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Using histological sections, the gonads of samples of yellow and silver eels of two populations were examined. The populations were previously analysed for growth and sex ratio. The histological structures observed are similar to those described in previous publications for the European eel, Anguilla anguilla and to those indicated for the Pacific eel, A. .japonica. Well differentiated gonads are present in the silver eels. In the yellow eels, ranging in age from 0 + to 2 + years and from a length of 20 cm to that at which they become silver, undifferentiated and both sex gonads are found. Histological evidence is presented which suggests that the ovary, found even in young and small eels, is completely differentiated at a very early stage. The testis-like gonad of the yellow eel is a more primitive, and possibly reversible. gonad which differentiates completely at the beginning of sexual maturation and the change to the silver phase.

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1. Freshwater ecosystems worldwide are experiencing native fish losses with severe threats to the conservation of freshwater biodiversity and ecosystem functioning, and the debate on whether the cause is biotic or abiotic disturbance is still open. 2. Temporal variation in fish assemblages was analysed over an 18 year period in 14 waterways of the lowland backwaters of the PoRiver in north-eastern Italy, which are important feeding, spawning and nursery sites for native fish. 3. In 1991, 14 native and eight exotic species were collected. In less than 20 years 10 native species underwent local extinction, three of which – Rutilus pigus, Rutilus aula, and Chondrostoma soetta – were endemic to the Padano-Veneto District in northern Italy. 4. Ordination of the data (MDS, CLUSTER, ANOSIM, SIMPER) showed a clear temporal gradient in fish community structure. After the establishment of the exotic predator Silurus glanis, some native species significantly declined in abundance and biomass (i.e. Alburnus arborella and Scardinius erythrophthalmus) or disappeared (i.e. Rutilus aula and Tinca tinca). Moreover, exotic species Cyprinus carpio, Ameiurus melas, and Carassius auratus from previous introductions, underwent significant changes in their abundance and biomass. No correlation was found between fish community structure and water quality parameters (BIOENV). 5. The success of exotic species, particularly S. glanis which thrived in this degraded habitat, seems to have led to the decline of native fish fauna in the canals of the lower portion of the Po River basin. Conservation strategies focusing on the containment of exotic species and habitat restoration are recommended

The paper describes a bioenergetic individual-based model of the growth of Tapesphilippinarum, a bivalve mollusc reared in the lagoons of the Northern Adriatic Sea. The model, which simulates the evolution of the size of an average individual, has been calibrated using different sets of experimental data collected in the lagoons of Venice and Goro, and has been validated against a 3 yr long, independent series of field measurements of the average size of a cohort. The formulations here proposed for describing the dependence of the gain and loss terms on water temperature and food ration have not been employed in relation to this species before, and allow one to apply the model also in eutrophic conditions. Possible applications of the model as a tool for site selection and management implementations are illustrated.

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The role of transition environments, such as lagoons or deltas, in shaping the geographical pattern of genetic variability may be investigated through the analysis of population structure in the species living in these habitats. Several fish species are strictly bound to deltas or coastal lagoons for at least part of their life cycle. This paper reports the analysis of mitochondrial DNA variability in several populations of Atherina boyeri (Risso, 1810) sampled along the north Mediterranean coasts from the Atlantic Ocean to the Black Sea. The results show a high genetic structure among populations, probably due to the deep fragmentation of the lagoon environments in which this species spends most of its life cycle. On the other hand, a clear phylogeographic pattern and a significant positive correlation between genetic and geographical distances support the hypothesis of gene flow among populations, probably due to an imprecise anadromic behaviour from the lagoons to the close coastal sea, which allows for an exchange of individuals between contiguous populations. Our results are in good agreement with those previously obtained on the same species by RAPD techniques. Our analysis among different Atherina genera are also compatible with the most recent classification of these species based on morphological and genetic characters.