Scalloped Hammerhead Sharks, Sphyrna lewini
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Description & Behavior
Scalloped hammerhead sharks, Sphyrna lewini (Griffith and Smith, 1834), are large hammerhead sharks with moderately high first dorsal fins and low second dorsal and pelvic fins. They can be distinguished by the broadly-arched front margin of their head that has a prominent median notch (see the ID photos on the great hammerhead page for a comparison between the great, scalloped hammerhead and bonnethead sharks). The side "wings" of their heads are narrow with their rear margins swept backward. Scalloped hammerheads are uniformly gray, gray-brown, or olive on their dorsal (upper) surfaces, fading to white on their ventral (lower) surfaces and their pectoral fins are tipped with gray or black ventrally. Most sharks encountered by divers average 2-2.5 m in length. Males mature at 1.5-1.75 m and reach an average of 3 m, females mature at about 2 m. Maximum length of this species is known to be at least 3.7 m, and it is thought that a few individuals may reach a length of over 4 m.
World Range & Habitat
Scalloped hammerheads, Sphyrna lewini, are coastal-pelagic, semi-oceanic sharks occurring over continental and insular (island) shelves and adjacent deep waters, often approaching close inshore and entering enclosed bays and estuaries. They are found in waters to about 275 m in depth. Huge schools of small migrating individuals move poleward in the summer in certain areas. Permanent resident populations also exist. Adults are solitary, in pairs, or schools; young are often found in large schools.
Feeding Behavior (Ecology)
Scalloped hammerheads, Sphyrna lewini, feed mainly on teleost (bony) fishes and cephalopods (squid, octopus, and cuttlefishes), also lobsters, shrimps, crabs, as well as other sharks and rays. In the Indo-West Pacific, stomach contents have been found to include sea snakes.
Scalloped hammerheads are viviparous, with a yolk-sac placenta. Pups are born following a 12 month gestation. Produces 15-31 pups, of 43-55 cm young in a litter. Pups occupy shallow coastal nursery grounds, often heavily exploited by inshore fisheries, and then migrate offshore as they mature.
Conservation Status & Comments
Scalloped hammerhead sharks are considered potentially dangerous to people but usually non-aggressive and shy when approached by divers. Under baited conditions scalloped hammerheads may make close approaches to divers but quickly lose interest and depart when they determine that the divers are not the source of the food odors.
Readily available to inshore small commercial fisheries as well as to offshore operations. Sold fresh, dried-salted, smoked and frozen; also sought for its fins and hides. Oil used for vitamins and carcasses for fishmeal. Lack of data on population trends makes it difficult to assess whether the high level of catches of this species at all life stages is having an effect on stocks, but some declines are reported.
The scalloped hammerhead shark is listed as Endangered (A2bd+4bd) on the IUCN Red List of Threatened Species:
ENDANGERED (EN) - A taxon is Endangered when the best available evidence indicates that it meets any of the criteria A to E for Endangered (see Section V), and it is therefore considered to be facing a very high risk of extinction in the wild.
References & Further Research
Research Sphyrna lewini » Barcode of Life ~ BioOne ~ Biodiversity Heritage Library ~ CITES ~ Cornell Macaulay Library [audio / video] ~ Encyclopedia of Life (EOL) ~ ESA Online Journals ~ FishBase ~ Florida Museum of Natural History Ichthyology Department ~ GBIF ~ Google Scholar ~ ITIS ~ IUCN RedList (Threatened Status) ~ Marine Species Identification Portal ~ NCBI (PubMed, GenBank, etc.) ~ Ocean Biogeographic Information System ~ PLOS ~ SIRIS ~ Tree of Life Web Project ~ UNEP-WCMC Species Database ~ WoRMS
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