New species and records of terrestrial Isopoda (Crustacea, Oniscidea) from Socotra Island, Yemen

Twenty-seven species of terrestrial Isopoda are recorded from Socotra Island (Yemen). One genus (Dumetoniscus) in the family Bathytropidae and three species (Serendibia cavernicola, Dumetoniscus graniticus and Platyarthrus alticolus) are described as new. A key to the 41 species of Oniscidea presently known from the Socotra Archipelago is given. Th e new records confi rm the high number of endemics present on this archipelago.


Introduction
Up to date 38 species of terrestrial isopods were known from the Socotra Archipelago (Ferrara andTaiti 1996, Taiti andFerrara 2004) with a very high number of endemic taxa and biogeographical affi nities with East Africa, the Oriental Region and, to a lesser extent, with the Mediterranean Subregion. Th ese data were based on a large amount of material collected mainly by S. Taiti during a visit to Socotra, Darsa, Samah and Abd al-Kuri islands in February 2000, and by A. Van Harten, W. Wranik, H. Pohl, K. Van Damme, P. de Geest and M. Apel in previous years.
Th is paper deals with the terrestrial isopods collected during a second visit of S. Taiti to Socotra Island in January 2003. During this trip investigations were carried out also in some parts of the island where no collection of Oniscidea had been previously made, such as the inland western and the extreme eastern areas. Twenty-seven species of terrestrial isopods are recognized in the present study, and one genus and three species are described as new.

Material and methods
Specimens have been stored in 75% ethanol and identifi cations are based on morphological characters. For each species citations referring to the Socotra Archipelago, the material examined, distribution and, when necessary, remarks are given. Th e new taxa are illustrated with fi gures prepared with the aid of a camera lucida mounted on Wild M5 and M20 microscopes. An identifi cation key, a list of species and an updated distributional map for all the oniscidean species of the Socotra Archipelago is presented.
Th e material is deposited in the collection of the Museo di Storia Naturale dell'Università, Sezione di Zoologia "La Specola", Florence (MZUF). Half of the material collected has been selected for future deposition in the planned Natural History Collection of Yemen (NHCY), and it is temporarily kept in MZUF.  Diagnosis. A species of Serendibia characterized by the absence of eyes, elongated body (about 3 times longer than wide), telson with widely rounded apex; antennule with elongated aesthetascs, antenna with fi fth article of peduncle not swollen, male pleopod 1 exopod with large triangular posterior point, and endopod with apical part pointed and bent outwards.
Description. Maximum length: ♂, 2.9 mm; ♀, 4.0 mm. Body elongated (about 3 times longer than wide) and colourless (Fig. 1A). Back with scattered long scalesetae (Fig. 1B); each pereionite with one nodulus lateralis per side (Fig. 1C), nodulus lateralis on pereionite 4 more distant from lateral margin than those on the other pereionites, b/c and d/c co-ordinates as in Fig. 1D; no visible gland pores. Cephalon (Fig. 1E,F) about 1.5 times wider than long, with very thin suprantennal line slightly bent downwards in the middle; eyes absent. Pleon narrower than pereion; pleonites without posterior points visible in dorsal view. Telson (Fig. 1G) about twice as wide as long, distal part with straight sides and very broadly rounded apex. Antennule (Fig.  1H) with third article equipped with 2 apical and 3-4 subapical long aesthetascs. Antenna (Fig. 1I) with fi fth article of peduncle not swollen; fi rst and third fl agellar articles subequal in length, second distinctly shorter; second and third fl agellar articles with a row of fi ve and seven aesthetascs, respectively. Buccal pieces as in all the other species of the genus, i.e. mandible with molar penicil simple, maxillule with outer branch bearing 7 simple teeth and inner branch with two short and thickset penicils, maxilliped with endite equipped with a big apical penicil and a long seta on outer corner. Pereiopods with no dactylar seta and with ungual seta fl agelliform. Uropod with protopod and exopod grooved on outer margin; insertion of endopod slightly proximal to that of exopod.
Etymology. Latin cavernicolus = cave-dwelling. Th e name refers to the locality (Kazekas Cave) where the type specimens were collected.
Remarks. Until now, the genus Serendibia included the type species Serendibia denticulata Manicastri & Taiti, 1987, from Sri Lanka, and three species from the Socotra Archipelago, i.e. S. vagans and S. fi liformis from Socotra Island, and S. samhaensis from Samha Island. Th e new species from Kazekas Cave described here belongs to the group of Serendibia from the Socotra Archipelago, characterized by the absence of eyes, the colourless body and shape of the endite of maxilliped which has a long seta on the outer corner. Th e specimens from Mega 1 Cave show some small  (Fig. 3A), the male pereiopod 1 carpus more enlarged (Fig. 3B), the male pleopod 1 endopod with longer and thinner apical part (Fig. 3C), and the male pleopod 2 endopod shorter (Fig. 3D) when compared with the exopod. With only one male examined from Mega 1 Cave, it is diffi cult to judge the importance of these diff erences, but considering the very similar shape of the male pleopod 1 exopod (compare Fig. 2C and 3C), we prefer to include also these specimens in S. cavernicola. Th e new species is readily distinguishable from all the other Socotran species in the genus by having the body proportionally wider (only 3 times instead of > 4.5 longer than wide), antenna with the fi fth article of the peduncle not swollen and more numerous number of aesthetascs on the fl agellum. It also diff ers from S. vagans in the suprantennal line bent down instead of straight, the male pereiopod 1 carpus enlarged, and longer posterior point of the male pleopod 1 exopod; from S. fi liformis in thinner and longer aesthetascs of the antennule, the male pleopod 1 exopod with triangular instead of rounded posterior point, and the male pleopod 2 endopod longer in comparison with the exopod. For the enlarged male pereiopod 1 carpus and the similar shape of the male pleopod 1 exopod, S. cavernicola seems to show closest affi nities with S. samhaensis, from which it is distinguished by the distal part of male pleopod 1 endopod which is not enlarged and not equipped with fl attened setae.
Diagnosis. Animals unable to roll up into a ball, runner type. Back with shallow tubercles and small T-shaped scale-setae; no glandular fi elds at lateral margins of pereional segments; one series of noduli laterales per side, nuduli on pereionites 4 and 7 clearly more distant from the lateral margins of the segments. Cephalon with frontal lobes well developed, lateral lobes obliquely directed downwards; suprantennal and frontal lines present. Posterior margin of pereionite 1 straight. Pleon slightly narrower than pereion; pleonites 3-5 with well developed posterior points. Telson triangular, reaching distal margin of uropodal protopod. Antennule of three articles with a tuft of aesthetascs at apex. Antenna with fl agellum of two articles, the fi rst longer than the second. Mandible with molar penicil dichotomized, 1+1 free penicils on the right mandible and 2+1 on the left. Maxillule with outer branch equipped with 4+6 (5 cleft) teeth; inner branch with 2 penicils and a posterior point. Maxilliped with endite bearing 3 large triangular spines at apex and no penicil. Pereiopods with fl agelliform dactylar and ungual seta on dactylus; distal margins of merus and carpus with a row of short scales. All pleopodal exopods with no respiratory struc- tures. Uropodal protopod with a triangular incision on lateral margin; endopod inserted proximally to exopod. Etymology. Latin Dumetum = bush + Oniscus. Gender masculine. Th e name refers to the habitat where the type species was collected.
Th e family includes all the genera of Crinocheta which are unable to conglobation, have T-or Y-shaped dorsal scale-setae, a two-jointed antennal fl agellum (except Monitus with three fl agellar articles), and no visible respiratory structures on the pleopods (except Bathytropa wahrmani Strouhal, 1968 which has Trachelipus-type uncoverd pleopodal lungs). As pointed out by Schmidt (2003) the monophyly of the family is questionable and in need of an accurate revision. Schmidt (2003) redefi ned the family only on the basis of one species in the type-genus Bathytropa, without taking into consideration all the other genera included until now.
Th e specimens from Adho Dimelho in the Hagghier Mts (see below) that we examined do not fi t in any of the genera included in the family and are described in the new genus Dumetoniscus. It is distinguishable from Bathytropa in the position of the noduli laterales, cephalon with medial frontal lobe triangular instead of quadrangular, presence of suprantennal line, pereion and pleon epimera less developed; from Neoptroponiscus in the medial frontal lobe less developed, triangular instead of quadrangular, the presence of a suprantennal line, the less developed pleon epimera and the triangular telson with straight sides; from Monitus in the two-jointed antennal fl agellum, the less developed frontal lobes on cephalon and epimera of pereion and pleon, and the triangular telson; from Papuasoniscus in the presence of a suprantennal line, diff erent disposition of the noduli laterales and the triangular telson. Th e new genus shows closest similarity with Laninoniscus from which it diff ers in the tuberculated instead of smooth dorsum, the more developed frontal lobes, and the presence of a distinct frontal line. Taiti  Diagnosis. A species of Dumetoniscus characterized by enlarged body shape (ca. twice as long as wide), cephalon with a triangular frontal medial lobe, triangular telson with straight sides, male pleopod 1 exopod triangular and endopod with apical part pointed and slightly bent outwards.

Dumetoniscus graniticus
Description. Maximum length: ♂ 3.0 mm; ♀, 4.8 mm. Body enlarged (ca. 2 times longer than wide) (Fig. 4A). Colour brown with yellow spots; pereionites 5-7 with postero-lateral corners pale (reddish in vivo?); antennae, pleopods and uropods brown. Vertex of cephalon and dorsal side of pereion with some rounded shallow tubercles; posterior margins of pereionites 5-7 and of all pleonites granulated; dorsum with small T-shaped scale-setae (Fig. 4B); co-ordinates of noduli laterales as in Fig. 4C. Cephalon (Fig. 4D-F) with frontal lateral lobes rounded and obliquely directed downwards, medial lobe triangular slightly protruding frontward compared with the lateral lobes; eyes very large, globose, with up to 16 ommatidia in the larger specimens. Pereionites with posterior margins straight. Pleonites 3-5 with falciform posterior points. Telson (Fig. 4G) triangular with straight sides, slightly surpassing posterior margin of uropo-   (Fig. 4H) with third article about three times longer than second with a tuft of about seven aesthetascs and a small point at the apex. Antenna (Fig. 4I) with fl agellum as long as fi fth article of peduncle; second fl agellar article ca. twice longer than fi rst, bearing one and two aesthetascs. Mandibles (Fig. 5A,B) with molar penicil consisting of about six plumose setae. Maxillule (Fig. 5C) with inner branch bearing two elongated setose penicils and a distinct triangular posterior point. Maxilla (Fig. 5D) with outer lobe smaller than inner one, two strong setae between the two lobes. Maxilliped (Fig. 5E) and uropods (Fig. 5F) as in the generic diagnosis.

Diagnosis.
A species of Platyarthus characterized by the dorsal ornamentation with two bosses on vertex and two prominent and two shallow ribs per side on pereionites 1-6 and two prominent ribs per side on pereionite 7, two paramedian ribs on pleonites 3-5, and very short telson with obtuse triangular distal part.
Etymology. Latin alticolus = living at altitude. Th e name refers to the collecting site of the species at ca. 1000 m elevation on the Hagghier Mts.
Remarks. Th e genus Platyarthrus currently includes 28 species (Schmalfuss 2003) mainly distributed in the Mediterranean area, with a few species from the Atlantic islands and west to Tajikistan. One species, P. acropyga Chopra, 1924, has been described from Chilka Lake, India. In the Socotra Archipelago only the introduced species Platyarthus schoblii was previously recorded (Taiti and Ferrara 2004).
Even if only two female specimens were collected, Platyarthrus alticolus sp. n. is worth of a description because of its peculiar dorsal ornamentation and the high probability of being authocthonous on Socotra Island. It belongs to the schoblii-group of species, characterized by the short telson and dorsal longitudinal ribs on the pereion. For the presence of four dorsal ribs per side on the pereionites 1-6 P. alticolus is similar to P. sorrentinus Verhoeff , 1931 from southern Italy. All the other species in the group have fi ve or six longitudinal ribs per side on pereionites 1-6. It is readily distinguishable from P. sorrentinus in having two prominent and two more shallow ribs per side instead of all with the same development, and in having two large bosses instead of ribs on the cephalic vertex (compare Pl. 8 fi g. 43 in Verhoeff 1931 for P. sorrentinus). Th is last character distinguishes the new species also from all the other species of the schoblii-group.
Distribution. Species endemic to the Socotra Archipelago (Socotra and Samha islands).
Distribution. Species endemic to Socotra Island. It has been collected on the Hagghier Mts above 1000 m elevation.
Distribution. Species endemic to Socotra Island.

Family Agnaridae
Genus Pseudoagnara Taiti  Remarks. As pointed out in Taiti and Ferrara (2004) this species shows a great variability in the colour pattern and dorsal granulation, while the male characters are very stable in the diff erent populations. Th e colour varies from pale cephalon, pereion, pleon epimera, telson and uropods, and dark pleonal tergites (Pl. 18 in Taiti and Ferrara 2004), to dark brown-grey with pale uropods and distal part of telson (Pl. 19 in Taiti and Ferrara 2004), to reddish as in some specimens from Samha (Pl. 20 in Taiti and Ferrara 2004) and western Socotra. Th e dark brown-grey specimens usually show more developed dorsal granulations. A molecular study of the diff erent populations is necessary to fi nd out if we are in front of a single variable species or to a complex of closely related species.

Distribution. Continental Yemen and Socotra
Island. On Socotra the species has certainly been introduced since it has been collected only in and around houses in Hadibo.

Somalodilloides pilosus Taiti & Ferrara, 2004
Somalodilloides pilosus Taiti & Ferrara, 2004: 298, fi  Remarks. Th ese specimens and those from SW of Qeysoh cited in Taiti and Ferrara (2004) have an uniformly pale instead of grey colour (see pl. 26 in Taiti and Ferrara 2004) as in the populations from Nojid, Wadi Ayhaft, Diksam, Hagghier Mts and Momi, while all morphological characters are identical. As in the case of Socotroniscus sacciformis, a molecular analysis is necessary to confi rm that the pale and grey forms belong to the same species.

Discussion
Twenty-seven species of terrestrial isopods from Socotra Island have been recorded in this contribution, including one new genus (Dumetoniscus) and three new species (Serendibia cavernicola, Dumetoniscus graniticus and Platyarthrus alticolus). Th e total number of Oniscidea known from the Socotra Archipelago is now 41 (Table 1), of which 35 occur on Socotra, three on Darsa, eight on Samha and fi ve on Abd al-Kuri.
During the visit of S. Taiti to Socotra in January 2003 also the inland western and the extreme eastern part of Socotra Island, from where no records were previously known, has been investigated. Th e recorded distribution of the oniscidean species in the Socotra Archipelago is shown in Fig. 8. With the three new species described in this contribution the number of species endemic to the Socotra Archipelago rise to 30, a very high number which demonstrates the long isolation and evolutionary history of the onisci-dean fauna of these islands. Th e new records confi rm the biogeographical conclusions already documented by Taiti and Ferrara (2004) with affi nities with the Afrotropical Region, in particular East Africa, with the Oriental Region and, to a lesser extent, with the Mediterranean Subregion. Of particular interest is the presence of the new species Platyarthrus alticolus. Th is species belongs to the schoblii-group which has a Mediterranean and Atlantic distribution. While the Mediterranean Platyarthrus schoblii, the other species in the genus present on the archipelago, is certainly introduced with human activities, the new species most probably originated on the island, as its presence only at high altitude on the Hagghier Mts seems to demonstrate. Similarly to Ligia dioscorides, P. alticolus may have originated from a taxon present when the Mediterranean was connected with the Indian Ocean, diverging from it after the closure of the connection.