Achelia assimilis (Haswell, 1884)

Arango, Claudia P., 2003, Sea spiders (Pycnogonida, Arthropoda) from the Great Barrier Reef, Australia: new species, new records and ecological annotations, Journal of Natural History 37 (22), pp. 2723-2772 : 2728-2729

Achelia assimilis (Haswell, 1884)

(figure 2)

Ammothea assimilis Haswell, 1884: 1026–1027 , pl. 55, figures 5–9; Loman, 1908: 59–60.

Achelia assimilis: Flynn, 1919a: 89–70 , pl. 22, figure 22–26; Stock, 1954: 97–100, figures 45, 46; 1994: 32–33, figure 9 (literature); Child, 1977: 440–441; 1988a: 289–290; 1988b: 2; 1991: 138; 1996b: 541; Bamber, 2000: 621.

Material examined. Turtle Bay, collected among Cladophora prolifera from intertidal rocky patches on a sandy beach during low tide (<0.5 m), 23 March 1997, 16 W, six X, 11 juveniles, one post-larva (coll. Otto); 5 October 1998, 18 W, 12 X; 14 May 1999, one W, one juvenile, eight adults; 12 July 1999, five W, three X; 4 May 2000, two W, 10 X; 1 July 2000, one W, one X, one juvenile. Rowes Bay , collected among C. prolifera from a rock ledge made of small boulders in a sandy-muddy environment during low tide (<0.5 m) , 17 April 1999, one W w/eggs, one X; 12 August 1999, one X; 25 September 1999, one X, one W; 1 June 2000, 24 W, 18 X, one juvenile. Coral Sea, Flinders Reef, amongst washings of algae, 12 m, 3 July 1999, one X. Pandora Reef, southern windward side of the reef, in rubble with turf algae and macroalgae Dictyota sp. and Laurencia sp. , 5 m, 15 July 1999, three juveniles. Great Palm Island , Cannon Bay, reef flat, on rubble and algae , 2 m, 4 February 1999, one juvenile. Goold Island , reef flat, in Sargassum spp ., 4 m, 15 March 2000, one W (coll. Diaz-Pulido).

Description. Trunk length 1.0– 1.4 mm, width across second crurigers 0.6–0.7 mm, third line of segmentation not clear, smooth, disc-shaped, crurigers touching; abdomen swollen at the base then curved upwards in the distal half. Anterior and posterior corners of the crurigers armed with one spine bearing a tubercle each. Three dorsodistal spines on scape. Palps eight-segmented, last segment twice as long as wide. Ovigers 10-segmented, fifth segment the longest, spines in the formula 2:2:2:2. Legs robust, short, first and second coxae of all legs with two pairs of spiny tubercles, femur and tibiae sub-equal in length, auxiliary claws threequarters length of the main claw. Cement gland a dorsodistal tube on mid-femur. Female: same as male but larger, femora swollen in adults and spines less strong and conspicuous. This species is known to be highly variable in diagnostic characters so an illustration of one of the specimens found is provided (figure 2).

Distribution. Described from Port Jackson in New South Wales, Australia, reported from Western Australia (Child, 1975) and more recently from Heron Island and Lizard Island in the southern and northern sections of the Great Barrier Reef, respectively (Child, 1990). The records from Townsville and nearby areas suggest a possible continuous distribution of the species in diverse shallow-water habitats of the GBR and perhaps the east coast of Australia. Achelia assimilis is mostly a southern hemisphere species. Records exist from Mozambique, Chile, Argentina and New Zealand. A corridor of distribution is suggested from New Zealand to Papua New Guinea to Malaysia, Indonesia and the Philippines which is the most northern location in the Pacific from where A. assimilis has been collected.

Remarks. The genus Achelia appears to be an artificial grouping of species with high intraspecific variation in their morphological characters. It is in strong need of revision, being a genus with a worldwide distribution and of relatively high abundance especially in littoral zones. Two forms of A. assimilis described by Stock (1954) exemplify the enormous variability of assumed species. These specimens agree with the description of the ‘large form’ but they are definitely smaller than those described (Stock, 1954). Achelia assimilis was predominantly found among intertidal tufts of Cladophora prolifera , being relatively abundant. It also occurs in coral reefs but in lower numbers.