What to expect when you're not expecting...
It's pretty simple to determine the sex of a shark. If it's got a pair of claspers it's a male, if it doesn't it's a female. Claspers are much like a pair of penises. They're the modified posterior of the pelvic fins and used to deliver sperm into the female cloaca during copulation.
Now imagine you're studying an elasmobranch fishery off India, and you find 6 pregnant MALE bigeye houndsharks. Yep, you read that right.
This was the reality for Alissa Barnes and colleagues while conducting landing site surveys along the northeastern coast of India. During their surveys, Barnes et al. caught and dissected seven immature, presumably male, bigeye houndsharks. They noticed, however, that they appeared to also show signs of pregnancy, denoted by bulging of the stomach area. Upon their dissection they found 6 of the 7 sharks were gravid with pups, at differing stages of development.
They concluded that the sharks were exhibiting "basic intersexuality," meaning they were showing both female and male reproductive organs.
Ain't nothin' but a houndshark - The bigeye houndshark (Iago omanensis) is a small deep water shark species found in the western Indian ocean, growing to only 37cm in length!
While very rare, it appears more and more hermaphroditic individuals are being found across many chondrichthyan (sharks, skates, ray and chimeras) species. This doesn't mean that it is becoming more common, but rather our ability to find these individuals may be improving (for example,. more research / fishing and better associated technologies). Regardless, hermphrodites have been found in:
- the lanternshark;
- the Port Jackson shark;
- the Pacific spadenose shark;
- the blacktip shark;
- the blue shark; and
- a number of ray species
just to name a few!
It is unclear what the cause or reason is. Some suggest it has arisen from environmental impacts, such as chemical pollution or radiation. Others suggest a natural adaptation to changing oceans. Regardless, there is a growing need to understand this phenomenon, particularly with regards to population monitoring and investigating whether human activity has led to these biological changes.
Hermaphroditic sharks aren't even the weirdest shark reproduction can get. In 2002, a female white spotted bamboo shark at the Belle Isla Aquarium in Detroit, Michigan, USA, laid a clutch of eggs from which two healthy pups hatched. The only thing was that she hadn't even touched the same water as a male in 6 years!
Much like in chickens, it isn't uncommon for the females of species like the whitspotted bamboo sharks to lay clutches of eggs in the absence of males. The eggs are simply unfertilised. This time though, two healthy female pups emerged, stunning the aquarists.
This isn't the only time this has happened either.
In 2008, a female blacktip reef shark was found to be carrying a near term pup when a necropsy was performed after her death. A female leopard shark in Australia gave birth to 3 pups back in 2016. In all cases, the females had be separated from males for years.
Even more interestingly, the pups were found to have exactly the same genetic material as their mothers. That is, these was no genetic input from a father. This is termed "parthenogenesis." In other words, the egg is not fertilised by a male, but the embryo grows regardless, giving rise to babies which are genetically identical to their mothers!
Experts believe this may arise when individuals are pushed into an "evolutionary corner." For example, in an aquarium with no males, the females may be forced to adapt to an asexual mode of reproduction in order to pass their genetics to the next generation. The downside is that there is a loss of genetic diversity, as the pups are genetically identical to their mothers, but the upside is that the species can likely push on in times of low population numbers and barriers between the sexes which may be encountered in the wild.