Updated: Apr 26
The hammerhead shark is absolutely iconic. There are not many people who would not recognise this bizarre shark. But can you actually name one of the eight species of hammerhead? And do you know why they have that weird-shaped face?
That's are indeed nine different species which we refer to as hammerheads (family Sphyrnidae). These include: the winghead shark (Eusphyra blochii), Carolina hammerhead (Sphyrna gilberti), smalleye hammerhead (Sphyrna tudes), the bonnethead shark (Sphyrna tiburo), scalloped bonnethead (Sphyrna corona), the scooped shark (Sphyrna media), the great hammerhead (Sphyrna mokarran), the smooth hammerhead (Sphyrna zygaena) and the scalloped hammerhead (Sphyrna lewini). You would probably recognise the scalloped hammerhead, as images of them aggregating in massive schools, 200-strong, make for excellent footage in nature documentaries (Gallagher et al, 2014).
Each different species of hammerhead has subtle differences in the physiology of their "cephalofoil" (the broadly symmetrical, dorso-ventrally compressed and laterally elongated, mallet-shaped projections from the side of the head). For example, bonnetheads have relatively short cephalofoils, which are quite rounded. The winged shark has relatively thin, long arms on its cephalofoil, whereas, the other hammerheads, have slightly wider, broad arms. Each also differs in the texture of the leading edge of the cephalofoil; some being smooth and some 'scalloped'. For example, the great hammerhead has a relatively straight leading edge to the cephalofoil (Mara, 2010).
But why did hammerheads evolve this bizarre (but fantastic!) head?
What is its function?
Several different theories have been put forward to explain why the these shark have hammer-shaped heads. Does the cephalofoil create lift or is important for prey manipulation, maybe? The hypothesis that has gained the most weight is the idea that the cepahlofoil provides the hammerheads an advantage in sensory perception. As the cephalofoil is formed by the lateral expansion of the olfactory (smell) and optic (vision) sensory regions, which confers several advantages to the hammerheads:
Firstly, the 'greater olfactory gradient hypothesis' suggests that the greater distance across a hammerhead's cephalofoil (compared to other sharks) provides them increased olfactory acuity (a better sense of smell), which means they are able to navigate towards the source of an odour accurately, when searching for prey.
Secondly, the 'enhanced binocular vision hypothesis' suggests that the elongation of the cephalofoil and the position of the eyes at each end, enhances binocular vision and provides the hammerheads with an enlarged visual field.
Finally (and this is the most widely recognised theory), the 'enhanced electrosensory hypothesis', suggests that the larger surface area of the cephalofoil allows for greater density of "Ampullae of Lorenzini" (jelly-filled pores responsible for detecting electrical fields), which confers the hammerheads remarkable electrosensory capabilities. The extended length of the ampullae pores along the cephalofoil, provide hammerheads greater sensitivity to electrical fields. This allows them to detect the electrical signal produced by muscle contraction in their prey, so they can find an animal even if it buried in sand! The increased distance across the cephalofoil also allows the hammerheads remarkable acuity to perfectly orient their body against an electrical field, which allows them to navigate towards prey very precisely. It is also thought that this ability may be responsible for hammerheads' remarkable long-distance navigation; allowing them to align their body against geomagnetic fields for directed swimming over incredible distances (Mara, 2010).
It seems hammerheads have evolved to become perfect hunters. But how well adapted are they?...
Because hammerheads are so specialised, there are now concerns that the same adaptations which made them so perfect in their environment when they first arose, may now be what has increased their vulnerability to overexploitation by human beings. Where a generalist species may be able to adapt to a changing environment, specialists are limited in the physiological and behavioural capabilities. This is known as an "evolutionary trap" (Gallagher et al, 2014).
The behaviour of hammerheads may be maladaptive in their modern, human-altered environment. Hammerheads group in large schools in order to find a mate and commercial fisheries often purposefully target these aggregations. Whatsmore, as some species of hammerhead undergo extensive migrations, they often cross over jurisdictional boundaries, which means they are especially exposed to unregulated extraction. When these migrations bring hammerheads into coastal regions, the are especially vulnerable to coastal fisheries
It has also been found that, even if hammerheads are freed when they are caught as "bycatch" in fisheries, this may not aid in population recovery. This is because their post-release mortality is especially high; they die as a result of the stress. In fact, it is thought that as many as 80% of individuals which are released will not survive (Gallagher et al, 2014).
Hammerhead sharks have a "K-selected life history strategy", meaning they grow slowly to a large size and have long life-spans, invest significant resources in reproduction and have few offspring. This is a trait they share with other shark species. However, in hammerheads this is particularly pronounced and makes them especially vulnerable to overexploitation, as it means they have a low capacity to recover after population declines.
Anthropogenic activity is altering the natural environment at an alarming rate; so rapidly that many species of animals will not be able to adapt. Hammerheads are experiencing dramatic population declines globally; exceeding 90% in some areas. This is predominantly due to overfishing, both in targeted fisheries and as "bycatch". The scalloped bonnethead is now considered 'near threatened', the small eye hammerhead and smooth hammerhead 'vulnerable', and the winghead shark is classified as 'endangered'. The great hammerhead and scalloped hammerhead are both classified as 'critically endangered', meaning they are seriously threatened with extinction (IUCN, 2020). We must work to make all our fisheries sustainable immediately, if we are to protect our wonderful species, such as the hammerheads, from extinction.
References Gallagher AJ, Hammerschlag N, Shiffman DS & Giery ST (2014). Evolved for extinction: The cost and conservation implications of specialization in hammerhead sharks. BioScience, 64:7, 619-624. Access online.
Mara KR (2010). Evolution of the Hammerhead Cephalofoil: Shape Change, Space Utilization, and Feeding Biomechanics in Hammerhead Sharks (Sphyrnidae). Graduate Theses and Dissertations, University of South Florida Scholar Commons, USA. Access online.