Smalleye Hammerhead - Biology and Ecology

Biology and Ecology

Four other species of hammerhead sharks overlap in range with the smalleye hammerhead: the small-sized scoophead and bonnethead, and the large-sized scalloped hammerhead and great hammerhead. There is little competition between these species because of their differing habitat and dietary preferences. The smalleye hammerhead is the dominant hammerhead in shallow muddy areas, where high turbidity limits the utility of vision (hence its smaller eyes). Adult males and juveniles of both sexes form schools of uniform body size; these schools do not appear to relate to reproduction or migration. Adult females are apparently solitary.

Young smalleye hammerheads under 67 cm (26 in) long feed predominantly on penaeid shrimp, mostly Xiphopenaeus kroyeri. Larger sharks feed mainly on bony fishes, especially ariid sea catfish and their eggs. The shrimp and the surface mucus layer and eggs of the catfish contain carotenoid pigments that appear to be the source of the sharks' golden color; it is uncertain whether the pigments in the catfish also ultimately come from the shrimp. Another shark species in the region, the yellow smooth-hound (Mustelus higmani), also feeds on shrimp and has a yellowish color, albeit not nearly as bright. This species has also been known to consume swimming crabs, squid, grunts, and newborn scalloped hammerheads. The smalleye hammerhead may fall prey to larger sharks such as the bull shark (Carcharhinus leucas), while smaller individuals may also be taken by bony fishes. Its coloration may provide camouflage. A known parasite of this species is the monogenean Erpocotyle schmitti; it may also serve as a host to common copepod ectoparasites such as Echthrogaleus coleoptratus, Pandarus saturus and P. cranchii.

Like all hammerhead sharks, the smalleye hammerhead is viviparous: when the developing embryos exhaust their supply of yolk, the depleted yolk sac develops into a placental connection through which the mother delivers nourishment. Mature females have a single functional ovary and two functional uteruses. Ovulation occurs at the same time as gestation, allowing females to bear young every year. The details of the smalleye hammerhead's life history vary across its range. Off Trinidad, reproduction occurs on a well-defined annual cycle with mating in August and September, and birthing in late May and June of the following year. The females carry between 5 and 12 pups for 10 months, and make use of food-rich, shallow coastal bays as nursery areas. The newborns measure around 30 cm (0.98 ft) long, and males and females attain sexual maturity at 80 cm (31 in) and 98 cm (39 in) long respectively. By contrast, smalleye hammerheads off the northern Brazilian state of Maranhão are substantially larger, with males maturing at over 92 cm (36 in) long and females at over 101 cm (40 in) long. As the litter size increases with female size, Maranhão sharks have been recorded carrying up to 19 pups. The seasonality of breeding also differs, with pregnant females found from June to October and January to April, and males in apparent reproductive condition from May to November and in March.