Smalleye HammerheadSphyrna tudes
This shark has an distinctly flat and arched cephalophoil, or "hammerhead", with unusually small eyes due to its preference for muddy coastal waters. It starts out as a bright gold or yellow orange color, and fades to a yellowish-gray with age, but retains some of its yellow-orange coloring at its head, fins, and underside. Its body is classically shark-shaped, with a tall dorsal fin and asymmetrical caudal (tail) fin, and typically grows to no more than 52 to 58 inches long.
Order - Carcharhiniformes
Family - Sphyrnidae
Genus - Sphyrna
Species - tudes
Common English names for the smalleye hammerhead include bonnet, curry shark, golden hammerhead, great hammerhead, hammerhead, and shark. Other names include cação-chapéu-armado (Portuguese), cação-martelo (Portugese), cambeva (Portuguese), cornuda de corona (Spanish), corunda ojichica (Spanish), kannankodi (Malayalam), kleinooghamerhaai (Dutch), krozza (Maltese), kurazza (Maltese), kurazza rasha zghira (Maltese), marteleiro (Portuguese), mikrozygena (Greek), peixe-martelo (Portuguese), pesce stampella (Italian), pez martillo (Spanish), pixximartell (Maltese), requin-marteau a petits yeux (French), and tubarão-martelo (Portuguese).
Importance to Humans
The smalleye hammerhead is one of the most abundant species along the east coast of South America and is therefore a significant portion of the by-catch of shrimp boats and other fisheries. While the fins and meat are sold, it is not considered to be of large economic importance.
Danger to HumansAccording to the International Shark Attack File there have been 17 unprovoked attacks by the genus Sphyrna. The actual hammerhead species is often difficult to pinpoint during an attack due to only slight variations between species morphology. There has only been one reported hammerhead attack in the smalleye hammerheads home range, the East coast of South America, and it is one of the few to be identified as a scalloped hammerhead, S. lewini. All sharks have the potential to be dangerous to humans if harassed, however the smalleye hammerhead likely poses no serious threat to humans.
In recent years, there has been a marked decline reported in artisanal catches of smalleye hammerheads in Trinidad implying that the population may have been impacted as a result of overfishing. Due to the pressures of overfishing and small litter sizes the smalleye hammerhead has been labeled as "Vulnerable" on the IUCN Red List of Threatened Species.
The IUCN (World Conservation Union) is a global union of states, governmental agencies, and non-governmental organizations in a partnership that assesses the conservation status of species.
Geographical DistributionThe smalleye hammerhead is found in the subtropical waters off the east coast of South America from Venezuela to Uruguay. The Orinoco delta located on the northeast coast of Venezuela is a common area of habitation, where it is thought to be the dominant species of hammerhead, as well as in the waters of northeastern Trinidad. Many reports of the smalleye hammerhead being found in the Mediterranean Sea or Gulf of Mexico are likely incorrect due to the confusion regarding its taxonomic name and can most likely be attributed to the great hammerhead (S. mokkarran) or the whitefinned hammerhead (S. couardi).
The smalleye hammerhead is known to inhabit muddy coastal waters not exceeding depths of 131 feet (40m) or more. Newborn sharks of this species reside in very shallow waters until they reach approximately 15.7 inches (40cm) in total length, at which point they will move out into deeper waters. Juvenile and adult male smalleye hammerheads tend to school together with other individuals of a similar size and are found abundantly at a depth of 88.5-118 feet (27-36m). The adult females, however, can be found in shallower waters of 29.5-59 feet (9-18m) and will rarely school.
The smalleye hammerhead, along with the other members of the Sphyrnidae family, has a distinctive, relatively long and narrow, prebranchial hammer or mallet shaped head. Their eyes are located slightly anterior to upper symphysis of mouth. However, unique to the smalleye hammerhead, is a deep indentation located centrally on the front of its head. One of the most distinctive features of the smalleye hammerhead is its proportionally small eye located on either side of its head.
The coloration of the smalleye hammerhead is one of its most distinguishing features. This shark's coloration can range from bright gold to a yellow-orange and its skin can have a metallic or iridescent hue. As the shark ages and reaches sexual maturity, the colors tend to fade; however, a yellowish grey color will still be visible on the underside.
The smalleye hammerhead has a narrowly arched mouth located ventral to its head. The teeth of this species are long and slender, with smooth or weakly serrated cusps.
The dermal denticles of the smalleye hammerhead are arranged in an alternating fashion. There are five distinct ridges along each serrated oval shaped denticle which cover the entire body of the shark.
Size, Age, and Growth
The smalleye hammerhead is one of the smallest members of the Sphyrnidae family. The maximum reported length of this species is 52.8 inches (134cm) total length for male specimens and 58.3 inches (148cm) total length for female specimens. Adult males of this species range between 43.3-52.8 inches (110-134cm) total length and adult females average 47.2-58.3 inches (120-148cm) total length. Smalleye pups, at birth, average around 11.8 inches (30cm) total length.
The diet of the adult smalleye hammerhead consists mainly of shrimp, sea catfish, swimming crabs, squid and scalloped hammerhead pups (S. lewini). Juvenile smalleye hammerheads generally feed on penaeid shrimp (Xiphopenaeus kroyeri) while the adult sharks feed on small bony fishes and sea catfish (Ariidae). The color of the sea catfish, their eggs and the shrimp are a very bold golden color. This has led scientists to postulate that there is a connection between the carotenoid pigment in the catfish and shrimp and the color of the shark.
The reproductive cycle of the smalleye hammerhead is thought to be annual, similar to the bonnethead shark (S. tiburo). Female smalleye hammerheads reach maturity at a length of 38.6 inches (98cm) total length and males by 31.5 inches (80cm) total length. Once the sharks are fully mature, ovulation and mating has been observed to occur during the months of August and September. This species of shark displays a viviparous method of reproduction which utilizes a yolk-sac placenta to nourish its young until parturition. Due to the limited data available regarding the life history of the smalleye hammerhead, the duration of the gestation period is not clear. However, scientists have hypothesized it to be between 10 and 11 months, with parturition likely occurring in May or June. The size of pups ranges from 11.8-12.2 inches (30-31cm) total length at birth and litter sizes range from five to twelve young. The presence of developing ovarian eggs in female smalleye hammerheads concurrent with near term embryos indicates that it is likely they will conceive shortly after parturition and continue a yearly reproductive cycle.
As one of the smaller members of the hammerhead family, the smalleye hammerhead is susceptible to becoming prey to some of the larger sharks in the area such as bull sharks (Carcharhinus leucas) and other hammerheads (Sphyrna spp.). Juvenile smalleye hammerheads are also preyed upon by large bony fish and other sharks.
The waters of eastern South America are inhabited, not only by the smalleye hammerhead, but by many different species of parasites. Commonly found in the area are various copepod species, including Echthrogaleus coleoptratus, Pandarus saturus and P. cranchii, which likely utilize the smalleye hammerhead as a host.
Historically, there has been much confusion regarding the scientific nomenclature for the smalleye hammerhead. The name Sphyrna tudes was first applied to a smalleye hammerhead specimen in France that was originally thought to be a great hammerhead. Due to this misnomer, for many years the great hammerhead was known as S. tudes. When the smalleye hammerhead was discovered off the coast of South America it was given the name S. bigelowi. Years later, when samples of the smalleye hammerhead were compared to the French sample of the misnamed great hammerhead and discovered to be conspecific, the names were changed. The great hammerhead was then changed to S. mokkarranand the smalleye hammerhead to its present name, S. tudes.
There are approximately 10 related species of hammerheads throughout tropical and temperate regions including the bonnethead (S. tiburo), great hammerhead (S. mokarran), scalloped hammerhead (S. lewini), and smooth hammerhead (S. zygaena).
Prepared by: Erin Gallagher