Thalassarachna basteri and T . affinis ( Acari , Halacaridae ) , history , characters , biology , and distribution

Thalassarachna basteri and T. affinis are known since more than a century. Though frequently found in shallow water areas, there exist only very few descriptions of their external characters. These and distinguishing characters are outlined, data of habitat, life cycle, feeding, and distribution are given. Adults and nymphs of T. basteri and T. affinis can be discriminated on the basis of the shape of the frontal spine and number of spines on leg I. Thalassarachna basteri is regularly found in cold-temperate and polar areas, both on the eastern and western coast of the North Atlantic whereas most records of T. affinis are from the warm-temperate eastern North Atlantic.


Introduction
Thalassarachna basteri (Johnston, 1836) and T. affinis (Trouessart, 1896) colonize shallow water substrata, tufts of multibranched algae and colonial organisms, namely hydrozoans, bryozoans, and bivalves (Mytilidae).The species are rather large-sized and abundant and can hardly escape to be noticed.The two species have been described more than a century ago.Though they are frequently mentioned in lists of species collected on the coast of the Northern Atlantic and its adjacent basins, there exist only few detailed descriptions.This paper will present the history of these species, with their different generic and specific names till they finally were called Thalassarachna basteri and T. affinis, offer descriptions and illustrations and summarize data of biology and distribution.

Material and Methods
The material described and illustrated below was collected by the author, Thalassarachna basteri in the Baltic and T. affinis in the Black Sea.The illustrations were done with a drawing tube.The legs, their segments and claws are numbered I to IV, from anterior to posterior.The leg segments are trochanter, basifemur, telofemur, genu, tibia, and tarsus.In the chaetotaxy formula of the legs the solenidia, famuli and parambulacral setae are excluded.Numbers in parentheses ( ) indicate rarely occurring variants, those in square brackets [ ] inform of the quantity of a given character state.
Abbreviations used in the descriptions are: AD, anterior dorsal plate; AE, anterior epimeral plate; ds-1 to ds-6, first to sixth pair of dorsal setae of idiosoma, numbered from anterior to posterior; GA, genitoanal plate; glp, (pair of) gland pore(s), numbered glp-1 to glp-3, from anterior to posterior; GO, genital opening; OC, ocular plate(s); P-2 to P-4, second to fourth palpal segment; pas, parambulacral seta(e); PD, posterior The most conspicuous difference between the specimens described by Gosse (1855a) and Lohmann (1893) is the length of the genua of legs I and II, they are shorter (Gosse 1855a: fig. 6) versus as long as (Lohmann 1893: fig. 5) the adjoining telofemora and tibia.On the basis of this difference, Lohmann (1907) proposed to split the genus Halacarus into two groups ('Artenkreise'), the one including all species with long genua ('Ctenopus-Gruppe'), the other the species with short genua ('Balticus-Gruppe').For the latter group Viets (1927) established the subgenus Halacarellus, whereas the species with long genua remained in the genus Halacarus.Schulz (1933) raised Halacarellus in the rank of a genus.Acarus basteri with its short genua was now called Halacarellus basteri (Johnston, 1836).Newell (1945) argued that Thalassarachna verrilli is identical with Acarus basteri and consequently Thalassarachna should have priority over Halacarellus.As the type material of T. verrilli is no longer in existence and amongst the residues from jars with hydroids, etc. collected by Verrill no mites similar to that described by Packard were found (Newell 1945), the synonymity of Thalassarachna verrilli and Acarus basteri was not accepted by all authors and during five decades both names Halacarellus and Thalassarachna were used in parallel.In 1997, the genus with two names was splitted into the two genera Thalassarachna, with the type species Acarus basteri Johnston, 1836, and Halacarellus, with the type species Halacarus balticus Lohmann, 1889 (Bartsch 1997a).To maintain stability in the usage of the names Halacarus and Halacarus ctenopus a neotype was designated and described Bartsch (2001Bartsch ( , 2003)).

Characters of adults.
Idiosoma.Length of females 864-1197 µm, of males 885-1105 µm.Dorsal and ventral plates widely separated by densely striated integument.Dorsum with four plates, AD, pair of OC and PD, and three pairs of platelets; a fourth pair of platelets covered by striated integument.Anterior dorsal plate with wide, frontal spine (Fig. 1A, B), its width at the base about 60 µm, length 30 µm; posteromarginal part of AD with delicate pores.Pair of glp-1 in margins of AD just anterior to the level of ds-1, glp-2 within striated marginal integument.Ocular plates 1.7-2.1 times longer than wide, with eye pigment and single cornea; posterior corner with gland pore and adjacent pore canaliculus.Posterior dorsal plate about quadrangular, slightly longer than wide, in ovigerous females (with striated integument dilated) reaching halfway between ds-4 and ds-5, in males PD slightly longer and extending to the level of ds-4 (Fig. 1A).Pair of ds-5 in female PD in anterior half (Fig. 1D), in male close to middle of PD.Marginal and median part of PD with faintly reticulate ornamentation and numerous minute pores, pair of longitudinal areas evenly and very delicately punctate, without reticulation (Fig. 1F).(Johnston, 1836).A -Idiosoma, dorsal, male.B -Idiosoma, lateral, male.C Idiosoma, ventral, male.D -Idiosoma, dorsal, female.E -Idiosoma, ventral, female.F -PD, female (right side showing reticulate ornamentation, left side numerous pores).(fsp, frontal spine) Scale = 100 µm.
Ventral plates uniformly punctate.AE with three pairs of ventral setae; PE with one dorsal and three ventral setae.Female GA slightly extending beyond large genital foramen; distance from anterior margin of GA to that of GO about 0.2 times of length of GO.GO mostly surrounded by three or four pairs of pgs, rarely five, six or even seven setae on either side of GO.Genital sclerites both anteriorly and posteriorly with two (to three) pairs of minute setae.Ovipositor extending beyond genital plate (Fig. 1E).Male genitoanal plate larger than that of female; interval between anterior margin of GA and of GO about 0.6 of length of GO (Fig. 1C).Genital foramen surrounded by approximately 110 perigenital setae (Fig. 1C); genital sclerites with 8-10 pairs of stout setae and two pairs of spurs.Female and male with three pairs of large internal genital acetabula.
Gnathosoma 1.5-1.7 times longer than wide.Tectum truncate (Fig. 2A).Rostrum shorter than gnathosomal base, extending to end of P-2.Integument of gnathosomal base delicately punctate.Basal pair of maxillary setae at the basis of the rostrum, apical pair in posterior third.Palps almost cylindrical, P-3 with medial spine (Fig. 2B).Three setae in basal whorl of P-4 slightly unequal in size.P-4 ending with two setulae and two spurs.Lateral surface of cheliceral basis faintly foveate (Fig. 2D).Cheliceral claw with about 25 minute tines (Fig. 2C).Legs.Two anterior pairs distinctly, two posterior pairs of legs slightly shorter than idiosoma.Number of setae on leg segments variable (Table 1).Leg I with conspicuous ventral spines; 35-70 µm long, at their basis almost 10 µm wide; telofemur I with three such spines; in general tibia I ventrally with three pairs of spines and anterior pair of setae (Fig. 2G ), rarely, as in female illustrated (Fig. 2E, F), with four or seven spines in addition to anterior pair of slender setae.Following tibiae with bristle-like ventral setae but no spines (Fig. 2H-J).Tarsus I with eight (rarely seven) dorsal setae, a short dorsolateral solenidion, a ventral spine, six to seven (rarely less) ventral eupathid setae, and a pair of doubled (rarely tripled) parambulacral setae.Claws on tarsus I shorter than on following tarsi; claws on tarsi II to IV with J-shaped pecten with numerous tines.
Colour.Integument almost transparent.Dorsum with three black eye spots.Gut content red-brown with dark spots (Fig. 3); colour highly depending on food items.In general a narrow, white bar present.

Characters of adults.
Idiosoma.Length of female 600-774 µm, of male 568-710 µm.Dorsal plates uniformly and delicately punctate, lateral areas with minute scattered pores.Plates separated by wide areas of densely striated integument.AD longer than wide; posterior margin arched, anterior margin with minute frontal spinelet, length of spinelet 5-8 µm, its width 15-22 µm., i.e. one-fifth or less than width of AD.OC oblong, 2.2-2.5 times longer than wide, with large cornea in anterolateral part.PD oblong, longer and wider than AD and about 1.3 times longer than wide.Female PD not reaching the level of ds-4 (Fig. 4A), male PD extending to or slightly beyond the level of ds-4.Pairs of ds-1 and ds-5 on AD and PD, respectively, ds-2 to ds-4 on minute sclerites in striated integument, ds-6 short, on anal plate.Internal platelets between each OC and AD and OC and PD covered by striated integument.Pair of glp-1 in lateral margin of AD level with insertion of leg I, glp-2 within marginal integument between legs II and III, glp-3 in posterior corner of OC, immediately anterior to pore canaliculus; fourth pair in posterior part of PD.  (Trouessart, 1896).A -Idiosoma, dorsal, female.B. Idiosoma, ventral, female.C -Genitoanal plate, female.D -Gnathosoma, ventral, male.E -Genitoanal plate, male.F -Idiosoma, ventral, male.Scale = 100 µm.
Ventral plates uniformly and delicately punctate.AE with three pairs of ventral setae; PE with one dorsal and three ventral setae.Female GA with three pairs of perigenital setae (Fig. 4C), anterior pair near anterior margin of GA.Length of GO half that of GA; interval between anterior margin of GA and that of GO distinctly less than half length of GO.Two (to three) pairs of sgs in anterior part of genital sclerites, two (to three) pairs in the middle part.Genital acetabula large, tube-like, three pairs present (Fig. 4C).Ovipositor at rest extending beyond anterior margin of GA.Male GA ovate; interval between anterior margin of GA and GO equalling length of GO (Fig. 4E); approximately 90 perigenital setae densely arranged around GO.Each of male genital sclerites with about six sgs.Genital acetabula smaller than in female.
Legs.Leg IV about as long as idiosoma, preceding legs shorter.Telofemora slender.Number of setae on telofemora, genu and tibiae highly variable (Table 1), none of setae long and spiniform (Fig. 5A-D).Tibia I to IV with 8, (6-)7(-8), 3(-4), 3(-4) ventral setae.On tibia I one (Fig. 5A) to two of ventromedial setae short and spur-like, all other ventral setae on tibia I and following tibiae bristle-like, tapering (in a Mediterranean specimen all eight ventral setae tapering, none spur-like).Tarsus I with eight (rarely 9) dorsal setae, dorsolateral solenidion, one ventromedial seta, approximately seven eupathidia, and pair of tripled parambulacral setae.Tarsus II with eight dorsal setae, dorsomedial solenidion and almost 10 ventral eupathidia (pas included).Tarsi III and IV generally with six dorsal setae, and one, rarely zero or two, ventral setae; tip of these tarsi each with pair of single pas.
Colour.Similar to that of T. basteri.

Taxonomic Remarks
The most marked differences between adult Thalassarachna basteri and T. affinis are: (1) the frontal spine.In adult T. basteri the spine is wide, about 30 µm long, at the basis 60 µm wide, slightly raised upward (lateral aspect).A prominent frontal spine is present also in nymphs and larvae.That spine in T. affinis is a short process on the anterior margin of the AD, rarely more than 10 µm long and 25 µm wide.In the juveniles the spine is still shorter.
(2) the shape of the PD.The PD of adult and juvenile T. basteri is only slightly longer than wide, in T. affinis about 1.3 times longer than wide.
(3) the ventral setae on leg I.In adults of T. basteri several of the ventral setae are long and spiniform, from telofemur to tibia most commonly three, one and six spines are present, respectively (Table 2).In T. affinis the ventral setae on telofemur and genu are slender, evenly tapering, on the tibia (none to) one or two of the ventromedial setae are spur-like.Deuto-and protonymphs of T. basteri have four (rarely three) ventral spines on tibia I, protonymphs two spines, respectively, deuto-and protonymphs of T. affinis one (rarely two) and one (rarely zero) spurs.Larvae of both species have bristle-like setae but neither spines nor spurs.
Thalassarachna hexacantha and T. longipes, with the above mentioned unverified records from the Black Sea (Caspers 1951;Vorobyeva & Yaroshenko 1979), bear a pair of ventral setae in the striated integument whereas in T. affinis and T. basteri this pair of setae is on the PE.In addition to this, T. longipes is characterized by its enlarged anal valves which extend beyond the anal sclerites.
Thalassarachna litoralis (Bartsch, 1976), known from Brittany, France (Bartsch 1976), is a species similar to T. affinis and its status not clear (cf.Bartsch 1998Bartsch , 2009).When T. litoralis was described, no detailed information of adult T. affinis was available and a careful comparison hence not possible.The holotype female (housed in the Zoological Museum, Hamburg) and a second female (in the author's collection) have an idiosomal length of 710 and 793 µm.Marked differences between T. litoralis and T. affinis are in the number and shape of the setae on the legs.On tibia I both species bear eight ventral setae, but in T. litoralis three of the ventromedial setae and one or two of the ventrolateral setae are short, spiniform (Fig. 5E, F), in T. affinis one or two, rarely none, of the ventromedial setae are spur-like, all other setae are long, slender, evenly tapering.Thalassarachna litoralis has four to six ventral setae on tibia II, the ventrolateral setae are slender (Fig. 5G), the basal one of the ventromedial setae is shorter and slightly wider than the other setae, in T. affinis there are mostly seven ventral setae, all tapering and increasing in length from the segment's basis onward (Fig. 5B).Tibiae III and IV of T. litoralis bear two to three ventral setae and tarsus I doubled parambulacral setae, tibiae III and IV of T. affinis in general three ventral setae and tarsus I parambulacral triplets.At present T. litoralis and T. affinis are expected to represent two species.
Table 2. Numbers of spur-like or spiniform setae on the leg segments telofemur, genu and tibia I of Thalassarachna affinis (from the Black Sea), T. basteri (from the Baltic), and T. litoralis (from the English Channel, France).The quantity of each state is given in square brackets [ ].
telofemur genu tibia T. affinis Habitat Thalassarachna species are exclusively benthic, as halacarids in general they cannot swim.Thalassarachna affinis and T. basteri have been extracted from a variety of substrates which include a system of microcavernes, from colonial hydroids, bryozoans, byssus filaments of bivalve molluscs (Mytilidae), and tubes of serpulid polychaetes, from mats of green, brown and red algae, the haptera of Laminaria sp.
(Phaeophyta), but also amongst sediment rich in debris.Juveniles and adults live in the same habitat, though juveniles are often more wide-spread than adults and are found in niches not occupied by the large-sized adults (Bartsch 2004a).The species are regularly found from the lower intertidal zone down to depths of at least 100 m.Presently deepest record of T. basteri is 105 m, of T. affinis 150 m.The two species may be abundant, 1800 individuals/m² at 3-6 S‰ are mentioned by Purasjoki (1947), 1600 individuals/kg Cystoseira sp.(Phaeophyta) by Makkaveeva (1966).Most records of T. affinis and T. basteri are from a salinity range of 10-32 ‰, T. basteri is still present in fairly large numbers in diluted seawater at 3-5 S‰, e.g. in the Gulf of Bothnia (Finland) and Matsalu Bay (Estonia) (Purasjoki 1947;Järvekülg 1965), and T. affinis was extracted from nearshore spring-water and slightly saline pools (Vitzthum 1932;Petrova 1973).

Feeding
Thalassarachna basteri is predaceous, preying on oligochaetes, polychaetes, small crustaceans, other halacarids and even its eggs (Lohmann 1889(Lohmann , 1893;;Kirchner 1969;Bartsch 1974).The prey is captured with the first pair of legs, the spines prevent an escape, the chelicerae, which can move to and fro within the rostral trough, pierce and cut the prey's integument and internal organs.After pre-oral digestion, the body content of the prey is sucked up (Kirchner 1969).Thalassarachna affinis is expected to have similar food items, though, in contrast to T. basteri, leg I bears bristle-or spur-like setae rather than spines (cf.Figs.2G  and 5A)

Reproduction
The two Thalassarachna species run through one larval and two nymphal stages until the adults, females and males, moult.Males produce elaborate spermatophores which include the sperm sac; the spermatophores are deposited on the substratum inhabited (Kirchner 1967(Kirchner , 1969)).Females pick up the sperm sac.The eggs are stuck into the surface of the substratum.Both T. affinis and T. basteri have a univoltine highly synchronized reproduction.In respect to T. affinis, Trouessart (1896) mentioned that adults were present only in winter time.More detailed data were presented by Makkaveeva (1966) who studied populations from the Black Sea.In the winter (January) only adults were found, females dominated the population, they were ovigerous, with 8-10 eggs per individual.The first juveniles appeared in February.In June and July only a few adults were present and the females held, if at all, just a single egg; the halacarid population was dominated by the nymphs which numerically hold almost 90 %.In October the first adults of the new generation had hatched and in October and November 25 % of the population were adults; in several of the females eggs were visible.
Detailed studies of the live cycle of T. basteri had been published by Lohmann (1893), Kirchner (1969) and Bartsch (1972).In populations from the Baltic Sea, eggs were deposited in early spring which in May and beginning of June hatched into larvae; a month later protonymphs appeared, and in August through October the deutonymphal stage dominated.Adults were present from beginning of November to end of May, they overwintered but died after reproduction in spring.In most of the months females numerically dominated over males and females had a longer life-span than males (Kirchner 1969;Bartsch 1972).Up to 15 eggs per female had been observed (Kirchner 1969); moribund females still contained one to three eggs.

Epizoans
Johnston (1836) already mentioned 'it [Acarus basteri] is much infested with infusory animalicus' and Gosse (1855b) found a peritrich ciliate (Vorticella) and several suctorians (Acinetae) on this halacarid species.The above mentioned T. litoralis was infested with Thecacineta, eight individuals fixed to marginal parts of the halacarid body.The loricae, 72-85 µm in length, 45-47 µm in diameter, are annulated with 22-24 horizontal ribs, the stalk is almost as long as the lorica, the slightly ovate macronucleus about 12 µm in length, and the more than 20 tentacles are arranged in a single bundle.Most likely it is a Thecacineta calix (Schröder, 1907) which is a cosmopolitan species found on a variety of taxa (Chatterjee et al. 2014).

Geographical distribution
Records of the genus Thalassarachna are at present restricted to northern seas.The genus is abundant in the North Atlantic and its adjacent basins (Barents Sea, White Sea, Baltic, Mediterranean, Black Sea), a single record is from the western part of the Northern Pacific (Makarova 1978;Bartsch 2009).At present 16 species are known (Bartsch 2009;Bartsch & Rybakova 2015).Records of the two species T. affinis and T. basteri are from the northern Atlantic and the Arctic.The most northerly records are from Greenland, Northern Norway (Gamvik) and the Barents Sea (Trägårdh 1905;Viets 1928b;Sokolov 1952;Nikitina 2013;unpublished record), the most southerly from the Mediterranean (Trouessart 1996;Viets 1940;Bartsch 1998;Durucan & Boyaci 2014).Records of the genus are from the lower tidal zone to deep sea basins (Bartsch 2009).
The species most wide-spread is Thalassarachna basteri.It has been taken on the coasts of North America, from Connecticut (US) to Quebec (Canada), and Europe, from Spain to Nowaya Zemlya (Newell 1945(Newell , 1947;;Viets 1928b;Sokolov 1952;Bartsch 1997b;Riesgo et al. 2010).Obviously it is a species of the temperate and Arctic region.A record from the Mediterranean is that of a single deutonymph which had been extracted from sediment between rhizomes of Posidonia sp., collected in September 1995 by T. Wittling in 6 m depth via SCUBA-diving in the Tyrrhenian Sea, near the island Giglio, in a bay which is opening to the northeast.The sample, preserved in ethanol, had been taken to Hamburg (Germany), there washed over a 100 µm sieve and the residue in the sieve scanned for halacarid mites.Contamination in the laboratory is unlikely as no material from the North Atlantic, North Sea or Baltic had been studied neither at

Table 1 .
Number of setae of trochanter to tibia I to IV and dorsal setae of tarsus I to IV of adults.The quantity of each character state is given in square brackets [ ]. d, dorsal.