Thalassarachna Packard 1871

Thalassarachna Packard, 1871

(Figs 76 –83)

Type species. Acarus basteri Johnston, 1836.

Adults. In both female and male GP and AP fused. Female GA with three to five pairs of pgs, genital sclerites with three to five pairs of sgs. Two to three pairs of gac often large and arranged in line (Figs 76, 81, 82; Thomae 1926: fig. 26; Bartsch 1976c: fig. 26, 1998b: fig. 122; Makarova 1978: fig. 6 GO female). Acetabula either almost equal-distanced or posterior pair slightly separated and enlarged. Ovipositor at rest extending beyond GO. Ovipositor with pair of basal and five pairs of apical genital spines; the latter in 2+3 arrangement, i.e. two anterior and three posterior pairs (Figs 77, 78). Genital spines spiniform, in T. basteri two anterior pairs of spines with delicate marginal denticles. Male GA with 40–160 slender pgs arranged scatteredly around GO. Male GO similar or slightly smaller than female GO. Genital sclerites with four to eight (nine) pairs of sgs, posterior pairs of sgs spiniform (Figs 79, 80). GO with two to three pairs of gac, these somewhat smaller than in female (Figs 79, 83). Acetabula in posterior part of GO, often immediately adjacent and with an opening to the exterior (Fig. 79; Makarova 1978: fig. 6, GO male; Bartsch 1996b: fig. 31). AE of adults without epimeral pores.

Juveniles. With a larval and one to two nymphal stages, in the former case only protonymphal stage present. GP and AP separated in both proto- and deutonymph. Deutonymphal GP with two pairs of pgs, two pairs of sgs and two pairs of internal gac (Bartsch 1976c: fig. 40). Protonymphal GP with single pair of internal gac; neither pgs nor sgs present. Protonymph with small, larva with distinct, tube-like epimeral pores.

Remarks. One of the ten female T. basteri studied has two pairs of basal but a single antero-apical pair of genital spines instead of vice versa. A similar arrangement is present in T. affinis (Makkaveeva 1966: fig. ovipositor, female)

The deutonymphal stage is suppressed in T. dissimilis (Bartsch, 1979b), assumedly also in T. hexacantha (Viets, 1927) and T. longipes (Trouessart, 1888). Of T. dissimilis both a female and a male were dissected from protonymphal skins (Bartsch 1979b: p. 28), in T. hexacantha populations from the White Sea, larvae, protonymphs and adults were present, but no deutonymphs (Nikitina 2013), and in respect to T. longipes the protonymphal stage was commonly taken (Newell 1947: p. 128), but obviously no deutonymphal. In these three species both females and males bear two instead of three pairs of gac.

The genus Thalassarachna is spread in the northern Pacific, Atlantic, and adjacent areas (Mediterranean, Black Sea, Baltic, Arctic Ocean); 14 species are described (Bartsch 2009a).