In a surprising development in marine biology, recurrent parthenogenesis has been documented for the first time in the shark Mustelus mustelus, also known as the common smooth-hound. This discovery, reported in the journal Scientific Reports, highlights these sharks’ ability to reproduce without the intervention of a male, a phenomenon that could have significant implications for the conservation of this species, which is classified as endangered by the International Union for Conservation of Nature (IUCN).

Parthenogenesis, or “virgin birth,” is a mode of reproduction where an egg develops into an individual without being fertilized, a phenomenon observed in various vertebrate taxa, excluding mammals. Although obligate parthenogenesis, where organisms reproduce only in this way, is rare, facultative parthenogenesis allows females to reproduce both sexually and asexually. This study provides the first report of this type in M. mustelus, revealing a potentially crucial adaptive strategy for the survival of this species in the changing marine environment.

The study was conducted at the Cala Gonone Aquarium in Sardinia, Italy, where two adult females of M. mustelus have been observed for 13 years in the absence of males of their species. Despite this absence, nearly annual production of offspring has been recorded, ruling out long-term sperm storage as a cause. Genetic analyses confirmed that these offspring were born through parthenogenesis, consistent with the mechanism of terminal automixis.

Female samples of Mustelus mustelus born by parthenogenesis in the Aquarium of Cala Gonone
Female samples of Mustelus mustelus born by parthenogenesis in the Aquarium of Cala Gonone. Credit: Giuseppe Esposito

Terminal automixis is a type of parthenogenesis where the egg fuses with one of its own polar bodies, resulting in offspring genetically similar to the mother, but with less genetic variability. This form of reproduction was observed in M. mustelus offspring, who also exhibited characteristics such as bite marks on their bodies, suggesting intrauterine cannibalism behavior, which might have contributed to the death of some offspring.

The ability of M. mustelus to reproduce asexually has significant implications for its conservation. This species, which inhabits the Mediterranean and other areas of the eastern Atlantic and western Indian Ocean, has suffered a significant population decline due to illegal fishing and the capture of sexually immature individuals. The IUCN estimates that its population could decrease by 50% in the coming decades, highlighting the need for effective conservation strategies.

Parthenogenesis offers a possible pathway to maintain the M. mustelus population in situations where males are scarce or absent, ensuring that reproduction can continue without relying on the presence of males. This phenomenon could be an evolutionary response to environmental pressures, allowing M. mustelus females to maximize their reproductive opportunities even in adverse conditions.

Although the mechanisms driving parthenogenesis in elasmobranchs (sharks and rays) are not fully clear, it is suggested that the reduction in male population could be a key factor. This hypothesis is supported by previous research, such as the study by Fields et al., which documented facultative parthenogenesis in the small-toothed sawfish, Pristis pectinata, a critically endangered wild vertebrate.

SOURCES

Esposito, G., Meletiadis, A., Sciuto, S. et al. First report of recurrent parthenogenesis as an adaptive reproductive strategy in the endangered common smooth-hound shark Mustelus mustelus. Sci Rep 14, 17171 (2024). doi.org/10.1038/s41598-024-67804-1

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