Micro-anatomical structure of the first spine of the dorsal fin of Atlantic bluefin tuna, Thunnus thynnus (Osteichthyes: Scombridae).

The first spine of the first dorsal fin (FS) of the Atlantic bluefin tuna (ABFT), Thunnus thynnus, is customarily used in age determination research because its transverse sections display well-defined growth marks. In this paper the FS structure was studied to explain its known dramatic age- and season-related morphological modifications, which are evidently caused by bone remodeling. Cross sections of samples from six adult ABFT were in part decalcified to be stained with histological, histochemical and immunohistochemical methods, and in part embedded in methyl-methacrylate to be either observed under a linear polarized light or microradiographed. FS showed an external compact bone zone and an inner trabecular bone zone. The compact bone zone consisted of an outer non-osteonic primary bone layer (C1) and an inner osteonic bone layer (C2). C1 was in turn characterized by alternate translucent and opaque bands. Evidence of spine bone remodeling was shown by the presence of osteoclasts and osteoblasts as well as by tartrate-resistant acid phosphatase (TRAP) positive bands at the boundary between old and newly formed bone. The examination of plain, i.e. not-fixed and not-decalcified, FS from 28 ABFT showed that the average thickness of C1 remained fairly constant during fish growth, whereas C2 increased significantly, indicating that the periosteal primary bone apposition is counterbalanced by the parallel bone remodeling occurring inside the compact bone zone. The present study revealed the structure of the ABFT FS and the pattern of its bone remodeling. Both of them underlay phenomena, never examined in detail before, such as the appearance followed by the progressive disappearance of growth bands.

Fin spine bone resorption in atlantic bluefin tuna, Thunnus thynnus, and comparison between wild and captive-reared specimens.

Bone resorption in the first spine of the first dorsal fin of Atlantic bluefin tuna (ABFT) has long been considered for age estimation studies. In the present paper spine bone resorption was assessed in wild (aged 1 to 13 years) and captive-reared (aged 2 to 11 years) ABFT sampled from the Mediterranean Sea. Total surface (TS), solid surface (SS) and reabsorbed surface (RS) were measured in spine transverse sections in order to obtain proportions of SS and RS. The spine section surface was found to be isometrically correlated to the fish fork length by a power equation. The fraction of solid spine bone progressively decreased according to a logarithmic equation correlating SS/TS to both fish size and age. The values ranged from 57% in the smallest examined individuals to 37% in the largest specimens. This phenomenon was further enhanced in captive-reared ABFT where SS/TS was 22% in the largest measured specimen. The difference between the fraction of SS of wild and captive-reared ABFT was highly significant. In each year class from 1- to 7-year-old wild specimens, the fraction of spine reabsorbed surface was significantly higher in specimens collected from March to May than in those sampled during the rest of the year. In 4-year-old fish the normal SS increase during the summer did not occur, possibly coinciding with their first sexual maturity. According to the correlations between SS/TS and age, the rate of spine bone resorption was significantly higher, even almost double, in captive-reared specimens. This could be attributed to the wider context of systemic dysfunctions occurring in reared ABFT, and may be related to a number of factors, including nutritional deficiencies, alteration of endocrine profile, cortisol-induced stress, and loss of spine functions during locomotion in rearing conditions.