To refer to this article use this url:

Contributions to Zoology, 68 (3) ..-.. (1999)

Consequences of the discovery of Stenasellus (Crustacea, Isopoda, Asellota) in the underground waters of Oman (Arabian Peninsula)

Stygofauna of Oman, 4*

Guy J. Magniez , Jan H. Stock (†)

Université de Bourgogne, Dept. Biologie Animale Bd. Gabriel, 21000, Dijon, France;

Institute for Systematics and Population Biology, University of Amsterdam P.O. Box 94766, 1090 GT Amsterdam, The Netherlands

Keywords: Arid climate, cohabitation of species, ecotones, gen. Stenasellus, , marine ancestors, paleobiogeography, Quaternary glaciations, recent expansion, underflow.


An overview of the present knowledge of the family Stenasellidae (Asellota: Aselloidea) is presented, focusing on the success of recent investigations of the underground waters of Oman (SE Arabian Peninsula). We take up the discovery of four new species of the genus Stenasellus : small species adapted to interstitial biotopes and larger ones typical of karstic aquifers. These species cohabit in ecotones between interstitial and karstic water systems. The putative presence of stenasellids in hypogean waters of Yemen and South Iran is suggested. The taxonomic status of the genus Stenasellus auct. needs to be reconsidered in the future A scenario to explain the origin and the presence of these Isopods in continental underground waters is advanced.


The family Stenasellidae Dudich, 1924

At present, this isopod family (Asellota: Aselloidea) includes 10 genera and more than 70 taxa. All stenasellids are anophtalmous and lack integumental pigment, but they possess pink or red haemolymphatic pigment and live in continental underground waters. The smallest of the species is 2 mm long, the largest 26 mm. First discovered in 1896, in the sink-hole near Padirac, southwestern France, they also have been collected in many other places, in the Pyrenees, the Iberian Peninsula (1924), the Balkan (1924), Turkmenistan (1949), West Africa (1938), the Zaire Basin (1951), East Africa (1966), the Indochinese Peninsula (1971) and the western hemisphere from Mexico and Texas (1972) (cf. Coineau et al., 1994; Henry et al., 1986; Lanza, 1966; Magniez, 1983).

The investigation of underground waters in Oman

Between March, 23, 1996 and April, 9, 1996, Prof. J. H. Stock and Mr. J.J. Vermeulen made a very accurate survey of underground waters in the region of Jabal Akhdar (between J. Nakhl, Rostaq and Nizwa, SW of the city Muscat, Oman). Sampling was done mainly in the coarse alluvium of temporary surface streams (wadis), using the Bou-Rouch pump system, and also in some springs and wells. Similar sampling was undertaken by Dr Wewalka, February 17-22, 1998. Stenasellids were present in 42 of the localities and point to the fact that the settlement of these stygobitic isopods is very important in this southeastern corner of the Arabian Peninsula (Fig. 1).


Fig. 1. Map with the localisation of the East African and Asiatic species of the genus Stenasellus Dollfus, 1897, car. emend. Racovitza, 1924.

The species of Oman: ecology and taxonomy

The four species

The study of these numerous samples (more than 42, as certain localities were sampled several times) show that 4 different species (Fig. 2) exist in this area. All belong to the genus Stenasellus Dollfus, 1897, car. emend. Racovitza, 1924, the same genus already represented in southwestern Europe, Kenya, Somalia and the Far East (Fig. 1) by some 34 taxa. These 4 new species are to be described in a separate paper (Magniez & Stock, in prep.). Their provisional appellations are:

Stenasellus sp. A -- average size 12 mm (may be longer), present in 10 localities;

Stenasellus sp. B -- average size 8 mm, present in 2 localities;

Stenasellus sp. C -- average size of adults 3-4 mm, present in 32 localities;

Stenasellus sp. D -- average size of adults 2-2.5 mm, present in 15 localities.

Some 28 of the localities harbor only one species, 11 others show the cohabitation of 2 species, and the remaining 3 localities contain 3 species together. Cooccurrences of stenasellid species were already known from cave waters, viz., Mexistenasellus wilkensi and M. parzefalli Magniez, 1972; and in interstitial biotopes: -in the phreatic layers of Guinea-Bissau or Ivory-Coast viz., Parastenasellus chappuisi (Remy, 1938) and Magniezia guinensis (Braga, 1950) or M. africana (Monod, 1948); -in the water-table of the Guadalquivir River valley, SW Spain, viz., Stenasellus escolai Magniez, 1976 and S. bragai Magniez, 1977. Nevertheless, we bring here the first record of the presence of three species in the same biotopes. This situation requires the study of each specimen of each sample, to avoid any confusion between adults of dwarf species with juveniles of large ones!


Fig. 2. Relative size of the adults of the four species of Stenasellus living in the underground waters of Oman.

Ecological relations

The study of the material shows that the samples of Stenasellus sp. C and Stenasellus sp. D include a large number of specimens, juveniles and adults. Among these, numerous females in reproductive intermolt, their brood pouches full of embryos, or empty after release of the larvae. This observation points to two facts: 1) We really know the size of these species (females of 2 mm of S. sp. D and females of 3 mm of S. sp. C are in reproductive stage). More important, 2) The underflow of the wadis represents the true biotope of these two species, as it harbours true populations. However, the large species, S. sp. A and S. sp. B are present only in a few number of localities. The composition of their samples is never representative of a population: we find them for the most part only as young stages (with sizes similar to adults of S. sp. C or S. sp. D ), and only a small number of larger subadults and “young adults”. We noticed that females with a brood pouch and even females with small oostegites are absent. We conclude two things. 1) The normal adult size of these species is larger than the size of the largest specimen captured. For instance, we think that for S sp. A, the maximum size could be similar to that of the karstic European form Stenasellus buili Remy, 1949 (14 mm for the male and 15-16 mm for the female). 2) The samples are not taken in the true biotope of these species, as they are not representative of a normal population. So, these samples are composed of erratic specimens, wandering in an ecotone, beyond their actual biotope. This region of Oman is rich in limestone an has caves with a karstic aquifer with even troglobitic fish in some of them. We can consider that the two large stenasellid species are actually karstic taxa and propose a scheme (Fig. 3) to explain the local occurrences together between the little interstitial and the larger troglobitic species. The communication between a karstic aquifer and an alluvial habitat is well known by hydrogeologists, and permits the expansion of stygobitic species from one type of underground aquifer to the other (cf. Magniez, 1979 and Fig. 4).


Fig. 3. Sampling with the Bou-Rouch pump (strongly magnified) in the alluvial deposits of a wadi. Local cohabitation of dwarf interstitial species and juveniles of large karstic species in the ecotone between free water of limestone cavities and interstitial water of alluvial gravel.


Fig. 4. Ecological relations between the four Stenasellus species of Oman; M = main biotopes, E = ecotones.

Taxonomic relations

Even though they are genetically independent (specific morphological characters and sizes quite different), these four species appear closely related to each other; obviously, they arose from a single original stock.

When descripting new species of Stenasellus from the Far-East (Cambodia, Borneo, Thailand, Sumatra), we were struck by their strong relationship with those of East Africa (Kenya and Somalia). The Oman species, geographically situated between the East-African and the Far-Eastern ones, fully belong to this taxonomic entity. Moreover, we can say that the species of East Africa, the Arabian Peninsula and may be the species Stenasellus asiaticus Birstein & Starostin, 1949 from a thermal brook in southern Turkmenistan represent a single center of settlement (Fig. 5).


Fig. 5. Paleogeographical map of the African and Arabian region during the upper Cretaceous, showing the putative installation and expansion of Stenasellus ancestor in continental ground waters (the colonisation of West Africa by ancestors of the African genera Magniezia, Metastenasellus and Parastenasellus is recalled, as given in Magniez, 1997, fig. 2).

So, the genus Stenasellus appears to be composed of three nuclei: 1) a western one in the Tyrrhenian region (Iberian Peninsula, S. France, Corsica, Sardinia, little part of W. Tuscany), 2) a central one: the horn of Africa (Messana, 1988) and Middle-East region, and 3) the Indochinese Peninsula and ancient Sunda continent (now reduced to islands) (Magniez, 1989, 1993). Thus the history of genus Stenasellus appears more complicated than that of West-African genera (Magniezia, Metastenasellus, Parastenasellus), which represent endemic taxa confined to paleobiogeographic units (Magniez, 1984, 1986, 1998).


Presence of stenasellids in Yemen and South Iran ?

Species of Stenasellus are present in Somalia + Kenya, the southeastern corner of the Arabian Peninsula and southeastern Turkmenistan. So we might ask if they are present in intermediate geographical regions. Yemen, southwestern Oman and southern Iran could harbour this fauna (Fig. 1). Ecological conditions indicate the existence of permanent underground aquifers, either as underflow in coarse grain alluvium (deposits of mountain streams), or in cavities in limestone, even at low altitude. This presence might be original and linked to the primitive expansion of the fauna after the colonisation, or, more recently, bound to a secondary expansion during intervals of wet climate or low sea-level.

This possibility concerns particularly southern Iran. We must not forget that, as late as 18,000 BP, the Persian Gulf was reduced to a series of small lakes and the Strait of Hormuz entirely emergent, offering a continuity of ground water between the Oman and Iranian aquifers. In order to reinforce this idea, we can invoke the existence of a similar system in the Far-East. Stenasellids are present in underground waters on the two shores of the Strait of Malacca, south on Sumatra, with Stenasellus covillae Magniez, 1987 and S. strinatii Magniez, 1991, and north on the Malayan Peninsula, with S. brignolii Pesce & Argano, 1981 and S. bedosae Magniez, 1991. However, recent exchanges of fauna across the Bab-El-Mendeb Strait don’t seem possible.

Unity or heterogeneity of the genus Stenasellus auct.

As stated above, the status of this genus, with its 17 taxa in southwestern Europe, 12 in the Middle East and 9 in the Far East, is difficult to understand. Two groups of species appear in the European stock: “virei” group and “breuili” group (Magniez, 1996). The “virei” group, Middle-Eastern and Far-Eastern species are so strongly related that we can imagine that they are derived from a same, but widely distributed marine ancestor of the Tethys that succeeded in three different passages to continental underground waters. It remains difficult to explain the present distribution, from Portugal to Borneo and to Kenya with a single settlement, as the paleogeography of the Mediterranean-Tethysian region is more complex than that of the African plates. The genus Stenasellus needs a general revision sometime in the future.

The best scenario for the evolution of Stenasellids in underground continental waters

We can summarize our hypotheses:

-The existence of a very low number of marine ancestors in the warm Tethys sea (these forms already anophtalmous, deprived of integumental pigment and adapted to life in sediments),

-The passage into continental, fresh or oligohaline underground waters (no intermediate step in surface waters), in a very few number of points (may be during Upper Cretaceous ?),

-The wide geographical expansion in all types of underground waters together with occurrence of subsequent speciation (during the major part of Tertiary Era), giving to the family a global distribution larger than the presently known one,

-The drastic reductions of these settlements, possibly destruction of entire species in two areas: in the temperate regions of the Northern Hemisphere by Quaternary glaciations (obvious in Europe, possible in North America and Asia, maybe with temporary reconquest of biotopes during Interglacial times), and in northern Tropical zone; with the development of arid climate (the case is obvious for West-African genera, possible for the Middle-Eastern settlement of Stenasellus),

-The recent expansion of some eurytopic species, adapted to life in cool water (Magniez, 1985), which rapidly colonized new interstitial biotopes developed in coarse post-glacial channels of alluvium, that is the case in southwestern Europe, in the underflow of the hydrographic basins north and south of the Pyrenees (Magniez, 1996).


Coineau N, Henry J-P, Magniez G, Negoescu I. 1994. Isopoda Aquatica. In: Juberthie Ch & Decu V, eds. Encyclopædia Biospeologica 1: 123-140.

Henry J-P, Lewis JJ, Magniez G. 1986. Isopoda: Asellota: Aselloidea, Gnathostenetroidoidea, Stenetrioidea. In: Botosaneanu L, ed. Stygofauna Mundi. Leiden: Brill, 434-464.

Lanza B. 1966. Stenasellus pardii sp.n. della Somalia e note sistematische sugli Stenasellinae (Crustacea Isopoda). Monit.Zool. Ital. 74: 221-256.

Magniez G. 1979. Les Sténasellides de France (Crustacés Isopodes Asellotes souterrains): faune ancienne et peuplements récents. Bull. Soc. Zool. Fr. (1978), 103: 255-262.

Magniez G. 1983. Biogéographie et paléobiogéographie des Sténasellides (Crustacés Isopodes Asellotes des eaux souterraines continentales). Mém. Biospéol. 10: 187-191.

Magniez G. 1984. Sur la biogéographie des Stenasellidae africains (Crustacés Isopodes stygobies). Mém. Biospéol. 11: 51-54.

Magniez G. 1985. Regressive evolution in Stenasellids (Crustacea Isopoda Asellota of underground waters). NSS Bull. 47: 118-122.

Magniez G. 1986. Répartition transsaharienne de Metastenasellus Magniez ; description d’une nouvelle espèce d’Algérie (Isopoda Asellota). Stygologia 2 : 267-275.

Magniez G. 1989. Les Stenasellidae (Crustacés Isopodes) et la vie insulaire. Mém. Biospéol. 16: 87-90.

Magniez G. 1991. Bons et mauvais caractères taxinomiques: exemple des Sténasellides d’Extrême-Orient. Mém. Biospéol. 18: 99-104.

Magniez G. 1993. Vie insulaire des Asellidae et Stenasellidae en Extrême-Orient. Mém. Biospéol. 20: 139-144.

Magniez G. 1996. Histoire des Stenasellidae ibéro-aquitains (Crustacea, Isopoda, Asellota, Aselloidea). Mém. Biospéol. 23: 173-179.

Magniez G. 1998. Réflexions sur les Asellidae et Stenasellidae d’Afrique du Nord. Mém. Biospéol. 24: 89-96 (1997).

Messana G. 1988. Stygobitic Isopods of East Africa. Biogeographia XIV: 113-124 (publ. 31/12/1990).

Racovitza EG. 1924. Diagnose des genres Asellus et Stenasellus et description de deux Stenasellus nouveaux. Bull. Soc. Sc. Cluj 2: 81-92.