separated by a large rhomboidal space, third joint orbiculocordate. Antennules transverse. Process outside the antennae transverse, triangular, set closely against first joint of antennae’. Rathbun (1918: 264), who revalidated Hemigrapsus , gave a more detailed definition of the genus: ‘Carapace broader than long, quadrate with anterolateral margins rounded and dentate; depressed; an oblique ridge runs inward and backward from the posterolateral margin. Front less than half width of carapace. Orbits of moderate size. Antennules folding obliquely. Tooth at inner angle of orbit well developed. Antenna filling the orbital hiatus; flagellum entering the orbit. The suborbital crest forms a tuberculated or striated stridulating ridge which scrapes against the distal end of the arm. Epistome well developed. Buccal cavity quadrate with the anterior corners rounded. Outer maxillipeds moderately gaping; merus broader but very little shorter than the ischium, and as broad as, or broader than long; its inner and outer margins convex, anterior margin excavate, with the palpus inserted toward its outer angle. Chelipeds equal or sub-equal, stout. Palms often with a patch of hair inside. Fingers hollowed out beneath in a shallow groove. Legs of moderate length, and almost unarmed. The abdomen of the male does not cover the sternum at its base.’ Currently there are 11 species in the genus Hemigrapsus Dana, 1851 , namely H. crassimanus Dana, 1852 (type species), H. crenulatus (H. Milne Edwards, 1837) , H. gibbus (Jacquinot, in Hombron and Jacquinot, 1843), H. longitarsis (Miers, 1879) , H. nudus (Dana, 1851) , H. oregonensis (Dana, 1851) , H. pallipes (H. Milne Edwards, 1837) , H. pencillatus (de Haan, 1835) , H. sexdentatus (H. Milne Edwards, 1837) , H. sinensis Rathbun, 1931 and H. spinosus (H. Milne Edwards, 1853) . This genus is morphologically quite variable and will need to be revised.

Comparison of larval morphological characters by Cuesta et al. (2000) revealed that zoea larvae of the two Atlantic genera Brachynotus and Cyrtograpsus Dana are distinct from all other Varunidae , including Hemigrapsus : ‘These two genera [ Brachynotus and Cyrtograpsus ] can be distinguished from the rest of Varunidae by the morphology of the telson, which only acquires one pair of serrulate setae on the posterior margin in the course of the development. In contrast, the rest of the Varunidae acquire two or three pairs of serrulate setae’ (Cuesta et al., 2000: 218). Re-description of the larvae of the two species of Hemigrapsus from New Zealand confirmed these morphological differences (Cuesta et al., 2001).

The conclusion of the present study is that the New Zealand species originally described by H. Milne Edwards (1837) as Cyclograpsus sexdentatus and C. crenulatus should currently be placed within the genus Hemigrapsus Dana, 1851 , which is distinct from Brachynotus de Haan, 1835 . The correct name for Cyclograpsus sexdentatus H. Milne Edwards, 1837 is therefore Hemigrapsus sexdentatus (H. Milne Edwards, 1837) .