Hammer Time

The hammerhead shark is absolutely iconic. There are not many people on the planet who would not recognise these bizarre sharks. Yet, did you know, there are actually many different species of hammerheads!? All unique. Could you name a specific species? And did you know that scientists now suspect that their distinctive hammer might actually be the Achilles heal that has driven these wonderful animals to the brink of extinction?

The great hammerhead is the largest of all the species of hammerheads (Image Credit: Martin Voeller / Shutterstock)

Out of 10

There are ten different species that we refer to as hammerheads. All the the family Sphyrnidae, these include the winghead shark (Eusphyra blochii), Carolina hammerhead (Sphyrna gilberti), shovelbill (S. Alleni) smalleye hammerhead (S. tudes), the bonnethead shark (S. tiburo), scalloped bonnethead (Sphyrna corona), the scooped shark (S. media), the great hammerhead (S. mokarran), the smooth hammerhead (S. zygaena) and the scalloped hammerhead (S. lewini) (Gallagher et al, 2014).

Probably the most recognisable of these species would be the scalloped hammerhead, as images of them aggregating in massive schools, 200-strong, make for excellent footage in nature documentaries (Gallagher et al, 2014).

Scalloped hammerheads form massive aggregations around seamounts (image Credit: Seawatch.org / WikimediaCommons)

Let's Get Hammered!

You can tell each species of hammerhead apart based on the shape of their face. Whilst all nine species have a dorso-ventrally compressed and laterally elongated mallet that projects out of their head, each species' "cephalofoil" is slightly different (Mara, 2010).

Each species of hammerhead has a slightly different shape to their cephalofoil (Mara, 2010).

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 (Mara, 2010).

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).

All these different features allow each species of hammerhead to fit perfectly into their particular ecological niche. The cephalofoil plays a role in lift and maneuverability for some, and also in enhancing sensory perception for many. To find out more about the specific roles the cephalofoil plays and how it evolved, check out Under The Hammer.

All nine species of hammerheads have a flattened, elongated face, known as a "cephalofoil" (Image Credit: Martin Voeller / Shutterstock)

The Right Tool for the Job?

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).

When in large schools, hammerheads are vulnerable to fisheries (Image Credit: EDGAR PHOTOSAPIENS / Shutterstock)

The behaviour of hammerheads may also 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 (Gallagher et al, 2014).

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).

They are so sensitive to stress that hammerheads can die even if they are thrown back after being caught in a fishery (Image Credit: David Clode / Unsplash)

Fighting Hammer & Tong

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 (Gallagher et al, 2014).

bonnethead sharks are classified as Endangered according to the IUCN (Image Credit: log park/iNaturalist, Source: IUCN, 2020).

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" (Gallagher et al, 2014; IUCN, 2020).

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. It would be an absolute tragedy if we were to lose these wonderful, iconic animals because of our own selfishness and greed.

Several threats have caused hammerhead shark populations to decline in the wild (Gallagher et al, 2014)

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.

IUCN (2020). Red List of Threatened Species. 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.

By Sophie A. Maycock for SharkSpeak