Sandy Plains: No Place to Hide
Considering that a large and long-standing commercial fishery off southern Australia takes it, the basic life history of the Longnose Sawshark (Pristiophorus cirratus) is remarkably poorly known. The belly flaps of this and a smaller, closely related species the Shortnose Sawshark (P. nudipinnis) are particularly highly regarded as a seafood delicacy, often featuring in Aussie fish and chips. Yet, other than that they superficially resemble the sawfishes, little is known about either of these sawsharks.
Just the Facts:
Habitat: Estuaries, Sandy Plains, Deep Sea
Depth: most commonly 120-480 ft (37-146 m) to maximum 1020 ft (311 m)
Distribution: South East Asian?, Western Australian, Southeast Australian/New Zealand, Northern Australian
Sawsharks and sawfishes both have a flattened, blade-like snout with teeth along the margin, but they differ in several important respects. Sawsharks (family Pristiophoridae) are true sharks, and thus their gill slits are located on the side of the head, while sawfishes (Pristidae) are rays, with gill slits located underneath the head. The rostral (snout) teeth of sawsharks alternate between long and short and are replaced throughout life, while those of sawfishes are all about the same length and if they become worn or broken are not replaced. Perhaps the most obvious difference between these two saw-bearers is that sawsharks have a pair of long barbels located about mid-way along the rostrum, while sawfishes do not. A sawshark may thus be readily distinguished from a sawfish by its characteristic Fu Manchu mustache.
The rostrum of the Longnose and other sawsharks apparently helps locate and subdue prey. The rostrum is equipped with lateral line and ampullary sensors, making it sensitive to vibrations and bioelectricity. The rostral barbels are highly mobile and studded with pressure and chemoreceptors, making them sensitive to touch and taste. Together, the rostrum and barbels form a forward-projecting prey-detector, enabling the Longnose Sawshark and its kin to scan the sandy plains for cryptic prey lying on or in the bottom sediments. Known prey of the Longnose Sawshark consists of bottom-dwelling crustaceans and small fishes, including the Rough Flutemouth (Fistularia petimba). The spatulate rostrum, flattened head, and enlarged knobs at the base of the skull enable the Longnose and other sawsharks to toss the head violently from side to side injuring small schooling fishes or sifting the fine, close-packed rostral teeth through bottom sediment to capture buried prey. Prey items that are impaled on the rostral teeth are removed by shaking the saw rapidly from side-to-side, then sucked into the mouth and crushed with the small, thorn-like teeth.
The Longnose Sawshark is the largest and most widely distributed of the Australian sawsharks. Although it is sometimes found close inshore in bays and estuaries, this species is most common in deeper offshore waters over sandy bottoms.
Longnose Sawshark pups are born during winter in shallow coastal areas. At birth, the long rostral teeth of this species are folded back possibly to make the process easier on the mother. Within hours of birth, the long rostral teeth become erect and later the smaller rostral teeth grow in between the larger ones. No data are available on growth rate of the pups, age at maturity of either sex, or longevity of the Longnose Sawshark although some sources speculate it has a maximum lifespan of 15 years or more.
The Longnose Sawshark is the basis for a considerable bottom trawl fishery off southern Australia, and is also taken as a bycatch of trawlers and gillnetters. The rostral saws of these sharks are often sold as ornamental curios. The flesh is of excellent quality and sold in fish markets as a delicacy. But given the low fecundity of this species and our very incomplete knowledge of its life history, the Longnose Sawshark fishery seems a recipe for population collapse.