Ovarian Structure, Function, and Asymmetry in Sexually Immature Small Cetaceans: Histological Analysis of Follicle Classes and Early Follicle Development

Small cetaceans serve as sentinel species for monitoring marine ecosystem health due to their vulnerability to bioaccumulative contaminants that may disrupt reproductive development. Despite this, there is a lack of validated reproductive toxicity markers, most notably for sexually immature individuals. This study assessed the feasibility of assessing ovarian follicle quantification in stranded sexually immature female harbour porpoises (Phocoena phocoena) and common dolphins (Delphinus delphis), using ovarian assessment criteria commonly applied in reproductive toxicity studies in other species. Ovarian tissues from 29 stranded individuals were processed, of which only those with adequately preserved bilateral ovarian sections (n = 18) were included in the follicle-count and morphometric analyses. Ovarian follicle classes, including naked ova, primordial, and primary follicles, were identified, counted, and measured. Results revealed significant left-right ovarian asymmetry in harbour porpoises, with the left ovary containing more combined follicle-class counts in both calves and juveniles, consistent with previously reported left-sided asymmetry in genital morphology and lateralised mating behaviour in the species. In contrast, common dolphins showed no evidence of ovarian asymmetry in sexually immature individuals. In both species, combined follicle-class counts declined sharply with increasing body length, while follicle diameter showed a positive association with somatic growth, indicating parallel patterns of early-life follicular depletion and enlargement. Naked ova were consistently observed across all body length groups in harbour porpoises, while none were reported in the ovaries of common dolphins. These results demonstrate that harbour porpoises and common dolphins differ in patterns of ovarian asymmetry and early follicular development, supporting species-specific baseline descriptions of ovarian morphology. Furthermore, quantification of ovarian follicle classes provides a reproducible histological endpoint that may help detect deviations from normal ovarian development associated with early-life exposure to environmental contaminants in stranded small cetaceans, provided tissue is adequately preserved.