Rhombognathus Trouessart, 1888

Bartsch, Ilse, 2006, A new species and new record of Rhombognathus from Singapore (Acari: Halacaridae), Zootaxa 1120, pp. 41-49 : 42-43

Genus Rhombognathus Trouessart, 1888

Rhombognathus scutulatus Bartsch, 1983

Rhombognathus scutulatus Bartsch, 1983: 413 –415, figs 46–57; Bartsch, 1993: 20, 21, fig. 1A–C; Bartsch, 1999: 354 –355, fig. 12F–H; Bartsch, 2000: 190; Bartsch, 2003: 273 –275, fig. 9A–D; Chatterjee, 1995: 284, figs 15–19; Chatterjee & De Troch, 2000: 187 –189, fig. 6A–F.

Material examined

One female, ZRC.ARA.489, 1 male, ZRC.ARA.490, Singapore, Strait of Singapore, Labrador Park (1°16’N, 103°48’E), patches of small brown and green algae from rocks, mid­tide area, 28 September 2004.

Diagnosis

Idiosomal length of female 360 µm, of male 322 µm. Dorsal plates foveate, fused to a single shield. Ventral plates fused. Area representing PD with single pair of setae. AE and PE generally each with one pair of adjunct setae. Female with five pairs of pgs and two pairs of sgs. Male with 19 plumose pgs, basilar setae included, the latter situated at 0.8. Male genital sclerites with two pairs of sgs. Gnathosoma 1.2 times longer than wide. Length ratio idiosoma:gnathosoma 1:0.23. Length:height ratio of telofemora 2.5–2.9. Leg chaetotaxy: leg I, 1, 2, 5–6, 5, 5, 4; leg II, 1, 2, 6, 5, 4; leg III, 1, 2, 3, 3, 5, 4; leg IV, 0, 2, 3, 3, 5, 3. Telofemora I to IV with (3–)4/(1–)2, 4/2, 3/0, 3/0 dorsal/ventral setae. None of ventral setae of genua enlarged and bipectinate. Tibiae I to IV with 2, 1, 1, 2 bipectinate setae. Paired fossary setae faintly plumose. Two basalmost setae on tarsus III adjacent. Claws rake­like with 16–19 tines

Remarks

Rhombognathus scutulatus is one of the most wide­spread Rhombognathus species in the Indo­West Pacific area ( Bartsch, 2003: fig. 11). There seem to be differences between populations of various localities, namely in the size of the claw pectines and their number of tines and in the length of the telofemora.

The largest number of tines, 24–28, was found in specimens from Rottnest Island, off Perth, Western Australia ( Bartsch, 1993). Specimens from the Great Barrier Reef, tropical Queensland, had 22–24 tines ( Bartsch, 2000), and those from the Dampier Archipelago, tropical Western Australia, 18–20 tines ( Bartsch, 2003). In the two specimens from Singapore claws I and II had 16–17 tines, claws III and IV 17–19 tines. The type material from the Philippines had 17–19 tines. A smaller number of tines was found in two females and males from the southern coast of Sri Lanka (unpublished record, material from Ahangama, from Halimeda (Chlorophyta) on a moderately exposed fringing reef flat, 28 February 1993, coll. H.­G. Müller); claws I and II of these specimens bear 13–14 tines, claws III and IV 15 tines. Specimens from India and eastern Africa ( Kenya) bear more than 17 tines ( Chatterjee, 1995; Chatterjee & De Troch, 2000). An increased number of tines may be correlated with life in very agitated water.

The telofemora in the specimens from Singapore, as well as in those from the Philippines, are slender, 2.5–2.9 times longer than high, whereas in specimens from western and eastern Australia (Rottnest Island, Dampier and Great Barrier Reef, respectively) the length:height ratio ranged from 2.1 to 2.5. Specimens from Sri Lanka had short telofemora, about twice as long as high. A similar wide range in the length:height ratio is else not documented in the genus Rhombognathus .

Distribution and Biology

Rhombognathus scutulatus is spread throughout the Indo­West Pacific area, from the Philippines in the east to Kenya in the west. The species lives in a variety of mid­tide, lowtide and shallow sublittoral habitats, on shores exposed to moderate as well as rather strong swell.