Dynomene guamensis, Mclay, 2001

Mclay, Colin L., 2001, Dynomenidae and Dromiidae (Decapoda, Brachyura) from Guam, Philippine Islands, Tonga and Samoa, Zoosystema 23 (4), pp. 807-856 : 814-818

Dynomene guamensis n. sp.

( Figs 2 ; 3B )

TYPE MATERIAL. — Holotype 11.0 × 9.7 mm, Piti Lagoon , Guam ( MNHN-B26476 ).

MATERIAL EXAMINED. — Guam. Piti Lagoon, 13°27’N, 144°47’E, 1-3.5 m, among dead coral, III.1997, coll. H. T. Conley, 1 11.0 × 9.7 mm.

ETYMOLOGY. — The specific name alludes to the island of Guam, the type locality.

SIZE. — Type 11.0 × 9.7 mm.

DEPTH AND HABITAT. — Type specimen collected from dead coral between 1-3.5 m.

DISTRIBUTION. — Only known from the type locality at Guam, North Pacific Ocean.

DESCRIPTION

Carapace wider than long, ratio of cw/cl= 1.13, shape sub-quadrangular, front gently rounded, anterolateral margins rounded, and posterolateral margins convergent. Posterior corner of carapace margin eroded to accommodate last pair of pereopods, middle of posterior border concave, accommodating first segment of abdomen between two strengthened lobes. Carapace surface and pereopods densely covered with long filiform and short plumose setae bent at right angles near tip (long setae 4.4 × length of short setae, 0.20 × cw). Setae on carapace do not obscure surface and filiform setae, on carapace, are not arranged in clumps but scattered over sur- face. Filiform setae much denser on pereopods than on carapace.

Carapace surface smooth except for prominent pair of convex laterally elongated swellings near mid-line, behind rostral area, and two small tubercles behind and to the side of each postorbital corner. Carapace regions not defined, short faint cervical grooves, arising from pits near mid-line, extend anterolaterally but do not reach carapace margins. Anterolateral carapace margin begins below level of postorbital corner, evenly rounded and armed with five equidistant acute teeth directed almost vertically. Preceding first tooth is a group of eight or nine small rounded tubercles that effectively lie below suborbital margin. Single posterolateral tooth, similar to anterolateral teeth, marks beginning of posterolateral margin.

Frontal margin continuous from U-shaped medi- an notch, thickened, initially armed with row of tiny tubercles that continue until about half-way across upper orbital margin where it becomes thinner and largely devoid of tubercles. Postorbital corner not produced, and only a faint notch separates suborbital margin visible in dorsal view.

First article of antennule much longer than wide, broadening distally and fitting closely beside epistome; second article at almost right angles to first, fitting beneath frontal margin; remaining articles folded into orbital cavity. First article of antenna moveable, much wider than long, beaked medially around urinal aperture, both lobes of beak similar in size; second article longer than wide, fitting closely alongside antennule, lateral distal corner produced as a lobe curving over base of eyestalk, medial corner less produced; third article, longer than wide, inserted at an angle between two lobes of preceding article; fourth article as long as wide, carrying flagella. Ratio of length of antennal flagella to cw= 0.5. Epistome triangular, concave, posterior margin with two eroded areas that accommodate distal extremities of operculiform mxp3. Bases of mxp3 widely separated by tip of sternum. The mxp3 palp setose along medial margin, two-thirds the length of ischium + merus; crista dentata composed of seven small teeth. Subhepatic area convex, tuberculated, bearing swelling with four or five larger tubercles. Sternal suture 4/5 complete, faintly marked in male. Female sternal sutures unknown. Male cheliped slightly built, scarcely longer than first pair of walking legs; merus triangular in cross section, superior border armed with six small acute tubercles, anterior inferior margin armed with three large acute tubercles paralleled by row of four small blunt granules, posterior inferior margin armed with four similar granules; carpus unarmed except for blunt spur on inner superior border; propodus devoid of granules, inferior margin straight; fixed finger without teeth, moveable finger with single small tooth mid-way, both fingers hollowed out internally, distal margins continuous, meeting only at tips. Two tufts of long setae, inserted mid-way along fingers, extend across gap between fingers.

P2-p4 decreasing in size posteriorly; borders of meri armed with acute tubercles; superior borders of carpi and propodi armed with similar tubercles; dactyli curved, inferior margins armed with seven to eight small spines increasing in size distally. Length of p3 merus about twice merus width and 60% of cl.

P5 reduced (29.3% of length of preceding pereopod), lying alongside posterolateral carapace margin, directed anteriorly, and reaching to about one-third of length of merus of preceding limb. Limb sub-chelate, propodus extended distally, dactyl claw-like as is typical for male dynomenids ( McLay 1999: 449). Fine structure of dactyl and propodal extension of male and female not available.

All segments of abdomen freely moveable, width of segments increasing distally, surface smooth. Male telson much wider than long, lateral margins of last abdominal segment occlud- ed to accommodate uropods that occupy almost 66% of lateral margin. Telson extends as far as mid-way between bases of p1. No effective abdominal locking mechanism: one small tubercle on each side of sternum opposite base of p2. When abdomen closed uropods lie beside these tubercles.

Five pairs of pleopods in male: first pair uniramous, forming a semi-rolled tube with dense setae around apical sperm aperture and soft medial lobe. Second pair biramous, endopod (i.e. gonopod) as long as first, basal half stout, calcified, narrowing distally, distal half horny and needle-like; armed with a series of six evenly spaced, tiny acute, inset sub-terminal spines, directed distally and spiraling around the shaft over 180° toward tip which ends with a terminal spine. Exopod welldeveloped, composed of one article, narrowing distally and reaching almost as far as proximal half of gonopod. Other three pairs of pleopods rudimentary and uniramous, decreasing in length posteriorly. Female characters unknown.

DISCUSSION

D. guamensis n. sp. can be distinguished from D. kroppi n. sp. because it has a higher cw/cl ratio 1.3 (vs 1.13 in D. kroppi n. sp.); five anterolateral acute teeth (vs six less acute teeth); two tubercles on the carapace behind the postorbital corner (vs none); subhepatic swelling bearing four or five blunt tubercles (vs none); superior borders of p2- p4 with acute tubercles (vs small blunt granules); and a pair of swellings behind frontal margin much more prominent than in any other species of Dynomene .

The structure of the second gonopod of D. guamensis n. sp. differs from that of D. kroppi n. sp. only in having a greater number of sub-terminal teeth. Apart from D. kroppi n. sp. the gonopods of D. guamensis n. sp. most closely resemble those of D. hispida and D. praedator . All dynomenid males have a rudimentary exopod on the second pleopod but in D. guamensis n. sp. it is much longer than in any other dynomenid. The exopod is composed of a single article, almost half the length of the endopod (i.e. gonopod) and proximally, it bears a few small setae. It may be that this individual is aberrant, just as we find many female Metadynomene tanensis (Yokoya, 1933) with aberrant first pleopods resembling those of males (see McLay 1999: 527-530).

The two new species described here are similar to Dynomene pugnatrix De Man, 1889 which is known only from the vicinity of Mauritius and Madagascar in the Indian Ocean. A distinctive feature of all three of these species is the shape of the chelipeds. They all have chelipeds that are very slender, largely unornamented, fingers gaping widely and not down-curved, and whose margins do not have well-developed teeth, as is usually found in other Dynomene species. D. pugnatrix is only known from two specimens, one from Mauritius (the type locality) from unknown depth, and the other one nearby from a depth of 90- 140 m. However, specimens of both D. kroppi n. sp. and D. guamensis n. sp. all come from shallow water (1-2 m) where specimens of D. praedator were also found. Despite having similar morphology, the two new species appear to differ from D. pugnatrix in their habitat, and they are geographically separated by a large distance. It will be interesting to see whether one or other of these species is present in intervening Indo-Pacific areas.

In both Dynomene kroppi n. sp. and D. guamensis n. sp., the crista dentata is composed of six and seven teeth respectively. The same number is also found in D. pugnatrix and is typical for species of the genus Dynomene where the number of teeth ranges from five to eight. Species of Hirsutodynomene also have a similar number of teeth, but in Metadynomene and Paradynomene there are around twice this number.

Figures

FIG. 2. — Dynomene guamensis n. sp., Guam, Piti Lagoon, 1-3.5 m, holotype 11.0 × 9.7 mm; A, dorsal view of right half of carapace; B, outer face of right cheliped; C, dorsal view of right cheliped (MNHN-B26476). Setae omitted. Scale bars: 2 mm.

FIG. 3. — A, Dynomene kroppi n. sp., Guam, Piti Reef, 1 m, holotype 11.9 × 9.7 mm (MNHN-B26474), dorsal view of whole crab, setae removed from right side of carapace, right third and left fourth leg not shown; B, Dynomene guamensis n. sp., Guam, Piti Lagoon, 1-3.5 m, holotype 11.0 × 9.7 mm (MNHN-B26476), dorsal view of whole crab, setae removed from right side of carapace, left second leg not shown. Scale bars: 4 mm.