Xanthoidea: Xanthidae, Pilumnidae, Menippidae and Trapeziidae) of Southern Oman

Tropical Zoology

ISSN: 0394-6975 (Print) 1970-9528 (Online) Journal homepage: https://www.tandfonline.com/loi/ttzo20

Brachyuran crabs (Xanthoidea: Xanthidae, Pilumnidae, Menippidae and Trapeziidae) of southern Oman

Peter J. Hogarth

To cite this article: Peter J. Hogarth (1994) Brachyuran crabs (Xanthoidea: Xanthidae, Pilumnidae, Menippidae and Trapeziidae) of southern Oman, Tropical Zoology, 7:1, 93-108, DOI: 10.1080/03946975.1994.10539244 To link to this article: https://doi.org/10.1080/03946975.1994.10539244

Published online: 01 Aug 2012.

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Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=ttzo20 Tropical Zoology 7: 93-108, 1994

Brachyuran crabs (Xanthoidea: Xanthidae, Pilumnidae, Menippidae and Trapeziidae) of southern Oman

PETER J. HoGARTH Department of Biology, and Tropical Marine Research Unit, University of York, York YOl 5DD, ·u.K.

Received 12 October 1992, accepted 23 April 1993

Thirty-eight species of xanthoid crab are recorded for the first time from Dhofar Province, southern Oman. The geographical range of three species [Nanocassiope alcocki (Rathbun 1902), Actumnus digitalis (Rathbun 1907), and Macromedaeus crassi manus (A. Milne Edwards 1867)] is substantially extended. The pattern of xanthoid species diversity is discussed in relation to the effect of the seasonal monsoon-driven upwelling, and compared with that of other parts of the Western Indian Ocean.

KEY WORDS: Brachyura, Xanthidae, Pilumnidae, Menippidae, Trapeziidae, Oman, Western Indian Ocean, biogeography, distribution.

Introduction 93 Collecting sites 94 Collections 94 Discussion . 103 Acknowledgements 107 References 107

INTRODUCTION

The marine fauna of Dhofar, southern Oman, is poorly known, despite being of considerable biological interest. For much of the year the coastline is affected by a seasonal upwelling system which subjects the shore to vigorous surf action and sea temperatures of 15-16 °C. Outside the upwelling season, the sea temperature may be in the region of 25-26 °C. The Arabian Sea upwelling and its effects are described by CuRRIE et al. (1973), ScHOTT & FIEux (1985), SAVIDGE et al. (1990), and SHEPPARD et al. (1992). The effects of this seasonal upwelling on the coastal marine ecosystem of Dhofar are profound. To investigate the ecological impact of the upwelling, surveys were instigated and carried out by the Tropical Marine Research Unit (TMRU) of the University of York. Two extended visits to the region were made during 1983, one before the period of most active upwelling (Phase I: March to May 1983), and one

Published online 01 Aug 2012 94 P .J. Hogarth

after (September and October 1983). A further visit was made during November 1985 (Phase III). The Anomura of Dhofar have been described by HoGARTH (1988), and a description of the area and its crustacean fauna in general has been presented by HoGARTH (1989). In this study information on the xanthoid crabs is presented. The classification of SERENE (1984) has been used, whereby the former family Xanthidae has been established as a superfamily Xanthoidea, comprising the families Xanthidae, Pilumnidae, Menippidae and Trapeziidae, corresponding to previous subfamilies. A few specimens have so far resisted identification, and may prove to be new species; this work will be published separately.

COLLECTING SITES

Four principal rocky shore collecting sites were chosen, as follows: (1) Mughsayl (16°52'20"N, 53°46'15"E). This site consists of an area of fissured, eroded and undercut sedimentary rock along the top of the shore, forming a promontory, and a flat shelving rock platform sloping gently from upper to lower shore. During Phase I (before the seasonal upwelling) the rocks were bare; during Phase II they were almost entirely covered in algae. (2) Raysut (16°55'40"N, 53°49'40"E). The principal Raysut site consists of sedimentary rocks, with undercut rock at the top of the shore and a shelving rock platform below. In addition, collections were made from a shelving pebble beach at Raysut, and from among boulders adjacent to the Raysut harbour mole. (3) Mirbat (16°58'20"N, 54°41'40"E). This is an area of igneous/metamorphic rock outcrops. Wave erosion has resulted in beaches of different forms to those of Mughsayl and Raysut. No rock platform is present, the shore consisting of a jumble of boulders, smaller stones, and bedrock. (4) Sadh (17°03'00"N, 55°04'25"E). The shore resembles that at Mirbat. Collections were made at Sadh itself, and also at Wadi Haart, approximately 19 km to the northeast. Additionally, small numbers of specimens were collected form more northerly parts of Oman, including Hasik, Ras al Hadd, Kuria Muria, Masirah Island, and in the vicinity of Muscat. The locations of these collecting sites are fully described in HoGARTH (1988, 1989).

COLLECTIONS Family Xanthidae

Actses polyscsnths (Heller 1861)

Mirbat Phase I (1 1'), Phase II (1 1'), Mughsayl Phase II (2 I' I', 1 juvenile); shallow sublittoral. Widely distributed in the Indopacific, from the Red Sea, East and southern Africa to Japan and Australia (SERENE 1984).

Actseodes tomentosus (H. Milne Edwards 1834)

Among stones and boulders, upper to lower shore: Raysut harbour mole, Phase II (1o, 11'), Mirbat Phase I (1 o, 1 1'), Phase II (1 o, 5 1'1', 1 juvenile) and Phase III (1 o). Distributed widely, from the Red Sea to southern Africa, Japan, Australia and Hawaii (SERENE 1984). Xanthoidea of southern Oman 95

ChloroJiella cytherea (Dana 1852)

Mirbat, Phase II (1 '1), Phase III (1 iS, 2 'I '1), shore, among stones; Sadh, Phase II (1 iS, 2 'I '1), Phase III (1 'I); shallow sublittoral, on living or dead coral. GUINOT (1966) describes the geographical distribution as including the Red Sea, Seychelles, Madagascar, Mauritius and Reunion. This distribution is uncertain, as the species has in the past been confused with C. nigra. It has certainly been recorded from Madagascar and the Comoros (SERENE 1984), Somalia and Djibouti (GAUL & VANNINI 1990).

ChloroJiella nigra (ForsskaJ. 1775)

Masirah, Phase I (2 iS iS); sublittoral (1 m) on branching corals. FoREST & GUINOT (1961) and SERENE (1984) discuss the variation in C. nigra and other species of the genus. Due to misidentifications in the past, the geographical distribution of C. nigra is uncertain, but the species probably occurs from the Red Sea, Madagascar and South Africa to Hawaii and Tuamotu (FoREST & GUINoT 1961, GALIL & VANNINI 1990).

Cymo andreossyi (Audouin 1826)

On branching corals in shallow sublittoral: Masirah (1 c), Mirbat (1 c) and Hasik (1 c), Phase I, Sadh (1 c) Phase II; on Stylophora. An obligate commensal of branching corals, most commonly Acropora and Pocillopora: recorded from the Arabian Gulf, Red Sea to South Africa, Japan and Australia (SERENE 1984).

Cymo melanodactylus De Haan 1833

Sublittoral, on Acropora Sadh, Phase I (1 c). Widespread Indo-West Pacific distribution, frequently occurring sympatrically with C. andreossyi; in the western India Ocean it has been recorded from Madagascar, Mauritius, Mayotte, Madagascar and Djibouti. As with C. andreossyi, this species is an obligate commensal of branching corals (SERENE 1984).

Epiactaea margaritilera (Odhner 1925)

Raysut, sublittoral at 10 m, Phase II (1 c). A little-known species that has in the past been confused with E. nodulosa, recorded from Aden and Djibouti, Mayotte and Aldabra to the Gulf of Siam and Australia (NoBILl 1906, GUINOT 1958, SERENE 1984). 96 P.J. Hogarth

Etisus snaglyptus H. Milne Edwards 1834

Intertidal among boulders, Mirbat, Phase III (1 o). Widely distributed in the Indo-West Pacific, from the Red Sea, Arabian Gulf, Seychelles and Madagascar to Japan and Australia (SERENE 1984, GALU.. & VANNINI 1990).

Etisus electra (Herbst 1801)

Mirbat, midclle and lower shore, Phase II (1 o), Phase III (1 o). Common and widely distributed, from the Red Sea and Gulf of Aden, Somalia, East and South Africa to Japan, Australia and Hawaii (SERENE 1984, GALU.. & VANNINI 1990).

Lachnopodus subacutus (Stimpson 1858)

Intertidal: Raysut Harbour mole, Phase II (1 o), Mirbat Phase III (1 'i'). Confusion over the identity of L. subacutus has been resolved by FoREsT & GUINOT (1961). The geographical distribution appears to extend from the Red Sea, Madagascar and Mauritius to the Ryukyu Islands and Samoa.

Leptodius exuatus (H. Milne Edwards 1834)

Very common under stones between tides; probably the most abundant xanthoid crab. Masirah (4 o o, 1 'i'), Mirbat (1 o) and Sadh (5 o o, 3 'i' 'i', 2 juveniles) Phase I; Mughsayl (1 o, 1 'i'), Raysut (38 oo, 18 'i"i', 5 juveniles), Mirbat (8 oo, 19 'i' 'i', 4 juveniles), Sadh (8 oo, 14 'i' 'i', 3 juveniles), Muscat (2 o o, 1 'i') Phase II, Mirbat Phase III (3 o o, 4 'i' 'i'). Widely distributed throughout the Indo-West Pacific region, from the Red Sea, East and southern Africa to Japan, Hawaii and Australia (SERENE 1984).

Leptodius gracilis (Dana 1852)

Lower shore, Ras al Hadd (1 'i') Phase I. Widely distributed in the Indo-West Pacific, from the Red Sea, Seychelles, Mauritius, Madagascar and Mozambique to Hawaii (SERENE 1984, GALn. & VANNINI 1990).

Liomera cinctimsna (White 1847)

Mirbat, Phase II (1 o). Scarce, among coral debris on lower shore and in immediate sublittoral. Xanthoidea of southern Oman 97

Distributed from Aden to East Africa, Japan, Australia, California and the Galapagos (SERENE 1984, GAUL & VANNINI 1990).

Liomers mgsts (H. Milne Edwards 1834)

Raysut Harbour mole (1 o), Mirbat (3 o o, 3 'i! 'i!), Sadh (1 o), among rocks and boulders; Phase II. Distribution: Red Sea, Djibouti, Aden and the western Indian Ocean, including Mauritius, Reunion, Chagos, Madagascar and South Africa (SERENE 1984, GAUL & VANNINI 1990).

Lybis plumoss Barnard 1947

Mughsayl, Phase II (1 o, 1 'i!). In crevice of rock platform. South Africa, Mozambique, Madagascar, Mauritius, Somalia, Aldabra and the Pacific (Moorea) (SERENE 1984, GAUL & VANNINI 1990).

Msuomedseus crsssimsnus (A. Milne Edwards 1867)

Middle and lower shore and shallow subtidal: Mughsayl Phase II (1 o, 4 juveniles), Mirbat Phases II and III (6 oo, 4 'i! 'i!). This species has been confused with other species of Macromedaeus, particularly M. quinquedentatus (BUITENDIJK 1960, SERENE 1984). Recorded geographical distribu tions must be viewed with reservations. SERENE (1984) believes the species not to have been found in the Western Indian Ocean. On grounds of carapace, cheliped, and male pleopod morphology, however, some of the specimens from Dhofar do belong to this species: for descriptions see under M. quinquedentatus below.

Mscromedseus quinquedentstus (Krauss 1843)

Middle and lower shore and shallow sublittoral: Mughsayl Phase II (1 o); Mirbat Phase I (1 juvenile), Phase II (2 o o, 1 'i!), Phase III (1 'i!). SERENE (1984) describes the nodular appearance of this species as "assez varia ble", a statement confirmed by the present material. Specimens of Macromedaeus from Dhofar appeared to fall into two distinct and non-overlapping morphological groups. In the first, carapace lobes 2M and 3M were more prominent, with a height: breadth ratio of 0.27-0.50; lobes 11, 31 and 41 were almost conical, with height and breadth approximately equal. In the second group, the height:breadth ratio of lobes 2M and 3M ranged between 0.17 and 0.27, while in lobes 11, 31 and 41 height was clearly less than breadth and the lobes were low and rounded. In individuals of the second group, a small subhepatic tubercle, situated anterior ly to the first of the principal anterolateral teeth is clearly visible from above. In the 98 P .J. Hogarth first group this tubercle lies below the first anterolateral tooth and is invisible from above. This feature is described by BUITENDIJK (1960). In members of the first group, the upper surface of the cheliped propodus is divided into conspicuous rather granular nodules, there are two longitudinal rows of relatively inconspicuous nodules on the outer palm, and conspicuous longitudinal grooves on the finger. In individuals of the second group the surface of the cheliped propodus is almost smooth, the only sculpture being in the form of shallow transverse depressions. The outer face of the palm is virtually smooth, and t~e fingers tend not to have conspicuous longitudinal grooves. The above morphological characteristics defining two groups of Macromedaeus accord with the descriptions of M. quinquedentatus and M. crassimanus, as described by BUITENDIJK (1%0). Moreover, the first pleopod of males of the first group closely resembled that of M. quinquedentatus, as illustrated in SERENE (1984). In males of the second group, the pleopods resembled that of M. crassimanus. Both species are therefore present in Dhofar. In view of confusion between the two species, described geographical ranges may be unreliable. SERENE describes M. quinquedentatus as occurring in Madagascar and from South Africa to Hawaii; TITGEN (1982) records it at Mauritius and GALIL & VANNINI (1990) in Somalia. Its occurrence in Dhofar therefore extends the known range within the Western Indian Ocean.

Medaeops neglectus (Balss 1922)

Shore, among stones: Raysut, Phase I (1 juvenile), Mughsayl, Phase II (1 'i'), Sadh, Phase II (1 c); from corals in shallow sublittoral. This species has been confused with Xantho disti,zguendus and Medaeus granulosus, and recorded distributions are therefore doubtful (GUINOT 1967). SERENE (1984) records the species from Madagascar, and indicates a distribution restricted to the Red Sea and western Indian Ocean.

Nanocassiope alcocki (Rathbun 1902)

Sublittoral; Sadh, Phase II (1 c). Typically found on the outer reef slope, at depths of up to 460 m; it has been recorded from the Seychelles and Maldives (SERENE 1984).

Psractaea rufopunctata (H. Milne Edwards 1834)

Sublittoral, on branching coral; Sadh, Phase II (1 c, 1 'i'). GUINOT (1969) describes a number of forms of P. rufopunctata in addition to the typical form, cited as P. rufopunctata rufopunctata. The present specimens do not differ significantly from the typical form. Paractaea rufopunctata (s.l.) is found throughout the Indopacific and Atlantic (GUINOT 1969, 1976; MANNING & HoLTHUIS 1981; SERENE 1984). Xanthoidea of southern Oman 99

Phymoclius granulatus (Targioni Tozzetti 1877)

Raysut Harbour mole, Phase II (1 o, 2 'i' 'i', 1 juvenile); Raysut (sublittoral at 10m) Phase II (1 o, 3 'i' 'i'); Mirbat Phase III (low shore) (4 oo, 9 'i' 'i', 2 juveniles); Sadh (sublittoral at 2 m, on live and dead Stylophora) (2 'i' 'i'). GUINOT (1964) suggested that the species was restricted to the Red Sea and Arabian Gulf, with records from the Seychelles and Madagascar requiring to be verified. More recently, the species has been recorded from Somalia (GALIL & VANNINI 1990).

Plloclius lll'eolatus (H. Milne Edwards 1834)

Shore: Mirbat, Phase I (2 o o, 1 juvenile), Phase II (2 o o, 1 'i', 1 juvenile) and Phase III (1 o); Sadh Phase II (1 o). Widely distributed in the Indo-Pacific Region, from the Red Sea and East Africa and South Africa to Australia, Japan and Hawaii (SERENE 1984, GAUL & VANNINI 1990).

Plloclius spinipes Heller 1861

Raysut Harbour mole, Phase II (3 oo, 1 'i', 1 juvenile); Mirbat shore, Phase II (1 juvenile), Phase III (2 oo, 1 'i', 1 juvenile); Muscat Phase II (1 'i'). Sublittoral: Raysut, Phase II (at 10m) (1 o, 1 juvenile), Sadh Phase II (on living and dead Stylophora) (7 o o, 4 'i' 'i', 7 juveniles). This species has been confused with P. pugil, leading to uncertainty over its geographical distribution. SERENE (1984) regarded P. spinipes (s.s.) as occurring in the Red Sea and Australia, with a similar species, described as Pilodius a££. spinipes more generally distributed in the western Indian Ocean.

Platypodia anaglypta (Heller 1861)

Shore, Mughsayl, Phase I (1 juvenile); Mirbat, Phase II (1 juvenile). Red Sea and Western Indian Ocean (Somalia, Madagascar, Mauritius, Chagos) to Japan and Australia (SERENE 1984, GALIL & VANNINI 1990).

Psaumis cavipes (Dana 1852)

Shore: Mirbat, Phase III (1 o). Indo-West Pacific; recorded from the Red Sea, Tanzania, South Africa, Mada gascar, Mauritius, Reunion, Mayotte, Chagos, Seychelles (SERENE 1984, GALIL & VANNINI 1990). 100 P .J. Hogarth

Xsnthias sinensis (A. Milne Edwards 1867)

Shore: Mughsayl, Phase II (1 'i'), Raysut Phase II (1 'i', 1 juvenile); Mirbat, Phase II (2 & &) and Phase III (1 &); Sadh Phase II (1 'i') and Phase III (1 & , 3 juveniles). Sublittoral at 10m: Raysut Phase II (1 &). A rare species; apart from the original type specimen from China, it has been known only from the syntypes from Karachi, and records from the Red Sea, Somalia and the Laccadives (SERENE 1984, GAUL & VANNINI 1990). In Dhofar it appears to be relatively common.

Zozymodes cavipes (Dana 1852)

Raysut (3 & & , 6 'i' 'i'), Mirbat (5 & & , 5 'i' 'i' , 2 juveniles) and Sadh (3 & & , 2 'i' 'i', 2 juveniles), under boulders and on rockflat; Phase II. Widely distributed in the Indo-West Pacific, from the Red Sea, Chagos and Aldabra to Japan and Australia) (SERENE 1984).

Zozymodes xanthoides (Krauss 1843)

Middle to lower shore, among stones and boulders, or in rock crevices: Masirah Phase I (1 'i') and Ras al Hadd Phase I (2 &&, 10 'i''i'), Raysut Phase II (19 &&, 8 'i''i', 15 juveniles). Common at Madagascar in interstices within reefs: known otherwise from the western Indian Ocean (Somalia and South Africa) and Red Sea (SERENE 1984, GALIL & VANNINI 1990).

Family Pilumnidae

Actumnus digitalis (Rathbun 1907)

Shore: Mirbat, Phase III (1 &). Apparently widespread in the Indo-West Pacific region, although the only record from the Western Indian Ocean so far appears to be from Mauritius (TAKEDA & MIYAKE 1969).

Actumnus setiler (De Haan 1833)

Sadh, Phase III (1 &). Widely distributed in the Indo-West Pacific, from the Red Sea, Arabian Gulf and East Africa to Australia, Japan and Tahiti (TAKEDA & MIYAKE 1969, TITGEN 1982). Xanthoidea of southern Oman 101

Pilumnopeus laevis Dana 1852

Mirbat Phase II (1 S, 6 'i' 'i'); Raysut Phase II (1 S); Muscat (al Bustan) Phase II (1 S); ?Sadh (label badly damaged and illegible) Phase III (1 S, 2 'i' 'i'). Distribution: Red Sea, Arabian Gulf and Pakistan (TITGEN 1982).

Pilumnus propinquus Nobill 1905

Shore: Raysut, Phase II (1 S). Distribution: Red Sea, Gulf of Aden, Arabian Gulf, Aldabra (GUINOT 1966).

Pilumnus savignyi Heller 1861

Low shore: Mirbat, Phase II (2 juveniles) and III (1 juvenile); Muscat (al Bustan) Phase II (1 juvenile). Distribution: Arabian Gulf, Red Sea, Gulf of Aden, Aldabra (GUINOT 1966).

Pilumnus vespertilio (Fabricius 1793)

Shore and sublittoral, to 10 m; Masirah Phase I (1 S, 1 'i', 1 juvenile); Sadh Phase I (2 'i' 'i'); Mughsayl Phase I (1 S, 1 'i'), Phase II (5 S S, 2 'i' 'i'); Raysut Phase I (1 'i'), Phase II (1 S); Mirbat Phase II (8 S S, 5 'i' 'i', 3 juveniles) and Phase III (2 S S, 1 'i', 2 juveniles). Common and widespread throughout the Indo-West Pacific Region, including the Red Sea and East Africa (TAKEDA & MIYAKE 1968, TITGEN 1982).

Parapilumnus quaclridentstus (De Man 1895)

Phase II: Mughsayl (1 S, 1 juvenile); Raysut (1 S); Mirbat (2 'i' 'i'); Muscat (al Bustan) (1 S): middle and upper shore. Distribution: Red Sea and Southeast Asia (Thailand, Malaysia, Borneo) (GUINOT 1966, M. TAKEDA pers. comm.).

Family Menippidae

Epixanthus frontalis (H. Milne Edwards 1834)

Sadh (1 S, 2 'i' 'i'); Mirbat (1 S, 1 'i'); Muscat (1 'i'): upper shore among stones; all Phase II. Widely distributed in the Indo-West Pacific, from South Africa, East Africa and the Red Sea to Australia, Melanesia, the Philippines and Japan (GARTH & KIM 1983, GALIL & VANNINI 1990). 102 P .J. Hogarth

Eriphia smithi McLeay 1838

Mughsayl Phase I (1 c, 1 '?, 1 juvenile); Raysut Phase I (4 '? '?, 2 juveniles); Sadh Phase I (2 c c, 1 '?), Phase II (1 c); Muscat Phase II (1 '?); Kuria Muria Phase II (1 '?). Distribution Indo-Pacific, including the Western Indian Ocean, Tanzania, Ma dagascar, Mozambique and South Africa (GAUL & VANNINI 1990).

Lydia tensx (Riippell 1830)

Shore, among boulders: Sadh Phase I (1 '?); Ras al Hadd Phase I (1 c); Hasik Phase I (1 '?); Mirbat Phase II (1 '?). Lydia tenax ha~ previously been recorded only from the Arabian Gulf and the Red Sea. Dhofar therefore fills in the distribution of this species between these two areas. Records from Zanzibar and other locations in the Western Indian Ocean have been questioned, since this species is similar to L. annulipes and has probably been confused with it. SERENE (1984) casts some doubt on the separation of the two species.

Pseudozius csystrus (Adams & White 1848)

Abundant under stones on upper and middle shore, Mughsayl Phase II (10 '? '?); Raysut Phase II (35 c c, 41 '? '?); Mirbat Phase II (4 co, 8 '? '?, 1 juvenile); Sadh Phase I (3 c c, 3 '?'?),Phase II (31 c c, 16 '? '?); among mangroves (Avicennia marina) near Muscat, Phase II (1 c, 1 '?). P. caystrus is a highly variegated species, which shows remarkable diversity of carapace pattern and colour. Its position within the family Menippidae is questionable (SERENE 1984). Widely distributed throughout the Indo-West Pacific (SERENE 1984).

Sphaerozius nitidus Stimpson 1858

A single juvenile, found on the Mughsayl shore (Phase II) is tentatively identified as S. nitidus. Widely distributed in the Indo-West Pacific (SERENE 1984).

Family Trapeziidae

Trapezia cymodoce (Herbst 1801)

Mirbat Phase I (1 c); Sadh Phase I (2 c c, 2 '?'?);Phase II (3 c c, 4 '? '?, 4 juveniles), Phase III (4 c c, 6 '? '?, 6 juveniles); Ras al Hadd Phase I (1 '?, 1 juvenile), Muscat (al Bustan) Phase II (1 c): sublittoral, on Stylophora and Acropora. An obligate commensal of branching corals: widespread in the Indo-Pacific, from the Red Sea and East Africa to Japan and Polynesia (GAUL 1987). Xanthoidea of southern Oman 103

Trapezia tigrina Eydoux & Souleyet 1842

Sadh (1 ~, 1 '1), Phase II; sublittoral, on branching coral. An obligate commensal of branching corals. Confusion between this and similar species of Trapezia was cleared up by GALIL & LEWINSOHN (1984): T. tigrina is widely distributed throughout the Indo-Pacific (GALIL 1987).

Tetrslia csvimtUJa Heller 1861

Sadh, Phase II (1 ~, 1 '1, 1 juvenile); sublittoral, on Acropora. An obligate commensal of branching corals. There has been considerable confu sion between various proposed species of Tetralia. The specimens from Dhofar correspond to Tetralia cavimana as described by GAuL (1987), corresponding to T. glaberrima as recognised by SERENE (1984), and divided by him into a number of subspecies. The distribution of T. cavimana (sensu GALIL 1987) is restricted to the Red Sea and Arabian Gulf; thus the present record from Dhofar links these two locations. Tetralia glaberrima (s.l.) is widely distributed in the Indo-West Pacific (SERENE 1984).

DISCUSSION

The presence in Dhofar of a total of 43 species of xanthoid (Xanthidae 28, Pilumnidae 7, Menippidae 5 and Trapeziidae 3) is recorded for the first time. In most cases the known geographical range of species is extended only marginally, but in a few cases (Nanocassiope alcocki, Actumnus digitalis and Macromedaeus crassimanus) the range extension is substantial. There is little sign of the high degree of endemicity recorded for the molluscs of the region by TAYLOR & SMYTHE (1985). The number of species collected in the pre-monsoon period (Phase I) was 17 (12 Xanthidae, 1 Pilumnidae, 3 Menippidae, 1 Trapeziidae). In the post-monsoon collec tions (Phases II and Ill), the numbers of species in these families were, respectively, 23, 7, 5 and 3, totalling 38. This apparent increase in species richness is in part attributable to ecological changes resulting from the monsoon-driven upwelling (Ho GARTH 1989), but may also be due to differences in intensity and techniques of collecting. In order to compare the composition of the Dhofar xanthoid fauna with that of other locations within the Western Indian Ocean, Ochiai Coefficients of Similarity were calculated where the Coefficient c

..J [(c +a) (c +b)]

(a is the number of species found at site A but not at site B, b the number of species at B but not A, and c the number of species recorded at both sites). The use of Coefficients of Similarity in faunal comparisons is discussed in CLIFFORD & STEPHEN- 104 P.J. Hogarth

SON (1975) and HOGARTH (1988). Comparisons were made only with respect to the Xanthidae, the other families of the Xanthoidea being represented by too few species for valid comparison. Information on distributions was obtained from GUINoT (1966), TrTGEN (1982), and the references cited for each species above, supplemented with the author's personal collections from the Maldives (publication in preparation). The xanthid faunal composition of Dhofar shows most similarity with, in order, that of Somalia, the Arabian Gulf, Mozambique, the Maldives, the Gulf of Aden, and the Red Sea (So' = 0.517, 0.509, 0.474, 0.459, 0.457 and 0.457, respectively; Total identity of species gives So' = 1, lack of any species in common So' = 0). Cluster analysis of the Coefficients of Similarity, using the group average method discussed by EvERITT (1974), is shown in Fig. 1. From the distribution of coral species, SHEPPARD & SHEPPARD (1991) (quoted in SHEPPARD et al. 1992) inferred the partial separation of the Western Indian Ocean into four subregions: (1) the Arabian peninsula; (2) East and southern Africa together with Madagascar and the other islands of the Western Indian Ocean; (3) Pakistan, western India and Sri Lanka; and

MAU li SEY

.---- SOM rl MAD '- RED EAF

I MOZ I SAF MAL

SRL ,--- WIN I I PAK ARG I I DHO

COM I I GAD CHA

I I I I I I I ~ I 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Fig. 1. -Cluster analysis of the distribution of species of Xanthidae at Western Indian Ocean locations, based on Ochiai Coefficients of Similarity. (Key to locations: MAUritius, SEYchelles, SOMalia, MADa· gascar, RED Sea, East AFrica, MOZambique, South AFrica, MALdives, SRi Lanka, Western INdia, PAKistan, ARabian Gulf, DHOfar, COMoros, Gulf of ADen, CHAgos). Xanthoidea of southern Oman 105

(4) the Maldive Islands, linked with parts of the Eastern Indian Ocean. Fig. 1 does not show strong clustering, but the pattern resembles that demonstrated by SHEPPARD & SHEPPARD. The most conspicuous anomaly is the Red Sea, which appears in its xanthid fauna to resemble Madagascar and East Africa more closely than the geogra phically closer areas of the Gulf of Aden and Dhofar. Fig. 2 shows the distribution of Coefficients of Similarity, a method previously used in HoGARTH (1988) in relation to Paguridea and Porcellanidae, and discussed by MAGURRAN (1988). This indicates the degree of heterogeneity in species distribution (parochiality) within a taxonomic group. The analysis includes only collecting loca tions where a minimum of 10 relevant species have been recorded. Data for Trapeziidae has been treated in the same way (Fig. 3). In this instance, the small number of trapeziid species found in Dhofar does not vitiate the analysis, which is based on other locations within the Western Indian Ocean, where a minimum of 10 trapeziid species has been recorded. With respect to the other families of Xanthoidea, too few collecting sites fulfilled this criterion for analysis to be justifiable. Tables 1 and 2 summarise the data displayed in Figs 2 and 3 and the comparable data relating to Paguridea and Porcellanidae already published (HoGARTH 1988). As a taxonomic group, Xanthidae are more cosmopolitan (more geographically homogen eous in species distribution) than pagurids, and less so than porcellanids. Trapeziidae are even more cosmopolitan than Porcellanidae. The relative degree of heterogeneity in species distribution of porcellanids and pagurideans does not appear to relate closely to larval dispersal ability (HoGARTH 1988). Unfortunately, published information on larval durations in the Xanthidae is too sparse for any conclusions to be drawn in relation to the present data.

Distribution of Coefficients of Similarity Xanthidae (17locations) 30 r------,

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Ochiai Coefficient Fig. 2. - Distribution of Ochiai Coefficients of Similarity for comparison of Xanthidae found at 17 Western Indian Ocean sites. (Only sites where 10 or more species have been recorded are included). 106 P .] . Hogarth Distribution of Coefficients of Similarity Trapeziidae (7 locations)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Ochiai Coefficient Fig. 3. - Distribution of Ochiai Coefficients of Similarity for comparison of Trapeziidae found at seven Western Indian Ocean sites. (Only sites where 10 or more species have been recorded are included).

Table 1. Summary of distribution of values of Ochiai Coefficients of Similarity for comparison of Xanthidae, Trapeziidae, Paguridea and Porcellanidae species found at Western Indian Ocean sites.

Mode Mean

Xanthidae 0.325 0.379 Trapeziidae 0.525 0.511 Paguridea 0.125 0.231 Porcellanidae 0.425 0.455

Table 2. Pairwise comparison of the distribution of Ochiai Coefficients of Similarity of Xanthidae, Trapeziidae, Paguridae and Porcellanidae in the Western Indian Ocean, as presented in Figs 2 and 3 (Xanthidae and Trapeziidae) and in HoGARTH (1988) (Porcellanidae and Paguridea).

Xanthidae Trapeziidae Paguridea

Trapeziidae 26.67 Paguridea 117.61* 86.29* Porcellanidae 37.17 26.27 104.86*

X' values (* = P < 0.001). Xanthoidea of southern Oman 107

It has been suggested (e.g. BARNES & HuGHES 1982) that high rates of speciation might be associated with ecological heterogeneity, hence low parochiality might be expected to be associated with ecological homogeneity. The fact that trapeziid crabs are all obligate commensals of branching corals may be seen as supporting this suggestion, since a relatively high degree of similarity between geographical locations is found in association with the high degree of ecological uniformity implied by an obligate association with corals.

ACKNOWLEDGEMENTS

I should like to thank members of the Tropical Marine Research Unit (TMRU) of the University of York, and in particular Drs Lynne Barratt and Rupert Ormond. The TMRU survey of Dhofar was carried out in association with the International Union for the Conservation of Nature and Natural Resources, Geneva, and in cooperation with the Council for the Conservation of the Environment and Water Resources, Muscat, on behalf of the United Nations Environmental Programme, and the Regional Office for the Protection of the Marine Environment, Kuwait. I gratefully acknowledge the help of a number of colleagues, particularly Ray Ingle and Paul Clark of The Natural History Museum, London for access to the Museum's collections and for lending material for comparison; Bella Galil of Israel Oceanographic and Limnological Research Ltd, Haifa, for assistance in identifying trapeziids, Masatsune Takeda for help with pilumnids, and the Museum National d'Histoire Naturelle, Paris, for the loan of specimens.

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