Nervous shark

Lore & Background

Australian ichthyologist Gilbert Percy Whitley originally described the nervous shark as a subspecies of Galeolamna greyi in a 1945 issue of the journal Australian Zoologist, giving it the name cauta, meaning 'cautious' in Latin, because of its skittish demeanor when confronted by people. Later authors recognized it as a distinct species in the genus Carcharhinus. The type specimen is the preserved skin and teeth of a 92 cm (36 in) long female caught in Shark Bay, Western Australia. Based on morphology, Jack Garrick suggested in 1982 that the nervous shark is closely related to the blacktip reef shark (C. melanopterus); this relationship was upheld in a 1992 phylogenetic study by Shane Lavery using allozymes and again in a 2011 study by Ximena Vélez-Zuazoa and Ingi Agnarsson based on nuclear and mitochondrial genes.

The nervous shark has a stout, spindle-shaped body, a short broadly rounded snout, and moderately large oval eyes with nictitating membranes. Its upper teeth are narrow and angled with coarsely serrated edges, while lower teeth are more slender and upright with finer serrations. The second dorsal fin is relatively large and high, and there is no ridge between the dorsal fins. The dermal denticles overlap and bear three horizontal ridges (five in larger individuals). The species is bronze to gray above and white below, with a white stripe on the flank and black edging on the leading margins of most fins and the lower caudal fin lobe. It is found over continental and insular shelves off northern Australia from Shark Bay to Moreton Bay, as well as off Papua New Guinea and the Solomon Islands, generally in shallow inshore waters to a depth of at least 45 m (148 ft), favoring mangrove-lined areas with sandy-muddy bottoms.

The diet consists mainly of small teleost fishes, with crustaceans and molluscs as secondary food sources, and it occasionally preys on semiaquatic snakes. The nervous shark is viviparous, with embryos nourished through a placental connection. Life history details vary with latitude: in Darwin Harbor, mating occurs from January to March and birthing in October and November after an 8–9 month gestation; in Shark Bay, mating occurs from late October to early November and birthing about a year later after an 11-month gestation. Females produce litters annually in Darwin Harbor and biennially in Shark Bay. Newborns measure 35–40 cm (14–16 in) long and are birthed in shallow nursery areas.

Reader's Guide

The nervous shark is notable for its timid behavior toward humans, which inspired its scientific and common names. It is harmless to humans and is occasionally marketed for food. The species is caught incidentally by coastal gillnet fisheries, such as the barramundi gillnet fishery off northern Australia, and may also be taken by line gear and prawn trawls. The Australian population does not appear to be threatened by fishing activities, and the International Union for Conservation of Nature (IUCN) has assessed it as Least Concern. Its life history varies with latitude, with differences in breeding season timing and gestation period between Darwin Harbor and Shark Bay, reflecting adaptation to local conditions. The nervous shark is among the most common sharks in Darwin Harbor, the Gulf of Carpentaria, and Shark Bay.

Did You Know?

• The nervous shark was originally described as a subspecies of Galeolamna greyi by Gilbert Percy Whitley in 1945.
• Its specific name cauta means 'cautious' in Latin, referring to its skittish demeanor toward humans.
• The nervous shark occasionally preys on semiaquatic snakes such as Cerberus rynchops and Fordonia leucobalia.
• Females in Darwin Harbor produce litters annually, while those in Shark Bay produce litters biennially.