Observations sur la biologie de Stenasellus virei (Crustacea Isopoda Asellota des caux souterraines)
St. virei has been bred in the laboratory for many years (1960-1974). Most of the St.v.hussoni were captured in karstic waters, near the Moulis subterranean laboratory. Some St.v.virei from the Padirac sink-hole; St.v.buchneri from Cantabrian caves; St.v.boui and St.v.virei from phreatic waters; and St.buili and St.breuili have also been bred. Since Stenasellids are unable to swim, very low aquariums are used, with a bed of cave clay, some calcareous stones, dead wood and dead elm tree leaves. Little depth of water is necessary. Stenasellus was originally carnivorous, being able to capture and devour living prey, such as Chironomid larvae, but the populations of cave waters have developed a different diet: silt, guano, plant remains..., because they have been often insulated from their original phreatic biocenosis. Nevertheless, the existence of cannibalism among them points out that the predatory behaviour has not completely disappeared. Adult St.virei can be fed with Cerophyl. Some observations on the burrowing activity and on the reactions to light, temperature and salt water have been made. All postmarsupial molts of Stenasellus occur in two steps (isopodian molts). The intramolt is extremely long (from 83 h 30 min. for the first molt of the free young), to 8-12 days, for the adult male and female, 14 days for female reproductive molts and 16-21 days for the molts of aged or senile individuals). The intermolts last from 2 1/2 months (first intermolt of the free young), to 9-12 months (non-reproductive ones of the adult) and 12-18 months (average: 15-16), for reproductive 9 intermolts. The normal lifespan of karstic subspecies of St.virei and related species must be estimated as 12 years (males) and 15 years (females). All these values are 10-20 times longer than these of an epigean Asellid of the same size (Asellus aquaticus). The reproductive cycle has been studied. The adult female is larger than the male. There is no precopulatory pairing (“nuptial ride”), contrary to epigean Isopods and Amphipods. It seems due to the fact that this group lived originally among interstices or burrows in alluvial deposits, where long coupling would be impossible. The successive bilateral copulation is very short (one hour) and takes place during the female reproductive molt. The female lays her eggs some 4-5 days after her anterior molt removal and laying may take 24 hours. The number of eggs (15-60, average 32, in St.v.hussoni) increases with the size of the female. The eggs of the Stenasellids are significantly larger than these of epigean Asellota. Intramarsupial development of St.virei takes 9-10 months at 9-11°C. As the reproductive female intermolt lasts 12-18 months, the female keeps her empty pouch fur 2-8 months, after the young leave it. In some caves, the laying season is the end of winter and beginning of spring; the liberation of the young occurs un autumn. A 9 reproductive intermolt is always followed by one or several genital-rest intermolts (each being 9-11 months long), so a female cannot lay eggs in each year: the minimal interval between successive reproductions of a single female is 2 years. It can reach 5 years or more, according to the scarcity of food-coming in the biotope. Intramarsupial life includes embryonic (5-5 1/2 months), then larval (4-4 1/2 months) stages. There are 2 intramarsupial larval molts (in one step, i.e. non-isopodian ecdysis). The liberation of the brood is very slow: average 28 days between the departure of the first and last young. Newly released young are 1,5 mm (St.virei), or 2,1 mm (St.buili) long. Postmarsupial larval stages include 3 intermolts and last some 11 months after birth. On the first intermolt, the young have no VII pereopods (=manca I + II stages); on the second, the VII pereopods are rudimentary (=manca III); on the third, the VII pereopods are normal, but shorter than VI pereopods. Juvenile development is very long. Size and age of puberty, in St. virei hussoni are: 4,5 mm and 4-5 years, fur the male and 6,5-7 mm and 6-7 years or more, fur the female. In the juvenile male, the morphogenesis of I and Il pleopods takes place normally on intermolts 4-9 and lasts 3 years or more. On intermolt 10, it seems that the male is able to mate.
Observations sur la biologie de Stenasellus virei (Crustacea Isopoda Asellota des caux souterraines).
International Journal of Speleology,
Available at: https://digitalcommons.usf.edu/ijs/vol7/iss1/8