SharkoFiles: Spinner Shark

The spinner shark is a fast and agile predator. Among the shark species, spinner shark method of feeding is unique by spinning out of the water in quick bursts of speed to catch a prey. The spinner shark’s body is slender and often mistaken for a Blacktip shark due to its grey or black tip fins. Even though spinner shark is not a deep sea swimmer, nevertheless, it prefers inshore waters not exceeding 350 feet deep. Spinner sharks have small narrow teeth designed for feeding on small fish such as sardines and herrings rather than larger prey marine mammals, which makes them pose no threat to humans. Larger sharks are potential predators of spinner shark, particularly on juveniles and those that are yet to become full adults. This shark is commonly targeted by commercial fishermen due to their fins, which are used for preparing shark fin soup, a local delicacy in some areas in Asia. Consequently, in the IUCN Red List report, this shark species are listed as near threatened.

The spinner shark’s body is very streamlined, which enables them to spin out of water at a quick burst to catch prey, hence their feeding strategy is enhanced by their body structure. It has a small narrow teeth suitable for feeding on small fishes, which are essentially the bulk of their diet. Spinner sharks belong to the order carcharhiniformes, of the family carcharhinidae and genus Carcharhinus with species name brevipinna.

The average life expectancy of spinner shark is between 15 to 20 years.

Morphology

The distinguishing features of the spinner shark include a pointed snout, which is as long or even longer than the width of the mouth. Also, the first dorsal fin is fairly small, protruding from over or just behind the free tips of pectoral fins. It has a slender caudal fin which has a narrow rounded tip. Overall, the shark has a slender shape body with a long pointed and small round eyes. The lower jaw of the spinner shark has a distinct notch along the trailing edge. At juvenile stage, the spinner sharks are often confused with the Blacktip shark. However, both are easily distinguished when they become adults since the anal fin of the adult spinner shark is black-tipped, while the anal fin of the adult Blacktip does not have these markings. The spinner shark has a gray to bronze coloration on the dorsal side, while the ventral surface is colored fading white.

The upper jaw of the spinner shark has narrow and erect cusps on broad bases that differs to the broader upper teeth present in the Blacktip shark. Furthermore, the first few teeth of the spinner shark are symmetrical and erect, while the outer three teeth are very low with finely serrated edges. The lower jaw teeth have more slender cusps on broad bases in contrast to the upper jaw teeth. The spinner shark has a tightly arranged dermal denticles, which overlap and conceal the skin. Rather than having long blades, the blades are broad and slightly raised. The spinner shark reaches an average size of about 6.4 feet, and maximum body size of 9.1 feet. Male spinner sharks attain maturity at a body length of 4 feet, while the females mature when they reach about 5 feet in body length. Compared to other shark species, the spinner shark has a relatively rapid growth and after reaching maturity continue to grow at around 2 inches per year.

Feeding

The spinner sharks primary food is the small pelagic teleosts such as sardines, lizardfish, stingrays, squid, mullet, bluefish, tunas, jacks and herrings. The feeding method of spinner sharks is unusual. The shark swims quickly through schools of prey (fish), while spinning around its body axis often out of the water to catch fish. Since the shark’s teeth is not adapted for cutting, it swallows prey whole.

Habitat

The spinner sharks have worldwide distribution ranging from subtropical, tropical to temperate regions, as well as from inshore to offshore waters over continental and insular shelves. In seawater, they are found at a depth ranging from 0 to 328 feet. The spinner sharks form schools, especially when feeding and are considered a highly migratory species of shark. They tend to swim near coastal habitats, hence making them vulnerable to human exploitation and habitat degradation. Spinner sharks prefer shallow waters around 98 feet, however, they can swim further as deep as 330 feet.

Reproduction

Similar to other requiem sharks, the mode of reproduction of the spinner shark is viviparous. The females can give birth to pups within the range of 3 to 20 pups in a single litter each year. The gestation period is between 11 to 15 months. The spinner sharks nurse their young ones in coastal areas like the bay, and beaches. Also, any high salinity estuaries not deeper than 16 feet are used as nursery grounds. Newborn spinner sharks grow relatively faster than other shark species. This shark reproduction begins at 12 to 14 years old.

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