Revision of the freshwater crabs of the Johora tahanensis (Bott, 1966) species group (Crustacea, Brachyura, Potamidae), with a key to the genus

Abstract The taxonomy of the potamid freshwater crabs of the Johora tahanensis (Bott, 1966) species group (Potamoidea) is revised. Seven species are recognised, all from Peninsular Malaysia and southern Thailand, three of which are described as new. The three new species were previously identified as J. tahanensis but can be distinguished by characters of the carapace, male first gonopod, and vulva. A revised key to the 18 recognised species of Johora Bott, 1966, is provided.

In recent years, the structure of the vulva has proven useful in potamid taxonomy for some groups, but for Johora species, its morphology appears to be relatively conservative. The one exception is J. thoi, where the lateral sternal vulvar cover projects obliquely anteriorly as a large triangular plate (Fig. 14C), quite different from the simpler structures of congeners (Fig. 14A, B, D-H).
G1 subterminal segment gradually tapering from broad proximal part to slender distal part, without distinct shelf-like structure along gently concave outer margin; terminal segment gently curved outwards (from median part of sternum), ca. half length of subterminal segment, surfaces with numerous short setae ( Fig. 8A-C); G2 slightly longer than G1, distal segment long, about half length of basal segment (Fig. 8D).
Remarks. Ng and Takeda (1992: 108) discussed the taxonomy of J. tahanensis at length, showing that the material from two parts of Taman Negara National Park in Malaysia belonged to two species and that the type series is mixed. Bott (1966Bott ( , 1970 and Ng (1987Ng ( , 1988 had regarded them as one taxon, incorrectly noting that the G1 structure was variable. The holotype of J. tahanensis was from Kuala Tahan in the state of Pahang, in the southern part of the national park, and the G1 of this specimen (in the Senckenberg Museum, Frankfurt) as well as the topotypic material we have from that location has a terminal segment which is gently curved (Fig. 8A). Material from the northern part of the park from the adjacent state of Kelantan had a G1 terminal segment that is straight (Fig. 8E). Ng and Takeda (1992) showed that the G1 differences of the Pahang and Kelantan specimens are consistent and the Kelantan material was referred to a separate species, J. hoiseni.
The material reported as "J. tahanensis" by Ng and Takeda (1992: 108) from and around Fraser's Hill in Selangor and Pahang should be now referred to J. booliati sp. nov. (see remarks for this species).
Distribution. Johora tahanensis is known from the tributaries around Sungei Tahan in southern Pahang, at the southern part of Taman Negara National Park (Fig. 15). It lives among rocks and submerged vegetation in the relatively fast flowing streams in the forest.
Conservation. The species is not under any immediate threat as it is found in Malaysia's oldest and largest national park. Ng and Yeo (2007) treated J. tahanensis s. str. as vulnerable as it is not known from a relatively wide geographic area but it was reappraised and regarded as of least concern in Cumberlidge et al. (2009)  (Figs 3C, 6C); outer surfaces of third maxillipeds with dense, long stiff setae; ischium subrectangular, with shallow median oblique groove (Figs 3C, 4C); ambulatory legs relatively longer, length to width ratio of merus of fourth ambulatory leg 3.0-3.2 (Figs 1C, 6A, 12C); G1 subterminal segment with broad proximal part, tapering suddenly to slender distal part, with distinct shelf-like structure along outer margin; terminal segment straight, long, slender, subequal in length to subterminal segment, surfaces with scattered short setae ( Fig. 9A-C, E-G); G2 shorter than G1, distal segment long, about two-thirds length of basal segment (Fig. 9D, H). Female pleon ovate; somites 3-6 progressively narrower; telson semi-circular (Fig. 13C). Vulvae large, on anterior half of sternite 6, adjacent to suture with sternite 5, lateral sternal vulvar cover triangular, partially overlapping sternite 5 (Fig. 14C).
Remarks. This is one of the largest species of Johora and is rivalled in size only by J. tiomanensis and J. counsilmani from Pulau Tioman. The G1 structure of J. thoi is distinctive and consistent, the long and straight terminal segment being evident even in small subadult specimens (Fig. 9E-G). The only other species with superficially similar G1s are J. singaporensis and J. michaeli sp. nov. from Singapore and mainland Terengganu, respectively. In these species, however, the G1 terminal segment is prominently shorter, being only two-thirds or less the length of the subterminal segment (Fig. 9I-K;Ng 1987: fig. 8A, B).
Distribution. Johora thoi is endemic to Pulau Redang, the largest island in the Redang Archipelago, a group of nine islands about 25 km off the northeastern coast of Peninsular Malaysia. The species has been recorded from most parts of the island, including its highest point (359 m), occurring in all clean waters there (Fig. 15). It lives under rocks and can be found along the stream banks at night.
Remarks. Ng and Takeda (1992) showed that the G1 differences of the material that had been identified as "J. tahanensis" by Bott (1966Bott ( , 1970Ng 1987Ng , 1988 belong to a distinct species, J. hoiseni (see remarks for J. tahanensis).
Distribution. Johora hoiseni is known from the drainages near Gua Musang in Kelantan, in the northwestern part of Taman Negara National Park (Fig. 15).

Johora thaiana
Remarks. The G1 structure of J. thaiana most closely resembles that of J. hoiseni in that the terminal segment is straight; but their subterminal segments differ. In J. thaiana, the subterminal segment gradually tapers from the broad proximal part to a slender distal section, without a shelf-like structure along the outer margin (Fig. 10A); in J. hoiseni, the distal part of the subterminal segment becomes slender more abruptly, resulting in distinct shelf-like structure on the outer margin (Fig. 8E). Biogeographically, the type localities of J. thaiana are less than 20 km from the J. erici sp. nov. in Jeli, Kelantan; but their carapace features and G1s differ markedly (Figs 2E, 10A-C versus Figs 2G, 7B, 11A-C, E-G).
Distribution. Johora thaiana is known from the forest tributaries in southern Thailand, near the Malaysian border at Kelantan (Fig. 15). Its distribution is adjacent to that of J. erici sp. nov. which lives in the highlands of northern Malaysia.
Etymology. The species is named after an old friend and mentor, the late Dr Lim Boo Liat , one of Malaysia's most eminent biologists and naturalists. In a remarkable career spanning 70 years, he has contributed immensely to biodiversity and conservation science in Malaysia; more importantly, he has inspired, mentored, and trained many generations of biologists, many who now lead science in Malaysia and Singapore.
Remarks. The large adult size of J. booliati sp. nov. and J. erici sp. nov. (carapace width in excess of 25 mm) with the pronounced epibranchial teeth allies them with J. tahanensis s. str., J. hoiseni, J. thaiana, J. thoi, J. tiomanensis, and J. counsilmani (the last two being endemic to Pulau Tioman, Peninsular Malaysia).
Johora booliati and J. erici resemble J. tahanensis, J. hoiseni, and J. thaiana in the carapace shape and general features. Their G1 structures, however, are different. In J. booliati and J. erici, the G1 subterminal segment is proportionately more elongate and the terminal segment is relatively shorter (Figs 10E-H, 11A-C, E-G), and significantly, the terminal segment has only scattered short setae (versus subterminal segment shorter with the terminal segment longer and the surface of the latter more densely covered with short setae in J. tahanensis (Fig. 8A-C). Both J. hoiseni and J. thaiana have relatively straight G1 terminal segments which are prominently setose (Figs 8E-G, 10A-C).
Johora booliati and J. erici can be distinguished from J. tiomanensis and J. counsilmani by their carapace being relatively more quadrate in shape (width to length ratio less than 1.2) and the adult carapace is distinctly wider than long (width to length ratio 1.3-1.4) in J. tiomanensis and J. counsilmani. In addition, the epigastric cristae of J. booliati and J. erici are distinctly anterior to the postorbital cristae and separated by a gap with the entire postorbital cristae sharp (Figs 2F, G, 7B) while in J. tiomanensis and J. counsilmani, the epigastric cristae are only slightly anterior of the postorbital cristae and separated only by striae (cf. Ng and Tan 1984: figs 1A, 2A;Ng, 1985: fig. 3). Most significantly, the G1 terminal segments of J. booliati and J. erici are proportionately stouter and less curved (Figs 10E-H, 11A-C, E-G); in J. tiomanensis and J. counsilmani, it is distinctly slenderer, more elongate and strongly curved (cf. Ng and Tan 1984: fig. 3a-c;Ng 1985: figs 2a, c, e, 4a-c). Johora booliati and J. erici differ from J. thoi in possessing slightly shorter ambulatory legs (length to width ratio of merus of fourth leg 2.7-2.8 versus 3.0-3.2), the lateral carapace surfaces are relatively more rugose, and the distinctly shorter and gently curved G1 terminal segment (Figs 10E-H, 11A-C, E-G); in J. thoi, this structure is diagnostic, being very elongate and straight ( Fig. 9A- The G1 structure of J. booliati closely resembles that of J. erici, with the terminal segments similar in shape and proportions. The G1 subterminal segments of the two species, however, differ in form, with that of J. booliati forming a shelf-like structure along the outer margin (Fig. 10E) while in J. erici, the tapering of the segment is gradual, and no shelf is visible (Fig. 11A, E). The carapaces of the two species are different in adult males and females. In J. booliati, the carapace has a more quadrate form because the anterolateral margin is only gently convex, even in the largest male and the posterolateral margin is straighter and gently converging, with the posterior carapace margin medially indented (even in females) (Fig. 2F). In J. erici, the carapace appears more ovate, with the anterolateral margin distinctly convex, and the posterolateral margin is medially concave and strongly converging, and the posterior carapace margin is entire (Figs 2G, 7B).
The G1 structure of J. booliati and J. erici is superficially similar to that of J. murphyi from southern Peninsular Malaysia, notably in the shape of the G1 terminal and subterminal segments (Ng 1986b: fig. 14a, b; 1987: fig. 6D, E; 1988: fig. 16D, E), but differs in having the distal part of the terminal segment more rounded and less sharply tapering (Figs 10E-H, 11A-C, E-G). The G1 of J. booliati is also similar to J. intermedia, which has a wide distribution in central Peninsular Malaysia. The G1 terminal segment of J. booliati and J. erici, however, is relatively stouter and less curved, and the subterminal segment narrows along the distal third to form a neck-like structure with the terminal segment (Figs 10E-H, 11A- fig. 1A-E). It is also noteworthy that both J. murphyi and J. intermedia are smaller species than J. booliati and J. erici, with adults not reaching carapace widths of 25 mm. Ng and Takeda (1992: 107) had specimens from Fraser's Hill as well as adjacent areas (Sungei Gumut,Peretak,Selangor,3°36'53.3"N,101°44'40.4"E;Sungei Sum [probably Sungei Sum Sum], near Genting Highlands, Pahang, 3°20'42.2"N, 101°51'12.0"E; Ulu Teranum, Teras, Pahang, 3°44'12.6"N, 101°47'29.5"E) which they identified as "J. tahanensis". On the basis of geography, they are probably all J. booliati as defined here.
Distribution. Johora booliati sp. nov. is known from highland streams in the central highlands of Pahang (Genting Highlands, Bukit Tinggi and Fraser's Hill) (Fig. 15). The distribution of J. booliati overlaps with those of J. intermedia and J. gapensis, and we can expect the taxa to be found together. One juvenile male of J. booliati (10.5 × 9.1 mm, ZRC 2020.0364) was in fact collected with two adult males of J. intermedia at Fraser's Hill (ZRC 2016.003).
Etymology. The species is named after the first post-independence director of the National Museum of Singapore (the renamed Raffles Museum) (1967)(1968)(1969)(1970)(1971)(1972)(1973), the late Eric Alfred. A very active freshwater ichthyologist, he collected many of the freshwater crabs in the museum. Eric was a good friend and provided wise counsel and help even years after he stepped down and took over the directorship of the Singapore Maritime Museum.
Remarks. Ng (1995: 250) commented that the G1 terminal segment of the males from Perak "seems to be somewhat longer and more slender" compared to typical J. tahanensis but incorrectly attributed it to variation. The differences are actually consistent, and it is here recognised as a separate species, J. erici sp. nov. See remarks for J. booliati sp. nov. for differences with allied taxa.
Distribution. Johora erici sp. nov. is known thus far only from highland streams in northern Perak and Kelantan (Fig. 15).
Etymology. The species is named after the last director of the Raffles Museum , the late Michael Tweedie, an intrepid collector of interesting animals from Malaysia. The author had the pleasure of knowing him, finally meeting him when the refreshed museum opened as the ZRC in 1988; and even after many years since retiring, he retained his great passion for his crabs, snakes, and fish.
Remarks. The holotype male of Johora michaeli sp. nov. is not fully adult and it is clear that it can grow larger, with the adult females reaching 30 mm in carapace width. The G1, however, remains diagnostic, with the terminal segment elongate and almost straight (Fig. 9I-K). While this somewhat resembles that of J. thoi which is known from the nearby island of Pulau Redang, that of J. michaeli is distinctly less elongate and slender, being only about two-thirds the length of subterminal segment. Specimens of J. thoi even smaller than the type of J. michaeli remain the same G1 terminal segment shape and proportions of adults ( Fig. 9E-G) so the differences observed here are independent of size. The vulva of J. thoi is diagnostic, the lateral sternal vulvar cover being triangular in shape (Fig. 14C); it is subtruncate in J. michaeli (Fig. 14H). The general shape of the G1 terminal segment somewhat resembles that of J. tahanensis s. str. (Fig. 8A-C), it is clearly straighter and slenderer in J. michaeli (Fig. 9I-K). It is also unlike that of J. hoiseni which has a proportionately and even straighter G1 terminal segment ( Fig. 8E-G).
The G1 of J. michaeli also resembles that of J. singaporensis but the terminal segment in this species is longer (Ng 1987: fig. 8A, B). In any case, the carapace of J. singaporensis is quite different from that of J. michaeli as it is a member of the J. johorensis species group (see remarks for the species group under remarks for the genus).
One large female (ZRC 1984.6794) had been collected from Gunung Padang, which is relatively close to the type locality in Sekayu Falls. Both are part of the same mountain system east of Lake Kenyir in Terengganu.
Distribution. Johora michaeli sp. nov. is known so far only from highland streams in central Terengganu (Fig. 15).
Conservation. The conservation status for J. michaeli should be regarded as vulnerable as it is known from a relatively well protected area that is not subject to development (see Cumberlidge et al. 2009).