Gammaropsis digitata (Schellenberg)

Appadoo, C & Myers, A. A., 2004, Corophiidea (Crustacea: Amphipoda) from Mauritius, Records of the Australian Museum 56 (3), pp. 331-362 : 356

Gammaropsis digitata (Schellenberg)

Eurystheus digitatus Schellenberg, 1938: 84 .—J.L. Barnard, 1965: 535, fig. 30.— Sivaprakasam, 1970a: 570, fig. 10.

Gammaropsis digitata J.L. Barnard, 1970: 178 , fig. 114.— Ledoyer, 1972: 239, pl. 54B.— Ledoyer, 1982: 227, fig. 82.— Myers, 1985a: 80, fig. 61–62.

Jassa sp. 1 Appadoo & Steele, 1998: 640.

Material examined. 1833, 21♀♀, 18 juv., AM P60858, from Amansia glomerata , Padina and Sargassum, Souillac (20°31'S 57°30.7'E), 8 April 1999; 4933, 13♀♀, 7 juv., AM P60859, from Sargassum sp. and Ulva reticulata, La Cuvette (20°00'S 57°34.2'E), 5 May 1999; 1333, 21♀♀, 5 juv., AM P60860, from Sargassum densifolium , Amansia glomerata and Digenia simplex, Souillac , 4 August 1999; 1433, 22♀♀, 12 juv., AM P60861, from mixture of Sargassum densifolium , Padina sp. , Digenia simplex and Cymodocea sp. , Roches Noires (20°6.2'S 57°44.5'E), 16 December 1999.

Diagnosis. Eyes oval. Male gnathopod 1, propodus 2.3× as long as broad, palm oblique defined by a weak protrusion bearing a robust seta. Gnathopod 2, carpus triangular, 1.2× as long as broad; propodus 2× as long as broad, posterior margin with a long slender proximal tooth and a short, stout, irregular distal protrusion forming a short, transverse palm; dactylus robust, 2.8× length of palm.

Female gnathopod 1, propodus with smoothly convex palm. Gnathopod 2 propodus slender, palm oblique.

Type locality. Nui , Tarawa .

Habitat. Gammaropsis digitata is very common in the north, east and south coasts of Mauritius and was collected at depths of less than 1 m. Three sites where it was very common are Souillac, Roches Noires and La Cuvette. This species lives amongst the red algae especially Amansia glomerata and Digenia simplex , algae which are very common at the wave-exposed sites such as Souillac and Roches Noires.

Distribution. Madagascar, Mauritius, India, Tarawa, Kiribati, Micronesia, Hawaii, Fiji.

Remarks. The material from Mauritius agrees with the description given by Myers (1985a: 80, fig. 61–62). This species is easily distinguished from Gammaropis species in the G. atlantica complex by the ovoid shape of the eyes, which have a purple-coloured core (in alcohol preserved material) surrounded by a lighter coloured ring of ommatidia.

The distinctive shape of the hyperadult male gnathopod 2 with a long proximal tooth on the posterior margin of the propodus helps to distinguish this species from all other Gammaropsis species from Mauritius. In younger males, the tooth is less well-developed and originates midway on the ventral margin of the propodus as compared to the proximal end in large adults. The gnathopod 2 of younger males superficially resembles that of Gammaropsis photisimilis Ruffo (1969) , which also has a medial tooth on the posterior margin of the propodus. However, young males of G. digitata have a very narrow cleft between the sharp tooth and the palm as compared to a round-bottomed excavation separating the more blunt tooth from the palm in G. photisimilis .

Ledoyer (1978) recorded 7 species of Gammaropsis and only Gammaropsis sp. ( G. atlantica complex) has been recorded again in this study. One of the reasons for this difference in species is probably due to the habitats sampled and also the depth of sampling; Ledoyer (1978) collected samples from depths up to 25 m and his substrates were mostly coral rubble.