Fig. 1

Cyst formation in Echiniscoides sigismundi induced by osmotic stress. a Dorsal view of the L1 cyst with two cuticular coats (c1 and c2). The animal has an abnormally formed pharyngeal bulb (pb). b The response of L2 to the osmotic stress series. The animal seemed ready to molt at the beginning of the experiment (t = 0 h). It entered a “tun” state in the hypersaline solution (70 ‰, t = 48 h); however, instead of molting upon return to normal salinity (16 ‰, t = 50 h), it synthesized an additional cuticle layer. The animal finally retracted into a tun-like state within the cyst (t = 72 h). Sclerified parts of the buccopharyngeal apparatus, e.g., stylets (st), are retained within the encysted animal. Note the extra set of claws (nc) in connection with the second cuticle (c2). c R2 seemed ready to molt at the beginning of experimentation (t = 0 h), but following exposure to a hyperosmotic solution (70 ‰) it synthesized yet an extra cuticle (t = 72 h). d R1 at t = 72 h represented the most advanced stage of encystment that was observed. The animal seemed ready to molt before (t = 0 h) exposure to a hyposmotic solution (3.5 ‰, t = 24 h). When returned to normal salinity (35 ‰) it synthesized what seemed to be two additional cuticles, and the animal thus appeared to be surrounded by three cuticular coats (c1, c2 and c3). It ultimately discharged stylets, placoids and pharyngeal bulb cuticle into c2, a feature not seen in the other cysts. bc bulb cuticle, c1–4 cuticles 1–4, cl claws, gu gut, nc new claws, pb pharyngeal bulb, st stylets