The scalloped hammerhead (Sphyrna lewini) is a fascinating shark species recognized by its distinct hammer-shaped head. This unique feature, from which it derives its name (the Greek word sphyrna translates to "hammer"), houses its eyes and nostrils at the tips of the extensions. The scalloped hammerhead, also referred to as the bronze, kinky-headed, or southern hammerhead, thrives in warm, temperate, and tropical coastal waters globally, between latitudes 46°N and 36°S, and depths reaching 500 m (1,600 ft).
Taxonomy and Distribution
Originally named Zygaena lewini, the scalloped hammerhead was later reclassified as Sphyrna lewini by Edward Griffith and Hamilton Smith in 1834. Over time, it received other scientific names, including Cestracion leeuwenii, Zygaena erythraea, Cestracion oceanica, and Sphyrna diplana.
This coastal pelagic species inhabits continental and insular shelves and nearby deeper waters, favoring warm temperate and tropical regions worldwide. While it can be found at depths exceeding 500 m (1,600 ft), it is most commonly observed above 25 m (82 ft). Scalloped hammerheads exhibit diel movement patterns, often staying closer to the shore during the day and venturing further offshore to hunt at night. Juveniles and pups find refuge in shallow coastal habitats like bays and mangroves, which offer protection from predators and nutrient-rich waters.
Research conducted by Misión Tiburón revealed that adult scalloped hammerheads migrate from the pelagic waters surrounding Cocos Island to the mangroves in Golfo Dulce, a tropical fjord on Costa Rica's Pacific Coast. This area serves as a pupping ground for females, with juveniles remaining in the mangrove's shallow root system for approximately three years.
Social Behavior and Habitat Use
Scalloped hammerheads are frequently observed in large schools, sometimes numbering in the hundreds, during the night, day, and morning. This social behavior likely facilitates more efficient foraging, particularly for larger and more challenging prey. Younger sharks tend to stay closer to the surface, while adults are found at greater depths. Deep water serves as both a refuge and a feeding ground for scalloped hammerheads.
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Despite their high metabolic rates, these sharks often exhibit sedentary behavior, allowing currents to carry them as they swim. This behavior influences their habitat selection and preferred depths.
Diet and Feeding Habits
Scalloped hammerheads are generalist and opportunistic predators, consuming whatever is available and abundant in their environment. While there is no dietary difference between male and female sharks, larger individuals tend to prey on larger organisms compared to smaller, juvenile hammerheads. Their diet primarily consists of fish, such as sardines, mackerel, and herring, with occasional consumption of cephalopods like squid and octopus. Younger individuals tend to feed in coastal waters, on benthic and neritic fish. There have also been reports of scalloped hammerheads preying on smaller sharks and rays, and even sea snakes, employing a "smash and grab" feeding technique. They either swallow their prey whole or disable it by biting.
A recent study highlighted the hammerhead's ability to hunt other shark species, particularly blacktip sharks. While this requires considerable energy, the payoff is significant. Consuming a blacktip shark, which can represent 20-25% of the hammerhead's body weight, can satisfy its energy needs for up to three weeks. This strategy proves more efficient than continuously hunting smaller reef prey. In times of scarcity, a single blacktip meal, supplemented with small fish, could sustain a hammerhead for up to two months.
Reproduction and Life Cycle
Sexual maturity in scalloped hammerheads is typically reached at a length of 240 centimeters (7.9 ft) or longer. Mature females exhibit significantly wider uteri compared to their maturing counterparts. The gestation period is approximately 12 months, with females giving birth to live young. Scalloped hammerheads produce large litters, ranging from 12 to 41 pups, likely due to high infant mortality rates. Like most sharks, they do not exhibit parental care.
Nursery grounds for this species are predictable and consistently used over the years, with individuals returning to their natal sites. However, these sites still experience high infant mortality due to limited resources, resulting in only the fittest surviving to maturity. Population recovery relies on reproduction rather than immigration.
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Conservation Status and Threats
The scalloped hammerhead is currently categorized as Critically Endangered by the IUCN Red List. Overfishing is the primary factor contributing to the decline in population numbers. These sharks are particularly vulnerable to fishing due to their slow growth, late maturation, and low fecundity. They are heavily targeted for their large size and fins, which are sold for US $100-120 per kilogram due to their high fin needle content.
Various conservation efforts are underway to protect this species, including using genetic information from fins sold in markets to track exploitation levels and identify areas where scalloped hammerheads are being caught. Protecting Sphyrna lewini nurseries is another crucial aspect of conservation.
Conservation Efforts
In many regions, fishing vessel management regulations have been implemented, prohibiting the transshipment, sale, or storage of sharks onboard. These regulations aim to protect not only the scalloped hammerhead but also the entire order of Carcharhiniformes. Scalloped hammerheads are also caught unintentionally as bycatch in longlines and nets, which significantly impacts their populations due to their tendency to migrate in large groups. The mortality rate for caught sharks is almost always 90%. Efforts are being made to reduce bycatch through the implementation of specific devices. Marine reserves are being established in various areas to facilitate the recovery of shark populations. Placing these reserves in breeding and nursery areas is crucial for providing a safe environment for pups to mature.
The Importance of Understanding Diet for Conservation
Understanding the dietary habits of scalloped hammerheads, particularly their reliance on larger prey like blacktip sharks, is crucial for developing effective conservation strategies. Protecting these sharks requires a comprehensive approach that addresses overfishing, bycatch, and habitat degradation.
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