Hammerhead Shark: Biology, Behaviour and Evolution

 The Scalloped Hammerhead sharks otherwise known as Sphyrna lewinischalloped hammerhead shark is part of the Sphyrnidae family, known for their distinctive hammer-like shaped head. Distinguishing features of this species is a broadly arched and hammer-shaped head that a prominent indentation in the middle and then two more flanking the middle indentation hence “scalloped” and pelvic fins that have straight rear margins. Attempting to blend in the scalloped hammerhead shark is brownish-grey to olive or bronze on the top of its body while the underside is white or pale yellow. The Sphyrnidae family has at least 10 species of sharks, 9 species in Sphyrna and 1 species in Eusphyrna both resemble with shovel-like or hammer-shaped heads. Hammerheads are cartilaginous fish where most of their skeleton is cartilage, except for their teeth. Hammerheads occupy tropical and temperate marine environments above continental shelves and near coastlines, migrating seasonally residing in the warmer waters by the equator and poleward during the winters (figure 5). However, years with warm El Nino hammerheads range is greatly expanded due to the warmer waters and they can travel hundreds of kilometers further than average (Kanesa M. Duncan, 2006).

Hammerhead sharks have a unique shape to their head which helps the shark in several ways one having an expanded/ flattened head helps them turn and raise their head quickly and sharply because its unique shape helping the shark get prey quickly and effortlessly. Next observed were the large nostrils that could help the shark have a better ability to sniff out its prey or follow a source of a scent, another idea is the extreme spacing of their eyes may provide a more extensive view of increase anterior depth perception and widen the horizontal field of view (D.M. McComb, 2009)Another key to their unique head shape is the underside of their head where electro-receptive organs which detect electrical impulses of prey, these receptors are so sensitive they can detect impulses of prey buried beneath the sediment (Stephen M Kajiura, 2002) . Lastly, in larger species of hammerheads, it has been observed that they use their unique head shape to pin prey down before biting them. Communication between sharks is mostly through body language and nares for detecting chemical cues or using their sensory organs called Ampullae of Lorenzini, which can detect prey and navigation using the changes in earth’s magnetic field that can be sensed. They can follow the geographical fault lines because recently cooled magma creates rock with a strong magnetic signal which animals are able to sense and follow to congregate in areas (Alex D Rygg, 2013).  Diets for these creatures vary between the different species but mainly consists of a wide array of fish, cephalopods, and crustaceans. Smaller species stick to eating crabs and crustaceans due to teeth that are thicker, flattened and better at crushing while the larger species of hammerheads eat stingray, larger fish, squid, and small sharks due to large sharp teeth. The Scalloped hammerhead shark commonly eats stingrays.  Majority of hammerheads eat in the shallow coastal waters, rarely brackish bays and estuaries, some travel to offshore waters to night feed or to find large social groups of hammerheads. These creatures have been observed being social creatures traveling in groups usually in sexually separated groups, juveniles learning and practicing, or during migration season when they travel in groups, and lastly in mating season (Holly A. Nace, 2011). Schools of sharks congregate near mating season (mostly females), where mature males then break into the groups in in a “S” formation using a torso thrust as the mature female sharks are in the middle of the groups (Peter A. Klimley, 2010). Female scalloped hammerheads range age of sexual reproductive maturity is 15 to 17 years and over 2 meters long, while in males its 6 years and around 1.5 meters in length. Mating season can vary with the region and they are thought to breed every other year, ranging in offspring from 12 to 41 pups, with a gestation period of 8 to 12 months. Pups that are born are usually about 20-28 inches in length feeding on small bony fish and crabs. Scalloped hammer head sharks are considered one of the larger reef fishes and a top predator can still be preyed on by great white sharks, killer whales, larger tiger sharks, and humans.

 Hammerhead shark juveniles change behavior and habitats as they grow. Juvenile sharks reside in shallow coastal waters and have been observed “sun bathing” in the shallow waters and tanning become a dark color. The heads of juvenile hammerhead sharks are rounded but eventually flatten out as they mature. The young sharks live in the shallow waters for the first 3 years of their lives this is when they are most vulnerable to predators, they then venture into deeper waters once their tolerance increases they are able to handle fluctuation in temperatures and low levels of oxygen. It is thought that the young sharks migrate to the deeper waters for richer food diversity or to join a school of hammerheads, they will attempt to assert dominance to establish a social rank within the school (figure 6). However, many of the schools of sharks disperse at night which is when the sharks go hunting. Researchers and scientist found this by tagging some of the juvenile sharks and tracking their movements.

Sharks are keystone species in the ecosystems cleaning the oceans and controlling populations.  Hammerheads like other sharks are important in their ecosystems by controlling the populations below them in the food chain, helping remove the weak and sick along with helping species diversity by keeping a balance between competitors. Being a predator hammerhead sharks alter prey’s spatial habitats by hunting, this, in turn, helps shift their diets and feeding strategies. Through the changing prey’s spatial habitats help maintain the coral reefs and seagrasses because the fish are adapting and moving locations all the time. Shark populations indicate oceans health, when shark populations are low has head to decline coral reeds, commercial fisheries, and seagrass beds. This is due to predatory fish other than the shark that are then able to overhunt on the herbivores. Without herbivores, microalgae thrive thus the coral suffers, making the waters become algae dominant which has adverse (Thomas A. s. Clarke, 1971). Hammerheads have a huge effect on the habitat and the biota living there because they are a key stone species.

The earliest fossil evidence of sharks or their ancestors are a few scales that were thought to be Chimaeras, a relative to the shark, dated to 450mya in the Late Ordovician period, but not teeth making researchers wonder if the ancestor of the sharks were toothless. Tracing living sharks, rays, and chimaeras researchers have predicted that by 420mya chimaeras had split off from the group. Shark-like teeth are discovered from an Early Devonian (410mya) belonging to Doliodus problematic, thought to have come from acanthodians or spiny sharks (figure 1). However, Acanthodians don’t like sharks they had diamond-shaped scales and spines in front of all their fins, the only resemblance they shared was a cartilage skeleton, skull, and jaw resembling sharks, and shark-like teeth. The Cladoselache evolved around the Devonian (380mya) which we recognize as our modern-day sharks, however, could have been part of the chimaera branch (figure 2). Cladoselache had torpedo-shaped bodies with forked tails and dorsal fins were active predators. Carboniferous period (began in 358 mya) known as the “golden age of sharks”, due to extinction even nearing the end of the Devonian which killed of almost 75% of species on land and marine. This extinction allowed sharks to thrive and disperse adapting pectoral fin skeletons, development anal fin, and new ventral element which evolved into different species such as the Stethacanthus, Helicoprion, and Falcatus. Near the end of the Permian Period (252 mya), there was another extinction that wiping out almost 95% of marine life were a small number of shark species survived. Early Jurassic period (195mya) the Hexanchiformes or six-gill sharks which are the oldest known group of modern sharks had evolved. They evolved to swim faster and protruding jaws that were flexible making it easier to eat prey bigger than themselves. During the Cretaceous period, two extinctions took place sharks were mostly affected by the second extinction where the larger species were killed off and the smaller deep-water species survived. These smaller deep-water species that survived are the ancestors of our sharks today is why sharks are considered living fossils.

 Hammerhead shark teeth found suggest their ancestors could have existed 45mya, however, molecular data suggest they appeared recently in the Miocene epoch (20mya). Like many other sharks have bones that are part cartilage making fossilizing sharks difficult, however, their teeth are pure bone and fossilize well. Hammerhead shark teeth have a defined notch on the outer side of the tooth which separates the crown and enameled shoulder also known as “hammerhead notch”. The teeth also have smooth-edged cusps and grow in multiple rows to replace teeth that have fallen out (figure 3). Scientist are using mitochondrial and nuclear DNA to create a family tree and track gene mutations. Finding there had been multiple examples of decreasing size in hammerhead shark which was thought to help retain juvenile traits and or neoteny (, K. M. Duncan, 2006). Sharks range in size of the cephalofoils which are flattened shaped heads with bulging eyes at opposing ends. They have evolved into different shapes such as the bonnethead shark or otherwise known as the shovelhead shark (Sphyrna tiburo) which has a shovel-shaped head, in comparison, there is the smooth hammerhead (Sphyrna zygaena), the great hammerhead (Sphyrna mokarran), and the wing-head shark (Eurphyra blochii).

 Scalloped Hammerhead sharks made it into Appendix II of the Convention on International Trade in Endangered Species list in 2013, different from the Endangered Species Act governments can certify that a catch is “sustainable” and is allowed for international commercial trade. However, the IUCN Shark Specialist Group consider the scalloped hammerheads great hammerhead shark, Wing head shark to be endangered species due to steep declines in populations (Miller, 1970). Researchers have found hammerhead populations have declined worldwide and are more severe declining populations than any other shark species. Laws have been put into place to help protect hammerheads however many determined fishermen find loopholes. During the mating season, scalloped hammerhead sharks aggregate making them an easy target for fishers who know their mating seasons and cycles. Hammerheads are migratory species they move through different political jurisdictions making it difficult to manage, along with commercial fishers catching hammerheads right outside of state waters. The fins of hammerheads are prized in the for their fins for shark fin soup which is a popular delicacy in China, but not shark meat because like most shark species they urinate through the skin making the meat taste of ammonia (Demian D. Chapman, 2010). Shark cartilage is used in cancer treatment, arthritis, psoriasis, wound healing, and inflammation of intestines (Hasan Monjurul, 2017). Another reason hammerhead populations are so low is because of their severe physiological stress from being caught, causing metabolic disturbances as well as physiological. Unfortunately, majority of hammerheads that are caught are predicted to die shortly after release. Due to hammerheads being damaged by stressors easily, studying the species can be difficult (Cliff G., 2010).

 The hammerhead shark is a unique creature and is therefore being hunted with natural causes adding to population decline. The shark’s unique nature as well as adaptations is specific to its region usually in shallower warmer waters, with increase in climate change and temperature changes makes me wonder if their habitat regions will expand. Although the populations are declining I think that could easily change with stricter laws on shark products on the market. A lot of people and cultures depend on the shark’s cartilage and fins making it hard to convince people to give up a tradition that has been going through generation after generation. There have been multiple documentaries about shark fin soup and the effects it has on the shark its self and the ecosystems that they are taken from, even with the documentaries though many people are not willing to give it up. Unfortunately, like mentioned earlier shark cartilage is used for many things in the medical field making it even harder to give up. Human disturbances to the sharks makes a huge impact on the ecosystem and marine biota living there, this in turn starts to affect the economy. There are multiple ways that humans are disrupting the sharks by cutting them up as well as destroying their habitats with tourism and human pollution in general like CO2 emissions amongst others.

Figures

Figure 1

Figure 2

Figure 3

(Figure 4) Phylogenetic tree of the scalloped hammerhead.

(Figure 5) Range map of scalloped hammerhead (Sphyrna lewini)

(Figure 6) School of hammerhead sharks

Bibliography

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• Clarke, Thomas A. “The Ecology of the Scalloped Hammerhead Shark, Sphyrna LewiniJ in Hawaii.” Scholarspace, manoa.hawaii.edu, Apr. 1971, scholarspace.manoa.hawaii.edu/bitstream/10125/4191/vol25n2-133-144.pdf.
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