Morphological rudimentation and novelties in stygobitic Cirolanidae (Isopoda, Cymothoidea

The diversity of cirolanid isopods, all of marine origin, in subterranean a quatic habitats, is remarkably high, 68 species being presently recognized (in 19 genera). These stygobites display, in clear contrast with the marine (i.e., non-subterranean) cirolanids numerous cases of rudimentation ("regr ession") but also a number of remarkable morphological and other evolutiona ry novelties, some of them unique in Isopoda, or even in Crustacea. Rudimen tation and novelties are the two components of troglomorphy, sometimes the limit between them being not sharp. These cases are examined under the foll owing headings: body shape; depigmentation and tegumental smoothness; absen ce of "molariform spines"; anophtalmy; elongation and slenderness of append ages; development of tactile (mechanoreceptory) equipment; development of c hemosensors ("olfactory organs"); rudimentation and novelty in uropods; rud imentation and novelty in pleopods; some particular structures in hypogean species; the propodial organs; volvation; paedomorphy; facts related to "K- strategy". Study of this array of facts contributes to a better understandi ng of adaption to hypogean life. In the introductory notes are briefly expr essed the author's opinions about mechanisms considered as underlying rudim entation and novelties respectively as responses to hypogean life, and abou t preadaptation considered as an essential notion for understanding coloniz ation of the Stygal, this being well exemplified by Cirolanidae. The remark able diversity of morphological and other adaptive traits of stygobitic Cir olanidae could be explained by the fact that the hypogean aquatic environme nt is much more complex than seen by traditional biospeology, animals displ aying a great variety of adaptive responses in order to cope with the const raints of this complex, rigorous environment.