Evolutionary biogeography of the Microparasellid isopod Microcharon (Crustacea) in the Mediterranean Basin

Abstract

The interstitial subterranean isopod Microcharon (Crustacea) is highly diversified within the Mediterranean. The present distribution of 70 species is considered the result of the joint history of this genus and its environment. This stygobiont is derived from marine surface ancestors. Both Plate Tectonics and the two-step model of colonization and evolution, the second phase of which represents vicariance during the Tethys regressions, enable an understanding of the evolutionary history of this monophyletic group. The single extant marine littoral species was separated from its sister group by the closure of the Gibraltar Straits during the Messinian. The drift of the Apulian plate, the AlboranKabylian-Calabrian fragment as well as the opening of the western Mediterranean through the translation of the Corsican-Sardinia plate could explain the divergences in the western and the eastern Mediterranean. The primitive sister groups in Morocco and in Spain would have been left by the Turonian Tethys regression. Such regressions of the Tethys embayments could have played a major role in the evolution of the genus from Turonian up to Tortonian in Norht Africa, and up to Pliocene and even to Pleistocene in Italian, Iberian and Greek zones. Several eastern European species may be a product of the Paratethys regressions. Everywhere, species with plesiomorphic characters are related to old regressions and vice versa. Local geological events could have resulted in further vicariance divergences. There is a good congruence between the evolutionary history of Microcharon and other stygobiont isopod, amphipods or syncarids living in the same regions studied in the Mediterranean.

Microcharon novariensis, a new microparasellid isopod from groundwater in Italy (Crustacea, Isopoda, Janiroidea)

“…According to the evolutionary trend of characters proposed by Dole & Coineau (1987) and Coineau (1994) within the genus Microcharon, the new species shows the following characters in plesiomorphic state: long sensorial seta on segment 2 of antennula, numerous pinnate setae on the outer endite of first maxilla and on the endites of second maxilla, relatively short pereopodal unguli, pointed sympod of the second male pleopod and female operculum armed with two setules. Microcharon novariensis sp.…”

“…Id). According to Coineau (1986Coineau ( , 1994, Dole & Coineau (1987), Pesce & Galassi (1988b), Galassi (1991), Galassi et al (1995a, b), Boulanouar et al (1995) and Yacoubi-Khebiza et al (1997), differences on these microcharacters may have a high significance in taxonomic distinction. Interestingly, the same differences with respect to the available descriptions of M. marinus were noticed also by Pesce & Galassi (1988b) for several Spanish populations, only dubitatively assigned to the nominate species.…”

“…Nevertheless, the new species shows only three spines on the distal segment of mandibular palp (apomorphic character state with respect to the condition of four spines on the same segment) and does not fit the diagnosis of the two monophyletic groups (the major and the profundalis groups) established by Galassi et al (1995a, b). With respect to the eastern Mediterranean group of Microcharon as defined by Coineau (1994), M. novariensis sp. n. shows only weak affinities with M. apolloniacus Cvetkov, 1964 Qatus group sensu Coineau, 1994) known from spring waters of Sozopol (Bulgaria) in the gross morphology of the first male pleopod, short in the distal third and with short hyaline edge not protruding beyond the distal part of the pleopod, in the presence of pectinate or denticulate setae on both maxillae, and in the relatively short pereopodal unguli.…”

“…With respect to the eastern Mediterranean group of Microcharon as defined by Coineau (1994), M. novariensis sp. n. shows only weak affinities with M. apolloniacus Cvetkov, 1964 Qatus group sensu Coineau, 1994) known from spring waters of Sozopol (Bulgaria) in the gross morphology of the first male pleopod, short in the distal third and with short hyaline edge not protruding beyond the distal part of the pleopod, in the presence of pectinate or denticulate setae on both maxillae, and in the relatively short pereopodal unguli. On the other hand, the new species differs from M. apolloniacus in the different morphology of the second male pleopod (sympod elongate, endopod with pointed truncated tip in M. novariensis sp.…”

“…In the new species, all these characters are present in a plesiomorphic state versus their apomorphic state in the western Mediterranean species, all with short sensorial seta on segment 2 of the antennula, long pereopodal unguli, and second female pleopod naked. For this reason, the new species occupies an enigmatic position in the phylogeny of the genus hypothesised by Coineau (1994), most probably because of a 'mosaic evolution' of characters.…”