The Isopods, with more than 11.000 described, terrestrial and aquatic species, are the second largest order of crustaceans. They are widely distributed across all types of habitats, from terrestrial to marine, fresh and ground waters. The aquatic taxa are grouped into seven sub-orders: Phreatoicidea, Anthuridea, Asellota, Calabozoidea, Sphaeromatidea, Valvifera and Cymothoidea. Except for the Valvifera, all the remaining sub-orders include genera and/or species which colonized, and still colonize, a broad spectrum of groundwater habitats. The most part of stygobitic taxa can be found among the Cymothoidea and Asellota.

The greatest part of groundwater isopods concentrate in the Caribbean and Mediterranean areas, whereas stygobitic Phreatoicidea show a typical gondwanian distribution.

As regards Italy, ground water isopods are represented by the suborders Asellota and Flabellifera, the former with the families Asellidae, Stenasellidae, Microparasellidae and Microcerberidae, the latter with two families, Sphaeromatidae and Cirolanidae.

The asellid isopods are widespread in surface fresh waters throughout the northern emisphere, reaching the southernmost limit in the regions of Mexico bordering on Guatemala; most species live, as well, in subterraean environments, and they generally show a very restricted or pin-pointed distribution.

Most of the ground water species seem to have been forced to leave surface habitats by temporary environmental changes, such as glaciations or droughts. For many stygobitic taxa of this family the starting point for subterranean evolution can be found among surface species existing today, but entire groups of species show no affinities with present day surface, oculate forms: this is generally considered as an indication that the second group is more arcaic.

A striking example of stygobitic subterranean asellids of the italian stygofauna is the recently discovered genus Chthonasellus, known on the single species C. bodoni, which lives in the hyporheic substrates of the Pesio river, near Como (Alpine province).

Two other genera of Asellidae are present in Italy: Asellus and Proasellus. Only one surface species of the former, which is an eurasian genus, has reached Italy: Asellus aquaticus. The genus Proasellus, on the other hand, is a typically peri-mediterranean taxon, with large number of stygobitic species living in Italy. Some belong to exclusively stygobitic groups of species, such as P. cavaticus and P. franciscoloi of the "cavaticus" group, which can be found in the Central European region, but probably of Alpine origin. Another group of italian species lacking epigean representatives and therefore boasting a long evolutionary history is the "pavani" group. It includes three species: P. pavani, P. acutianus and P. deminutus. The close affinities among these species and the others of the same group suggests a biological distribution that once embraced Corsica, as well as the karstic systems of Slovenia, with remarkably complex palaeogeographical implications.

At the same Proasellus genus belong also some italian species with different degree of adaptation to groundwater life, and closely related to the surface species Proasellus banyulensis (?).

The first specimen of stenasellid isopods was brought to light in rather bad conditions after an adventurous exploration of the Gouffre de Padirac at the end of the last century. It led to the description of the first species of stenasellids, Stenasellus virei, which long remained a "mysterious object" for scientists, until later discoveries were made.

At present, approximately sixty species, subdivided into ten genera, have been identified: all are completely stygobitic and have thermophilic tendencies; distribution ranges from central America to the Far East, with a large number of species living in Europe and Africa.

The above characteristics attest to the extremely archaic nature of this group. The evident affinities between American and African forms constitute a good example of the complexity of this group's palaeogeographic history. In fact, their distribution and the filogenetic relationships can only be explained by going back to the Turonian events that gave rise to the Atlantic Ocean.

Three species of stenasellids are presently known for Italy: Stenasellus racovitzai, Stenasellus assorgiai and Stenasellus nuragicus. S. racovitzai was discovered in 1925 in the muddy bottom of a lake in the "Grotta del Danese", near Grosseto (Tuscany). This isopod is rather large and brilliant red coloured, au unusual characteristic for a stygobitic species, but a rather common one for a stenasellid. The other two species of the same family, which are very similar each other, live in southern Sardinia, but in different caves.

The family Microparasellidae is present in Italy with six species, five of which belong to Microcharon, a pantropical genus widespread in Europe and in the Mediterranean basin, the other belongs to the genus Angeliera.

Microcerberidae, recently ascribed to the Asellota, is the last family of the suborder Asellota living in ground (interstitial) waters of Italy with the genus Microcerberus. Two euryhaline species of this genus, Microcerberus remanei and Microcerberus arenicola, are widespread along the sandy coastlines of Italy; another species, Microcerberus ruffoi is a typical fresh water species, living in the interstitial substrates of the Adige river basin (Alpine province).

Both families Sphaeromatidae and Cirolanidae (Flabellifera) embrace directly sea-originated species; the former includes several species of the genus Monolistra, the latter, beside typically marine taxa, contains species which secondarily colonized subterranean environments in different parts of the World. Two species of this family are at present known in Italy, viz.Sphaeromides virei and Typhlocirolana moraguesi.

[Numbers between brackets according to the stygofaunal provinces of Italy]
[Endemisms marked with an asterisk]

Family Asellidae
Genus Asellus

Asellus aquaticus cavernicolus Racovitza 1925 (1)

Genus Proasellus

Proasellus adriaticus Argano & Pesce 1979 (3) *
Proasellus acutianus Argano & Henry 1972 (3) *
Proasellus amiterninus Argano & Pesce 1979 (3) *
Proasellus cavaticus (Leydig 1871) (1)
Proasellus deminutus (Sket 1959) (1)
Proasellus faesulanus Messana & Caselli 1995 (3) *
Proasellus franciscoloi (Chappuis 1955) (1) *
Proasellus gardinii (Arcangeli 1942) (1) *
Proasellus intermedius Sket 1965 (1)
Proasellus ligusticus Bodon & Argano 1982 (1) *
Proasellus micropectinatus Baratti & Messana 1990 (3) *
Proasellus patrizii (Arcangeli 1952) (2) *
Proasellus pavani (Arcangeli 1942) (1) *
Proasellus slavus (Remy 1948) (1)
Proasellus vignai Argano & Pesce 1979 (3) *
Proasellus vulgaris (=deminutus?) (Sket 1965) (1)

Genus Chthonasellus

Chthonasellus bodoni Argano & Messana 1991 (1)

Family Stenasellidae
Genus Stenasellus

Stenasellus assorgiai Argano 1968 (2) *
Stenasellus nuragicus Argano 1968 (2) *
Stenasellus racovitzai Razzauti 1925 (2,3) *

Family Microparasellidae
Genus Microcharon

Microcharon angelicae Pesce & Galassi 1988 (3) *
Microcharon arganoi Pesce & Tetè 1977/78 (4) *
Microcharon marinus Chappuis & Delamare Deboutteville 1954 (2,3,5)
Microcharon nuragicus Pesce & Galassi 1988 (2) *
Microcharon silverii Pesce & Galassi 1988 (2) *

Genus Angeliera

Angeliera phreaticola Chappuis & Delamare Deboutteville 1954 (3)

Family Microcerberidae
Genus Microcerberus

Microcerberus arenicola Chappuis & Delamare Deboutteville 1954 (3)
Microcerberus remanei Chappuis & Delamare Deboutteville 1952 (3,4)
Microcerberus ruffoi Chappuis 1953 (1)

Family Cirolanidae
Genus Sphaeromides

Sphaeromides virei Brian 1923 (1)

Genus Typhlocirolana

Typhlocirolana moraguesi Racovitza 1905 (5)

Family Sphaeromatidae
Genus Monolistra
Subgenus Monolistra

Monolistra caeca Gerstaeckere 1856 (1) *

Subgenus Microlistra

Microlistra schottlaenderi Stammer 1930 (1) *

Subgenus Typhlosphaeroma

Typhlosphaeroma bericum (Fabiani 1901) (1) *
Typhlosphaeroma boldori Brian 1931 (1) *
Typhlosphaeroma lavalensis Stoch 1984 (1) *
Typhlosphaeroma pavani Arcangeli 1941 (1) *
Typhlosphaeroma racovitzai Strouhal 1928 (1) *

A selected World bibliography on subterranean aquatic isopods can be found in: Botosaneanu L., N. Bruce & J. Notenboom (1984). Isopoda:Cirolanidae. Stygofauna Mundi: 412.422; Sket B. (1984). Isopoda:Sphaeromatidae. Stygofauna Mundi: 423-427; Botosaneanu L. (1984).Isopoda: Anthuridea. Stygofauna Mundi: 428-433; Henry J.-P., Lewis J. J. & G. Magniez (1984). Isopoda: Asellota: Aselloidea, Gnathostenetroidoidea, Stenetrioidea. Stygofauna Mundi: 434-464; Coineau N. (1984). Isopoda: Asellota: Janiroidea. Stygofauna Mundi:465-472; Coineau N. (1984). Isopoda: Microcerberidae- Stygofauna Mundi:473-497; Van Lieshout S. E. N. (1984). Isopoda: Calabozoidea. Stygofauna Mundi:480-481; Knott B. (1984). Isopoda: Phreatoicidea. Stygofauna Mundi: 486-489; Coineau N., Henry J.-P., G. Magniez & I. Negoescu (1994). Isopoda Aquatica.Encyclopaedia Biospeologica. Societè de Biospéologie, Moulis, Bucarest: 123-140.

An up-date regarding the italian isopods, as a whole, can be found in: Argano a.o. 1995. Crustacea Malacostraca. II (Tanaidacea, Isopoda, Amphipoda, Euphasiacea). In: Minelli A., Ruffo S. & La Posta S. (eds), Checklist delle specie della fauna italiana, 30. Calderini, Bologna.

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