Abstract
Squalus acanthias is a commercially exploited small coastal shark, highly sensitive to overexploitation due to a low reproductive potential and a low intrinsic rate of population increase. We collected specimens caught by commercial bottom trawls from April 2005 to January 2007 in the Adriatic Sea and investigated reproductive traits of the species in ovaries of 68 immature females (total body length range: 26.1-70.1 cm). Histomorphometric analyses included measurements of the growing follicle diameters, the width of the follicular layer and the width of the zona pellucida. We analysed 2700 ovarian follicles and grouped them into four categories: small previtellogenic, medium previtellogenic, large previtellogenic and small vitellogenic follicles. The growth of follicles was positively correlated with the body size of females (r=0.726; p<0.001). We documented significant changes in the thickness of the follicular layer and zona pellucida as follicles increased in size. Small vitellogenic follicles started to appear in small proportion (around 1% of follicles) in subadult females with the minimum size of 60 cm total body length. Yolk droplets first emerged in follicles larger than 600 μm in diameter while vitellogenesis was evident in follicles ranging 1.2-2.0 mm in diameter when both follicular layer and zona pellucida were the widest (41.1 μm and 49.1 μm, respectively).
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