Histopathological events throughout the development of turbot (Scophthalmus maximus L.) larvae.

Histopathological observations in cultured turbot (Scopthalmus maximus L.) larvae are described in varying developmental stages. No striking changes were found during the endogenous feeding period. After the introduction of live food, the main lesions encountered were atrophy and degeneration of the digestive system, but these lesions were not associated with high mortality rates in the culture system during the early stages. Subsequently, during the stages of organogenesis, progressive bacterial colonization of the digestive tract was observed, accompanied by severe enteritis, especially in the more mature larvae. High mortality rates were detected in the rearing system at this time. In severe cases, necrosis of pancreatic acini and hepatocytes were observed also. Swimbladder alterations such as non-inflation, malformation and bacterial invasion via the pneumatic duct were frequently observed at all stages of development. It is suggested that infectious agents may play an important role in the high mortality often encountered in cultured turbot larvae.

Influence of the age of algae fed to rotifers (Brachionus plicatilis O.F. Müller) on the expression of mixis in their progenies.

The sequence of the appearance of mixis in the rotifer Brachionus plicatilis was followed among the descendents of amictic rotifers transferred from a high salinity media (40 S) to a low one (9 S). All the neonates that hatched from the amictic eggs, after being transferred to a low salinity, were amictic. Each one of these neonates was cultured individually and its offspring removed periodically every 8-10 h. It was observed that throughout their reproductive phase, these parental females retained their potential to produce either mictic or amictic offspring. All the first produced neonates developed into amictic females, but among those produced later, three patterns were prevalent. The prevalent pattern (type A) was one in which the probability of a neonate being mictic increased towards the middle of the parents' reproductive phase and was followed by a slow decline. In the second pattern (type B), the probability of a daughter being mictic was constant throughout the parents' reproductive phase. It is suspected that the quality of food supplied to the rotifers determines the appearance of patterns, A, B or C. It is postulated that the innate capacity of rotifers to undergo mixis is genetically controlled, while its expression is modulated by environmental conditions.