Thalassogamasus, Makarova, 2019

Makarova, Olga L., 2019, North Pacific versus North Atlantic: a case with species of the amphiboreal littoral mite genus Thalassogamasus gen. nov. (Parasitifomes, Mesostigmata Parasitidae), Zootaxa 4647 (1), pp. 457-485 : 458-460

Thalassogamasus gen. nov.

Type species. Thalassogamasus sidortschukae sp. nov., new species.

Diagnosis. The new genus is based on adult female and male material representing two newly described species, and on the illustrated descriptions of Parasitus (Eugamasus) lindrothi Sellnick, 1974 (female) and Parasitus (Neogamasus) anderssoni Sellnick, 1974 (male). Adults of Thalassogamasus are immediately distinguished from those of other genera of the subfamily Parasitinae by having a homogenous dorsal idiosomal chaetome, a narrow hypotrichous opisthonotal shield (normally with only setae of J and Z series) in combination with hypertrichy of the soft cuticle in laterocaudal areas of the opisthosoma, paraxial (al) setae on palp femur and genu entire, and the peritrematal shield strongly reduced and posteriorly free. In addition, the female has the anterior extension of the epigynium extremely attenuated, tightly abutting the medial margins of metasternal shields, and the epigynal shield fused with the ventrianal shield. The male completely lacks a tritosternum, and has a sternitigenital shield nearly completely to fully separated from the ventrianal shield, a deeply cleft hypostome, and the cheliceral arthrodial membrane fringelike.

Description

Small or middle-sized, yellowish mites. Idiosoma oval, its anterior margin truncate, posterior margin roundish ( Figs 7 , 49 , 57 ). Idiosomal shields weakly sclerotised, very finely punctate, mostly reticulate. Most idiosomal setae of moderate size with attenuated hair-like tips.

Idiosomal dorsum. Dorsal shields rather narrow, lateral soft cuticle with numerous setae ( Figs 22 , 49 , 57 ). Podonotal shield with 15–17 pairs of setae and 6–10 pairs of pore-like structures, fused anteriorly with rudimentary peritrematal shields or not. Opisthonotal shield with 9–11 pairs of setae and 6–9 pairs of pore-like structures. Soft cuticle of opisthonotal region with 18–31 pairs of setae.

Idiosomal venter. In female, tritosternum with elongate base and pilose laciniae free for most of length, their fused part ciliate ( Figs 9 , 70 ). In male, tritosternum absent. Female sternal shield and male sternitigenital shield with broad, well-developed angular endopodal extensions between coxae I–II ( Figs 18 , 28 , 50 , 58 , 80 ), other endopodal extensions of shield smaller, acute; gland pores gvb rudimentary. Presternal platelets indistinct or distinct; triangular or rhomboid, and free, abutting or consolidated with sternal shield. Female sternal shield with setae st 1–3 and two pairs of lyrifissures (iv 1, 2); metasternal shields with lateral incisions, setae st 4 and one pair of lyrifissures (iv 3), posterior half of shields hidden under epigynium ( Figs 20 , 52 ); entire genitiventrianal shield separated from metapodal platelets (mpd) laterally ( Fig. 50 ) or fused with them ( Fig. 18 ), bearing setae st 5, 4–6 pairs of opisthogastric setae, 1–2 pairs of lyrifissures and gland pores gv 2 with 1–3 openings. Female epigynal shield extremely attenuated anteriorly ( Figs 20 , 52 , 81 ), tightly abutting paraxial margins of metasternal shields ( Figs 18 , 50 ); endogynium with two thick sclerotised masses of different size (posterior mass much bigger, Figs 21 , 52 ). Male with expansive sternitigenital shield incorporating all endopodal sclerites ( Figs 28 , 58 , 85 ), bearing setae st 1–5 and lyrifissures iv 1–3, mostly or completely separated from ventrianal shield; genital funnel under anterior margin of sternitigenital shield projecting as roundish genital lamina (g.l.) covering distinct solid genital sclerite, g.s. ( Figs 29 , 63 ). Male opisthogastral sclerotisation more expansive than in female, the ventrianal shield abutting or consolidated with exopodal strips behind coxae IV, encompassing or not metapodal platelet sigillae, bearing 7–8 pairs of opisthogastric setae ( Figs 28 , 58 ). Gland pores gv 3 small, outside shield; cribrum insignificant; anal opening, postanal (post) and paraanal (par) setae small. Exopodal sclerites between coxae II–III and III–IV rudimentary. Peritreme not extending beyond mid-level of coxa I anteriorly, entire or fragmented ( Figs 19 , 28 , 33 ). Peritrematal plates reduced, posteriorly free, their pore-like structures (lyrifissures ip 1–2 and gland pore gp 1) present ( Figs 19 , 51 ) or absent ( Figs 28 , 58 ).

Gnathosoma . Gnathotectum basically triramous ( Figs 13 , 26 , 60 ) or pentaramous ( Figs 25 , 55 , 62 ), with large smooth median process and variably denticulate shorter lateral projections, associated with salivary styli. In female, subcapitulum of normal form, corniculi (co) of moderate size ( Figs 17 , 73 ). In male, hypostome deeply cleft, corniculi smaller than in female, inserted on large peduncles ( Figs 27 , 71 , 84 ). Internal malae gradually tapering to tip, with lateral margin fimbriated basally and medial margin smooth. Setae hp 1–3 smooth attenuate, pc barbed ( Figs 17 , 27 ) or scabrous ( Figs 71, 73 ). Deutosternum narrow, with 9–13 rows of small denticles (2–15 denticles in each row), laterally delimited ( Figs 71, 73 ) or free ( Fig. 17 ). Chelicerae relatively large, dorsal seta and pilus dentilis (p.d.) of ordinary form, arthrodial membrane (a.m.) fringed ( Figs 8 , 24 , 68, 69 , 82 ). In female, fixed digit basically with four teeth, sometimes with additional row of small denticles following the apical hook ( Figs 8 , 68 ); movable digit with three teeth; sometimes some intervals between large teeth on both digits with files of minute denticles ( Fig. 68 ). In male, fixed digit basically with two large teeth (distal tooth truncate), sometimes with additional row of small denticles following the apical hook ( Fig. 69 ); movable digit with one tooth ( Fig. 83 ) or two teeth of different sizes ( Figs 24 , 69 ) and simple, evenly wide trunk of spermatotreme (sptr); both digits sometimes with obliquely truncated tips ( Figs 24 , 69 ). Palpi with normal setation as described for Gamasina by Evans (1964); internal seta (in) of trochanter barbed ( Figs 17 , 73 ), longer than smooth external seta, ex (in male, external seta inserted on slightly elevated base or significant apophysis, Figs 27 , 71, 72 ); palpfemoral seta al and palpgenual setae al -1 and al -2 entire, more or less spatulate distally, palpfemoral seta al sometimes distally oblique, scabrous ( Figs 17 , 73 ); palptarsal apotele clearly three-tined.

Legs. Legs of moderate length with relatively small claws. In male, leg I longer than idiosoma, in female they are equal in length. Basitarsus of leg IV similar in form to those of legs II and III ( Fig. 16 ) or distinctly swollen ( Figs 56 , 59 ). Setae of legs heterogeneous, some ventral and lateral setae on genu and tibia of legs II–IV thickened. Tarsi II–IV with large blunt spine-like setae and long attenuated setae ( Figs 15, 16 , 23 , 31 , 56 , 59 , 67 ); apical setae ad -1 and pd -1 clearly shorter than ambulacrum. Ambulacrum I with short pedicel, claws I slightly smaller than claws II–IV; tarsi II–IV sometimes with short pedicel. Paradactyli (pd) of legs II–IV short ( Figs 15 , 45 ), sometimes almost invisible, lobes of pulvillus (plv) apically roundish. Tarsus I distally with six rod-like sensory setae. Complement of setae on segments of legs I–II–III–IV in general typical for Parasitidae as presented by Evans (1963) for all instars; in female and deutonymph: femora (2 3/2, 2/2 2) (2 3/1, 2/2 1) (1 4/1 0) (1 4/1 0); genua (2 3/2, 3/1 2) (2 3/1, 2/1 2) (2 2/1, 2/1 1) (2 2/1, 3/1 1); tibiae (2 3/2, 3/2 2) (2 2/1, 2/1 2) (2 1/1, 2/1 1) (2 1/1, 3/1 2); sometimes femur I with 12 setae (av -1 absent). In male, leg II thicker than others ( Figs 4, 5 ), with strong finger-like apophyses on femur (with axillary process of different sizes) and tibia; genu with one or two smaller apophyses; tips of all apophyses striated ( Figs 31 , 66 , 87 ).

Etymology. The name of the genus combines two Greek words, i.e. “thallasis” meaning “of the sea”, and ” Gamasus ” being the common old name of numerous mesostigmatan mites, and is intended to refer to a group of gamasine mites inhabiting seashore habitats. The proposed name is masculine in gender.

Remarks. Adult members of Thalassogamasus possess one attribute that is unique for the subfamily Parasitinae , namely a combination of oligotrichy on the dorsal shields, especially of the opisthonotal shield, with moderate hypertrichy of the laterocaudal soft cuticle. A similar state is rather common for deutonymphs of the parasitine genera Parasitellus Willmann, 1939 , Poecilochirus G. & R. Canestrini, 1882, and Coprocarpais Hrúzová & Fenďa, 2018. However, the opisthonotal shields of those deutonymphs include setae of series J, Z, S (versus only setae of series J and Z in Thalassogamasus ), and in adult mites, most of laterocaudal setae come to be on the dorsal shields, whereas in Thalassogamasus , this reduced set of dorsal shield setae is retained in adults of both sexes. Also, the dorsal shields of deutonymphs and adults of Parasitellus , Poecilochirus , and Coprocarpais bear heterogeneous chaetomes, whereas in Thalassogamasus , all dorsal setae of these instars are uniform in size and needle-like in form. Among Parasitinae , an extremely reduced chaetome of dorsal shields (opisthonotal shield with only 5–7 pairs of setae of series J and Z) is known only in members of the genera Psilogamasus Athias-Henriot, 1969 , and Taiwanoparasitus Tseng, 1995 ( Ma & Lin, 2005, 2019; Hrúzová & Fenďa, 2018), as well as in Gamasodes nudus Tseng, 1995 and Parasitus truncatus Tseng, 1995 (original generic combination), for which hypertrichy of the laterocaudal soft cuticle was not mentioned.

Both sexes of Thalassogamasus have reduced, posteriorly free peritrematal shields. This attribute occurs rarely in adult members of Parasitinae and is commonly associated with a general reduction of idiosomal sclerotisation. This state is noted, for example, in Psilogamasus and Taiwanoparasitus , some Vulgarogamasus (including Vulgarogamasus halophilus ), and is also found in an undescribed genus from the Caucasus highlands, whereas in most other Parasitinae , the peritrematal shields are commonly consolidated with the ventrianal shield in females (rarely in males) or are incorporated in the holoventral shield in males.

Females of Thalassogamasus have a broad epigynium whose anterior margin is extremely attenuated into a long narrow spike. A similar epigynial form is known among species of Gamasodes Oudemans, 1939 ( G. spiniger Trägårdh, 1910 ; G. aequipilis Athias-Henriot, 1980 ; and an undescribed species from Mongolia) and of Cornigamasus Evans & Till, 1979 (e.g., C. ocliferius Skorupski & Witaliński, 1997), but these genera are very clearly separated morphologically from all other Parasitinae .

Males of Thalassogamasus possess a combination of two significant, possibly functionally connected features, namely, a strongly or fully reduced tritosternum and a deeply cleft hypostome. These attributes are recorded also in males of the genera Phorytocarpais Athias-Henriot, 1979 ; Rhabdocarpais Athias-Henriot, 1981 ; Saprogamasus Willmann, 1949 and Coprocarpais, but the dorsal setae in deutonymphs and adults of these mites are heterogeneous (versus homogenous in Thalassogamasus ). Moreover, all developmental instars of Phorytocarpais and Rhabdocarpais have the paraxial seta al on the palp femur deeply divided (versus entire in Thalassogamasus , Saprogamasus and Coprocarpais), and the male movable cheliceral digit has a baculiform arthrodial membrane (versus fringe-like in Thalassogamasus and Coprocarpais). It is important to note that males of some Cornigamasus species are known to possess a strongly reduced tritosternum, but this condition is not accompanied by any hypostomatic modification.

Thus, there seem to be no grounds to relate Thalassogamasus closely to any other genus of Parasitinae , other than to note that it belongs to the “ Coleogamasus - Neogamasus branch” of Parasitus s. l. by Tikhomirov (1969) and that only the coprophilous Coprocarpais shares the greatest number of its important characters.

During the differential diagnostics of Thalassogamasus , the morphology of another littoral Parasitinae species was analysed, namely Parasitus kempersi Oudemans, 1902 . Since its first description, it has been placed in other genera as Gamasus (Gamasus) kempersi (in Berlese, 1906) , Eugamasus kempersi (in Holzmann, 1969) or was referred to the subgenus Coleogamasus of the genus Parasitus (in Tikhomirov, 1969, 1977). In the last keys ( Hyatt, 1980; Karg, 1993) this species became mentioned as Parasitus kempersi . The morphology of its adults is in full agreement with the diagnosis of the genus Phorytocarpais ( Athias-Henriot, 1979, 1980), which allows generation of this new combination, Phorytocarpais kempersi (Oudemans, 1902) comb. nov.

Figures

FIGURES 7–17. Thalassogamasus sidortschukae, female. 7. Dorsal idiosoma; 8. Chelicera; 9. Tritosternum; 10–14. Variant forms of gnathotectum; 15. Tarsus II; 16. Tarsus IV; 17. Subcapitulum.

FIGURES 49–56. Thalassogamasus kurilensis, female. 49. Dorsal idiosoma; 50. Ventral idiosoma; 51. Peritrematal shield; 52. Genital opening region; 53. Anterior part of epigynium; 54, 55. Variant forms of gnathotectum; 56. Tarsus IV.

FIGURES 57–65. Thalassogamasus kurilensis, male. 57. Dorsal idiosoma; 58. Ventral idiosoma; 59. Tarsus IV; 60–62. Variant forms of gnathotectum; 63. Genital orifice region; 64, 65. Variant forms of genital sclerite.

FIGURES 22–27. Thalassogamasus sidortschukae, male. 22. Dorsal idiosoma; 23. Tarsus IV; 24. Chelicera; 25, 26. Variant forms of gnathotectum; 27. Subcapitulum.

FIGURES 66–73. Thalassogamasus kurilensis. 66, 69, 71, 72. Male; 66. Leg II, anterolaterally; 69. Chelicera; 71. Subcapitulum; 72. Palp trochanter; 67, 68, 70, 73. Female; 67. Tarsus II; 68. Chelicera; 70. Tritosternum; 73. Subcapitulum.

FIGURES 18–21. Thalassogamasus sidortschukae, female. 18. Ventral idiosoma; 19. Peritrematal shield; 20. Metasternal and epigynal structures; 21. Sclerites of endogynium.

FIGURES 28–31. Thalassogamasus sidortschukae, male. 28. Ventral idiosoma; 29. Genital orifice and presternal region; 30. Leg II without coxa and tarsus, ventrally; 31. Leg II without coxa, anterolatrally.

FIGURES 80–87. Thalassogamasus lindrothi comb. nov. 80–82. Female; 80. Ventral idiosoma; 81. Metasternal and epigynal structures; 82. Cheliceral digits; 83–87. Male; 83. Cheliceral digits; 84. Subcapitulum; 85. Ventral idiosoma; 86. Dorsal idiosoma; 87. Femur, genu and tibia II (after Sellnick, 1974; modified).

FIGURES 32–38. Thalassogamasus sidortschukae, deutonymph. 32. Dorsal idiosoma; 33. Peritrematal shield; 34. Ventral idiosoma; 35. Chelicera; 36–38. Variant forms of gnathotectum.

FIGURES 39–45. Thalassogamasus sidortschukae. 39–42, 44. Protonymph; 39. Dorsal idiosoma; 40, 41. Variant forms of gnathotectum; 42. Ventral idiosoma; 44. Ambulacrum II; 43. Larva, ambulacrum III; 45. Female, ambulacrum III.

FIGURES 1–6. Littoral members of the subfamily Parasitinae. 1–3. Females; 4–6. Males; 1, 4. Thalassogamasus sidortschukae;; 2, 5. Thalassogamasus kurilensis; 3, 6. Vulgarogamasus halophilus (Sellnick, 1957). Scale 100 μm.