Pycnogonum coninsulum, Bamber, 2008

Bamber, Roger N., 2008, A new species of Pycnogonum (Arthropoda: Pycnogonida: Pycnogonidae) from Hong Kong, Journal of Natural History 42 (9 - 12), pp. 815-819 : 815-818

Pycnogonum coninsulum sp. nov.

( Figure 1 )

Material

One male, holotype (registration no. NHM.2003.9); Conic Island Cave, Hong Kong, 22 ° 219540 N 114 ° 239220 E; sample B6, sediment 18% gravel, 25% sand, 57% silt/clay, depth 17 m. Coll. N.J. Evans and P.F. Clark, 21 October 2002 .

Description

Small Pycnogonum , trunk length (anterior of cephalon to posterior of fourth lateral processes) 1.6 mm, leg span approximately 3.6 mm. Integument surface sparsely and minutely pitted (as indicated for legs on Figure 1D ), white in life. Trunk ( Figures 1A, B ) fully segmented, glabrous, tapering from anterior to posterior, lateral processes short. Cephalon hexagonal, 40% of trunk length, domed ocular tubercle about half as high as wide, bearing four eyes, integument slightly swollen posterior to ocular tubercle but not raised into a tubercle, posterior margin raised slightly in dorsal midline, no lateral process tubercles; second and third trunk segments each two thirds as long as cephalon, second as wide as half trunk length, with posterior margin raised slightly in dorsal midline, low lateral process tubercles (‘‘humps’’), third with posterior mid-dorsal tubercle and lateral process tubercles about as high as wide, higher than ocular tubercle; fourth trunk segment half length of second, no mid-dorsal tubercle but dorsal lateral process tubercles larger than those on third segment. Abdomen truncate, naked, not articulating, held just below horizontal, slightly wider at mid length, reaching back as far as distal edge of coxa 2 of fourth legs.

Proboscis naked, barrel-shaped, not tapering, without tubercles, as long as cephalon; oral glands (sensu Staples 2002) not seen (may be present, but not distinguished in such a small animal).

Oviger ( Figure 1C ) of seven articles plus claw, articles 3, 4 and 5 subequal and longest, claw strong, with two dorsal denticulations.

Third leg ( Figure 1D ) with low swellings bearing scale-like rugosity on dorsal and ventral margins of coxa 1, ventral margins of coxae 2 and 3, dorsal margins of femur, tibiae and propodus; few dorsal and most ventral spines with distal split, not bifurcating (detail, Figure 1D ). Coxa 1 wide, with paired dorsal spines, one split; coxae 2 and 3 ventrally spinose, coxa 3 half length of coxa 2; femur longest article, with large proximoventral swelling (‘‘spur’’), dorsally spinose with dorsodistal spine three times as long as other spines; tibia 1 0.75 times as long as femur, one ventral and four dorsal spines plus longer dorsodistal spine; tibia 2 half length of femur, 1.25 times as long as wide, two dorsal and two ventral split spines, no longer dorsodistal spine; tarsus short, quadrangular, with seven slender ventral spines; propodus slender (three times as long as wide), with five dorsal and eight ventral slender spines; main claw half length of propodus; auxiliary claws absent. Male gonopores tiny, ventral on coxa 2 of fourth legs. Coxal glands (sensu Staples 2002) not seen.

Measurements (mm)

Trunk length 1.6; width across second lateral processes 0.83; proboscis length 0.66; abdomen length 0.29; oviger total length 0.38; third leg, coxa 1 0.17, coxa 2 0.2, coxa 3 0.1, femur 0.35, tibia 1 0.26, tibia 2 0.18, tarsus 0.06, propodus 0.28, claw 0.15.

Etymology

Named after Conic Island, from the Latin ‘‘conos’’, a cone, and ‘‘insula’’, an island.

Remarks

The present small species fails to key out in Stock’s (1966) key to the genus. Although a number of species have been described since then, it is unattributable to species described earlier than 1966.

With a seven-articled oviger, this small species would fit into the subgenus Retroviger of Stock (1968), although with both presence and absence of auxiliary claws, of tapering proboscides and of other features in all three of his subgenera, their monophyly is unlikely and thus their status as subgenera hardly tenable.

The character is, however, valuable in comparing similar species (where males are known). Most recently Staples (2002), in describing the Pycnogonum species of Australia, reviewed species with seven-articled ovigers while describing as new P. clarki Staples, 2002 , and redescribing P. aurolineatum Flynn, 1919 owing to his discovery of the first males of that species.

There are eight known (or postulated) species of Pycnogonum with seven-articled ovigers and no auxiliary claws, namely P. crassirostre Sars, 1888 , P. aurilineatum , P. gibberum Marcus & Marcus, 1963 , P. guyanae Stock, 1974 , P. ornans Stock, 1992 , P. musaicum Stock, 1994 , P. calculum Bamber, 1995 and P. clarki . All possess inter alia, dorsal tubercles on the midline of the cephalon, unlike the present species.

In fact the distribution of dorsal trunk tubercles in Pycnogonum coninsulum sp. nov. is unusual, in that their size increases posteriorly. Thus the first to fourth lateral processes bear no tubercle, a low hump, large and larger tubercles, respectively, while in the dorsal midline only trunk segment 3 has a tubercle, segments 1 and 2 being merely raised posteriorly. There is no postocular, nor fourth segment tubercle. This disposition of the dorsal tuberculation is unique within the genus, and serves most readily to distinguish P. coninsulum from the other similar, small species without auxiliary claws, even in the absence of males, although it is possible that the female of P. coninsulum may not have quite the same disposition of dorsal tubercles.

The only species of Pycnogonum previously recorded from Hong Kong, Korean or Chinese waters are P. daguilarensis Bamber, 1997 (from Cape d’Aguilar, Hong Kong) and P. uedai Nakamura and Child, 1983 (from Korea and Japan). These are easily distinguished from P. coninsulum as both possess auxiliary claws.

Figures

Figure 1. Pycnogonum coninsulum sp. nov., male holotype. A, trunk and proboscis, dorsal, with most coxae and entire left second leg; B, trunk, lateral (ventral profile and oviger diagrammatic, obscured by legs); C, oviger; D, third right leg with detail of split spine. Scale line51 mm for A and B, 0.13 mm for C, 0.3 mm for D.