Taxonomy of the Cryptopygus complex. I. Pauropygus - a new worldwide littoral genus (Collembola, Isotomidae)

Abstract In this paper, we describe the new genus Pauropygus gen. n. which includes three minute species, blind and unpigmented, living in interstitial littoral habitats in tropical or subtropical countries. Two of these species are new to science (type species Pauropygus projectus sp. n. from New Caledonia and Pauropygus pacificus sp. n. from China); the third one, originally described in the genus Cryptopygus (Cryptopygus caussaneli Thibaud, 1996), has a larger pantropical distribution. We synonymize here Cryptopygus riebi Barra, 1997 from South Africa with Pauropygus caussaneli. Two paratypes of the Mexican species Cryptopygus axayacatl Palacios & Thibaud, 2001 turned also to be Pauropygus caussaneli, while the holotype and remaining paratypes of this species support its placement in Proisotomodes. Among the Cryptopygus complex, Pauropygus gen. n. is easily recognized by characters of mouthparts (presence of two large projections on pleural fold, basolateral field with 6 chaetae, modified mouthparts) and reduced sensillar chaetotaxy (tergal sensilla 2-3,0-1/0-1,0-1,1-2,1-2,1-3, microsensilla reduced in number: 00/0-100, with sensilla situated in p-row on the abdomen). Small size, absence of eyes and pigment are also shared by all its species. The three species belonging to the genus differ by sensillar chaetotaxy.


Introduction
The species of the subfamily Anurophorinae with furca and two last abdominal segments fused are widely distributed in the world. If special remarkable detail had not been found in such species, they were usually assigned to genera Isotomina Börner, 1903 (mostly in Northern Hemisphere) or Cryptopygus Willem, 1902 (mostly in Southern Hemisphere). In the course of the development of taxonomy in the family Isotomidae the question of division of these two artificial taxa became important. So far Isotomina, Cryptopygus and several formally similar genera have been given some discussions and historical reviews (Deharveng 1981;Potapov 2001;Rusek 2002;Deharveng et al. 2005;Potapov et al. 2009, Jordana et al. 2009). Most forms, however, of this so-called "Cryptopygus complex" did not find their final generic position and therefore even modern monographs and catalogues continue to use Cryptopygus in a wide sense (Fjellberg 2007;Danyi & Traser 2008;Furuno et al. 2000, and others). At convenience, we accept the traditional understanding of the complex -it means Abd.V and VI fused (vs. separated in Proisotoma complex). We believe that fusion/separation of Abd.V and VI is really a convenient feature to separate these two large groups of taxa of which the latter one is more common in Northern Hemisphere while the former mostly occupies Southern Hemisphere. In the present paper, we describe a new genus for three species, two of which are new to science and share several remarkable morphological modifications that are probably adaptations to interstitial seashore habitats.
Diagnosis. Blind small-sized Anurophorinae with two last abdominal segment fused, modified mouthparts including remarkably modified pleural fold, and first segments of antenna set together on frontal part of head.
Name derivation. The name is derived from the Greek suffix -pygus which points to the fusion of the abdominal segments and from the Greek prefix pauro-which refers to the reduced chaetotaxy, particularly reduced number of sensilla on the body.
Affinity. The new genus is established mainly due to the unusual position of antennae on head and the strongly modified mouthparts. In mouthparts, the V-shaped pleural folds and the presence of 6 chaetae on basolateral fields of the labium are especially important; these characters were never seen in the family before, except for the latter one that was mentioned for part of the Algerian population of Isotominella geophila sensu Jordana et al., 2009; normally, the pleural fold looks like a weak swelling proximal to the maxillary outer lobe, and basolateral field of mouth cone has 5 chaetae in species of the Isotomidae family (Fjellberg 1984(Fjellberg , 1999. Other modifications of mouthparts, like swollen labrum, unequal labial papilla, reduced plate of outer lobe of maxilla, are more common features. The sixth chaeta of basolateral field has unclear derivation; since it has weaker socket than the other five, it is probably one of the sublobal hairs which has migrated from the sublobal plate to more posterior area of head and grouped together with basolateral chaetae. Together with two finger-like extensions of pleural folds this chaeta probably makes lateral parts of head more functionally important. The projected position of antennae and modified mouthparts are probably adaptations to an interstitial life between small sand grains on the beach and to feeding on particles suspended in water. Projections on different parts of body are well known among species living in contact with salt water in genera Archisotoma Linnaniemi, 1912, Anuridella Willem, 1906, Xenylla Tullberg, 1869, Hypogastrura Bourlet, 1839, Friesea Dalla Torre, 1895 As an unusual feature for the group, the species of the new genus show considerable reduction of sensillar chaetom. In Pauropygus projectus sp. n. all medial sensilla on Th.II-Abd.IV and lateral sensilla on Th.III-Abd.II are lost, while in P. caussaneli and P. pacificus sp. n. it is the posterior and lateral parts of body that lost sensilla. In spite of differences between number of sensilla on body (2,0/0,0,1,1,3) and (3,1/1,1,2,2,3) among Pauropygus species, the general pattern of their distribution and differentiation is kept.
Pauropygus is closely related to Isotominella Delamare Deboutteville, 1948 after the redescription of Jordana et al. (2009). The identity of the type specimens of Isotominella geophila Delamare Deboutteville, 1948 (Ivory Coast) which were not seen by Jordana and specimens from Algeria on which the redefinition was based remains somewhat doubtful. The two genera share simple maxillary palp, two prelabral chaetae, posterior position of sensilla on tergites, and general appearance of furca. Other shared characters (blindness, absence of foil chaetae, sensillar equipment of antennae, microsensillar set 10/100) are less significant. Apart from the two characters mentioned above, Pauropygus sp. n. differs from Isotominella in more differentiated tibiotarsal chaetotaxy (presence of tenent and spiny chaetae), shape of PAO (flat and broad vs. oval), number of sublobal hairs and e-guards (3 vs. 2, and 7 vs. 5, respectively). Isotominella also has a rather common set of sensilla on body (33/22223) while it is reduced in Pauropygus. At last, the new genus is strictly restricted to seashore sands, while Isotominella geophila prefers soil. We also examined specimens of Isotominella geophila from Algeria kindly provided for us by Jordana and surprisingly concluded that females and males also differ in antennae on head which are positioned almost like in Pauropygus in males and set apart in females. The crenulation of basal part of dens was stressed by both Delamare Debouteville (1948) and Jordana et al. (2009) as one, if not the main diagnostic characters of the genus Isotominella. We consider that this character is of low taxonomical value since it strongly depends on mounting of the animal on slide. We have also seen other specimens of Isotominella from Eurasia (Ukraine and China). They did not show the dimorphism of Algerian populations and represent at least one more species of the genus. The comparison of Pauropygus gen. n. with other genera of the Cryptopygus complex is given below. Description. Size 0.5-0.6 mm. White, without eyes. Cuticle with thin hardly visible primary hexagonal granulation ("smooth"). PAO flat, roundish, not constricted, about 1.5 as long as inner edge of U.III and shorter than width of Ant.I (Fig. 11). Sublobal plate of maxillary outer lobe small with 3 hairs grouped together, palp simple. Pleural fold with one chaeta (as common for the family) and two high projections (Fig.  3). Labral chaetotaxy as 2/554, middle part of labrum swollen, chaetae in two apical rows set on wide papillae, edge of labrum weakly developed (Fig. 5). Labium with 3 proximal, 6 basolateral and 4 basomedian chaetae and a complete set of papillae (A-E) and guards (16). Papillae B, D and E projected considerably forward, papillae A and C partly reduced and fused with B and D, respectively (Fig. 4). Ventral side of head with 6-7+6-7 postlabial chaetae. Ventrolateral chaetae of head and postlabial chaetae delimit an almost unbroken unsetaceous area. Maxillary head elongated, with four well visible enlarged lamellae of which two have long cilia and two have fine serration. Two remaining lamellae possibly as small weakly serrated projections set in a common cluster at base of claw (Fig. 6). Maxillary claw reduced, single-tipped, finger-shaped, with some weak teeth at the head which are visible in lateral view only (not shown on figure). Pars incisiva of mandible slender, apically with four weak teeth, basal part of pars molaris with two strong hooks (Fig. 7).

Pauropygus projectus
Ant.I with many chaetae (more than 20), 1 ventro-basal microchaeta (bms; dorsal bms not differentiated), and 2 thick ventral sensilla (s). Ant.II with 3 rather large bms and 1 thick laterodistal s. Ant.III without bms and with 5 distal s of which two inner as thick and short as outer ones. Male antennal "spurs" unknown. Sensilla on Ant.IV weakly differentiated, as common for the family, subapical organite pin-like and small, subapical microsensillum short and curved (Figs 11,12).
Unguis of normal shape, without inner tooth, two lateral teeth forming a weak tunica partly covering dorsal edge of unguis (Fig. 10). Upper subcoxa of Leg I-III with 1,1,4 chaetae, lower subcoxa with 1,10,11 (one individual studied). Ti.I-III with basic set of chaetae (21-22, 22-23, 28), T-chaetae absent. Chaetae on tibiotarsi with irregular distribution, outer side with higher number of chaetae than inner side. Chaetae C7 (inner part of tibiotarsi) either lost or migrated laterally. Modification of chaetae x and B5 on Ti.III in males unknown (no males). Distal tibiotarsal tenent chaetae on Ti.I-III (1-2-2) well developed, not clavate, about 1.5 as long as U.III. Each tibiotarsus with one (sometimes two on Leg II) additional tenent chaeta at middle part. Ti.III with two stick-like chaetae (A6, A7) in distal ring. Ti.I,II with similar thickened chaetae but much less developed. One or two inner chaetae of distal ring of Ti.I-III shorter than others. Three chaetae of distal ring A (two tenent chaetae and one nearby) set apart from distal edge of tibiotarsus, unlike other chaetae of the ring (Fig. 10). Pretarsus with two chaetae, inner one shorter. Ventral tube with 4+4 laterodistal and 2 posterior chaetae. Tenaculum with 4+4 teeth and 1 chaeta. Anterior furcal subcoxa with 13-15 chaetae, posterior with 6-7. Furca of medium size. Anterior side of manubrium with a pair of distal chaetae, posterior one with 8+8 chaetae on the main part and 3+3 on the laterobasal lobes (one individual studied). Manubrial thickening with a pair of additional inner teeth. Lateral parts of manubrium with 1+1 chaetae. Dens slender, anteriorly with 12 (more rarely 11) chaetae. Posterior side of dens slightly crenulated in the medial part, with 5 chaetae of which 3 basal and 2 at the medial part (short inner and long outer) set together on wide papillum. Mucro slender with two teeth of which the subapical one is larger, lamellae not differentiated (Figs 13, 14). Ratio of manubrium : dens : mucro = 2.8-3.4 : 4.5-4.6 : 1. Anal lobes without microchaetae.
Affinity. Pauropygus projectus sp. n. differs from P. caussaneli in number of sensilla on body (20/00113 vs. 31/11221, see also the genus affinity section), relative size of inner and outer sensilla in AIIIO (outer ones smaller than inner ones in P. caussaneli), number of posterior chaetae on ventral tube (2 vs. 4), serrated chaetae on upper and lower subcoxae (smooth in P. caussaneli), more differentiated chaetal equipment of legs. Besides, P. projectus sp. n. is larger in size, which may explain higher number of chaetae on Ant.I, parts of legs, axial group of tergites and postlabial area than in P. caussaneli (see descriptions for details).
Name derivation. The species has well developed "projections" on the body as: swollen labrum, v-shaped pleural folds, extended labial palp, and antennae projected ahead.
Distribution. Only from Loyalty Islands in New Caledonia.
Pauropygus caussaneli (Thibaud, 1996)  Redescription. White, without eyes. Size up to 0.4 mm. Cuticle smooth. PAO about 1,9 as long as inner edge of U.III and shorter (0.7) than width of Ant.I. Outer (Fig. 20) and inner mouthparts principally as in P. projectus. Ventral side of head with 5+5 postlabial chaetae. Ant.I with 14-15 chaetae, 1 ventro-basal microchaeta (bms; dorsal bms not differentiated), and 2 thick ventral sensilla (s), short and long. Ant.II with 3 bms and 1 thick laterodistal s. Ant.III without bms and with 5 distal s of which two inner thicker and longer than outer ones. Male antennal "spurs" present on Ant. II, III and basal part of Ant.IV.
Distribution. Probably widely distributed on sandy seashores of tropics and subtropics. Recorded from the coast of Indian and Atlantic Oceans (Africa, Central America). Description. Size about 0.4 mm. White, without eyes. Cuticle smooth. PAO about 1.7 as long as inner edge of U.III and somewhat shorter (0.8-0.9) than width of Ant.I. Outer (Fig. 23) and inner mouthparts principally as in P. caussaneli. Ventral side of head with 5+5 postlabial chaetae. Ant.I with about 18 chaetae, 1 ventro-basal (bms) microchaeta (dorsal bms not differentiated), and 2 thick ventral sensilla (s), short and long. Ant.II with 3 bms and 1 thick laterodistal s. Ant.III without bms and with 5 distal s of which two inner thicker and longer than outer ones. Male antennal "spurs" present.
Affinity. The complete absence of microsensilla on body of Pauropygus pacificus sp. n. is the second reported case among Isotomidae (so far only known in Proisotoma minima). This peculiarity remains the only difference between P. pacificus sp. n. and P. caussaneli. Taking into consideration also the different distribution ranges of these species we prefer to consider them as two independent species. Name derivation. The species is probably distributed in the sands all over the Pacific coast of China.
Distribution. Known from two distant localities on Pacific coast of China.

Taxonomical remarks on genera similar to Pauropygus
Examination of numerous species of the Cryptopygus complex revealed that six genera of this complex share the posterior position of sensilla on the three first abdominal segments. Like in other taxa of Isotomidae, as for instance in Proisotoma complex (Potapov et al., 2006) and in species groups of Folsomia (Grow & Christiansen, 1976;Deharveng, 1979), the position of medial sensilla on abdominal tergites (within or well in front of posterior row of chaetae) is taxonomically very relevant at supraspecific level. The posterior position of sensilla is shared by several genera of the Cryptopygus complex and not necessarily indicates close relationships, but rather might be a result of convergent evolution. It is for instance the case in the genus Micrisotoma Bellinger, 1952 which also belongs to Cryptopygus complex and shows relationships with Hemisotoma Bagnall, 1949 in which sensilla are in mid-tergal position (type species Isotomina thermophila).
To figure out the relation between these genera, we propose below a key to the genera of the Cryptopygus complex with posterior position of sensilla on three first abdominal segments. Pauropygus projectus sp. n. lost posterior sensilla and can be identified by pleural fold; the loss of mentioned sensilla is probably shared with several species of Arlea but descriptions do not give information on these characters. The position of the odd genus Appendisotoma is not fully decided: six species studied by us (A. abiskoensis (Agrell), A. bisetosa Martynova (types), A. sibirica Stebaeva (types), and three species from Far East Russia and North America) have 11 apical chaetae on tibiotarsi while other four (A. stebayevae (Grinbergs), A. montana (Martynova), A. juliannae (Traser et al.), and one species from Kazakhstan) show common set for the subfamily (7 chaetae). We include the genus Appendisotoma in the key since it formally matches the diagnosis of Cryptopygus complex. PAO complex, with lobes. Well-marked cylindrical foil-chaetae at the end of abdomen. Anterior medial sensilla on Abd.V present (Fig. 27)  The key is based on examination of three species of Pauropygus, four of Proisotomodes (one undescribed), two of Isotominella (one undescribed), eight of Appendisotoma (two undescribed), and one of Micrisotoma.