Brief natural history of the Bathynellacea
Bathynellacea are small crustaceans, between 0.5 and 3 mm in size, with an almost cylindrical body and vermiform shape, without eyes or pigmentation. All known species live in the interstitial environment (stygobionts), either in cave groundwater or phreatic water close to epigean rivers and aquifers. Only 2 species that live free in Lake Baikal seem to be the exception, but it is interesting to note that they live on the deep dark beds of the lake. They lack free swimming larvae and do not swim well as adults, preferring to crawl amongst the grains of sand.
Bathynellaceans females lay one egg at a time. The embryonic development lasts for 9 months (Antrobathynella stammeri) and the newly emerging form is similar to the adult, except that it has fewer legs (1 to 4) than the adult. The young forms successively acquire legs, up to 8, in the following moults. The development leads to adults which look like larvae lok, and this has been related with their colonization of the interstitial environment (Coineau, 2000).
They are distributed widely in all the subterranean waters of the world, except at the Poles. At present we know of 2 families, 60 genera and almost 200 species.
There is no known fossil species, but the geographic distribution suggest an origin not later than the upper Paleozoic (Schram, 1977).
The morphology of the adult bathynelids can be summarized by the following characters:
· absence of cephalic caparace;
· presence of 8 thoracic and 5 abdominal free segments;
· eyes and statocysts absent;
· thoracopods 1 to 7 biramous;
· thoracopod 8 male transformed in copulatory organ and female reduced;
· petasma absent;
· furca always present, and
· uropod with pleotelson not forming tail-fan.