Repeat shark parthenogenesis report adds to smooth-hound case: full analysis

A new paper in Animals adds another documented case of facultative parthenogenesis in the common smooth-hound shark (Mustelus mustelus), extending an unusual reproductive story first detailed in Scientific Reports in July 2024. The earlier paper described the first genetically verified parthenogenesis in this species, based on repeated births from two female sharks kept without conspecific males at Cala Gonone Aquarium in Sardinia. The new report says an April 2024 event produced two offspring from a single parthenogenetic reproductive episode, strengthening the idea that this is a recurrent phenomenon rather than an isolated accident. (nature.com)

The background is what makes the case stand out. In the 2024 Scientific Reports study, the sharks had been housed for 13 years without males, yet the aquarium still observed a nearly annual production of young. The authors documented births in 2020, 2021, and 2023, and noted a likely but untested earlier case in 2016. Because some shark species can store sperm, the team used 13 species-specific microsatellite loci to test whether long-term sperm storage could explain the births. Their analysis instead found offspring homozygosity at informative markers and no non-maternal alleles, supporting parthenogenesis via terminal fusion automixis. (nature.com)

That matters scientifically because the common smooth-hound is not just any shark. The 2024 paper identifies M. mustelus as an IUCN-listed endangered species and notes concerns about illegal fishing and projected population decline. The authors also argue that the repeated, alternating pattern between the two females makes long-term sperm storage increasingly implausible. In other words, the new Animals report is landing in the context of an already strong longitudinal case that these females can switch to asexual reproduction under some circumstances. (nature.com)

The broader shark literature gives that claim some context, but also some caution. The Scientific Reports authors noted that documented shark parthenogenesis cases from 2001 to 2023 were mostly observed in captivity. Smithsonian Ocean, citing shark biologist Toby Daly-Engel, notes that parthenogenesis may be more common than once thought, but hard to detect in the wild. At the same time, researcher Jennifer Wyffels has pointed out that shark reproduction is not straightforward even when males and females are present, and that parthenogenesis does not necessarily mean it is an optimal or preferred strategy. (nature.com)

For veterinary professionals, the practical takeaway is less about novelty and more about case management. In elasmobranch collections, unexplained pregnancies or births in female-only groups should prompt a differential that includes facultative parthenogenesis, not just historic breeding or sperm storage. Confirming the diagnosis requires molecular testing, and the clinical or husbandry picture may be mixed: in the 2024 report, all offspring were female, several had bite marks, one had deformities, and only one remained alive three years later. That combination raises familiar questions around neonatal viability, maternal or conspecific aggression, enclosure dynamics, and whether parthenogenetic offspring face higher developmental risk because of reduced genetic diversity. (nature.com)

There’s also a conservation angle, though it should be handled carefully. The authors suggest recurrent parthenogenesis could represent an adaptive reproductive strategy in the absence of males. That is their interpretation, and it is plausible given the repeated events, but outside experts have also emphasized that asexual reproduction in sharks is not necessarily beneficial at the population level because it reduces genetic diversity. For endangered species, that means parthenogenesis may demonstrate reproductive flexibility without offering a straightforward conservation solution. (nature.com)

Why it matters: For aquatic veterinarians and managed-care teams, this case reinforces the need for reproductive surveillance, sample retention, and access to genetic diagnostics when births occur under apparently impossible circumstances. It also suggests that long-term all-female housing does not eliminate reproductive events, which has implications for population planning, neonatal care, necropsy protocols, and communication with pet parents or the public when rare births occur. More broadly, these reports may help clinicians and researchers distinguish true parthenogenesis from sperm storage, failed breeding assumptions, or undocumented male exposure. (nature.com)

What to watch: The next step is whether the Animals report provides enough additional genetic and husbandry detail to clarify repeatability, offspring outcomes, and mechanism, and whether similar cases emerge in other shark species or institutions with routine molecular follow-up. (nature.com)