﻿Two new species of Tmethypocoelis Koelbel, 1897 (Decapoda, Brachyura, Dotillidae) from Sulawesi, Indonesia

﻿Abstract Tmethypocoelis Koelbel, 1897, is a central Indo-West Pacific genus of small intertidal, soft sediment dotillid crabs that includes five recognised species. Two new species, Tmethypocoelissimplexsp. nov. and T.celebensissp. nov., are here described from Sulawesi, Indonesia. Tmethypocoelissimplexsp. nov. is found on the west coast of Central Sulawesi, while T.celebensissp. nov. occurs in the north-eastern part of Sulawesi. Both new species differ from each other and known congeners by the male cheliped, male pleon, and male first gonopod characters. The differences in gastric mill morphology further confirm the two species as new. The distinct water current patterns in the Makassar Strait and the Maluku Channel might have contributed to the evolution of these two sibling species.


Introduction
Crabs of the dotillid genus, Tmethypocoelis Koelbel, 1897, are small and found on intertidal mudflats and estuarine mud or sandy-mud banks, often extending into low salinity (Dutreix 1992). Tmethypocoelis is unique among confamilials by having a long narrow apical styliform projection on the eyestalks that extends beyond the cornea (Davie 1990).
The first species to be described was Tmethypocoelis ceratophora (Koelbel, 1897) from Hong Kong. Although it was initially placed in Dioxippe de Man, 1888, Koelbel (1897Koelbel ( , 1898 remarked that its unique characters deserved the recognition of a new subgenus Dioxippe (Tmethypocoelis) Koelbel, 1897. Five species are currently known in Tmethypocoelis: T. ceratophora (Koelbel, 1897) from China; T. koelbeli Davie, 1990, from Northern Territory, Australia (Davie 1990;Davie and Kosuge 1995); T. odontodactylus Davie, 1990, from Madang, Papua New Guinea (Davie 1990; Davie and Kosuge 1995); T. choreutes Kosuge, 1995, from Japan (Davie andKosuge 1995); T. liki Murniati, Asakura, Nugroho, Hernawan & Dharmawan, 2022, from Papua, eastern Indonesia (Murniati et al. 2022. The previous records of T. ceratophora from Japan and Indonesia (Dutreix 1992;Huang et al. 1992;Murniati 2015) would appear to be misidentifications, and the Japanese specimens have since been described as T. choreutes Davie & Kosuge, 1995. The specimen first recorded from Lombok, Indonesia, is also likely to be a new species and will be further discussed in a subsequent paper with other potential new species from the Indonesian region following ongoing revisionary work on this genus.
Fieldwork by the first author to investigate the systematics of the Dotillidae Stimpson, 1858, of Indonesia has resulted in the discovery of populations of two species occurring on opposite coasts of Sulawesi Island, Indonesia, here described as both new to science and compared with the previously known species of Tmethypocoelis.

Morphology
The crabs were observed, measured, and photographed using either a stereo microscope (Olympus SZ) connected with a digital camera (Olympus E-330), or a Leica Z6 microscope connected to a computer using LAS Core v. 4.13 software. Measurements in millimeters (mm) were of carapace width (cw, measured across at the widest point) and carapace length (cl, measured from the mid-front to the midposterior margin). Smaller specimens and body parts were examined under a Nikon SMZ 800 stereo Microscope equipped with a camera lucida drawing tube. Drawings were made by hand and enhanced using a Wacom drawing pad and Adobe Illustrator CC2015 software.
Morphological terminology for the teeth of the gastric mill follows Davie et al. (2015). The G1 and teeth on the gastric mill were removed, fixed in glutaraldehyde and cacodylate buffer, serially dehydrated in ethanol (50%, 70%, 85%, 90%, 100%), and vacuum-dried using TITEC VC-96N for 10 minutes. Each prepared sample was then mounted on a specimen stub and coated with gold at 5-8 mA for 5 min using ion coater (Dewi and Purwaningsih 2020). The detailed photos of the teeth plate of the gastric mills and gonopods were captured using a scanning electron microscope (SEM), JEOL JSM-IT 200, at an accelerating voltage of 5 kV. The photographs of the teeth plate are presented with posterior portion upper-most.
Description. Carapace ( Fig. 2A, B) pentagonal, weakly convex along mid-dorsal line, weakly convex laterally, ca. 1.7× as wide as long. Dorsal surface smooth, lateral portion with granules, regions semi-defined; epigastric lobe poorly defined. Cervical grooves well-marked. Cardiac region with slight central depression. Branchial regions sloping; protobranchial, mesobranchial and metabranchial regions well-defined. Subbranchial region bulging, bearing regular setae and tubercles. Carapace widest between exorbital angles. Intestinal and branchial borders poorly defined. Lateral margin of carapace recurved, with row of tubercles and short stout setae. Posterior margin weakly concave, ca. 0.53 distance between exorbital angles; fine ridge parallel to posterior margin forming broad rim. Front with lateral border moderately converging, width at base ca. 0.24× distance between exorbital angles, ca. 0.21 at anterior margin; frontal angle rounded; anterior margin with small central blunt prominence (Fig. 3B). Exorbital angle triangular, acute, directed forward; anterior margin with microscopic tubercles, lateral margin glabrous; one short tubercular ridge parallel to supraorbital margin;  posteriorly followed by broad U-shaped sinus. Second anterolateral tooth slightly acute, slightly shorter than exorbital angle. Posterolateral facet well-defined by crest originating anteriorly from base of exorbital angle (Fig. 4A). Supraorbital margin sinuous, sloping backward, microscopically tubercular. Infraorbital margin with medial notch; pterygostome with inwardly directed oblique channel; inner portion consisting of two granular ridges separated by shallow channel; upper ridge granular, with one row of setae; lower ridge granular, granules larger than those of upper ridge, without setation; outer portion granular, concave, ending in broad notch below exorbital angle (Fig. 4B).
Male chelipeds stout, long, equal. Merus cross-section triangular; standing higher than exorbital angle (Fig. 3C); lower margin covered with granules extending entire length, granulation branched sub-medially into two rows (Fig. 5A); upper margin narrower proximally, wider distally, with irregular rows of granules on distal half, proximal portion smooth (  5F). Palm bulky, ca. 1.4× wider than long; upper margin with one row of granules, distinct groove extending below granular rows forming clear granular string (Fig. 5G); lower margin granular, granulation branched into two rows medial to distal portion (Fig. 5H); inner surface irregularly granular, upper granulation extending over median portion, curved to sharply cut upper margin of outer surface and base of fingers, lower granulation extending near lower margin from proximal portion to base of pollex (Fig. 5M); outer surface distinctly granular over upper half to base of pollex, lower half smooth (Fig. 5N). Fingers shorter than palm, broadly gaping at base; curved inwards, expanded distally forming spooned-tip; cutting margins evenly serrated; inner margin at tip of both fingers with short row of stout setae. Pollex short, triangular, cutting margin evenly dentate; long flat enlarged dentate tooth over most of length, ca. 0.4× as wide as palm; inner surface smooth (Fig. 5M); outer surface granular proximally parallel to cutting margin (Fig. 5N); lower margin granular nearly whole length (Fig. 5H). Dactylus ca. 0.6× as wide as palm; cutting margin evenly dentate; one enlarged wide convex tooth over proximal half; inner surface granular from proximal to median portion near cutting margin, one clutch of granules proximally near upper margin (Fig. 5M); outer surface with 1 row of granules medially, densest on proximal portion of surface, granulation extending nearly entire length, irregular granulation near cutting margin, a single row of spaced tubercles medially, parallel to upper margin (Fig. 5N); upper margin with median row of granules, narrower distally matching shape of upper margin (Fig. 5O).
Habitat. Tmethypocoelis simplex sp. nov. lives in estuarine conditions on both sandy and muddy substrata ( Fig. 9). At Towale Village, it inhabits sandy substrates alongside other ocypodoids, Austruca annulipes (H. Milne Edwards, 1837) and Scopimera intermedia Balss, 1934, but also in muddier areas where it co-occurs with Tubuca dussumieri (H. Milne Edwards, 1852). At Tosale Village, it was typically collected on sandy substrates. While not collected, it was also observed along a small muddy canal near local residences. It was recorded approximately 1 km further upstream beyond the estuary in non-tidal area.
Etymology. The name simplex refers to the simple form of the cheliped dactylus that lacks a conspicuous outer subdistal dorsal projection, a character that is characteristic of other described species.
Description. Carapace (Figs 10A, 12A) pentagonal; weakly convex laterally and longitudinally; ca. 1.6-1.7× wider than long. Dorsal surface smooth, regions semi-defined; epigastric lobe poorly defined. Cervical grooves, well-marked; cardiac region slightly depressed. Branchial regions sloping, protobranchial, mesobranchial and metabranchial regions well-defined. Carapace widest between exorbital angles. Sub-branchial region bulging, bearing regular setae and tubercles. Intestinal and branchial regions well-defined. Posterior margin weakly concave, ca. 0.6× distance between exorbital angles; fine ridge parallel with posterior margin forming broad rim. Lateral margin recurved with row of tubercles and short stout setae. Frontal margin rounded, mod- erately convergent, basal width ca. 0.19× distance between exorbital angles, ca. 0.13× at anterior margin; anterior margin with small central blunt prominence (Fig. 11A). Exorbital angle triangular, acute, directed forwardly; anterior margin with microscopic tubercles, lateral margin slightly tubercular; posteriorly followed by broad U-shaped sinus. Epibranchial tooth less acute, slightly shorter. Posterolateral facet well-defined by a crest originating anteriorly from base of exorbital angle (Fig. 12A). Supra-orbital borders sinuous, sloping backward, microscopically tubercular. Infra-orbital border with medial notch; pterygostome with inwardly directed oblique channel. Inner part of infra-orbital border with two granular ridges separated by shallow channel; upper ridge with row of setae; granules on lower ridge larger than that of upper ridge, without setation. Outer part of infra-orbital border granular, concave, ending in broad notch below exorbital angle (Fig. 12B).
Male chelipeds stout, long, equal. Merus triangular in cross-section; standing higher than exorbital angle (Fig. 11B); lower margin with two rows of granules extending whole length of margin (Fig. 13A); upper margin narrowing proximally, with irregular rows of granules on distal half (Fig. 13K); outer margin with one row of granules extending whole length (Fig. 13L); upper surface flattened, ovate smooth tympanum, scattered long setae around tympanum, more setation distally, microscopically tuberculate (Fig. 13I); lower surface flattened, nearly smooth, with scattered granules, lacking   (Fig. 13J); outer surface convex, tympanum smaller than that of upper surface, evenly distributed granules and setae (Fig. 13B). Carpus shorter than merus, elongate, ca. 1.4× longer than wide; upper and lower margins tubercular (Fig. 13C, D); outer surface rectangular, scattered microscopic granules near upper and lower margins, median portion without microscopic tubercles (Fig. 13E); lower surface smooth, with one longitudinal row of granules, one patch of tubercles on proximal part (Fig. 13F). Palm bulky, ca. 1.3× longer than wide; inner surface granular over upper half, with granules extending over upper margin and curved to sharply cut upper margin of outer surface, distally with one row of regular granules reaching pollex, smaller granules near lower margin, median portion smooth (Fig. 13M); outer surface distinctly granular over upper half reaching to base of pollex, lower half smooth (Fig. 13N); upper margin with one row of granules, distinct groove extending below granular rows forming clear granular string (Fig. 13G); lower margin with granulation extending to lower part of inner surface (Fig. 13H). Fingers shorter than palm, lacking obvious basal gape, curved inwards, spooned-tip; cutting margins evenly serrated; inner margin at tip of both fingers with short row of stout setae. Pollex short, triangular, cutting margin slightly oblique, without large differentiated tooth or lobe, ca. 0.5× as long as palm; inner surface nearly smooth, one row of granules over proximal half (Fig. 13M); outer surface granular parallel to cutting margin, granules with similar size (Fig. 13N); lower margin granulated only along proximo-medially (Fig. 13H). Cutting margin of dactylus with teeth, larger teeth over proximal half, smaller teeth over distal half, without median lobe; inner surface with one row of granules parallel to upper margin, granulation extending from proximal to distal, one patch of granules proximally (Fig. 13M); band of granules on outer surface near cutting margin and junction to palm, one tubercular ridge extending medially parallel to upper margin (Fig. 13N); upper margin with row of tubercles terminating with triangular upturned tooth subdistally (Fig. 13O).
Etymology. The species name is derived from the type locality. Celebes is the former name of Sulawesi Island, one of the great islands in Indonesia.
Remarks. The two new species described here differ from each other and from the other known species by numerous characters compared below. In general, the species of Tmethypocoelis are all extremely similar in general morphology with only small differences in male cheliped shape and dentition (Table 1), differences in the apical setal ornamentation of the male first gonopod, and sometimes differences in the proportions of the somites of the male pleon.
A comparison of male first gonopod setation patterns of described species suggests that the possession of two or three of markedly elongated apical setae (Figs 8C-E, 16C, D; Davie 1990: fig. 2), common to both Tmethypocoelis simplex sp. nov. and T. celebensis sp. nov., is so far shared with T. liki from Papua and T. koelbeli from the Northern Territory, NW Australia. Therefore, these four species may be more closely related to each other than they are to T. ceratophora, T. choreutes, and T. odontodactylus, which all share a coronet of shorter more evenly sized stout setae on the tip of the G1. A more thorough analysis of relationships within the genus will be undertaken as part of a larger revision of the genus, and with the help of DNA sequencing data.
Both Tmethypocoelis simplex sp. nov. and T. celebensis sp. nov. differ significantly from T. koelbeli in the shape of the male pleon, with that of T. koelbeli being relatively narrower, and in particular Pl5 being more constricted proximally ( Table 2). The pleons of T. simplex sp. nov. and T. celebensis sp. nov. are similar, however, both Pl6 and the telson are slightly proportionately wider in T. celebensis sp. nov.
Tmethypocoelis simplex sp. nov. differs from T. celebensis sp. nov., T. koelbeli, and T. liki in the form and number of the apical setae of the G1. The G1 of T. simplex sp. nov.  Davie and Kosuge 1995: fig . 1 A, C) Finely tubercular ridge on the upper margin continues evenly distally; outer surface has separate ridge ending in prominent triangular subdistal protrusion (see Davie and Kosuge 1995: fig. 1 B, D) Full length medial granulate ridge; superior granulate crest terminating subdistally in strong upturned tooth (Davie 1990: fig. 3C typically has two or three very long setae apically (Fig. 8C-E), and subapically there are three shorter stout setae on the outer margin increasing in length distally, and four short, downwardly reflexed setae on inner lobe. The G1 of T. celebensis sp. nov. has two or three very long recurved setae apically (Fig. 16B-D), and subapically there are also three stout setae on the outer margin, though the proximal seta is much smaller and less prominent than on T. simplex sp. nov., and also four or five short, downwardly reflexed setae on the inner lobe. The G1 of T. koelbeli similarly has two long apical setae but lacks a row of outer subapical setae and has a row of five short distally pointed setae on the inner lobe (Davie 1990: fig. 2). The G1 of T. liki has one long and five short apical setae (Murniati et al. 2022: fig. 20C).

Discussion
Tmethypocoelis for many years included only the type species Tmethypocoelis ceratophora (Koelbel, 1897), which was believed to be widespread from Hong Kong, China, Japan, and south to Lombok in Indonesia (Tesch 1918;Dutreix 1992;Huang et al. 1992;Murniati 2015). In recent years, however, four new species have been described (see Introduction), and the distribution of T. koelbeli has become more restricted (Davie 1990;Davie and Kosuge 1995;Murniati et al. 2022). Nevertheless, the genus is outwardly morphologically relatively homogenous, with only small differences among the species most obviously related to chela dactylar tooth shape, and differences in the apical setae of the G1. This is particularly exemplified by the separation of the pseudocryptic T. choreutes, that had long been confused with T. ceratophora, but the morphological differences were shown to correlate with the evolution of a different male courtship waving display (Davie and Kosuge 1995). The relative morphological homogeneity within the genus is also an indication that Tmethypocoelis species have undergone relatively recent speciation based around small-scale biogeographic restrictions. With the complex evolving paleogeography of land-connections and sea-level changes throughout the Indo-Malaysian Archipelago over the last two million years, it can be expected that the genus may have speciated much more than previously thought. Careful collecting across a broad range of areas within the region and more careful observations of populations, including finer scale morphological investigations, behavioural analyses and genetic studies are indeed revealing this pattern, and further new species will be described by the present authors as part of ongoing revisionary work.

General morphology
It is interesting to note that the tympani on the anterior and posterior surfaces of the pereiopods are essentially the same between the present two new species; and while the tympani are very similar on both faces of P2 and P3, on both P4 and P5 the anterior tympani are markedly smaller in size with P5 the smallest; the posterior tympani on P2 and P5 are much larger and cover a proportionately similar surface area to the first two pereiopods (Figs 7,  15). This is simply an observation, and no physiological explanation can be offered.

Feeding morphology
Speciation in ocypodoid crabs seems to have commonly involved variations in structures related to feeding and adaptations to different sediment particle sizes or food types on which each species feeds. For example, both the setation of the second maxillipeds and the shape of the grinding plates inside the gastric mill, have proven useful in distinguishing closely related species (Davie et al. 2015). The second maxilliped has specialized "spoon shaped" setae for sorting organic matter and microorganisms from the sand (e.g., Icely and Jones 1978;Vogel 1984;Colpo and Negreiros-Fransozo 2013). Spoon-tipped setae mostly occur on the inside margins of the second maxillipeds where they hold sand grains that are then brushed by short stiff setae on the outer faces of the first maxillipeds. Such setal structures have been well studied especially in species of Dotilla Stimpson, 1858 and Uca Leach, 1848 sensu lato, and vary according to the preferred substrate particle-size composition, and the distribution of the species on the shore (Icely and Jones 1978). Murniati and Wowor (2017) were the first to use second maxilliped setation to successfully separate three species of Tmethypocoelis occurring in Indonesia, and to help infer micro-ecological niche separation.
Although, the second maxillipeds have not been examined as a part of the present species descriptions, the gastric mill structure has been included, and equally shows that the two new species described here have adapted to different dietary requirements (e.g., see Kosuge and Davie 2001), and this is the first study to report species-specific gastric mill differences within Tmethypocoelis (Fig. 17A-D). The main trunk of the urogastric ossicle is noticeably broader in T. simplex sp. nov. than in T. celebensis sp. nov.; in both species, the medial tooth plate is simple and without defined ridges. However, the lateral margins are evenly convex in T. simplex sp. nov., versus more quadrate margins with anterior lobes discrete, prominent, and rounded in T. celebensis sp. nov.; the propyloric ossicle is also flatter and broader in T. simplex sp. nov., versus narrower and more protruding in T. celebensis sp. nov. In lateral zygocardiac teeth, there are also significant differences: in particular, T. simplex sp. nov. has nine accessory teeth that have a broader thicker brush of apical setae than T. celebensis sp. nov., which has only eight accessory teeth, each with a narrower brush of apical setae. While these differences would need a more specialized study to understand the dietary implications, nevertheless, it is apparent that although Tmethypocoelis species are deposit feeders, the lack of fine transverse median ridges on the urocardiac ossicle (as one would find on deposit feeding ocypodids such as Uca; see Icely and Nott 1992) would seem to indicate that the particulate organic matter that they are consuming does not require fine grinding, and the brushes of setae and fine scales on the accessory teeth of the zygocardiac ossicles serve more of a brushing function.

Biogeography
Tmethypocoelis species are essentially estuarine animals, living abundantly on estuarine mud flats and able to tolerate low salinities. Davie (1985) and Davie et al. (2010) have postulated that a short larval life in combination with local hydrological factors may be enough to lead to the allopatric separation of two geographically close taxa. Davie and Kosuge (1995) remarked that Tmethypocoelis ceratophora and their new species T. choreutes are separated by the relatively narrow strait between Taiwan and the Japanese Yaeyama Islands. Davie et al. (2010) described a similar disjunction between the Chinese/Taiwanese Mictyris brevidactylus Simpson, 1858, and their new species Mictyris guinotae Davie, Shih & Chan, 2010, as well as citing a number of other similar cases of closely related sibling species on either side of the aforesaid strait. They concluded that the Ryukyus appear to be much more influenced by the main Kuroshio Current in contrast to the continental coastline, which is impacted mainly by the South China Sea Current and westerly flowing Kuroshio Branch Current (Jan et al. 2002), and that the deep-water strait between Taiwan and the Yaeyama Islands plays an important additional role in the local circulation patterns of the region, so as to become an effective barrier for species that may have rapid larval development and/or abbreviated life cycles. Thus, this narrow passage of water has functioned as a barrier against genetic flow between the two regions, and allowed the allopatric speciation of sibling taxa. In the case of the two new species described here, T. simplex sp. nov. and T. celebensis sp. nov., there is good evidence that a similar pattern of local current flow may have led to their separate evolutionary development (Fig. 19). Sprintall et al. (2019) have published an interesting map of current circulation patterns within the Indonesian Archipelago that shows the southerly flowing Mindanao current forming a counter-clockwise circulation flow within the Celebes Sea as well as flowing south through the Makassar Strait. The Celebes Sea is bordered to the east by a shallow ridge and island chain extending northwards from the tip of north Sulawesi to Mindanao in the southern Philippines; whereas the east coast of Sulawesi, home to T. celebensis sp. nov., is instead under the influence of an off-shoot of the North Equatorial Counter Current that flows southwards through the Maluku Channel. There are, therefore, two major separate southerly flowing current systems on either side of the island of Sulawesi, and this appears to effectively separate larval dispersal of both species to the east and west coasts, respectively.
While no genetic clock estimates have yet been applied to the two species studied here, it can be presumed that their evolutionary separation has been recent, i.e., within the last 2 million years. This time-frame has precedent in other recent speciation events that have occurred in the Indo-West Pacific region. For example, based on the molecular clock of COI mutation rates suggested by Schubart et al. (1998), Ragionieri et al. (2009Ragionieri et al. ( , 2010Ragionieri et al. ( , 2012 found that the sister sesarmid species Neosarmatium africanum Schubart, 2012, andN. meinerti (De Man, 1887) became isolated between 1.6-1.96 ± 0.34 mya (1.6% divergence); and similarly, Lai et al. (2010) estimated that the portunids Portunus armatus (A. Milne-Edwards, 1861) and P. reticulatus (Herbst, 1799) became established around 0.78-2.5 mya, based on a 1.8% CO1 divergence.
Given our assumption that species separation has been caused by differences in circulation patterns, then it is important to understand the geological history of the Indonesian archipelago and the geological changes that have led to the current shape of the island of Sulawesi. Nugraha and Hall (2018) have studied the Late Cenozoic palaeogeography of Sulawesi, and it is clear that it is only within the last 2 million years (since the Early Pleistocene) that, Sulawesi began to resemble its present form ( Fig. 20A-C). By the Early Pleistocene, paleogeographic change across Sulawesi included the rise of high mountains and the rapid subsidence in offshore basins; much of the North Arm and most of the southern South Arm appear to have emerged although the northern part of the South Arm was still a shallow marine area. Between 1.8-1.0 mya subsidence in the southern SE Arm continued, and by 1 Mya, Sulawesi was very similar in form to the present. The inter-arm basins were close to their present depths of 1.5 to 2.0 km. The North Arm was largely emergent, and there was a land connection between the North Arm and western Central Sulawesi as the Neck elevation increased. Therefore, this pattern of recent island emergence and sea basin separation supports our contention that speciation of T. simplex sp. nov. and T. celebensis sp. nov. began as a vicariant event within the last 2 million years.
Studies of survivorships of pelagic larvae under various salinity regimes, and analyses of genetic structure among different island populations throughout the Indonesian Archipelago will provide exciting insights into the speciation of coastal crabs and the evolutionary impacts of paleogeography throughout this region.