Early development of Zingel streber

Zingel streber eggs contained a relatively large yolk (1 . 63-1 . 83 mm dia meter after activation) and were strongly adhesive. The egg envelope was th ick and not transparent. Embryos hatched very late-from 14 to 19 days after activation. Embryonic development was long (23 days at mean water temperat ure 14 .3 degrees C). The circulatory system appeared early, being characte rized by a complex anastomosing vitelline system and by well-developed segm ental vessels. This suggests that Z. streber embryos can exploit available oxygen sources very efficiently, unlike Gymnocephalus spp. whose respiratio n efficiency does not increase dramatically until the first larval step. Sk eletal development is also described and discussed. High investment into th e quality of sexual products, relatively fast development of sense organs a nd swimming abilities, way of spawning, long embryonic period, short durati on of two vulnerable steps, and early development of spiny structures on th e head provide the embryos and larvae with relatively efficient protection against predators. On the other hand, low fecundity together with highly sp ecialized habitat requirements make Z. streber a vulnerable species that is very sensitive to perturbations in its environment. (C) 2000 The Fisheries Society of the British Isles.