The First Known Case Of A Skin-Less Shark

jhon pablo

As if 2020 couldn’t get any weirder, what if we told you that a shark with no skin-related structured (so no epidermis, stratum laxum, dermal denticles, or teeth) was discovered? Seems like something straight out of a science fiction movie. Except, it’s not. The first case of lack of skin-related […]

As if 2020 couldn’t get any weirder, what if we told you that a shark with no skin-related structured (so no epidermis, stratum laxum, dermal denticles, or teeth) was discovered?

Seems like something straight out of a science fiction movie. Except, it’s not.

Galeus melastomus, also known as the blackmouth catshark, is a small elasmobranch distributed in the eastern Atlantic from Norway to Senegal on the outer continental shelves and upper slopes. Here, they mainly feed on invertebrates, cephalopods, small pelagic bony fish, and other small elasmobranchs. This shark is caught in fisheries within their range and have very low or no commercial value. Back in July 2019, one individual stood out and became a scientific megastar. The specimen was caught by trawl in central‐western Mediterranean waters in July 2019 at about 1,640 feet (500 meters) deep off Cape Carbonara in South Sardinia. A free‐swimming immature shark, it measured 302 millimeters in total length (TL) and weighed 82.7 grams in total mass (TM).

“The first reaction was a big surprise,” said lead author Antonello Mulas. “Since the specimen was so strange, at first we checked for a possible report of a new species for the Mediterranean, but it was clear that this was not the case.”

According to the researchers, this captured shark was female and had developed gonads. As the article explains, shark skin is responsible for multiple functions, and therefore classified as a fundamental organ. The most external of skin structures are the tooth-like dermal denticles which protects sharks from their surrounding environment, predators and ectoparasites. Shark dermal denticles also reduce potential mechanical abrasions and reduce friction with water by altering the hydrodynamic flow on their body surface. The mucus secreted by the cells of the epidermis layer is also considered the first part of the shark’s immune system, preventing any infectious microbes from colonizing. “When we realized that it lacked dermal denticles and teeth, we had a lot of questions. The first and more important was: how could it survived to such a severe malformation? Dermal denticles – and teeth! – are fundamental for so many reasons that we are led to think that their complete lack is potentially fatal or at least very ‘penalizing,’” explained Mulas.

But the mystery surrounding this shark just continued to grow as they examined this specimen. The skin was thin, smooth and translucent, due to lack of the dermal denticles, and was particularly reduced around the gill openings, the nictitating membrane and the ampullary pores. Given the numerous functions performed by their skin, the lack of dermal denticles, epidermis, and stratum laxum, is likely to have caused the shark to modify its swimming, increased its energetic cost, and ultimately slowed the shark down. “The histological section showed that the malformation was even more severe than we thought but, at the same time, the stomach contents and the vertebral centra analysis showed us that this catshark was perfectly capable to feed on its typical prey and that, according to the species growth curve, it followed a normal growth,” said Mulas.

How in the world was this shark still alive? When asked, Mulas said the team had a few hypotheses. “Maybe the particular organization of the stratum compactum, with the collapsed collagen fibers, provided some protection against the environment and the pathogens and the specimen was able to prey because this species ingests its prey whole, so the teeth lack is not so penalizing,” he commented.

One would also think this shark would have a weakened immune system due to the lack of skin-related structured! But is that what was seen? Although this kind of morphological abnormality is potentially fatal, the observations published suggest that the specimen was in good health and naturally developed. “Our observations lead to think that this abnormal condition did not totally compromise either the swimming skills or the functionality of sensory structures like the ampullary pores. The lack of teeth did not influence the feeding abilities, probably because the blackmouth catshark ingests its prey whole,” write the authors.

It is unclear just how much this abnormality has impacted the shark’s overall behavior, physiology, or ecology. Mulas and the co-authors wish they could have seen this particular animal in its environment, so directly observe it. However, this find has raised many future research questions, such as figuring out why this happened. “Was this malformation ‘natural’ or was it induced by some teratogenic factor like ocean acidification, temperature rise or pollution? It is very difficult to answer but, considering that [other] malformations in sharks are being reported more frequently, it is very interesting (and important) to investigate,” said Mulas. “In any case, that famous Hamlet verse, ‘There are more things in heaven and earth, than are dreamt of in your philosophy’ is true!”

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