Arhodeoporus, Camactognathus, Plegadognathus, and Winlundia (Acari: Halacaridae), re-evaluation and geographical distribution

Arhodeoporus, Camactognathus, Plegadognathus, and Winlundia are very similar in their general shape, they share numerous characters but differ in the position of the solenidion on tarsus II, and also in the number of gland pores and the chaetotaxy of the legs. Comparison with other halacarid genera proved, the position of the solenidion is stable within a species and genus, no variants are known, whereas in respect to the other characters, various states within a genus are documented. Because of differences in the position of the solenidia, the genus Arhodeoporus Newell has to be splitted. Species of Arhodeoporus s.str. have the solenidion on tarsus II dorsolateral in position; the genus includes the two species once described by Newell (1947), namely A. arenarius and A. submarinus. The other species have this solenidion on the dorsomedial flank of claw fossa. The species once referred to as the bonairensis group are moved to the re-established genus Plegadognathus, the other species are included in the new genus Maracarus. Camactognathus shares with Arhodeoporus s.str. the position of the solenidion on tarsus II but differs in other characters; the genus is solidly diagnosed. The differences between Winlundia and Plegadognathus are small, with an increase of knowledge of the species, Winlundia may turn out to be a synonym of Plegadognathus. The five genera have records from tropical and temperate coastlines but not from polar waters. Maracarus and Plegadognathus are spread all around the globe whereas Arhodeoporus s.str. and Winlundia are restricted to the north-eastern Atlantic and south-eastern Pacific, respectively.

The definition presented by Newell (1947: 173ff) said, the subgenus Arhodeoporus includes Copidognathus s.l. species in which (1) tibia I has two pairs of ventral setae (instead of three unpaired setae); (2) rostrum is wide at base and scarcely extends beyond middle of P-2 [second palpal segment]; (3) rosette pores are absent, and (4) OC [ocular plates] are drawn out into a long, posterior, caudiform projection (in the species known at present). Characters they share with Copidognathus are (Newell 1947: 129): 'Palpi attached to capitulum laterally; bases of palpi separated by a distance considerably greater than width of P-1 [first palpal segment], and clearly visible in ventral view. Palpi distinctly 4segmented. P-4 [fourth palpal segment] usually longer than P-2, slender, tapering; attenuate distal portion longer than basal portion. P-2 with a single, usually long dorsal seta. P-3 [third palpal segment] without setae; P-4 with 3 setae on thick basal portion. Palpi distinctly geniculate. Genital suckers internal'. Three of the four above mentioned discriminating characters of Arhodeoporus are expected to include a wide range of states, depending on mode of life, feeding, substratum, actually Newell (1947: 34) already had stated 'the form of the ocular plates may be a useful key character, but its use as a subgeneric character is more of an impediment than an aid in the taxonomy of the groups'. Accordingly, the only difference that could be used to distinguish between Arhodeoporus and Copidognathus was the number of ventral setae of tibia I, four (two pairs) in Arhodeoporus but three in Copidognathus. Later Newell (1984: 202) added, Arhodeoporus female with two pairs of subgenital setae, male with either four or five pairs.
In the same paper, Newell (1984) described the new genera Camactognathus and Winlundia, both very similar to Arhodeoporus. Characterization of Camactognathus was (Newell 1984: 201, 202): dorsal gland pores not enlarged; adanal setae in margin of posterodorsal plate; female of type species with one pair of subgenital setae; palps inserted laterally; third palpal segment without seta; fourth segment with three basal setae; genu II shorter than telofemur or tibia; tibiae I and II with four ventral setae; solenidion posterior in position on both tarsi I and II.
The diagnosis of Winlundia reads (Newell 1984: 197, 198): Palpi four-segmented, attached laterally to gnathosoma, P-2 with one seta; no seta on P-3; P-4 with three setae; genu of leg II much shorter than telofemur and tibia II; tibia I with three dorsal and four ventral setae; ambulacra I and II similar in form, solenidia I and II both on posterior aspect of tarsi (but on page 201, position of solenidia on tarsi I and II posterior and anterior in position, respectively, and in the tabular key on page 12, solenidia on tarsi I and II equal a posterior:anterior combination); tarsus I with two ventral setae; adanal setae on anal papilla; female with two pairs of subgenital setae, male with five pairs (in type species W. filistoma, but three pairs in W. forcipata).
Plegadognathus, another genus most similar to Copidognathus and Arhodeoporus, was introduced and described by Morselli (1981). Its characters (page 266) are: dorsum with postocular plates; palps four-segmented, laterally attached; P-4 with two basal setae; tarsi and tibiae I with two and four ventral setae, respectively.
In the beginning of the 1970s three new species were added to Arhodeoporus, A. mammillifer Newell, 1971, taken in the south-eastern Pacific, and A. brevocularis Bartsch, 1973 andA. lineatus Bartsch, 1973, both from the north-eastern Atlantic (Newell 1971;Bartsch 1973b). The two last-mentioned species nicely agreed with the generic characters outlined for Arhodeoporus arenarius and A. submarinus. The only serious difference was, the solenidion on tarsus II of A. brevocularis and A. lineatus was on the dorsomedial fossa membrane, in contrast to the dorsolateral position of the north-western Atlantic and Pacific species mentioned by Newell (1947Newell ( , 1971. Halacarus gracilipes Trouessart, 1889, by Newell (1947 included in the genus Arhodeoporus, agreed with A. brevocularis and A. lineatus in that the solenidion was on the dorsomedial fossa membrane (Bartsch 1973b(Bartsch , 1977a. This difference raises the questions, is the position of the solenidion on tarsus II of generic importance? Can the solenidion move from one side to the other? What is the correct position in present-day known Arhodeoporus, Camactognathus and Winlundia species, and do they belong to just one genus?

RESULTS
The parsimony analysis is prepared on the basis of 30 Arhodeoporus s.l. species, three Camactognathus and two Winlundia species, the genus Copidognathus is used as outgroup. Discrepancies within a description or differences between description and examined species are: A. bonairensis (Viets, 1936): In contrast to the figures in Viets (1936: fig. 44) and Sarma and Chatterjee (1993: figs 7 and 8), telofemora I and II bear four and three setae, respectively, tibia I seven setae, tibia II three and tarsus I two ventral setae. Though not mentioned in the description (Viets 1936;Newell 1947;Sarma and Chatterjee 1993;Otto 2000a;Bartsch 2009b), a pair of gland pores is present in the posterior part of the PD, immediately adjacent to the medial costae. The pores are similar to the cup-like alveoli of the costae, hence difficult to recognize. These pores have also been found in A. bucculentus Bartsch, 1977 and A. wadjemupis Bartsch, 1997. As illustrated in Bartsch (2009b: fig. 12) (though not in Sarma and Chatterjee 1993, figs 2 and 3), the AE bears a pair of epimeral pores. Re-examination of P. bonairensis (from Western Australia and Singapore) proved that females and deutonymphs have two pairs of internal genital acetabula, instead of a single pair as mentioned by Bartsch (2015b).
A. clypeatus Otto, 2000: The characters given for the male are size and number of pgs, else it is said (page 5) 'other characters as for A. caudatus'. Males of the latter species have five pairs of sgs but according to Otto (2000a: fig. 3e), there are three pairs of sgs on the male genital sclerites of A. clypeatus.
A. mammillifer Newell, 1971: The solenidion on tarsus II is said to be posterodorsal in position (dorsolateral) (Newell 1971: 27) but the species is so very similar to a group of others (eclogarius group), that the statement by Newell (1971) is expected to be a lapsus.
A. submarinus Newell, 1947: Newell (1947 mentioned one seta on trochanter IV, in material studied (from Rhode Island, US) no seta was found.
Winlundia Newell, 1984: The description of the position of the solenidion on tarsus II is contradicting (cf. Newell 1984: 12, Key Group 2000198, 199, 200, 201). Examination of slides with W. forcipata Newell 1984 and W. filistoma proved, the solenidion is dorsomedial in position ( Figure 1A), and both species have a pair of epimeral pores ( Figure 1D and G).
The tree ( Figure 2) reveals several clusters of species. Camactognathus species, which had been included in the analysis, turn up as a basal clade. One of the following clusters holds the species bonairensis, bucculentus, disparilis, labronicus, robustus, wadjemupis but also the two Winlundia species, another cluster includes a species group with caudatus, clypeatus, corallicolus, lizardensis, longirostris, mactanus, psammophilus, tanzanicus as well as with gracilipes, minor, perlucidus, subtilis and leptopus, longicrus, mammillifer, mirabilis, mooreus. The latter cluster should also include eclogarius, as proven by reexamination of the holotype (Table 1). The two Arhodeoporus species A. arenarius and A. submarinus share a branch with the two last-mentioned clusters though have a separate position.
The tree largely supports the natural groups within the genus Arhodeoporus s.l. proposed by Bartsch (1983a), namely the bonairensis, eclogarius, gracilipes, and longirostris group. The species of the bonairensis group share external characters, e.g. number of gland pores reduced, porose areolae of dorsal and ventral plates not evenly punctate but with cup-like alveoli or foveae, tibiae I with no more than seven setae, tibia II with six setae. On the basis  of these characters, species of the bonairensis group can be separated from the other species of Arhodeoporus s.l. The group also includes A. thyreophorus which had been excluded from the data matrix.
Arhodeoporus s.l. eclogarius is characterized by its coarsely reticulated dorsal plates, the four pairs of gland pores opening subapically on conspicuously raised, pointed cones, the legs being long and slender, tibiae I and II bearing eight setae, and each of tarsi III and IV a large ventral seta but a single pas, the medial one. These characters are found in all species of the eclogarius group.
Characters of the gracilipes group are, dorsal plates with reticulate ornamentation, AD with pair of circular porose areolae and PD with porose costae, PE with two dorsal setae, tibiae I and II with eight setae, tibiae I to IV with 0, 2, 1, 0 pectinate se-tae, and tarsi I to IV with 3, 1, 0, 0 ventral setae.
The presently known species of the longirostris group have punctate but else rather smooth dorsal and ventral plates, costae are lacking, major parts of the plates are almost evenly punctate, tibiae I and II bear eight setae, on both tibia III and IV one of the two ventral setae is bipectinate, and tarsi I to IV have 3, 0, 0, 0 ventral setae.
The two species A. minusculus and A. nanus are on a basal branch, characters they share are, ornamentation of plates rather weak, genua I and II with no more than two setae, tibia IV with two slender ventral setae, and both tarsus III and IV with a small ventral seta.
Arhodeoporus arenarius and A. submarinus are close to species of the gracilipes and eclogarius groups but distinctly differ from these in the posi-tion of the solenidion on tarsus II. The two species have, same as Camactognathus and Copidognathus species, the solenidion of tarsus II in a dorsolateral position, in the other above mentioned species the solenidion is in a dorsomedial one.

DISCUSSION AND CONCLUSION
Classifications of Halacaridae (Halacaroidea) have been prepared mainly on the basis of external morphological characters of adults, rarely juvenile stages or internal characters have been included, e.g. number of genital acetabula and number and shape of female genital spines (Viets 1927;Newell 1947Newell , 1984Bartsch 1983bBartsch , 2015bAbé 1998). The spermatopositor has as yet been excluded but its shape, though species specific, demonstrates intrageneric similarities. An example of similarity is its shape in species of the Agauopsis brevipalpus group (Bartsch 2015a, c). Sufficient information on spermiogenesis and genetic data are lacking. Many of the external characters are correlated with the environment and mode of living and feeding, this means both, similar features may be found in different genera, evolved independently, and one can expect a range of characters states within a given genus. In parsimony analyses important phylogenetic characters may be masked by a large number of irrelevant ones. An example of morphological similarity is demonstrated by species of the Arhodeoporus s.l. eclogarius and Copidognathus tricorneatus group (cf. Bartsch 1997c). Tarsi III and IV of Copidognathus and Arhodeoporus species in general have a pair of parambulacral setae, on tarsus III one seta is flattened, the other eupathid, on tarsus IV either both are flattened or one flattened, the other slender ( Figure 3C and E), but in a few species of the C. tricorneatus group the lateral pas is moved to a ventral position (cf. Bartsch 1973a: fig. 12, 1973b: fig. 14), just as in A. s.l. eclogarius ( Figure 3A) and the other species of this group. Species of both groups (eclogarius and tricorneatus) have slender legs, striking similar reticulate ornamentation on the dorsal plates, a PD with a pair of narrow medial costae, demarcated porose areolae on the ventral plates, and a divided posterior cornea on the OC. The same arrangement of setae on tarsi III and IV, namely a single param-bulacral but one ventral seta, is also found in Simognathus species, three examples, Simognathus abnormalus Otto, 2000, S. gibberosus Bartsch, 1994, and S. gracilis Bartsch, 1994) (Otto 2000b, Bartsch 1994. Newell (1947) listed four characters which could be used to discriminate between the new subgenus Arhodeoporus and Copidognathus s.str., one was the absence of rosette pores. Raised areolae with typical rosette pores, namely an alveolus, in deeper layers of the procuticula surrounded by canaliculi (pore channels), opening to the exterior via an ostium (Newell 1947;Crowe and Camara 1973), are present only in species which live in a marine environment (at least 28 0 / 00 ), here amongst surface substrata, e.g. amongst coralline algae, colonial organisms or upper sediment layer. Species generally inhabiting low saline environments have less cuticularized plates, less pronounced costae, canaliculi seem to open directly to the exterior, rosette pores, if present, have often the shape of porose foveae or polygons. The plates of interstitially living species are rather uniformly ornamented, reticulated or foveate, with or without small pores.
Also a reduction of the size and number of setae is expected to be the result of chemical, physical and biological parameters. Copidognathus species with reduced numbers of setae on the genua and tibiae are generally from sandy deposits, examples are C. cribellus Bartsch, 1993, C. psammobius Bartsch, 2008and C. viridulus Bartsch, 2009(Bartsch 1993a, 2008, 2009b. Isobactrus species which live between and feed on delicate algal covers exposed to longterm desiccation have a lower number of setae than congeners from almost permanently wet habitats. One can expect that during periods of desiccation, when the mites become dehydrated, food intake and digestion are interrupted. Though speculative, as data of inorganic and organic components in interstitial water of sandy deposits as well as feeding rates during unfavourable periods are not known, the reduction of number of setae on the legs as well as thickness and ornamentation of the integument is expected to be correlated with life at permanently reduced diets. Will say, due to reduction in the exoskeleton, number of setae included, the species could compensate a restricted supply of or-559 FIGURE 3: A -Maracarus eclogarius (André, 1959), female: tarsus IV, lateral (medial pas obscured, medial claw omitted); B -Maracarus kunzi (Bartsch, 1987), female: tarsus II, medial. C-E: Maracarus brevocularis (Bartsch, 1973), male: C -tarsus III, ventral (dorsal setae in broken line); D -GO and perigenital setae; E -tibia and tarsus IV, ventromedial. F -Plegadognathus bonairensis (Viets, 1936) ganic and inorganic matter. Such compensation has certainly evolved independently in different genera. The faint ornamentation and reduced number of setae of the two psammobiont Arhodeoporus s.l. minusculus and A. nanus may be due to the environment and the separate basal position on a phylogenetic tree just be an artificial one. Newell (1947Newell ( , 1953Newell ( , 1984 drew attention to setae of the tarsi and said 'The chaetotaxy of the tarsi is especially important at the generic level, in particular with regard to the placement of the solenidion (bacillum) on tarsus I and II' (Newell 1984: 7). The shape and position of these setae seem to be very conservative. They are present in all instars, from larva to adult. There is no reliable evidence of a major change in position and shape of the solenidia from larva to adult, and no report of any anomaly on the right or left leg. The few reported differences are obviously due to lapsus when interpreting or recording what had been seen on the microscopical slide. In all halacarids studied, the solenidion on tarsus I is dorsolateral in position, it may be clavate, as in all Isobactrus species (Bartsch 2003a: fig. 4A-E), or setiform, as in the majority of halacarids, it is shorter than the fossary setae ( Figure 1H). Adjacent to the solenidion is a famulus, in shape and length similar to the solenidion, e.g. in Metarhombognathus and Rhombognathides (Newell 1953: fig. 2B and C) or reduced, even to a small, cap-like structure, but recognizable due to the afferent canal within the often enlarged fossa membrane. Almost all halacarid species have a solenidion on tarsus II, but here either dorsolateral, dorsal or dorsomedial in position. The solenidion may be similar to the one on tarsus I, e.g. in Isobactrus (Newell 1953: fig. 2A; Bartsch 2003a: fig. 4F-J), but often it is setiform ( Figures 1A,  I, 3B). Position and shape is the same in all instars. Many species have a fourth dorsal seta on tarsus III (commonly present in Copidognathus, Halacarus, Isobactrus, Rhombognathus species), the length of this seta is often somewhat less than that of the three fossary setae and in contrast to these never tapering, always ending bluntly and including a distinct internal lumen. In species with this sensory seta, by Newell (1984) called proximodorsal seta, it is present in all stages from larva to adult. In a few species a fourth (or more) tapering dorsal seta is present which is no sensory seta but an additional dorsal seta. In Isobactrus levis (Viets, 1927), the dorsal fossary seta is duplicated on all tarsi. Thalassarachna larvae have no more than three dorsal setae on the tarsi (Bartsch 2015d: fig. 2H), only the following instars may have an increased number of setae, all of them are tapering.
When regarding phylogeny, the position of the solenidion is a significant character, certainly more important than the ornamentation of plates or number of setae as these may be correlated with the environment and similarities are expected to have evolved independently in different genera and lineages. Because of the different position of the solenidion on tarsus II, the former genus Arhodeoporus s.l. has to be splitted. Arhodeoporus s.str. includes A. arenarius and A. submarinus, both with the solenidion on tarsus II dorsolateral in position. The other species, which have the tarsus II-solenidion dorsomedial in position, are transferred to the reestablished Plegadognathus Morselli, 1981 and to Maracarus n. gen. Camactognathus is considered a separate, valid genus.
Plegadognathus includes the species of the bonairensis group, namely bonairensis, bucculentus, disparilis, labronicus, robustus, wadjemupis, and thyreophorus. The dorsal and ventral plates (André 1959: fig. 13 D and V) of the last-mentioned species show an ornamentation typically found in P. bonairensis, hence P. thyreophorus may be a junior synonym of P. bonairensis. Compared to the other species once united in the genus Arhodeoporus s.l., the gland pores of Plegadognathus are small, not on cone-like projections, there are three (or less?) pairs of glands present, not four pairs, the dorsal plates are neither uniformly reticulated nor have punctate areolae, instead there are cup-like pores or subsurface alveoli with a few canaliculi opening to the exterior, and tibia I bears seven instead of eight setae.
For the remaining species, when the two Arhodeoporus s.str. and the seven Plegadognathus species are removed, a new genus has to be erected, namely Maracarus n. gen., with the type species Maracarus gracilipes (Trouessart, 1889). Maracarus has the solenidion on tarsus II dorsomedial in po-sition ( Figure 3B) (in contrast to Arhodeoporus s.str.) and there are four pairs of gland pores (in contrast to Plegadognathus), these open to the exterior via a very distinct, sclerotized canal which is at least twice as long as wide ( Figure 3G) (versus gland pores not on cones, canal rather inconspicuous, not longer than wide).
Winlundia shares a cluster with species of the bonairensis group. Newell (1984: 198) already noted that there are several points of similarity between the type species of Winlundia and the species now called P. bonairensis, amongst others the arrangement of the porose areolae on the ventral plates of W. filistoma. The most striking difference between species of the two genera is the length of the gnathosoma. In Winlundia filistoma and W. forcipata it is very slender, in the type species the gnathosoma is 3.3 -3.8 times longer than wide. In Plegadognathus species the length:width ratios lie between 1.2 -1.7:1, in the type species of Plegadognathus the gnathosoma is 1.2 times longer than wide, in P. bonairensis 1.6 -1.7 times. The length of the gnathosoma is no reliable generic character. Intrageneric different length:width ratios are documented. Most Agaue species have a long, slender gnathosoma, in A. circellaris Bartsch, 1999 the gnathosoma is four times longer than wide, in A. scita Bartsch, 1999 only two times, the rostrum is 3.8 times longer than the gnathosomal base (A. circellaris) versus both similar in length (A. scita) (Bartsch 1999a, b). The gnathosoma of Copidognathus guttatus Bartsch, 1977 is 4.3 times longer than wide, in C. lutarius Bartsch, 2003 only slightly longer, the rostrum of C. guttatus is 1.4 times longer than the gnathosomal base, that of C. lutarius only half that length (Bartsch 1977b(Bartsch , 2003b. Many Lohmannella species have a slender gnathosoma, in L. fukushimai Imamura, 1968 it is longer than the idiosoma, its rostrum almost twice as long as the gnathosomal base, whereas in L. kervillei Trouessart, 1894 the length of the gnathosoma is less than half that of the idiosoma and the length of rostrum less than that of the base (Imamura 1968;Bartsch 1977c). Moreover, halacarids demonstrate an allometric growth, legs and gnathosoma of adults are longer and more slender than those of larvae (Bartsch 2007a). Another discriminating character mentioned by Newell (1984) is the number of ventral setae on tarsus I, two setae in the two Winlundia species (Figure 1A and G), two to three in Plegadognathus species, both the type species P. labronicus as also P. bonairensis and P. disparilis have two setae (Morselli 1981: fig. 2Z1; Bartsch 1997Bartsch : 270, 2009b fig. 16). Intrageneric differences in the number of ventral setae are known. Werthella species bear either three (W. crinata Bartsch, 2005), two (W. terella Bartsch, 1993) or just one ventral seta (W. atlantica Bartsch, 1986) on tarsus I (Bartsch 1986(Bartsch , 1993b(Bartsch , 2005. A reduction of the assumedly plesiomorphic state with three ventral setae is no unique phenomenon. Characters Winlundia and Plegadognathus species share are a reduced number of gland pores, generally one pair on AD, OC and PD ( Figures 1B and C), seven and six setae on tibiae I ( Figure 1G) and II, respectively, and often no more than three setae on genua I and II. Adults of both genera are expected to have two pairs of internal genital acetabula, in W. filistoma one pair is wide, one slender ( Figure 1E), in P. bonairensis both are delicate ( Figure 3F). Noteworthy is that in Winlundia forcipata the medial pas on tarsus II is more slender than the eupathid lateral one ( Figure 1I). In W. filistoma the pas only slightly differ in width (Figure 1A). In P. bonairensis the lateral pas is distinct, eupathid, the medial pas slender; in most of the Maracarus species the pas of either side are equalsized. At present, Winlundia is retained as a valid genus, though studies on more material may prove it to be a junior synonym of Plegadognathus.
As to Camactognathus, the result of the character compilation and parsimony analysis supports the position as a genus. Discriminating character states of Camactognathus, in contrast to Arhodeoporus, Plegadognathus and Maracarus n. gen. are, epimeral pores lacking (versus present in the other genera); both pairs of maxillary setae on the rostrum (versus rostrum with a single pair); solenidion on tarsus II in dorsolateral position (shared with Arhodeoporus s.str.); tarsus I with a single ventral seta (versus two or three setae in the other genera).

Arhodeoporus
Type species -Arhodeoporus arenarius Newell, 1947 Idiosoma -Dorsal plates AD, OC and PD with faint, uniform ornamentation. Dorsum with four pairs of gland pores and six pairs of setae, gland pores not on raised cones. AE with pair of epimeral pores and three pairs of setae, pair of PE each with one dorsal and three ventral setae. Female GA with three pairs of pgs, each genital sclerite with (one to) two sgs. Male GA with numerous pgs scatteredly arranged around GO, genital sclerites with four to five pairs of sgs.
Gnathosoma -Somewhat longer than wide. Rostrum triangular. Basal pair of maxillary setae on gnathosomal base, following pair on rostrum. Palps four-segmented, laterally attached. P-2 with one dorsal seta, P-4 with three setae in basal whorl; no seta on P-3.
Deutonymph -With two pairs of pgs and two pairs of internal acetabula.

Camactognathus Newell, 1984
Type species -Camactognathus grossipes Newell, 1984 Idiosoma -Dorsum with (two? to) three pairs of gland pores and (five? to) six pairs of setae. Gland pores small, not on raised cones. Epimeral pores lacking; AE with three pairs of setae, pair of PE each with one dorsal and three ventral setae. Female GA with three to four pairs of pgs, each genital sclerite with one pair of sgs. In male, each genital sclerite with two to three sgs.
Deutonymph -Genital plate with two pairs of pgs and two pairs of internal acetabula.
Idiosoma -Dorsal plates AD, OC and PD often with punctate areolae but major parts uniformly ornamented, often reticulated. AD and OC each with one pair, PD with two pairs of gland pores, pores generally opening to exterior via conspicuous internal canal, this being two or more times longer than wide ( Figure 3G and I), aperture often situated apically or subapically on domes or cones. Dorsum with six pairs of idiosomatic setae. AE with pair of epimeral pores and three pairs of setae, pair of PE each with one or two dorsal and three ventral setae. Female GA with three (rarely four) pgs on either side of GO, genital sclerites with two, rarely three pairs of sgs. Male GA with numerous pgs scatteredly arranged around GO, genital sclerites generally with five pairs of sgs, rarely with three, four or six setae on either sclerite. Adults with two pairs of internal acetabula ( Figure 3D). Gnathosoma -Longer than wide. Rostrum generally slender, triangular. Basal pair of maxillary setae on gnathosomal base, following pair on rostrum. Palps four-segmented, laterally attached. P-2 with one dorsal seta, P-4 with three setae in basal whorl; no seta on P-3.
Deutonymph -Genital plate with two pairs of internal acetabula and two pairs of pgs ( Figure 3H).

Plegadognathus Morselli, 1981
Type species -Plegadognathus labronicus Morselli, 1981 Idiosoma -AD, OC and PD with raised areolae which include cup-like foveae or alveoli, remainder of plates coarsely pitted, foveate or reticulate. AD and OC with pair of gland pores, PD with (zero? or) one pair of gland pores near posterior margin; pores not on cones. Dorsum with pairs of ds-1 to ds-6. AE with epimeral pores and three pairs of setae, pair of PE each with one dorsal and three ventral setae. GA of female with three (rarely four) pairs of pgs, genital sclerites with two pairs of sgs, rarely a single pair, GO with two pairs of internal acetabula ( Figure 3F). Male GA with numerous pgs, genital sclerites with five, rarely four pairs of sgs.
Gnathosoma -Slightly longer than wide. Rostrum triangular. Basal pair of maxillary setae on gnathosomal base, following pair on rostrum. Palps four-segmented, laterally attached. P-2 with one dorsal seta, P-4 with two (?) or three setae in basal whorl; no seta on P-3.
Deutonymph -Genital plate with two pairs of pgs and two pairs of internal acetabula.

Winlundia Newell, 1984
Type species -Winlundia filistoma Newell, 1984 Idiosoma -Dorsum with three pairs of gland pores, one on AD, OC and PD, and (five? to) six pairs of dorsal setae. Gland pores not on cones. AE with epimeral pores and three pairs of ventral setae, pair of PE each with one dorsal and three ventral setae. Female GA with three pairs of pgs and each genital sclerite with two sgs. Male genital sclerites with three to five pairs of sgs; GO with two pairs of internal acetabula ( Figure 1E).
Deutonymph -Genital plate of W. filistoma with two pairs of pgs and two pairs of internal acetabula.

DISTRIBUTION AND BIOLOGY
Records of Arhodeoporus, Camactognathus, Maracarus, Plegadognathus, and Winlundia (Table 2) are plotted on a map (Figure 4). The genera are wide-spread in tropical and warm-temperate, but rare in cold-temperate and lacking in polar waters. The absence or low numbers of records may partly be due to the restricted collecting activities in these regions, e.g. in the southern Atlantic, but at least along the northern cold-temperate Atlantic and Pacific coast, the halacarid fauna has been studied intensely. About 50 species are recorded from the north-western Pacific, from Hokkaido and northern part of the Sea of Japan to the Bering Strait (Sokolov 1952;Makarova 1972aMakarova , b, 1974Makarova , 1977Makarova , 1978 Newell, 1984 Southeastern Pacific: Chile (Punta Arenas, Punta Caldera) Tidal and subtidal; coarse sand, kelp holdfasts Newell 1984 Camactognathus tesselatus (Morselli and Mari, 1982) Mediterranean and Black Sea: Italy (Leghorn coast) and Turkey (off
The knowledge of the fauna along the American Atlantic coast is more meagre. A list of species from Massachusetts (Cap Cod) to the eastern edge of New Foundland includes 21 halacarid species but just one Arhodeoporus (Newell 1947;Bartsch 1982aBartsch , 1997a. From Massachusetts to South Carolina two Arhodeoporus species are listed. A record of Maracarus gracilipes from Nova Scotia (Anderson 1933) is excluded, it is in need of verification.
Arhodeoporus, with just two species, is according to present records restricted to north-eastern North America. Plegadognathus has records from tropical and warm-temperate waters, there are no from cold-temperate coastlines. Seven species are described but P. thyreophorus is expected to be a junior synonym of P. bonairensis. The latter species is spread in tropical waters, in the the Caribbean Sea, the Pacific and Indian Ocean. The wide geographical distribution raises the question: is it just one or several cryptic species? More studies are necessary. An unnamed species, found on the Galapagos Island, Santa Cruz, Bahia Academy, rockpool, is the male by Bartsch (1977b: 60-61, figs 234, 235, 237, 241) erroneously identified as 'Arhodeoporus bonairensis'. In the meantime, males of P. bonairensis have been found and described (Sarma and Chatterjee 1993;Bartsch 2009b) and it is proven, there is no sexual dimorphism in the ornamentation of the dorsal and ventral plates. Camactognathus is known from both the south-eastern Pacific and the north-eastern Atlantic and the adjacent Mediterranean and Black Sea. The genus Maracarus is most rich in species; 18 of the 23 presently described taxa can be attributed to one of the species groups eclogarius, gracilipes or longirostris. Records of the six species of the eclogarius group are at present restricted to the tropical and warm-temperate Pacific and Indian Ocean (Gulf of Suez). Included in Figure 4 is an undescribed record from the South China Sea, Peninsula Malaysia (off Mersing, coral rubble, 1 -3 m; 12-17 April 1991, coll. H.G. Müller). Most wide-spread is the gracilipes group. Of the four species described, three have records from the North Atlantic, one from the West Pacific. A larva of Copidognathus sensu Viets, mentioned from southern Africa, Lüderitz Bay (Bartsch 1972), is a Maracarus species. Characters of that larva are: AD with pair of circular, raised areolae, PD with pair of porose costae, remainder of plates coarsely reticulated; dorsum with four pairs of glands which open via narrow canals on small domes. Tarsi I to III with 4/3, 4/1, 4/0 dorsal/ventral setae (solenidia included); solenidion on tarsus II dorsomedial in position; claws with pectines. The larva is most similar to the gracilipes group. The M. longirostris group includes eight species, they are concentrated within the Indo-West Pacific region. At present there are no records from other parts of the world. Most descriptions of the species are based on rather few individuals, characters may vary (e.g. the punctation of the plates, from coarse to delicate with intermediate stages) or be differently interpreted (e.g. the hyaline, ovate spot posterior to the distinct sperical lens ( Figure 3I), can it be called a cornea or not?, or the slightly thickened basal seta on tibia I, is it pectinate or not?). The one or other species may turn out to be an ecovariant.
Most of the species mentioned above have been taken in shallow water, from near the low water edge to a depth of almost 500 m. The abyssal record listed by Trouessart (1896), namely Halacarus gracilipes quadricostatus, is no variety or subspecies but belongs to Copidognathus (cf. Bartsch 1991b). Still there is no reason to expect that the four above mentioned genera are restricted to the continental shelf and upper bathyal. A species of Arhodeoporus s.l. recorded from the Mozambique Channel, from 700 m (Bartsch 1982b), can be ignored, it was a decaying protonymph which most likely had been washed into this depth.
According to present records, the majority of Arhodeoporus, Camactognathus and Maracarus species live within sandy deposits, in surface and sub-surface layers, an exception, species of the eclogarius group are expected to be epibenthic. Plegadognathus is found amongst surface deposits, sand and pebbles on the ground or an epifauna (sponges, hydrozoans, bryozoans) and -flora (corallines).
Psammophilous halacarids are known to bear suctorians, examples of infested genera are Actacarus, Halacarellus, Copidognathus (Abé 1997;Bartsch , 2003c and consequently, it is not surprising that suctorians have been found on Maracarus. Figure 3J shows the gnathosoma of M. corallicolus Otto, 2000 with eight suctorians (Praethecacineta sp.) attached to it, another three suctorians were found on each of the two telofemora I. The thecae of the suctorians are 40 -48 µm long, 21 -26 µm wide, their stalks 27 µm long. The mite is from medium coarse, shallow water sediment.