Great Hammerhead

I am so excited to introduce you to this species-- because our good friend, and Behind the Finsinterviewee, Sebastian Kraft is going to be talking about this animal (and a few others in the future)! See his interview here.

Size
Like the name suggests, the great hammerhead, Sphyrna mokarran, is *ta daa* the largest species in the hammerhead family. It can measure up to 6 meters, but the majority of individuals are smaller. There are two main distinctive features to these animals: the tall, scythe-like dorsal fin, which is also the biggest fin of the shark, and the shape of the head, which has a more rectangular outline compared to other hammerheads. Like other hammerheads, they bare their young in a very unique way: they reproduce via placental viviparity, kind of in the way mammals do (although it’s not exclusive to hammerheads). The structures involved are similar to those of mammals, but they have a different developmental origin: in these sharks, once the yolk sac is depleted, it transforms into an umbilical cord and placenta. They can give birth to 13 to 42 pups, after a gestation period that can last from 7 to 11 months. The pups are born at a size of 50 to 70 cm.

The unique shape of the dorsal fin is enough to guess the species in this picture. This spotted eagle ray clearly did not know this and got a little too close while trying to identify the shark. Credit: Ralph De Bie/Caters News

​These are coastal and semi oceanic sharks, found in tropical and subtropical waters of all oceans, from very shallow waters of a meter in depth down to 80 m. Here, they feed on a wide variety of animals, from crustaceans to fishes, but they are famous for being avid predators of stingrays. And eating stingrays comes along with a sharp, nasty problem. Yup, you guessed it: barbed stingers. On top of that, these can also be venomous, but hammerheads don’t seem to care much, because they are frequently found with several lodged in the mouth and throat. One shark was found with almost 100 of them. They have been observed pinning them down with their head and holding them down to the bottom, then circling it and biting off the fins to hinder the ray from escaping. Clever girl (Jurassic Pun intended).

Can’t really blame the poor stingray for trying to defend itself if this is what’s coming at it.

​Now, let’s talk a little more about the head.
The laterally elongated head of a hammerhead is called a cephalofoil, and the shape varies depending on the species. But it’s not just aesthetics: several purposes have been hypothesized for the evolution of such a peculiar head shape, mainly to boost the senses and to aid in hunting. While swimming, sharks move their head laterally (a movement known as yawing), gaining a broader visual field, because this way they cover areas that would otherwise be blind spots. Frontally, the sight of each eye overlaps with this movement, generating an area of binocular vision which greatly improves their depth perception. This study neatly looked into this (doi: 10.1242/jeb.032615). You can test this for yourself by trying to reach for an object while covering one of your eyes (do this before doing it with both eyes so your brain doesn’t have previous experience to rely on).
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The cephalofoil also works like a metal detector for prey. The ampullae of Lorenzini are electro-perceptive cells, present in chondrychthyans and some bony fishes like sturgeons that perceive electric fields at a short distance, through openings filled with a conductive gel. They are distributed over the ventral surface of the head. This way they can “see” visually-hidden prey, for example a flounder buried under the sand, since sharks can pick up very weak signals, such as heartbeats. Thanks to the shape of their heads, hammerheads have a larger sensing surface relative to other sharks.

This ability of sharks in general has been studied for many years, like in the classic work entitled “The Electric Sense of Sharks and Rays” of A.J. Kalmijn, published in the year 1971 in the Journal of Experimental Biology.
Some studies have looked into the way sharks pick up odors from their surroundings (like this one 10.1016/j.cub.2010.04.053), noting that sharks tend to turn to the side from which the odor comes first, in disregard of its concentration. Following the plume in this way allows the shark to home in to the source of the odor. As with the other senses, hammerheads take their smelling game up a notch. Their nostrils are wider apart and therefore more sensitive to smaller angles of incoming odor plumes, better resolving the direction from which it comes.

All we’ve talked about so far serves to the purpose of navigation and finding their prey. After finding it, if they have to go on a high-speed chase, the cephalofoil is of help here too. It improves the maneuverability and makes sharp turns easier, allowing the shark to get a better chance at getting to that delicious, weaponized sea pancake.

Ouch. Photo: https://tinyurl.com/y6u7x367

​While on the subject of making swimming easier, the large dorsal fin serves a curious purpose in this. It has been found that great hammerheads actually swim up to 90% of the time on their sides, in an angle of 50-75° (a video here). This reduces the energetic cost of swimming. Here’s how it works: in the water, a shark is constantly facing and generating different forces. One of these is the shark’s natural tendency to sink, which is counteracted by another force called lift (“L” in figure XX), generated by the pectoral fins. By rolling on its side, the dorsal fin of the great hammerhead functions as a pectoral fin, but of greater surface and hence greater lift force.

This figure shows that the lifting surface while swimming tilted (b’) is larger than the lifting surface while swimming upright (b). Source: DOI: 10.1038/ncomms12289

​Threats
Its IUCN Red List conservation status is Endangered, because it is highly sought for its fins and is often caught as bycatch. The high fishing pressure has greatly reduced their numbers, since, like virtually all elasmobranchs, these animals are long lived, of late sexual maturity and don’t give birth to many pups. Additionally, hammerheads are especially sensitive to capture and handling related stress and have high post release mortality.
 

WHAT DID YOU LEARN ABOUT THIS ANIMAL?

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