Development of European bitterling in the gills of freshwater mussels

The development of bitterling embryos within the unique environment of a freshwater mussel's gills requires a departure from typical cyprinid embryological development. Eggs are large (2·6×1·7 mm), illustrating the low risk of predation, and elliptical; a response to unionid gill morphology and a way of increasing the transfer rates of respiratory and excretory products to and from the tissues. The yolk sac develops elongated lateral processes during early ontogeny; these secure the embryo into the host's interlamellar space. Once the larva is capable of movement (8·2 mm) the lateral processes are lost and the larva becomes less dependent of the host's gills for the provision of oxygen. Hatching (3·3 mm) and pigmentation of the blood (6·4 mm) occur relatively early; this may increase the rate of oxygen supply to the tissues. Pigmentation of the eyes and appearance of the melanophores occurs relatively late (7·4 mm and 7·9 mm, respectively); embryos are not required to detect or avoid predators. Bitterling larvae generally emerge from the host once the yolk sac has been consumed (10·5 mm); this may mark a change in respiratory and nutritional requirements.