Adult representatives of freshwater crab families examined Gecarcinucidae: Maydelliathelphusa lugubris (Wood-Mason, 1871), ♀ CW 55.8 mm, Nepal, coll. S. Rana, 29 May 2008, NMU 29.V.2008 B (Fig. 7). Potamidae: Larnaudia chaiyaphumi Naiyanetr, 1982, ♀ CW 51.0 mm, Chetsanoi waterfall, Saraburi Province, Thailand, coll. Pongrat Dunrongsoiwattana, 18 February 2006, NMU 18.II.2006.e (Fig. 8). Potamonautidae: Acanthothelphusa niloticus (H. Milne Edwards, 1837), ♀ CW 61.2 mm, Cheprock, Kenya, coll. R. Mujinga, December 1979, NMU 12.07.1979.1 (Fig. 9). Deckeniidae: Hydrothelphusa agilis A. Milne Edwards, 1872, ♂ CW 47.8 mm, Lalangina River, Province de Fianarantsoa, Madagascar, coll. J. Elouard & S.M. Goodman, 17 November 1993, FMNH 5729 (Fig. 12A, B); Globonautes macropus (Rathbun, 1898), ♂ CW 29.0 mm, Bong County, Liberia, coll. G. Duncan & J. Momo, NMU 17 July 1988 (Fig. 12C); Deckenia mitis Hilgendorf, 1898, ♂ CW 36 mm, Kiono Swamp, Kibno Forest Reserve, Tanzania, coll. Trefor Williams, British Tanzanian Expedition, NMU III.1990 (Fig. 12D). Pseudothelphusidae: Guinotia dentata (Latreille, 1825), ♀ CW 83.5 mm, River near Marigot, Dominica, West Indies, NMU 4.1.2004 (Fig. 10). Trichodactylidae: Sylviocarcinus pictus (H. Milne Edwards, 1853), ♀ CW 32.6 mm, Tucurui, Rio Tocantins, Brazil, coll. O. Odinetz Collart, 1985, NMU 1985.1 (Fig. 11). Hatchlings of freshwater crab families examined Gecarcinucidae: Holthuisana biroi Bott, 1969, Papua, Indonesia, NHM 1940.VII.15.19-28 (Fig. 4). Potamidae: Johora singaporensis Ng, 1986, Merolodge, Singapore, coll. 11 December 2014, ZRC 2019.0654 (Figs 5, 6B). Potamonautidae: Acanthothelphusa niloticus (H. Milne Edwards, 1837), Nile near Samannud, Loat collections, NHM 1908.1.9.1; Rotundopotamonautes loveni (Colosi, 1924), El 94b, Tributary of the River Nanafuna, Mount Elgon, Uganda, coll. Trefor Williams, NMU TRW 8.1.1961 (Fig. 6A). Adult representatives of marine crab families examined Thoracotremata: Gecarcinidae MacLeay, 1838: Discoplax gracilipes Ng & Guinot, 2001, ♀ CW 58.7 mm Hinagoanan Cave, Visayas, Bohol, Panglao, Panglao Island, Philippines, coll. P.K.L. Ng, 3 March 2003 (Figs 13A, 14A, B); Cardisoma armatum (Herklots, 1851) ♀ CW 60.4 mm, Liberia, coll. R. Sachs, 2008, NMU 2008a (Fig. 14B–D). Grapsidae MacLeay, 1838: Goniopsis pelii (Herklots, 1851), ♂ CW 30.3 mm, Taigbe West, near Kamsar, Boké Préfecture, Guinea, coll. N. Cumberlidge, 30 April 2005, Guinea Rapid Assessment Program 2005, NMU 30.IV.2005 (Fig. 14E, F). Sesarmidae Dana, 1851: Labuanium politum (De Man, 1888), ♀ CW 29.6 mm, Loboe River, Bohol, Philippines, coll. P.K.L. Ng, 28–29 July 2003, ZRC 2003.0380 (Fig. 14G, H). Varunidae H. Milne Edwards, 1853: Eriocheir sinensis H. Milne Edwards, 1853, ♀ ovig. CW 65.5 mm, River Thames, Erith, UK, coll. P.F. Clark, no further details (Fig. 14I, J). Heterotremata: Epialtidae MacLeay, 1838: Doclea ovis (Fabricius, 1787), ♀ CW 48.0 mm, Gulf of Thailand, Pattaya, Chonburi Province, Thailand, coll. N. Cumberlidge, 24 October 2004, NMU 24.10.2004 (Fig. 14K, L). Portunidae Rafinesque, 1815: Callinectes sapidus Rathbun, 1896, ♀ CW 157 mm, east coast of USA, purchased from suppliers, 16 April 2019, NMU 16.4.2019 (Fig. 3). RESULTS The typical brachyuran uniramous appendage comprises a coxa, a basis, and an endopod divided into up to five articles (ischium, merus, carpus, protopod, and dactylus; Fig. 1A). In all freshwater and marine crab families examined here, the mandible consisted of four divisions: the large white biting jaw (coxa) and a three-articled mandibular palp comprising the basis and two endopod articles (Tables 2 and 3). The coxa and basis are as expected for a typical crustacean appendage, whereas the number of endopod articles of the freshwater crab mandibular palp is reduced from five articles to two [here referred to as the proximal endopod article (A1) and the distal endopod article (A2)] (Fig. 1B–D). It is not possible, however, to establish whether these two endopod articles are equivalent to the first two endopod articles (i.e. ischium and merus) and the carpus, protopod, and dactylus are not expressed or whether the two palp endopod articles represent unknown fused combinations of the five endopod articles. This is not the only example where the endopod of a crustacean mouthpart has a reduced number of articles, because marine brachyuran larval stages have a second maxilliped with a three-articled endopod whose divisions cannot be named to correspond to the parts of the five-articled endopod (Clark and Ng 2004: fig. 9a, b). Examination of the internal musculature of the mandibular palp using stained palps and computed tomography scanning was aimed at delimiting palp article divisions (Boxshall 2004). Three distinct muscle bands were detected, one inside each of the three palp articles: the basis, A1, and A2 (Figs 2A, B, 6A, B). Other studies of marine crab early zoeal larval stages have used internal mandibular palp morphology to establish the number of palp divisions during development, where the palp is initially undifferentiated and non-functional (Clark and Cuesta 2015) but becomes functional for food processing in the megalop as a three-articled palp, e.g. Pilumnus sluiteri De Man, 1892, Actumnus setifer (De Haan, 1835), and Austinograea rodriguezensis Tsuchida & Hashimoto, 2002 (Clark and Ng 2004a: fig. 11a, b; Clark and Ng 2004b, fig. 11b, c; and Demidow et al. 2021, fig. 5B, respectively). These authors describe the megalop mandibular palp as having a basial article, from which originates a single two-articled ramus (endopod) comprising a proximal and terminal article. Similar observations of the mandibular development of freshwater crabs cannot be made because they reproduce by direct development and lack free-living larval stages, but the mandible of first-stage freshwater crab hatchlings is always fully developed for their species (see Cumberlidge et al. 2021). The internal musculature of the mandibular palp of freshwater crab hatchlings belonging to two families [Potamonautidae, Rotundopotamonautes loveni (Fig. 6A) and Potamidae, Johora singaporensis (Fig. 6B)] indicated that the basis and A1 were indeed separate palp articles and not a single fused article. This observation was possible for these two families because the specimens were well preserved and their cuticle was essentially transparent, but the poor condition of the hatchlings of other preserved hatchlings used in this study [e.g. Holthuisana biroi Bott, 1969 (Gecarcinucidae), Acanthothelphusa niloticus (H. Milne Edwards, 1837) (Potamonautidae), Neostrengeria lindigiana (Rathbun, 1897) (Pseudothelphusidae), and Dilocarcinus pagei Stimpson, 1861 (Trichodactylidae)] made their internal palp musculature difficult observe. The first mandibular article: the coxa The coxa is the proximal stem of the brachyuran limb (Boxshall 2004) that in the mandible forms the calcified enlarged jaw, which is expanded towards the mid-line to produce a sharp cutting edge (incisor) along its inner margin (the mandibular gnathobase), and a low mound dorsally (the molar). The coxal article of the mandible lacks endites, epipods, and gills. In freshwater crabs, the coxa shows little variation and consequently does not provide useful diagnostic characters. The second mandibular article: the first palp article, the basis The basis is the first article of the three-part mandibular palp arising from the pedestal of the coxa (Figs 3–14) and is a short, simple limb article that lacks endites, epipods, and gills. The suture that marks the junction between the basis and the first endopod article (A1) is either well defined (Fig. 13A, B) or obscure (Fig. 7C, D) but was always detectable in every specimen included in this study when direct panoramic observations of the basis were made at high magnification (Figs 3–14). In freshwater crab specimens for families with a bilobed mandibular palp, the often-obscured suture between the basis and A1 gives the superficial appearance that the basis and first endopod article form a single part. This has, no doubt, given rise to many authors erroneously describing the mandibular palp as being two-segmented. The third mandibular article: the second palp article (A1) The proximal article that arises from the basis is the first of the two endopod articles of the palp (A1) and is elongated, undivided, and follows the curvature of the upper margin of the coxa. The basis is separated from A1 by a suture that is always present but in some families is difficult to detect. A1 is widened distally by a semi-circular projection on the distal ventral margin, here termed the ‘palp auricle, PA’, a reference to its rounded earlike shape (Fig. 1B–D). The palp auricle is positioned behind the coxa on its inner face when the palp is in its resting position, where only the upper margins of the two endopod articles can be seen from the frontal view of the mandible. Morphologically, A1 of the palp does not vary between brachyuran species and is consequently not exploited as a diagnostic taxonomic character. The fourth mandibular article: the third palp article (A2) The distal article of the palp is the second of the two palp endopod articles (A2), which is referred to as the terminal article and is long, tapering, and fringed with setae. A2 is separated from A1 by a distinct suture that is always prominent. In marine crabs, A2 lacks any additional accessory structures (such as a lobe or a ridge), a condition that is typically described as ‘simple’. In the freshwater crab families, A2 is highly informative, because it exists in several different states depending on the family and is often used as a higher-level taxonomic character. The terminal article (A2) in some freshwater crab families is simple, e.g. Potamidae (Larnaudia chaiyaphumi; Fig. 8), Potamonautidae (Acanthothelphusa niloticus; Fig. 9), Trichodactylidae (Sylviocarcinus pictus; Fig. 11), and some Deckeniidae. A mandibular palp with a simple terminal article was also found in the marine crab families Epialtidae (Doclea ovis; Fig. 14K, L), Portunidae (Callinectes sapidus; Fig. 3), Grapsidae (Goniopsis pelii; Fig. 14E, F), Sesarmidae (Labuanium politum; Fig. 14G, H), and Varunidae (Eriocheir sinensis; Fig. 14I, J). The bilobed mandibular palp of other freshwater crab families has a terminal article (A2) that possesses a large subequal accessory lobe, i.e. the accessory structure of Ng et al. (2008), e.g. Gecarcinucidae (Maydelliathelphusa lugubris; Fig. 7), Pseudothelphusidae [Guinotia dentata (Fig. 10A, B) and Holthusiana biroi (Fig. 4)], Epiloboceridae, and Deckeniidae [Hydrothelphusa agilis (Fig. 12A, B), Globonautes macropus (Fig. 12C), and Deckenia mitis (Fig. 12D)]. In these families with bilobed palps, the terminal article (A2) resembles that found in those species with a simple terminal article and rests behind the coxa on its inner face. The accessory structure is a large, setaefringed lobe that is slightly wider than the terminal article, but usually only half its length; it arises along the entire junction between A1 and A2 and rests on the upper margin of the coxa, overhanging its outer face (Figs 4, 7, 10, 12; Klaus et al. 2002).

Published as part of Cumberlidge, Neil, Krajenka, Grace E., Clark, Brett & Clark, Paul F., 2024, A review of primary freshwater crab mandible morphology: terminology and systematic importance (Crustacea: Decapoda: Brachyura), pp. 1-19 in Zoological Journal of the Linnean Society 202 (4) on pages 5-10, DOI: 10.1093/zoolinnean/zlae044, http://zenodo.org/record/14764203