You might assume that an animal as mighty and ferocious as the great white shark (Carcharodon carcharias) is completely indestructible... But that is simply not the case. These sharks are vulnerable to injury, just like the rest of us! It is very common to see extensive scarring on white sharks, especially on their snout, where they have taken a few hits from seals fighting for their lives (seals can pack quite a punch - they are not as cute and defenceless as you might think!). Recently, scientists in Mexico even found that great white can suffer serious injuries from squids, which are not too happy about being the shark's next snack! So how on earth can a creature without a backbone injury a mighty white shark?
Let's Eat!
Guadalupe Island, off the coast of Mexico, is home to large aggregations of great white sharks, which migrate into the area to target seasonally abundant seal prey. Despite coming into the area looking for seal dinners, white sharks are "generalist" predators and have quite a varied diet. When they are young, white sharks feed predominantly on fish, squid and octopi, and will even snack on smaller shellfish. It is only when they reach a critical size of 3 metres total length, that they start to also incorporate mammals, like seals, dolphins and whales, into their meal plans (this is known as an "ontogenetic diet shift") (Klimley, 1985).
Calamari Entree
In Mexico, there are several species of squid which white sharks can snack on: neon flying squid (Ommastrephes bartramii), purpleback squid (Sthenoteuthis oualaniensis), sharpear enope squid (Ancistrocheirus lesueurii) and the giant squid (Architeuthis dux) (Becerril‑García et al, 2020). All of these species can be quite large - reaching more than a metre in length. Individual jumbo squid (Dosidicus gigas) also known as the Humboldt squid or Red Devils, have even been known to reach sizes as large as 2 metres long!
Researchers studying the white sharks in this region in Mexico noticed that many of their sharks had unusual scars and injuries on their bodies... so they decided to take a closer look. They described scars, mostly on the head and trunk of the sharks, which had distinctive sucker marks. Some of these scars were as long as 60 cm! Based on the location of these scars, they suspected that they were caused by a large squid, like the Humdoldt squid, defending itself as the shark attempted to eat it (Becerril‑García et al, 2020).
Spineless, but Not Harmless
Unlike mammals and fish, squids do not have a skeleton (they are "invertebrates"), therefore, their muscles cannot pull on solid bones to create power. Squids swim by undulating their tentacles whilst simultaneously expanding and contacting their mantle (the calcified protective layer which covers the main bulk of the squid's body); which draws water in, before forcing it back out to create forward thrust.
The suckers which run all along their tentacles and arms, allow them to grasp objects by creating a vacuum; the sucker is placed against a surface to create a tight seal and water inside the squid is pumped away to create pressure behind the sucker. This gives the squid an incredibly tight grip! (Becerril‑García et al, 2020)
It is quite remarkable to think that an animal without a skeleton, generating muscular contraction through the displacement of water could be so strong! No jaws... no teeth... just the sheer power of suction... With enough power to cause significant injuries to one of the oceans most formidable sharks. Incredible! (Becerril‑García et al, 2020)
Way Down We Go
These findings are particularly interesting to scientists, because it tells us a little something about how and where great white sharks hunt in Mexico. The squids in this region generally occupy a deep layer of the ocean (between 200 and 600 metres deep) during the day and then come into shallower waters to hunt at night. This is known as a "diel vertical migration" (Becerril‑García et al, 2020).
The great whites around Guadalupe Island are known to hunt seals at the surface during the day, and then to spend time at depths greater than 100 metres throughout the night. So their battle scars suggest they are probably performing nocturnal dives to deeper depths in order to hunt the squid (Becerril‑García et al, 2020).
This tells us that Guadalupe island is not only important to white sharks because it offers them seals for food, but that the sharks target a wider range of prey in the area than we previously thought (Becerril‑García et al, 2020).
References
Becerril‑García EE, Bernot‑Simon D, Arellano‑Martínez M, Galván‑Magaña F, Santana‑Morales O & Hoyos‑Padilla EM (2020). Evidence of interactions between white sharks and large squids in Guadalupe Island, Mexico. Scientific Reports,10, 17158. Access online.
Klimley AP (1985). The areal distribution and autoecology of the white shark, Carcharodon carcharias, off the west coast of North America. Southern California Academy of Sciences, Memoirs, 9, 15-40.
By Sophie A. Maycock for SharkSpeak
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