Metaplax gocongensis, Davie & Xuan, 2003

Davie, Peter J. F. & Xuan, Nguyen van, 2003, A New Species Of Metaplax (Crustacea: Brachyura: Varunidae: Cyclograpsinae) From Vietnam, Raffles Bulletin of Zoology 51 (2), pp. 379-386 : 380-384

Metaplax gocongensis , new species

( Figs. 1A–C , 2 , 3 )

Material examined. – Holotype – male (29.0 x 20.6 mm) ( QMW26683 ), My Loi Commune , Vai co River, ca. 40 km south of Ho Chi Minh City, Go Cong Province, Vietnam, 106 39’E 10 28’N, in dike wall of tiger prawn pond near river bank, coll. Nguyen van Xuan, 28 Jul.2002.

Paratypes – 2 males (29.1 x 20.7, 18.9 x 14.2 mm)( ZRC), Phu Thanh Dong commune, ricefields near bank of Tan Thoi Island , Cua tieu River (Mekong River estuary), ca. 70 km south of Ho Chi Minh City, Vietnam, 106 42’E 10 17’N, coll. Nguyen van Xuan, 2 Feb.2002 ; 1 female (22.8 x 17.2 mm)( QMW26684 ), data as for holotype ; 4 males (19.0 x 14.4, 18.4 x 13.8, 16.4 x 12.4, 15.2 x 11.5 mm), 1 female (20.0 x 15.1 mm)( QMW28800 ), data as for holotype ; 2 males (21.4 x 16.1, 18.1 x 13.7 mm), 1 female (19.4 x 14.7 mm)( MNHN), data as for holotype ; 3 males (24.1 x 17.6, 18.3 x 13.5, 15.7 x 11.7 mm), 1 female (17.9 x 13.4 mm)( UAFV), data as for holotype .

Description. – Carapace rectangular, greatest width across second anterolateral teeth; lateral margins moderately convex, armed with 3 lateral teeth behind exorbital tooth, exorbital tooth broad, rounded, bluntly pointed anteriorly, first and second lateral teeth clearly marked, first similar in length to exorbital angle, second much smaller, third reduced to a small notch, sometimes a fourth may be indicated by a minute break low on the posterolateral margin. Lateral margin edged with row of short feathered setae. Carapace 1.32–1.41 (mean 1.34) times broader than long, largest males of present material comparatively broader at 1.41, while smallest male and female 1.32. Carapace surface covered in minute, smooth granules, and more-or-less microscopic setae; three short oblique, granular, setiferous rows on posterolateral region, first beginning on third anterolateral tooth, second in from fourth lateral tooth, and fifth above base of last walking leg. Front (measured at anterolateral corners) ca. 0.27 times maximum carapace width, deflexed, broadly triangular, distinctly notched medially, broad shallow sulcus running posteriorly separating low, indistinct, postfrontal lobes. Dorsal carapace regions well indicated, with gastric, cardiac and intestinal regions strongly demarcated. Infraorbital margin forming a lobulated stridulatory ridge, in combination with chitinous crest on inner border of merus of cheliped. Male crest consisting of 15-18 lobes or rounded tubercles, first lobe very long, prominent, and sinuous, occupying entire inner half of orbit, this followed by group of three relatively small blunt tubercles, followed after a small space by the remaining blunt tubercles that become gradually and progressively smaller. Female infraorbital border with about 16 (15-17) relatively even tubercles becoming smaller laterally but not obviously differentiated in form one from another.

Chelipeds equal, large and robust. Merus obviously protruding beyond anterolateral margin of carapace, but not markedly elongated as in some species; posterior face of merus armed with remarkable strong, rigid, black spines, forming a regular broad tooth comb near superior border, smaller spines scattered proximally and extending as a second less prominent row obliquely across the lower face; posterior border subdistally rounded, without distinct subdistal tooth or spine; lower border coarsely granulate. Anterior border of male merus with long, low, dark brown, chitinous crest on anterior part of lower proximal third; in female this stridulatory crest is less obviously raised and chitinous, and placed at distal end of anterior border. Carpus with inner angle granulate, but not produced. Palm with outer surface minutely, and evenly granular, granules becoming slightly more prominent and tipped with chitinous points along upper and lower margins and on fingers; without obvious setae; inner surface of palm more coarsely but less densely granulate, except for relatively smooth patch behind base of dactylus. Cutting margins typically serrated, but in mature males the fixed finger has a long raised tooth over proximal half, and dactylus has 1 or 2 more prominent tubercular teeth at about proximal third; large males also develop an obvious basal gape between the fingers. Fixed finger moderately deflexed, slightly less than half length of palm.

Second and third pairs of walking legs longer than first and fourth pairs, second pair the longest and ca. 1.6–1.7 times carapace width. Merus of third leg ca. 3.5 times longer than wide; anterior and posterior margins more nearly subparallel. Setae sparse on merus but becoming long, thick at distal end of carpus and on propodus and dactylus, although less conspicuously on last leg.

Male abdomen with telson abruptly narrowed, longer than wide, only gradually tapering, shorter than penultimate segment; sixth segment the longest, ca. 1.3 times wider than long. G1 stout, moderately curved; apical process corneous, oblique, placed on sternal face; palp well differentiated and not hardened. Apical setae simple, brush-like, obscuring structural detail. Eggs, prior to development of eye-spots, dark brown; diameters measuring from 280 x 266 microns to 306 x 280 microns.

Colour. – Carapace and legs generally pale brown to muddy grey; chelae of male a uniform dull reddish brown, chelae of female somewhat brighter reddish orange.

Etymology. – The specific name gocongensis , refers to the Go Cong province, Vietnam, the type locality where the species flourishes.

Ecology and economic importance. – This species seems to be restricted to the intertidal silty-mud zone of the banks of rivers or ricefields subjected to tidal influence where salinity ranges from 0– 15 p. p.t. During the rainy season, the water may be completely fresh for up to six months, while in the dry months salinity will reach 10– 15 p. p.t. on high tides. Since about 1994, suitable habitat for this species has been progressively diminishing because of increasing human settlement and urbanisation, but mostly because of the government’s program to enhance rice production by changing the natural river banks in order to prevent the intrusion of saline water into the adjacent ricefields.

Metaplax gocongensis , new species, is a gregarious species, that digs burrows in intertidal silty mud. The burrow is cylindrical, slightly oblique, and has no ramifications; large males (29.5 mm c.b.) have a burrow about 24 mm in diameter, and about 550 mm deep, of which the bottom 250 mm is flooded. Females become egg-bearing at about 15 mm carapace width, while the largest ovigerous female collected reached 25.4 mm carapace width.

Historically, each year on the fifth day of the fifth month of the lunar year, and marking the beginning of the season of heavy rain in south Vietnam, great numbers of this species have emerged from their burrows, to head into nearby wet vegetation, freshwater pools, or water filled buffalo tracks, and for a 24 hour period, have engaged in a synchronised mass moulting. During this day, the local people have traditionally collected large quantities of these soft-shell crabs for making a special regional dish named mam cong lot (salty moulted-crab). The carapace, abdomen, stomach and gills are discarded, and the rest of the crab is preserved in saltwater. This special dish is highly regarded by old people of the Gocong Province where this species has flourished, but also by some gourmets in Saigon (now Ho Chi Minh City). Relatives of the second author still living in this area make this salty mam cong every year for gifts. Other people sell this food to passengers on the ferry-boat Myloi of Gocong Province. Since about 1995, however, this extraordinary annual synchronised mass moulting has been seriously affected by climatic changes. In some years the wet season has continued through until January, with the result that the crabs have moulted earlier than usual. This has occurred during some days of the third or fourth month of the lunar year, and involved fewer than normal numbers. In other years, such as in 2001, the dry season persisted longer than 8 months and the mass moulting failed to happen at all. On 26 April 2003, there was an opportunity to purchase 176 individuals of M. gocongensis , new species, at the market of Phu Thanh Dong commune (the quantity offered for sale that morning was about 3 kg). These had been collected during this year’s synchronised mass moulting event. In total there were 164 soft–shell females, five hard females, but only seven soft-shell males. It seems probable that the mass moulting could be followed by synchronised copulation with the soft-shell females although this has not been directly observed.

Throughout the year these crabs are also collected as hardshelled crabs, by digging them from their burrows. This is mainly done by children from poor families, for eating at home, or for sale, or for providing them to duck farmers as live food for the ducks. Others may also exploit them to sell in the markets in Ho Chi Minh City. In areas where people have left the countryside as part of the rural exodus to the city, and there are no traditional duck farms, this species flourishes and may cause a reduction in the rice yield by cutting the roots of the newly transplanted rice plants while burrowing, or by tearing the young plants with their claws, although they do not eat them. Since 1996, the rice-fields along the banks of the Vai Co River (adjacent to the My Loi Ferry station), that are influenced by semi-diurnal tides, have been converted to shrimp pond culture, and Metaplax gocongensis , new species, continues to burrow into the dike walls of the ponds.

An interesting opportunity for ecological information occurred on 28 July 2002 when one of the shrimp ponds belonging to the My Loi community was drained for harvesting. After the shrimps were harvested, ten local children were allowed to excavate all available borrows in the four dike walls surrounding the pond, to collect the crabs for home consumption. The dike walls covered a total area of 2815 m 2. The total yield of all species weighed about 1.6 kg, with the size of crabs collected ranging from 16–25 mm carapace breadth. Of 142 individuals, nine were a Parasesarma species, 16 were a Perisesarma species, and the remaining 117 belonged to Metaplax gocongensis , new species. Similarly, from a sample of 66 individuals collected on 6 April 2002, in the Binh Khanh commune, about 25 km southeast of Ho Chi Minh City, 56 belonged to Metaplax gocongensis , new species, consisting of 24 ovigerous females, 7 non-ovigerous females, and 25 males. So far, no other species of Metaplax has been found to occur sympatrically with M. gocongensis , new species.

Remarks. – This species has its closest allies amongst the Metaplax species with less than 20 lobules/tubercles on the male infraorbital crest. These include Metaplax indica H. Milne Edwards, 1852 , M. longipes Stimpson, 1858 , M. occidentalis Pretzmann, 1971 , M. sheni Gordon, 1931 , M. takahashii Sakai, 1939 , and M. tredecim Tweedie, 1950 . Metaplax gocongensis , new species, differs from all of these by the uniquely apomorphic presence of prominent black, slender, fixed spines on the outer face of the merus of the chelipeds.

Of all the above species M. gocongensis , new species, most closely approaches M. tredecim Tweedie, 1950 , in the similar number (12-14) lobules or tubercles on the infra-orbital ridge but in M. tredecim these lobules decrease in size and length more-or-less progressively from the inner orbit outward, and there is not an obviously much larger and longer inner lobe. The different pattern of lobulation can be seen by comparing Fig. 1B, C , with Fig. 1D, E . Metaplax tredecim also has only two lateral teeth behind the external orbital angle (versus three), and the meri of the walking legs are more slender (merus of P4 ca. 4.3 times longer than wide versus 3.5). For this study we re-examined the holotype male of M. tredecim (ZRC 1971.10.13.1), from Labuan, coll. M.W.F. Tweedie, 1950, as well as other paratypes, and a further male (QMW17215), from Pantai Meragang, Brunei, coll. S. Choy, 9 Apr. 1989.

Metaplax sheni Gordon, 1931 , is similar to M. gocongensis , new species, in having the innermost lobule of the infraorbital ridge by far the largest, but even so, it is still not as long and pronounced as that of M. gocongensis , new species; it also has 15-19 lobules or tubercles similar to the 15-18 on M. gocongensis , new species. However, M. sheni is a smaller species, apparently less than about 15 mm carapace breadth Gordon, 1930, 1931; Tweedie, 1936, and material examined for this study held in the ZRC from Johore, Malaysia (ZRC 1964.8.12.81-88; 1969.9.12.3-6; 1969.12.18.3) and Singapore (ZRC 1964.8.12.89; 1964.8.12.90; 1969.9.30.3)), and the adult males have a prominent broad submedian differentiated tooth on the cutting edge of the dactylus of the cheliped (only a low tubercle in this position in M. gocongensis , new species). In addition, M. sheni has the cheliped meri considerably more elongated beyond the anterolateral margins; four (versus three) lateral teeth behind the exorbital angle; and the walking legs with longer meri (merus of P4 c. 4.2 times longer than wide) and with their anterior margins more convex. In M. gocongensis , new species, the merus of P4 is ca. 3.5 times longer than wide, and the anterior and posterior margins more nearly subparallel.

Metaplax occidentalis Pretzmann, 1971 , was originally described as a subspecies of M. indica H. Milne Edwards, 1852 . Pretzmann (1971) stated that it differs from Metaplax indica by the following: suborbital border beginning with five small granules, followed by five more, becoming larger and more tubercular; then two oval, vesicular structures, followed closely by three smaller teeth; fingers of chela are somewhat longer, and more strongly bent downward; carapace lateral margins with a large exorbital tooth, followed by a similar sized second tooth, and a third tooth in the form of an indistinct projection; carpus of cheliped bearing distinct, inwardly placed spine; G1 without forwardly directed apical spine. Despite superficial similarities it seems clear that both taxa must be reproductively isolated to have led to the number of differences described above, and in particular, differences in the male gonopod apical structure. We therefore here attribute full species status.

We have not examined specimens of M. occidentalis and M. indica , but from their published descriptions and available figures, both have fewer lobes on the infraorbital crest (10– 12), and these are conformed differently from M. gocongensis , new species, beginning with 4-5 small tubercles on the inner part of the orbit, versus the first lobe being very long and occupying the entire inner half of the orbit in M. gocongensis , new species. Also unlike M. gocongensis , new species, both these species have greatly elongated chelipeds, with long meri and propodi.

Metaplax longipes Stimpson, 1858 , is problematic. It appears almost certain that it is a senior synonym of M. takahashii Sakai, 1939 , however M. longipes remains poorly defined. Originally described from Hong Kong, four males were subsequently recorded from there by Shen (1940a: 236), and a further four males from Fujian, and Zhejiang Provinces by Shen (1940b: 95). Lee (2001) recorded M. longipes from the small Taiwanese controlled islands of Kinmen and along the coast of mainland China but not from the main island of Taiwan. Shen & Dai (1964) noted that M. longipes had 9 or 10 lobes on the infraorbital ridge, slightly more than the seven originally recorded by Stimpson. Dai & Yang (1991) have recorded what they thought to be M. longipes from Guangdong, Fujian, and Zhejiang Provinces, China, but their key character for this species is the presence of 11-15 tubercles on the infraorbital crest of males. Strangely however they have two separate figures of the infraorbital ridge, one with 17 lobes, the other with nine. The shorter ridge with fewer lobules is presumably the “typical” form for M. longipes . It seems possible that the specimens with the more numerous lobes, may represent a separate or new species. Stimpson (1907: 98), in his English description of M. longipes , clearly stated “Infra-orbital margin seven-lobed, lobes rounded, smooth, glossy, decreasing in size outwardly, the innermost lobe being the largest and most projecting, somewhat curving downward.” This is also a perfect description of the infraorbital crest of M. takahashii as originally figured by Sakai in his description of his new species from Taiwan. In fact there do not appear to be any conflicts in the respective descriptions of these two species, except that M. takahashii supposedly has four lateral teeth behind the external orbital angle while M. longipes has only three, however the indentations that mark the posterior teeth can be very indistinct, and may have been discounted or overlooked by Stimpson. Sakai (1939) failed to mention or draw comparisons with Stimpson’s species and therefore it seems that he did not take this species into account when deciding on the novelty of his specimens. Davie (1992) recorded M. takahashii from Hong Kong (specimens re-examined here: males, 2 females (QMW16467), from Mai Po, New Territories, Hong Kong, coll. R. Choi, 16 Dec. 1987), but also failed to adequately check the identity of the little known M. longipes . He was also confused by the reports of the infra-orbital ridge of M. longipes being armed with 15-17 lobes and tubercles. Given its name, it could be presumed that M. longipes has relatively long legs, and from the original measurements given by Stimpson, from the figure of Shen & Dai (1964), and from the colour picture of Lee (2001: 115), the length of P3 is c. 2.1-2.3 times the carapace breadth, compared with c. 1.6-1.7 times for M. gocongensis , new species, i.e. still distinctly shorter.

Distribution. – At present only known from southern Vietnam.

Figures

Fig. 1. Metaplax gocongensis, new species, holotype male, 29.0 mm c.b. (QMW26683), a. dorsal view; b. infraorbital stridulatory crest; c. anterodorsal view of front, orbit and infraorbital crest. Metaplax tredecim Tweedie, male, 16.3 x 12.0 mm (ZRC 1971.10.13.1, Labuan, M.W.F. Tweedie, 1950), d, infraorbital stridulatory crest; e. anterodorsal view of front, orbit and infraorbital crest.

Fig. 2. Metaplax gocongensis, new species, holotype male, 29.0 mm c.b. (QMW26683). a. frontal view showing chelipeds; b. male abdomen; c. posterior face of merus of right cheliped showing characteristic black spination.

Fig. 3. Metaplax gocongensis, new species, holotype male, 29.0 mm c.b. (QMW26683), male first gonopod.