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Sharks on Seamounts

By Nikki Harasta, Marine Conservation Institute Science Intern


Sharks are incredibly important components of many different marine ecosystems. Unfortunately, sharks often enter our consciousness only when a shark attack on a beachgoer makes the news. Take a closer look at the numbers however, and you’ll see that they’ve been given a bad rap. The U.S. only averages about 16 shark attacks per year, with less than one death occurring per year. In contrast, lightning strikes claim about 41 lives each year. In fact, you’re more likely to die from a falling coconut, an ant attack, or even a vending machine than from a shark attack! Despite the sharks that you most frequently see on the news, many shark species spend most or all of their time in habitats far from beaches, including pelagic offshore waters and deep-sea environments.


Many of the larger sharks we fear are largely pelagic – they spend a lot of time in the open ocean spanning wide migratory distances. In 2003, female great white shark, Nicole, completed the first documented cross-ocean migration from Africa to Australia and back (a total distance of about 12,400 miles). A number of sharks also call the deep sea their home, using seamounts, ridges, and other seafloor features as nurseries, feeding grounds, and migratory pathways.


Unfortunately, even in these remote open-ocean and deep-sea habitats, sharks are still at considerable risk from human impacts. Shark finning, overfishing and by-catch have landed a number of sharks and rays (class: Chondrichthyes) on the IUCN Red List of Endangered Species. Due to the fact that they generally birth few offspring, take a long time to reach sexual maturity, and live relatively long lives, sharks are highly susceptible to over-fishing. Commercial and recreational fisheries that intentionally catch sharks, or incidentally hook them as by-catch, pose a huge threat to shark populations. A staggering 70 million sharks are killed by humans every year, with 17% of species classified as ‘threatened’ by the IUCN.


Keep reading to learn about three sharks that live, feed, or congregate in or around seamount habitats.


Sixgill Sharks

A bluntnose sixgill shark (Hexanchus griseus) swims past the remotely operated vehicle (ROV) Beagle on a joint expedition with Marine Applied Research & Exploration, NOAA, and Marine Conservation Institute. Image courtesy of MARE/NOAA.


Bluntnose sixgill sharks (Hexanchus griseus) are often observed in deep-sea habitats up to depths of more than 6,000 feet. Also known as cow sharks, they grow to over 20 feet in length and may weight up to 1,300 pounds. These sharks have an ancient lineage, and still closely resemble their ancestors from the Triassic Period – over 200 million years ago! As evidenced by their scientific name (Hexanchus – the prefix ‘hex’ meaning six), they have the unusual feature of having six gills – one of only three living sharks that do (most modern shark species have five, while a few have seven). Sixgill sharks hunt a wide array of prey including fish, rays, squid, crabs, seals, and even other sharks. They perform a classic example of vertical migration, typically resting near the seafloor and then rising through the water column in search of food. Like many shark species, they are ovoviviparous meaning that they produce eggs but rather than laying them, the eggs are allowed to develop and subsequently hatch within the mother. Unlike with human embryos, all of the nutrition for the growing shark embryos comes from the yolk sac of their egg. However, in addition to carrying the eggs, the mother also provides one important service for her growing young – gas exchange. Once the eggs hatch inside of the mother, the young sharks are ‘born’ in litters as large as 108 baby sharks!


Hammerhead Sharks

A smooth hammerhead shark (Sphyrna zygaena) swims in the productive offshore waters in Southern California. This species is listed as Vulnerable by the IUCN. Image courtesy of NOAA.


Hammerhead sharks (Sphyrnidae) are 500–1,000 pound oceanic carnivores that range from 13–20 feet in length. These communal fish are often seen grouped in schools during the day, but they prefer to hunt in solitude at night. As an apex predator, the hammerhead shark plays a top-down regulatory role in marine ecosystems. The distinguishing feature of hammerheads is their elongated, flattened head, which give them a better visual range for hunting and contain sensory organs that detect the electrical fields of their prey. These sensors also give hammerhead sharks a unique connection to seamounts. They help them detect the magnetic fields emitted by lava flows from nearby seamounts, enabling them to navigate miles of open ocean to concentrated areas of their cohorts. By using seamounts and other deep-sea features for navigation, hammerhead sharks form well-established “superhighways” to meet in large groups to pair for mating. One seamount in the Sea of Cortez was once discovered to have a massive aggregation of over 500 mating hammerheads!


Goblin Sharks

A 15-foot-long goblin shark caught (and released) by a shrimp fisherman in the Gulf of Mexico. Image courtesy of Carl Moore/NOAA.


Goblin sharks (Mitsukurina owstoni) are among the oldest living sharks. These ‘living fossils’ have been around for over 125 million years, typically inhabiting the deep sea at depths of up to 4,000 feet. Their elongated snout contains sensory organs including the ampullae of Lorenzini, a highly developed organ that allows sharks to detect electric fields in water. The ampullae are a series of gel-filled canals, each containing a connection to a pore in the skin. Each one contains a large number of electroreceptor cells capable of detecting extremely small voltage differences. Animals produce small amounts of electric currents due to muscle contractions, allowing goblin sharks to hunt effectively in extremely deep, dark waters. Goblin sharks are believed to feed primarily on small fish and squid, snatching their prey with their extruded teeth (as many as 115 teeth!). Their jaws have the incredible ability to rapidly snap forward to grab prey. Two pairs of elastic ligaments in their lower jaw are naturally held under tension, allowing the shark to snap its teeth forward just like releasing a stretched rubber band.


Conserving Sharks

As commercial fishing activity brazens forward, the necessity for conservation strategies to protect sharks from further decimation becomes more and more pertinent. Protecting seamount from the threats of shark fishing industries will help preserve important nursery, feeding, and migratory habitats for sharks. Urgent conservation efforts, including more marine protected areas, are needed to prevent the loss of vast numbers of shark species in the next decade, with cascading ripples on the rest of our delicate marine ecosystem.