Tunebia, Mendoza, Jose Christopher E. & Ng, Peter K. L., 2011

Mendoza, Jose Christopher E. & Ng, Peter K. L., 2011, The Polydectinae Dana, 1851, of the Philippines, with description of a new genus for Lybia hatagumoana Sakai, 1961 (Crustacea: Decapoda: Brachyura: Xanthidae), Zootaxa 3052, pp. 51-61 : 55-56

Tunebia gen. nov.

Lybia — Sakai 1965: 162; 1967: 77; 1976: 503 (in part). — Guinot 1976: 69 (in part). — Serène 1965: 26; 1968: 88; 1984: 25 (in part). — Tan & Ng 1994: 738 (in part). —Ng et al. 2008: 201 (in part).

Veles Števčić, 2011: 137 (pre-occupied name).

Type species. Lybia hatagumoana Sakai, 1961 , by present designation. Gender feminine.

Other species included. Lybia tutelina Tan & Ng, 1994 . Type locality: New Caledonia.

Diagnosis. Carapace longitudinally sub-ovate, length equal or sub-equal to width; dorsal surface granulate; regions well defined, 2M, 3M, 4M entire, branchial region divided into several areolets, 1P bissected by short, longitudinal furrow; front wide, about 0.4 times maximum carapace width, truncate, distinctly bilobate; anterolateral margin continuous with posterolateral margin, both margins distinctly denticulate to granulate, anterolateral margin with 3 low, denticulate lobes, including exorbital angle, third lobe not clearly distinct from postero-lateral margin; posterior margin granulate. Eyes large, corneas well developed. Basal antennal article with medial extension; antennal flagellum long, setose. Epistome narrow, central region broadly triangular. Endostome without prominent crests. Third maxillipeds narrow, inner margins denticulate. Anterior thoracic sternum coarsely granular; sternites 1, 2 fused, separated from sternite 3 by distinct suture, posterior margin of sternite 2 lined with round granules; sternite 3 separated from sternite 4 by deep, transverse depression; sternite 4 inflated, posterior region with deep median depression anterior to and continuous with sterno-abdominal cavity. Chelipeds slender, much shorter than ambulatory legs (P2–P5); P2–P4 subequal in size, P5 distinctly smaller, all granular, setose. Male abdomen proportionally short, somites 3–5 immovably fused but sutures visible. G1 short, distal tip not bifurcate, but with broad, prominent apical lobe. G2 about one-third G1 length.

Etymology. This new genus is named in honor of Japanese carcinologist, Professor Tune Sakai. It is an arbitrary combination of his first name and Lybia . Gender feminine.

Remarks. Guinot (1976), in her revision of the Polydectinae , recognized three species groups within the genus Lybia : one group consisting of L. tessellata and L. edmondsoni Takeda & Miyake, 1970 ; another consisting of L. leptochelis and L. plumosa ; and a third with only L. denticulata Nobili, 1906 . This grouping was done on the basis of the outline of the carapace, the condition of the carapace anterolateral margins, the first and third maxillipeds, the male thoracic sternum, ambulatory legs, male abdomen and G1. She, however, did not treat the other known species at that time as she did not have specimens of them. Serène (1984: 25) agreed with this grouping. Tan & Ng (1994: 738), in their description of a new species, L. tutelina , opined that it, together with the closely related L. hatagumoana , formed yet another grouping within Lybia . They, however, deferred from taking any further action.

As a good series of specimens of L. hatagumoana was available for this study, and the differences between this species (also with L. tutelina ) and other Lybia species were very obvious, we intended to establish a new genus for these two species. As the present paper was being finished, however, Števčiċ (2011: 137) established a new genus, Veles , for L. hatagumoana , and provided a short diagnosis: “Subcircular, slightly wider than long; front thick, rather wide, projecting beyond orbits, bilobed, median notch distinct, antennular sinus small; lateral margin continuous with posterolateral one; ambulatory legs stouter and longer than chelipeds; abdomen in both sexes relatively narrow and 7-segmented; first gonopod slightly curved, apex simple (not bifid).” He did not provide information on material examined or any discussion about why it was necessary to establish a new genus for L. hatagumoana , how the new genus can be distinguished from Lybia and other polydectines or whether it included other species. As it turns out, Veles Števčiċ, 2011 , is a junior homonym of Ve l es Bangs, 1918 (Aves), Veles Pakhorukov, 1981 (Arachnida) , and Veles Sugiyama, 1997 (Radiolaria) , and is therefore unavailable. As such, we retain our original name, Tunebia gen. nov., for these two species.

Tunebia gen. nov. can be distinguished from Lybia (type species: Grapsus tessellatus Latreille , in Milbert, 1812) (cf. Guinot, 1976) by the following features: 1) the sub-ovate outline of the carapace, where the anterolateral margin is continuous with the posterolateral margin, and which also tends to be proportionally longer and narrower ( Figs. 2 A, 3A) (carapace with sub-hexagonal outline, anterolateral margin distinct from posterolateral margin; carapace obviously wider than long in Lybia ); 2) the distinctly denticulate lateral and posterior margins of the carapace ( Figs. 2 A,3A) (smooth in Lybia ); 3) the P2 being nearly as long and robust as the P3, P4 (P2 distinctly smaller than P3, P 4 in Lybia ); 4) absence of prominent crests on the endostome (present in L. tessellata and L. edmondsoni ); 5) the anterior thoracic sternum of the male being coarsely granulate ( Figs. 2 B, 3D) (anterior thoracic sternum smooth in Lybia ); 6) thoracic sternite 4 inflated, with a deep, median depression just anterior to the sterno-abdominal cavity ( Figs. 2 B,3D) (thoracic sternite 4 flat, without prominent median depression in Lybia ); and 7) the G1 is not bifurcate distally and has a broad and straight apical lobe, without long, plumose, subterminal setae ( Fig. 3 I, J) (distally bifurcate, apical lobe narrow and curved, with several plumose, subterminal setae in L. tessellata and L. edmondsoni ).

Sakai (1967) and Guinot (1976) considered the monotypic Prolybia Ward, 1933 (type species: Prolybia australiensis Ward, 1933 ) as a junior synonym of Lybia , and this was followed by subsequent workers ( Serène 1984; Ng et al. 2008; De Grave et al. 2009). Ward (1933: 386) thought that Prolybia differed from Lybia in the form of the carapace, front, orbits, third maxillipeds, epistome and ambulatory legs. As no specimens could be examined for the present study, only the figure provided by Ward (1933: pl. 21 figs. 3, 4) could be used for comparison. Prolybia australiensis actually resembles species of Lybia such as L. caestifera , L. leptochelis and L. plumosa , in having a subhexagonal carapace, a moderately produced front, and trilobate carapace anterolateral margins. Only the ambulatory legs, being shorter relative to the carapace, seem to differentiate this species from the others. Prolybia australiensis is a poorly understood species, however, and is known only from the holotype, a female, deposited in the Australian Museum (Sydney). A close examination of the holotype and additional material is needed to ascertain the true systematic position of this species. Nonetheless, Tunebia gen. nov. differs from Prolybia in having 1) a subovate outline of the carapace (subhexagonal in Prolybia ); 2) a much more produced front (less produced in Prolybia ); 3) denticulate carapace lateral and posterior margins (vs. smooth in Prolybia ); and 4) the carapace width equal or only slightly greater than the carapace length (carapace distinctly broader than long in Prolybia ). All the above mentioned differences are significant enough to recognise Tunebia as a distinct genus. This was also supported by a recent molecular study of the Xanthidae which shows that T. hatagumoana is in a separate clade from Lybia s. str. ( Lai et al. 2011).

Tunebia gen. nov. contains L. hatagumoana (type species) and L. tutelina (cf. Tan & Ng 1994: 738, figs. 1, 2), these two species being quite unique and easily separable from species of Lybia . Both have been found in relatively deeper waters than species of Lybia , with all specimens collected from depths no less than 70 m (Sakai 1961; Baba & Noda 1993; Tan & Ng 1994), whereas Lybia . s. str. tends to be collected from shallower subtidal habitats ( Sakai 1976; Takeda & Miyake 1970).

Figures

FIGURE 2. Tunebia hatagumoana (Sakai, 1961), male, 3.9 × 3.9 mm (ZRC 2011.0772). A, dorsal view; B, ventral view.

FIGURE 3. Tunebia hatagumoana (Sakai, 1961); A – H, male, 3.2 × 3.1 mm (ZRC 2011.0773); I – K, male, 3.9 × 3.9 mm (ZRC 2011.0772). A, carapace, dorsal view; B, anterior and buccal area, ventral view; C, right mxp 3, external view; D, anterior thoracic sternum, ventral view; E, abdomen, external view; F, right chela, external view; G, left P 2, dorsal view; H, left P 4, dorsal view; I, J, left G 1, external view (I), internal view (J); K, right G 2, external view. Scale bars: A, B, G, H = 1.0 mm; C – F, I – K = 0.5 mm.