Developmental abnormalities in elasmobranchs (sharks and rays) are rarely documented, with reported cases primarily involving fin or cranial deformities. Monozygotic twinning, the formation of identical twins from a single zygote, is particularly rare in these species and has been observed overwhelmingly in viviparous elasmobranchs. Here, we document a rare case of monozygotic twinning in an oviparous shark, the Small-spotted Catshark Scyliorhinus canicula (Linneaus, 1758). This case was characterized by two embryos connected to a single yolk sac via separate yolk stalks within the egg capsule. The embryos exhibited normal development until mortality at developmental Stage 31 (Ballard et al., 1993) or stage 4 (Musa et al., 2018), likely due to stressors such as oxidative stress and allostatic overload, resulting from shared resources within the capsule. This is the first confirmed instance of mortality in monozygotic elasmobranch twins, highlighting the plausible challenges of polyembryony in oviparous elasmobranchs. These findings underscore the importance of understanding reproductive abnormalities and their implications for fecundity, particularly in light of ongoing anthropogenic pressures that threaten elasmobranch populations globally.
This paper is describing a new finding of Carcharhinus plumbeus (Nardo, 1927) caught at the insular shelves off the Sazan Island at the depth of 120 m, which is the southernmost record for a sandbar shark in the Adriatic Sea. The individual had a partially healed umbilical scar and is considered as neonatal. Examined shark appeared in good body condition with showed no macroscopic signs of diseases or macroscopic parasites. Detailed morphometric measurements are compared with four neonate and juvenile specimens from the north and central Adriatic Sea and are provided in the paper. Despite it is not possible to establish a certain conclusion on whether our specimen was born in the studied area or elsewhere, presented finding provides the very first evidence that neonatal sandbar sharks can be found along the coast of Albania. Knowing the exact locations and movements of both neonates and juveniles is of high importance for the effective long-term in-situ conservation.
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