Biology of the Crocodile Shark
|Field Marks||Body slender and spindle-shaped; eyes huge (more than half the greatest height of snout); gill slits long, extending onto top of the head; mouth angular and subterminal (snout length greater than eye diameter), jaws highly protrusible; teeth long, slender and prominent, without basal cusplets; dorsal fins and anal fin small; caudal peduncle with precaudal pits and weak keels; caudal fin asymmetrical, with strongly developed lower lobe (nearly 2/3 length of upper lobe). Color dark brown above, sometimes with a few dark blotches on the flanks and ventral surface (some individuals reported with a whitish blotch on either side of head, between jaw corner and first gill slit); fins with narrow translucent or white margins.|
|Size||Pups are about 16 in (40 cm) long at birth; most specimens have been about 3 ft (1 m) in length and about 9-13 lb (4-6 kg) in weight; maximum length about 3.6 ft (1.1 m).|
|Range||Nearly circumtropical in distribution, although not yet recorded from the western Atlantic.|
|Habitat||Rare to locally abundant in oceanic waters, usually found far from land but sometimes occurring inshore near the bottom; epipelagic to mesopelagic, known depth range from the surface to at least 1 940 ft (590 m); circumstantial evidence suggests this species is a member of the DSL community, exhibiting a diel pattern of vertical migration toward the surface at night and away from it during daylight hours.|
|Feeding||Little data available; dentition and protrusible jaws combined with firm body musculature, narrow caudal peduncle with keels, and strongly forked caudal fin suggest it preys on moderately large and active oceanic prey; known stomach contents include bristlemouths (Gonostomatidae) and lanternfishes (Myctophidae), squid beaks, and shrimps.|
|Reproduction||Ovoviviparous, with uterine oophagy; 2-4 pups per litter (usually 4, two from each uterus); no data on pupping season or nursery grounds.|
|Age & Growth||Males mature at a length of about 29-43 in (74-110 cm), females at 35- 40 in (89-102 cm); no data on age at maturity r longevity for either sex.|
|Danger to Humans||Never implicated in attacks on humans and generally regarded as harmless to people due to its small size, non-cutting teeth, and oceanic habitat; strong jaws and powerful jaw muscles should invite respect while handling, as this species snaps strongly and vigorously when captured.|
|Utilization||Primarily caught incidentally by Japanese pelagic longline fisheries, details are sketchy but apparently this species is often caught on tuna longlines but discarded due to its small size and meat that is deemed unsuitable for the Japanese market; liver is very large and rich in squalene, and is hence of potential value.|
|Remarks||The smallest lamnoid, the diet and life history of this species is poorly known; catches should be preserved and reported.|
In September 1985, AT&T installed the world's first deep-sea fiberoptic communications cable. The cable linked two of the Canary Islands, Gran Canaria and Tenerife. In October of the same year, system monitors indicated that power transmission in the cable had shorted out about 6 miles (10 kilometres) from Tenerife at a depth of about 3,300 feet (1,000 metres). Similar faults occurred elsewhere along the cable in January 1986, March 1986, and April 1987. Each time communications service was interrupted, AT&T had to raise the cable, snip out and replace the damaged portion. This is a very expensive undertaking, and if AT&T planned to replace its trans-Atlantic cables with similar technology that proved just as prone to failure, the company stood to lose many millions of dollars.
Examination of the damaged cables indicated that in all cases, shark bite had caused the failures. So AT&T sponsored an extensive (and expensive) program of research designed to assess for main aspects of the problem: 1) what kinds of shark were responsible for biting their cables, 2) why were the sharks biting the cables, 3) which locations were most likely to result in shark-bite damage to the cables, and 4) what was the most cost-effective way to prevent shark-bite damage to the cables. The results yielded many surprises, not least of which was who the main culprit was. Scores of shark teeth and tooth fragments were removed from the recovered cable segments. Two were apparently from an open ocean carcharhinid, probably the Oceanic Whitetip (Carcharhinus longimanus), a few were tentatively identified as having belonged to Goblin Sharks (Mitsukurina owstoni), but the vast majority — a total of over 50 teeth and fragments — were identified as having come from a rare and little-known species: the Crocodile Shark.
Growing to a length of just over 3.5 feet (1 metre), the delightfully named Crocodile Shark is the smallest living lamnoid. The species' vernacular name is derived from one of its Japanese names — Mizu-wani — which means, literally, "water crocodile". The name reflects not only its prominent, spike-like teeth, but also this scrappy little shark's habit of snapping vigorously when removed from the water. Its jaws are short, heavily constructed, and powerfully muscled. Japanese fishermen who have suffered the misfortune of underestimating the Crocodile Shark's reach attest that it can bite very hard. But, if left unmolested in its natural habitat, it is probably not particularly dangerous to people.
Probably the most completely oceanic of lamnoids, the Crocodile Shark exhibits many features that resemble those of mesopelagic dogfishes of the order Squaliformes. It has very large eyes, relatively small fins, and a long body cavity filled with a very large, oily liver. The liver comprises up to 20% of the Crocodile Shark's total weight and is very rich in low-density lipids and the hydrocarbon squalene. Since these compounds are significantly less dense than seawater and are virtually incompressible, the large liver may act as a kind of 'hepatic float' — affording the shark near neutral buoyancy over a large range of depths. This attribute allows it to hover nearly motionless in the water column, conserving energy while waiting for prey in the vast blackness of the deep-sea. As in many mesopelagic creatures — including the closely related Megamouth Shark (Megachasma pelagios) — the Crocodile Shark may be a vertical migrator, remaining at depths of 1 000 feet (300 metres) or more by day, ascending near the surface at night, and returning to the depths before dawn. This circadian pattern of depth distribution may enable the Crocodile Shark — like the mesopelagic squaloids it resembles — to remain close to its prey within the DSL's. The similarities displayed by the Crocodile Shark and the deep-sea dogfishes provide fine examples of evolutionary parallelism, the development of similar adaptations to similar environmental challenges.
The Crocodile Shark wields highly protrusile, trap-like jaws armed with smooth-edged, grasping teeth. It appears to feed heavily on small, deep-sea fishes such as bristlemouths (family Gonastomatidae) and lanternfishes (Myctophidae), but is also known to take oceanic squids (several families) and mesopelagic shrimps. The Crocodile Shark's large eyes suggest that it is a visual hunter, perhaps specializing in bioluminescent prey or spotting it near the surface when silhouetted by the silvery glow of the Moon. However it locates its prey, the Crocodile Shark learns its carnivorous ways early. Like most — perhaps all — extant lamnoids at late stages of development, the Crocodile Shark is known to practice in utrero oophagy (egg-eating). Interestingly, four pups are usually born per litter, two per uterus. How more than one pup per uterus survives this intense sibling rivalry is one of the more perplexing mysteries about the Crocodile Shark.
The AT&T deep-sea shark expedition off the Canary Islands, posed as many new mysteries as it solved. For example, neither open ocean carcharhinids nor the Crocodile Shark are known to feed on the ocean bottom. It was therefore presumed these predators inflicted their damage while the cable hung in the water column as it was being deployed. As to why the sharks were biting AT&T's cable, it was presumed that their electric field simulated that of prey and elicited the attacks. Unfortunately, laboratory experiments using Chain Dogfish (Scyliorhinus retifer) and Lemon Sharks (Negaprion brevirostris) produced inconclusive results.
Since at least some of the shark bites apparently occurred in mid-water as the cables were being set, determining which locations and depths were most vulnerable to attack was not simply a matter of identifying where along the cable the bites occurred. So the research team went fishing to determine which kinds of sharks occurred at what depths. Using deep-sea longlines, the team caught 211 sharks of 13 species. The largest of these was a 12-foot (3.7 metre) Bluntnose Sixgill Shark (Hexanchus griseus) weighing some 800 pounds (360 kilograms). Such catches suggest that deep-sea sharks are much more abundant and diverse off the Canary Islands than anyone suspected. Although at least five of the species caught — the Bluntnose Sixgill, Portuguese Dogfish (Centroscymnus coelolepis), Kitefin Shark (Dalatias licha), Greenland Shark (Somniosus microcephalus), and the Little Sleeper Shark (Somniosus rostratus) — not one Crocodile Shark was caught. In any case, most of these sharks were caught between 1,600 and 8,200 feet (500 and 2,500 metres). Since this represents a substantial range of depths, AT&T opted to physically protect their cables against shark-bite by including a layer of steel tape under a coating of dense polyethylene. The cables have been functioning well ever since.
Although AT&T's shark research cruise failed to turn up any Crocodile Sharks, it has added substantially to our knowledge of deep-sea sharks of the Canary Islands and nearby Mid-Atlantic Ridge. It provided a rare opportunity for scientists to explore one of the least known parts of our planet, extended the known depth ranges and diets of several deep-sea shark species, produced several museum specimens of rare sharks, and even yielded several potential new species. Perhaps most startling and thought-provoking, the expedition discovered that there is virtually nowhere on Earth that is so remote that it is beyond the reach of human impact: a False Catshark (Pseudotriakis microdon) hauled up from the lightless depths contained a softdrink tin, a whole pear, several boiled potatoes, and a pack of cigarettes. As is so often the case in fishing, the one that got away is not nearly as important as the stories we can tell afterward.