Heteromysis (Gnathomysis) Bonnier et Perez 1902

Heteromysis (Gnathomysis) Bonnier et Perez, 1902

Gnathomysis Bonnier et Perez, 1902: 117 (originally as a genus).

Chiromysis.—Hilgendorf, 1878: 845, partim.

Heteromysis (Gnathomysis).— Băcescu, 1968: 235, 236.— Băcescu & Bruce, 1980: 70.— Bowman & Orsi, 1992: 739.— Vannini et al., 1993: 190.— Bravo & Murano, 1996: 483.— Bamber, 2000a: 133.— Wittmann, 2000: 287; 2008: 370.— Wittmann & Abed-Navandi, 2019: 81.— Hanamura & Kase, 2001a: 15, 17, 18.— Lowry & Stoddart, 2003:446.— Daneliya, 2012: 146.— San Vicente & Monniot, 2014: 340.

Type species. Gnathomysis gerlachei Bonnier et Perez, 1902: 117, by monotypy.

Diagnosis. Eye without distomedial spine (spines) or tubercles. Antennular peduncle segment 3 with nonflagellated and non-modified setae. Ischium of pereopod 1 endopod nearly as long as wide, triangular; medially with denticles. Merus of pereopod 1 endopod semilunar in crosssection, sulcate medially; without distomedial process; its medial margin without flagellated spiniform setae, and lateral margin with smooth setae. Carpopropodus of pereopod 1 rather massive, longer than or as long as merus, with medial spiniform setae, and without long serrated paradactylary setae. Penis tubular, without setae. Pleopods unmodified in both sexes. Uropodal endopod longer than exopod.

Comparison. Gnathomysis is uniquely characterized by the exceptionally developed pereopod 1 endopods, which have the broad, nearly triangular, serrated ischium and the semilunar in cross-section merus; the uropodal endopod is longer than the exopod. From Neoheteromysis, in addition to the mentioned unique features, Gnathomysis differs by the penis that has no setae (present in Neoheteromysis), and the absence of the male pleopod modifications. Comparison with Olivemysis can be found here in the relevant subgeneric section.

Remarks. Bonnier & Perez (1902) described Gnathomysis as a genus, and, in fact, as the type genus for a new family, which they did not name. They provided a rather detailed description of the genus, which was distinguished by the particularly enlarged pereopod 1 endopod (gnathopod) and the rudimentary pleopods in both sexes. W. M. Tattersall (1922) clearly recognized a heteromysine in it and synonymized Gnathomysis with Chiromysis, and it was considered so (at least by W. M. Tattersall, 1951) until Băcescu (1968) re-established the name under the subgeneric status. He considered the pereopod 1 as a maxilliped 3 (later he abandoned this term). Its endopod was so enlarged that M. Băcescu thought it had a reduced exopod (repeated by subsequent authors: Bowman & Orsi, 1992; Bravo & Murano, 1996). He also noticed that the uropodal endopod was longer than the exopod in Gnathomysis, compared to other subgenera, where it is shorter. Later (Băcescu & Bruce, 1980) he also added a “half-moon-like” shape of the pereopod 1 merus and roughly triangular shape of the ischium with the serrations to the diagnosis. Hanamura & Kase (2001a) preferred not to distinguish subgenera within Heteromysis at all, but rather considered groups, particularly naming Gnathomysis species as Heteromysis harpax group. Wittmann (2008), in turn, accepted the subgenera and added some negative features to the diagnosis of Gnathomysis: the absence of modified setae on the antennular peduncle and pleopods, and the absence of the sternal processes. Earlier I studied specimens of H. (G.) harpaxoides and found that the pereopod 1 exopod is equally developed as other exopods, looking contrastingly short only in comparison with the tremendously enlarged endopod. Also the males had the short conical sternal processes, especially developed on the sternites 1−3 (Daneliya, 2012). I mentioned that the uropodal endopod was as long as the exopod, but previous authors were more correct indicating that the endopod was indeed slightly longer than the exopod. Here I provide a revised diagnosis of the subgenus, comparing it with the other subgenera.

Băcescu & Bruce (1980) informally distinguished two groups in the subgenus based on the structure of the pereopod 1 endopod: one with the merus finely serrate only on the posteromedial margin, bearing spiniform setae in the distal part, and the carpopropodus having two sets of the spiniform setae, proximal and distal; and another group with only rougher serrations on both antero- and posteromedial margins of the merus, without spiniform setae and the carpopropodus with one set of medial spiniform setae along the margin. The latter group is confined only to the Australian waters.

Distribution. The species of Gnathomysis so far have been recorded exclusively from the West Indo-Pacific Region.

Habitat. Specialization for commensalism with hermit crabs (Vannini et al., 1993).

Composition. The subgenus Heteromysis (Gnathomysis) includes four species: Heteromysis (Gnathomysis) gerlachei (Bonnier et Perez, 1902), Heteromysis (Gnathomysis) harpax (Hilgendorf, 1878), Heteromysis (Gnathomysis) harpaxoides Băcescu et Bruce, 1980, Heteromysis (Gnathomysis) stellata Băcescu et Bruce, 1980. Among them one species, H. (G.) harpaxoides, is recorded from the Tasman Sea.