Shark of the Day – Sphyrna zygaena smooth hammerhead30.03.2020
He looks like a fairy tale animal. A unicorn. A shark with a hammer structure at the top of its head. Meet the hammerhead shark, and find out why this shape is useful, and what has been happening to the world’s hammerheads in the last decade.
The Sphyrnidae family includes 10 species, all but one of which belongs to the genera Sphyrna. The scientific name is from the Greek “sphyra” which means hammer. They are commonly found in all oceans, and their size varies by species from a half-meter shark to the largest 6-meter shark. The smooth hammerhead is the second largest species and reaches about 4 meters as an adult.
Three of the species are cosmopolitan, including in the Red Sea and the Israeli Mediterranean: The smooth hammerhead, Sphyrna zygaena (Mediterranean); scalloped hammerhead, Sphyrna lewini (Eilat); and great hammerhead Sphyrna mokarran (Eilat).
Biology – or – why do they look like this?
Hammerheads are usually characterized by a very high back fin, and of course a flattened and laterally extended head. Their eyes are located on both edges of the hammer, allowing for a much wider range of vision than eyes located on a face. A hammerhead, unlike other sharks, can also look down.
In addition, as we know, both sharks and skates have a sixth sense for detecting electrical currents. This sensing is done with a network of organs called the ampullae of Lorenzini which are located in sharks throughout the head area. For hammerheads, these organs are particularly developed, and the entire hammer shape is filled with electric sensing ampoules.
Hammerheads are relatively large predators. Most of them feed on large fish. Smooth hammerheads feed on large fish, squid, skates, and smaller or younger sharks, even those belonging to the same species.
The special head shape and electric sensing allow the shark to use its head as a “minesweeper” to locate skates hidden in the sand.
Of all the hammerheads, the smooth hammerhead is the most widely distributed shark. It is a cosmopolitan shark commonly found in oceans worldwide, with a relatively high tolerance for temperature changes. This species is present in the Mediterranean Sea and even in Israel. It has never been documented in the Red Sea.
Smooth hammerheads are relatively shallow swimmers, often cruising close to the surface or in the top 20 meters, with short descents of up to 200 meters. Sharks of this species are usually solitary, but during the migratory season, herds of hundreds of young, and even herds of thousands have been observed off the coast of California.
The breeding cycle in hammerheads includes a long pregnancy, followed by birth, as with their gray shark relatives. Other shark families spawn or lay eggs.
Pregnancy in smooth hammerheads lasts for almost a year; the mother carries a relatively large litter of pups (20-50), and gives birth in shallow coastal areas called nursery grounds. Babies are born about 50-60 cm in size. Males will reach sexual maturity at about 2 meters in size and females around 2.7 meters. They are known to have a life span of around 20 years. This life cycle puts them among the fastest reproducers among all shark species, and the fastest by far of the other hammerheads.
Due to their overall size, and the size of their fins, hammerheads are a highly sought-after species for shark fin trading and for fishing in general. Smooth hammerheads are usually caught from shore by nets or baited hooks on longlines. Sharp historical declines of this species have been documented in the Atlantic, but since the implementation of management and outreach programs, these declines have slowed or stopped, and in some regions of the world there have even been increases in population sizes (such as in the Indian Ocean). For this reason, and because the smooth hammerhead has a relatively fast breeding rate for sharks, this species is listed as only “vulnerable” in the IUCN Red List (International Conservation of Nature Association). A relatively low rating for sharks, and especially for hammerheads.
In the Mediterranean, on the other hand, there are hardly any conservation and management plans for sharks, and we expect the decline and eventual disappearance of all kinds of hammerheads in the sea. In Israel, hammerheads used to be observed in large bands of several dozen individuals in the spring and fall around the coastal power stations (Hadera, Ashdod, and Ashkelon). Observations collected under the “Sharks in Israel” project show that over the past decade, the reports have all but disappeared, apart from occasional individual sitings (one in 2013, and one in 2016). The situation in the rest of the Mediterranean is similar, with observations almost nonexistent, and the species is ranked as “critically endangered” in the Mediterranean by the IUCN Red List.
Despite the fact that hammerheads are considered predators, in 2018, researchers at the University of California found that Bonnethead sharks eat seagrass. Due to reports of herbivorous sharks, a laboratory experiment was conducted in which they measured how much seagrass hammerhead sharks would eat, and how efficiently their bodies could break it down. Researchers determined that this species of hammerhead can consume up to 60 percent of their diet from vegetation and observed no adverse effect on their growth rate and development following this diet!
Despite the exciting headlines, these sharks are not, of course, vegetarians, but are defined as omnivores, and yet, this is a groundbreaking discovery of omnivorous sharks; species that previously were considered absolute predators or plankton filter feeders.
Remember: All sharks and skates are protected in Israel and their fishing is prohibited by law.
Adi Barash is a doctoral student at the University of Haifa’s School of Marine Sciences, and chair of the Sharks in Israel Association.
Translated by Daphna Shapiro Goldberg.
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