INACHIDAE

Ng, Peter K. L., Forges, Bertrand Richer De & Jones, Georgina, 2013, Taxonomy and ecology of the Cape Town Spider Crab, Macropodia falcifera (Stimpson, 1858) (Crustacea: Decapoda: Brachyura: Inachidae), Zootaxa 3626 (3), pp. 391-396: 391-392

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http://dx.doi.org/10.11646/zootaxa.3626.3.7

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scientific name

INACHIDAE
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Family INACHIDAE  MacLeay, 1838

Macropodia falcifera Stimpson, 1858  ( Figs. 1–3View FIGURE 1View FIGURE 2View FIGURE 3)

Stenorhynchus falcifer Stimpson, 1858: 219  .—Miers 1886: 6, pl. 1 fig. 1.—Doflein 1904: 70, fig. 6.—Stimpson 1907: 22, pl. 3 fig. 8.—Lenz & Strunck 1914: 273.

Macropodia falcifera  —Stebbing 1910: 284. — Balss 1913: 109.—Odhner 1923: 26. —Barnard 1950: 14, fig. 2 b–f.—Kensley & Buxton 1984: 191.—Griffin & Tranter 1986: 33 (key).

Material examined. South Africa, False Bay, 24 m, living on a Leptogorgia palma (Pallas, 1766)  (Anthozoa: Gorgonacea  : Gorgonidae  ), coll. G. Jones, 14 August 2010: 1 male (18.65 × 11.10 mm) (carapace bearing bryozoa and serpulids), 1 ovigerous female (17.33 × 9.87 mm) (ZRC).

Diagnosis. Relatively large size (ca. 16 mm carapace length). Carapace pyriform with long rostrum ( Figs. 2 View Figure , 3 View Figure A). Rostrum composed of 2 appressed smooth sharp spines ( Figs. 2 View Figure A, 3 A). Dorsal surface of carapace evenly granular; regions distinct; raised gastric, cardiac regions each with strong long sharp, gently anteriorly curved spine; branchial region slightly inflated; hepatic region forming conical expansion with granule-like spine ( Fig. 2 View Figure A). Ocular peduncle long with ovoid cornea, appearing more tapering from dorsal view ( Fig. 2 View Figure B, C). Epistome very long ( Figs. 2 View Figure B, 3 A). Antennular fossa large, slightly splayed; basal antennal article fused to carapace, side of fossa; interantennular septum with short sharp spine ( Figs. 2 View Figure B, 3 A). Basal antennal article with single distal tooth ( Figs. 2 View Figure B, C, 3 A). Basal antennular article with 3 or 4 low spines; antennules folding slightly obliquely outwards ( Figs. 2 View Figure B, C, 3 A). Third maxilliped pediform; inner margin of ischium serrated, outer surface with 7 spines; merus ovate with 3 long inner lateral spines and 5 short median spines; carpus, propodus, dactylus elongated, longer than merus; exopod slender ( Fig. 3 View Figure C). Anterior thoracic sternum longitudinally elongated; sternites 1–5 fused, with only lateral clefts between sternites 3, 4 visible; surface of sternite 4 depressed; proximal part of sternite 4 with small cluster of small sharp tubercles ( Figs. 2 View Figure B, 3 B); sternite 5, 6 with 2 or 3 sharp tubercles on each side; surfaces of sternites 5–7 finely granular. Press-button male abdominal locking mechanism prominent, peg-like, on median part of thoracic sternite 5. Male abdomen with 5 free somites, with somite 6, telson completely fused; somites 3–6 each with median tubercle ( Figs. 2 View Figure B, 3 E); somite 1 longitudinally broad, somite 2 narrow. Male chelipeds elongated, inflated: propodus rounded, smooth; single spine on upper margin of carpus; strong distal spine on dorsal margin of merus ( Fig. 2 View Figure B). Ambulatory legs relatively very long, thin; first leg longest; meri with strong distal spine on distal margin; dactylus styliform on first, second legs, ventral margin setose but not armed; dactylus falcate on third, fourth legs, ventral margin lined with 8 or 9 strong curved spines on distal half and 6–8 sharp tubercles on proximal half ( Figs. 2 View Figure B, 3 D). G 1 relatively stout, gently sinuous, distal part bent outwards ( Fig. 3 View Figure F–H).

Female. The carapace is proportionately slightly broader and more rounded compared to the male and agrees very well with the type female figured by Stimpson (1907: pl. 3, fig. 8). It has a large dome-shaped abdomen with somite 6 and the telson completely fused. The median portion of each somite has a raised tubercle. The ovigerous female abdomen is dome-shaped.

Remarks. This species was described by Stimpson (1858) on the basis of one small female specimen collected from shallow waters in Simon's Bay (a small bay inside the larger False Bay, near Cape Town). Barnard (1950: 14, fig. 2 b–f) examined 36 specimens and gave a supplementary description with figures of the epistome, antennae, antennules, G 1 and ambulatory legs. Griffin & Tranter (1986: 33) only mentioned the species in their key to the Indian Ocean species, comparing it with M. formosa Rathbun, 1911  , and M. intermedia Bouvier, 1940  (see also Griffin 1974; Kazmi & Tirmizi 1995).

Although Stimpson (1858) named the species for its prominently falcate third and fourth ambulatory dactyli, none of the specimens examined and attributed to M. falcifera  by previous authors have a dactylus as strongly curved as described and figured by Stimpson (1858: 219; 1907: 22–23, pl. 3 fig. 8). Miers (1886) and Barnard (1950) both discussed this; and the dactylus of the present specimens agree with their observations and figures. A strongly falcate third and fourth ambulatory dactylus is a feature of members of the genus Achaeus Leach, 1817  , but members of this genus have short rostrums. The long rostrum and general carapace form of Stimpson’s species agrees best with members of Macropodia  , and it is almost certainly for this reason that subsequent authors have referred their specimens to Stimpson’s species despite the difference observed in the dactylus form. Macropodia falcifera  is easily distinguished from the two other Indian Ocean species, all of which are characterized by a long rostrum, presence of only one spine on the antennal basal article and with 14–16 spines on the ventral border of the dactyli of the third and fourth ambulatory legs. The types of Stimpson are almost certainly lost (Evans 1967; Deiss & Manning 1981) but M. falcifera  is a relatively distinctive species that is unlikely to be confused with other taxa for the time being. A neotype designation is therefore unnecessary at this stage.

Although most previous authors cite the year of publication as 1857, this is incorrect. Volume nine of the Proceedings of the Academy of Natural Sciences of Philadelphia (which published Stimpson’s paper) was dated December 1857 but it was not available until early 1858 (see Manning & Holthuis 1981: 377). The species was inadvertently not listed in the world compendium of Brachyura by Ng et al. (2008).

The only Macropodia  species recorded from the Pacific Ocean is from deep water in the Tasman Sea, M. trigonus Richer  de Forges, 1993.

Ecology. Macropodia falcifera  has so far been found only along the South African coast from False Bay in Cape Town to East London. It lives at depths below 15 m and has been observed as deep as 35 m, apparently preferring cooler water. Sea temperatures at this depth range range from 12 ° C at the western end to less than 25 ° C at the extreme eastern edge of its distribution. It is usually seen on gorgonians (although this may be because it is more easily spotted), but has also been observed on reef substrates as well as the sand and shell debris surrounding reefs. It is often found covered by hydroids, bryozoans, serpulids or algae, occasionally to the extent that it is almost completely camouflaged and indistinguishable from the background. It is not a commonly found, but this may be the result of its very effective camouflage. The crabs have been observed by some divers cleaning the gills of catsharks (family Scyliorhinidae  ). When disturbed, crabs raise their white-tipped pincers at divers.