Increasing importance of the hybrid striped bass, Morone saxatilis x M. chrysops, as a food fish grown in recirculating aquaculture systems and of striped bass as a sport and commercial fish has prompted further investigation of its general biology, including its respiratory system. We report here transmission electron microscopic (TEM) data on secondary (respiratory) gill lamellae of hybrid striped bass (15-30 cm length) maintained on pelleted ration in a production-style recirculating water system at low and production densities. Lamellae were collected from the medial region of gill arches and rapidly fixed in Na cacodylate buffered (pH 7.4) cold 5% glutaraldehyde/4% formaldehyde. The general architecture and most normal cell types of the lamellae resembled that of other teleostean species, and mucus cells, epithelial cells and cartilage cells were non-distinctive. Pillar cells were atypically flattened within the lamellae. Chloride cells showed a more dense distribution of the cytoplasmic tubular system and less branching than reported for many other teleosts. Particularly prominent, electron dense cytoplasmic granules were observed in marginal, lamellar blood vessel endothelial cells. Fish kept in a high population density, recirculating water system showed a number of reversible, gill pathologic changes such as interfilamental cell hyperplasia, inflammatory cell infiltration, lamellar fusion, etc. Ultra structurally, the pillar cells became disrupted, subepithelial lymphatic spaces enlarged, central blood spaces contained inclusions, and several types of cytoplasmic alterations were seen in outer epithelial cells of secondary lamellae.

This data from non-crowded, normal fish serve as a background for interpreting gill changes in hybrid striped bass, and the adverse reactions in the high density population attest to the high sensitivity of the respiratory tissue in these cultured fish.