The Invisible Shark: Squatina squatina
Squatinidae (angelsharks) are the second most endangered family of sharks. There are 24 species, where 3 are listed as Critically Endangered and 4 are data deficient on the IUCN Redlist. Angelsharks are unique species of sharks as they have a large flattened body with enlarged pelvic and pectoral fins which give them a ray-like resemblance. One well studied angelshark species is the Atlantic Angelshark (also known as Monkfish) which is one of the Critically Endangered angelshark species. Once commonly found throughout the North Eastern Atlantic, they are believed to have disappeared from their former range. Currently, their main stronghold is in the Canary Islands but in recent years there has been an increase in records off the Welsh coast. They are benthic species occurring close inshore (5m) within the intertidal zone or down to a depth of 150m. Like all angelshark species, S.squatina is a large flat bodied shark which grows up to 2.4m long. Angelsharks are ovoviviparous (pups are born alive), with litters ranging from 7 to 25. When born pup lengths range from 24 to 30cm. They are highly adapted to their life as benthic ambush predators. But, what makes them the perfect ambush predator?
Life in the sand or mud:
Like all angelsharks the Atlantic Angelsharks are patient predators, where they use their enlarged fins to bury themselves in the sand or mud waiting for prey to come in range for it to ambush (Figure 2). Their diet primarily consists of flatfish and other benthic fish, also prey on rays, crustaceans and molluscs. Once buried in the sediment their cryptic colouration completes their camouflage. Additionally, angelsharks have small barbels present on their snout which helps in detecting electrical signals which are emitted from other organisms. Of all senses, the sense of sight is dominantly used for hunting. With eyes being positioned on the top of their flattened head it allows Angelsharks to view potential prey which comes within the striking range.
Once a suitably sized prey enters the striking range; the shark will rapidly snap its head upwards where it pharynx then expands. Its jaws then protrude and a suction force is then created which draws the prey into its mouth (Figure 3). The strike can be complete within a tenth of a second. It’s needle-like grasping teeth also ensures that its prey doesn’t escape.
Fig.3. Ambush and feeding of Angelsharks, example of Squatina squatina ambushing prey in the taken Canary Islands. video by the Angelshark Project
Being sedentary isn’t always the best:
Due to their sedentary lifestyle angelsharks are susceptible to fishing pressures such as benthic trawling (also known as bottom trawling), where they are frequently caught as bycatch. Being caught as bycatch may be one of many impacts that have lead to the decline of the species throughout its natural range. Additionally, late maturity, low fecundity and biannual reproductive cycle of the species make it difficult for recovery following a steep decline in population.
To sum it all up:
Angelsharks are perfect ambush predators with a variety of adaptions such as cryptic coloration and enlarged flat bodies that allow them to remain invisible within the sediment. Unfortunately, the species are Critically Endangered and much of their ecology and life history remains unknown. To help understand more about 3 of the species, Atlantic Angelshark (Squatina squatina), Sawback Angelshark (Squatina aculeata), Smoothback (Squatina oculata) within the Eastern Atlantic and Mediterranean Angelshark Conservation Strategies were created by working with a variety of stakeholders i.e. fishers and divers.