Halacarus actenos Trouessart, 1889

Bartsch, Ilse, 2011, Halacarus socius (Acari: Halacaridae), description of the male and diagnoses of species of the Halacarus actenos group, Zootaxa 2800, pp. 18-40 : 26

Halacarus actenos Trouessart, 1889

Halacarus actenos: Trouessart (1889: 239–240) ; Lohmann (1893: 83–85, pl. 10, figs 1–2); André (1946: 60–62, figs 28–30); Newell (1947: 87–91, figs 94–102); Bartsch (1980: 38–39, figs 2–15).

Diagnosis (on the basis of specimens from the French Channel coast). Length of female 785–879 µm (750 µm according to Trouessart 1889), of male 555–670 µm (650 µm according to Trouessart 1889). Epicuticula on plates and legs delicately reticulate. Frontal spine medium-sized and extending slightly beyond level of middle of P-2. Length of spine 0.4 and 0.1 times that of AD and idiosoma, respectively. Pair of glp-1 at 0.6 relative to length of AD. Posterior part of AD slender, its shape similar to that of frontal spine. Pairs of ds-1 anterior to glp-1. Ocular plates almost completely covered by cornea. Canaliculus in striated integument close to glp-2. Pairs of ds-5 and ds- 6 immediately adjacent to glp-4 and glp-5, often on same minute sclerite. In female, anterior pair of pgs about level with posterior angle of PE. Distance from pair of pgs-1 to margin of female GA about 0.8 times interval between anterior margins of GA and GO. Male GA wide, anterior margin somewhat triangular or rounded, extending anteriad slightly beyond level with end of PE. Pair of pgs- 1 in striated integument, its distance to anterior margin of GA less than half distance between margins of GA and GO; 68–83 pgs around GO. Distance between anterior margin of GA and GO less than the latter’s length.

Length of gnathosoma twice its width. Rostrum longer than gnathosomal base, reaching to level of end of P-2. Basal seta of P-2 at 0.7. Spine on P-3 with blunt tip. Trochanters I to IV with 1, 1, 2, 2 setae, basifemora I to IV with 2, 3–4, 2, 2 setae. Telofemur, genu and tibia I with 2, 2, 4 spiniform and tapering ventral setae. Ventral spine on telofemur I in basal part at 0.4. Both ventromedial setae on tibia II and that seta on genu II distinctly bipectinate. Tarsi I to IV with 2, (2–)4, (2–)4, (2–)4 ventral setae, tarsi III and IV with (3–)4 and 3 dorsal setae. Paired claws with accessory process with about four delicate tines.

Remarks. The species is characterized by its frontal spine being of moderate length, the ds-5 and ds-6 being adjacent or sharing the minute sclerite with the glp-4 and glp-5, respectively, and by its generally four bristle-like ventral setae on tarsi II to IV.

Newell (1947) presented a record of this species from Florida ( US). Apart from the size, one female (519 µm) and one male (422 µm), the specimens are similar to the northeastern Atlantic individuals. A difference, the frontal spine of the female is more slender and longer than in the European form, but the frontal spine of the male has the same shape as that of the European form. Unfortunately all distal segments of the legs are lacking in the two adults available. The trochanters I to IV bear 1, 1, 2, 2 setae, the basifemora I to IV 2, 4, 2, 2 setae. Because of the smaller size of the western Atlantic species Bartsch (1980) raised doubts whether the individuals from the coast of Florida are conspecific with H. actenos . More material is necessary for detailed comparison in order to verify or reject a conspecifity.

Distribution and ecology. Spread along the eastern North Atlantic from Cape Verde to Ireland ( Spain, France, UK, Eire, The Netherlands). Most records are from lower tidal and subtidal algae and sediment. The identity of specimens from the warm-temperate western Atlantic (Florida) is in need of confirmation. In general, the halacarid fauna of the temperate and tropical western and eastern Atlantic is similar on genus level but different on species level. Fifteen Halacarus species are known from the North Atlantic (Mediterranean included), nine are from a depth range of lower tidal to 100 m. Two of the three species known from the western Atlantic coast have amphiatlantic records, but of both ( H. actenos and H. ctenopus Gosse, 1855 ), the identity of the specimens from the opposite side of the Atlantic Ocean is questioned. The ‘old’ records of H. ctenopus from Europe have turned out to base on mis-interpretations ( Green and MacQuitty 1987; Bartsch 2001) and as to more recent records (e.g. by Riesgo et al. 2010), a re-examination of the identity of individuals is recommended.