First spine of Atlantic bluefin tuna cranial dorsal fin: structure and bone resorption in wild and captive-reared specimens

The Atlantic bluefin tuna (ABFT), Thunnus thynnus, is one of the most promising fish candidate for aquaculture. The ABFT, a highly migratory fish, is provided with a full set of fins, which play different roles in the swimming mechanics. The two dorsal fins are supported by spiny rays (spines) at the front and soft rays (rays) at the rear. The aims of the present study were to investigate the structure of the first spine of the cranial dorsal fin and to compare its bone apposition/resorption patterns in wild and captive-reared individuals. Cross-sections of this spine were cut by a low speed diamond saw; some of them were ground, polished and mounted on glass slides, whereas others were fixed in 4% paraformaldehyde, decalcified in 10% EDTA, dehydrated in ethanol and embedded in paraffin. Total, compact bone and reabsorbed bone surfaces were measured on undecalcified sections. Paraffin-embedded sections were further cut into 10-μm sections, which were stained with histological and conventional histochemical methods. Spine cross sections are characterized by several peripheral concentric compact bone layers delimiting an internal trabecular zone. New bone tissue is continuously apposed at the periphery of the spine while a progressive bone resorption occurs on the inside. The fraction of spine compact bone progressively decreases as fish age and size increase. The rates of spine bone resorption were significantly higher in captive-reared specimens. This abnormality adds to a number of anomalies occurring in reared ABFT and already reported in the literature