This month we’re taking a look at some truly swell sharks, the Swell Sharks!

Swell Sharks are a group of catsharks belonging to the family Scyliorhinidae. Their most unique feature is probably their threat response: they are able to expand their bodies to twice their normal size by swallowing water! This wedges them into their hiding spot, making it more difficult for predators to bite them from inside.

There are 18 different species of swell shark. In this article, we will focus on two of them, the Swell Shark (Cephaloscyllium ventriosum) and the Australian Swell Shark (Cephaloscyllium laticeps).

The Swell Shark (Cephaloscyllium ventriosum)

Biofluorescence

In 2019, scientists discovered the molecules responsible for a special ability of the swell shark – their biofluorescence. In the dark, special amino acids in their skin reflect the moonlight, appearing bright green in the darkness. This has been found to be species specific, and sex specific, and therefore this unique adaptation may function to help sharks species recognise each other or even potential mates. It may also play a role in camouflage.

Australian Swellshark, Cephaloscyllium laticeps

The Australian Swellshark is also sometimes known as the Draughtboard Shark due to its colouration: It has 11 brown ‘saddles’ that alternate with blotches on its flanks, forming a pattern resembling that of a checkerboard.

Oviparity

Like many other species of Swell Shark, the Australian Swellshark is oviparous. This means that the adult swell shark lays an eggcase with the embryo inside. Depending on the species, the shark may lay two at a time. These eggcases contain a developing embryo and a yolk. Before hatching, the embryo can feed on this yolk for sustenance as it grows. Once fully developed, the embryo hatches out as a fully formed miniature version of the adult shark.

Australian Swell Sharks have a particularly interesting eggcase: cream-coloured and flask shaped, this eggcase has 19-27 transverse ridges (lined horizontally across the eggcase). As with most catsharks, there are long curly tendrils on either end too.

Status

Although the Swell Shark is listed as Least Concern globally on the IUCN Red List of Threatened Species, some species of are listed as Critically Endangered. Take, for instance, the Whitefin Swellshark, endemic to southeastern Australia. Much of its habitat overlaps with areas of intensive fishing effort – as such, although not a target species, they were and still are frequently caught as bycatch. According to the IUCN, populations have reduced by >80% over the past three generations,

Scientific Name: Cephaloscyllium ventriosum

Family: Scyliorhinidae

Maximum Size: 110cm

Diet: Small crustaceans, cephalopods and fish

Distribution: Eastern Pacific, most commonly found at 5m to 40m depth.

Habitat: Usually found in rocky areas of kelp beds.

Conservation Status: They’re not typically targeted for food as their meat is generally considered to be of poor quality. They are however, often caught as bycatch in gillnets and trawls.

IUCN Red List Status: Least Concern

Image Credits: 

biofluorescence (Sparks, J. S.; Schelly, R. C.; Smith, W. L.; Davis, M. P.; Tchernov, D.; Pieribone, V. A.; Gruber, D. F., CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons)

laticeps (Mark Norman / Museum Victoria, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons)

Eggcase 1 ‘Cephaolscyllium ventriosum’ (vagabondvince310, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons)

Eggcase 2 ‘Cephaloscyllium laticeps eggcase’ (Museum Victoria, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons)