Deep Sea: the Twilight Zone and Beyond
Until relatively recently, the Cookiecutter Shark (Isistius brasiliensis) was known as the “Cigar Shark”, in reference to its smallish size, overall shape and the dark band around the gill slits. Its genus name derives from Isis, the Egyptian goddess of light, referring to this species’ bright green bioluminescence. But the Cookiecutter’s current vernacular name refers to its nasty habit of removing neat plugs of flesh from large pelagic creatures.
Just the Facts:
Habitat: Open Ocean, Deep Sea
Depth: surface to 11,500 ft (3,500 m)
Distribution: Arctic, Antarctic, North Pacific, Central Pacific, South Pacific, Tropical Eastern Pacific, North Atlantic, South Atlantic, Caribbean, Amazonian, West African, Central South Indian, Southeast Australian/New Zealand, Northern Australian, Japanese
The Cookiecutter Shark combines many specialized features that enable it to carve out a living in the deep-sea. Like many mesopelagic sharks, it has an elongated body cavity filled with an enormous liver comprising as much as 35% of its total weight. The Cookiecutter’s liver is perfused with low-density oils which render the shark nearly neutrally buoyant over a wide range of depths and thereby saves energy by freeing it from the need to swim constantly to avoid sinking. It has very large eyes — the better with which to see potential prey — and a short, broad caudal fin that is ideal for rapid bursts of acceleration over short distances — that is, ambushing prey from close range.
To lure prey within range, the Cookiecutter Shark relies on its brilliant luminescence. Complex light-producing organs called “photophores” are scattered over the Cookiecutter’s entire body and are especially richly distributed on its belly and lower surfaces. A main function of this pattern of bioluminescent organs is to eliminate an animal’s shadow as seen from below, a common mesopelagic anti-predatory strategy known as “counter-illumination”. But there is a very curious fact about the distribution of photophores on the Cookiecutter Shark’s undersurfaces: they are completely absent from the region under the throat between the gill slits. It has recently been proposed that this dark patch that is bordered by luminescent organs may mimic the search image of many upward-looking pelagic predators. Thus, when a would-be predator approaches what appear to be a small shadow of a potential prey animal, it is brought within the striking range of the insidious Cookiecutter Shark and the predator has become prey.
The sheer variety of large pelagic creatures parasitized by Cookiecutter Sharks in this way is astonishing (see the accompanying sidebar). A member of the complex biological community known as the “Deep Scattering Layer”, the Cookiecutter moves surfaceward at night and returns to deeper water before dawn. Creatures that also inhabit this community, such as large squids and the Megamouth Shark (Megachasma pelagios), may encounter Cookiecutters at any time of the day — as can deep-diving cetaceans, such as the Sperm Whale (Physeter macrocephalus). But many shallower-dwelling oceanic creatures may be parasitized at night, when the glow-in-the-dark sharks move en masse into the upper layers of the open ocean.
Humans who, by accident or by design, enter the open sea are not immune from attacks by Cookiecutter Sharks. The body of a drowned fisherman recovered off the coast of Oahu, Hawaii, in July 1992 had two Cookiecutter bites to the lower back. These bites are believed to have been inflicted post mortem, but there are a few documented reports of people — including shipwreck survivors and, in one case, an underwater photographer — being attacked in the tropical open ocean by schools of blunt-snouted and extremely ferocious foot-long (30-centimetre long) “fish” that neatly sliced out circular plugs of flesh about an inch (2.5 centimetres) in diameter. The fish responsible may well have been Cookiecutters. This frightening possibility certainly puts the romantic notion of a moonlight swim in a whole new light.
How does the Cookiecutter Shark manage to remove such neat plugs of flesh from its prey? The oral anatomy of this little shark is at once bizarre and gruesomely efficient. The Cookiecutter has unique suctorial lips that glom onto and help create a good seal against the body surface of its prey. Suction is created primarily by the large, highly mobile basihyal (‘tongue’), which elevates in the shark’s throat much like the compactor of a municipal garbage truck. Thus firmly secured, the Cookiecutter bites: its broad, greatly enlarged lower teeth make the initial incision as its spike-like upper teeth function like a many-pronged fork to grasp onto the prey securely. Each lower tooth is shaped like the nib of an old-fashioned pen (the sort that users dip into inkwells before writing) and has a base that interlocks with adjacent teeth. Collectively, the lower teeth of the Cookiecutter form a ‘bandsaw’ like arrangement with a continuous cutting edge. Through a combination of oscillatory jaw movements and violent bodily twisting, the Cookiecutter Shark removes a conical plug of flesh from its victim.
Like other sharks, the Cookiecutter replaces its teeth continually. But what makes the Cookiecutter Shark unusual is that it apparently replaces each lower tooth band as a single unit. This ensures that the Cookiecutter always has a sharp set of lower dentition available. Perhaps even more remarkable, shed lower tooth bands are often swallowed — possibly as a way of recycling calcium and phosphates in the nutrient-poor deep-sea environment it inhabits.
A parasite may be defined as a predator that eats its prey in units of less than one. In our biased view of living things, we may abhor parasitism as a lowly way to earn a living and shudder at the gruesome possibility that a school of Cookiecutter Sharks could easily remove numerous plugs of our flesh. But there can be little debate that parasitism is a highly successful mode of life and that the little, glowing Cookiecutter Shark is among its most remarkably efficient practitioners.
Large Animals Known to Have Been Parasitized By Cookiecutter Sharks
There is a sort of morbid fascination about just how many different kinds of large creature have been parasitized by the Cookiecutter Shark (Isistius brasiliensis). Below is a conservative list, trolled from the scientific literature as well as my own observations at sea. Undoubtedly the Cookiecutter has parasitized many more species than listed here. If the full tally of this little shark’s victims were available, I wouldn’t be surprised if virtually every animal species inhabiting tropical open oceans has — at one time or another — been bitten by the opportunistic Cookiecutter Shark.
Neon Flying Squid (Ommastrephes bartrami)
Escobar (Lepidocybium flavobrunneum)
Opah (Lampris regius)
Pacific Pomfret (Brama japonica)
Giant Trevally (Caranx ignobilis)
Amberjack (Seriola dumerilli)
Rainbow Runner (Elegatis bipinnaulata)
Dolphinfish (Coryphaena hippurus)
Wahoo (Acanthocybium solanderi)
Skipjack Tuna (Katsuwonus pelamis)
Yellowfin Tuna (Thunnus albacares)
Albacore (Thunnus alalunga)
Kawakawa (Euthynnus affinis)
Blue Marlin (Makaira nigricans)
Striped marlin (Tetrapterus audax)
Sailfish (Istiophorus platypterus)
Swordfish (Xiphias gladius)
Goblin Shark (Mitsukurina owstoni)
Megamouth Shark (Megachasma pelagios)
Blue Shark (Prionace glauca)
Dugong (Dugong dugon)
Northern Elephant Seal (Mirounga angustrirostris)
Southern Elephant Seal (Mirounga leonina)
Hawaiian Monk Seal (Monachus schauinslandi)
Leopard Seal (Hydrurga leptonyx)
Guadeloupe Fur Seal (Arctocephalus townsendi)
New Zealand Fur Seal (Arctocephalus forsteri)
Humpback Whale (Megaptera novaeangliae)
Sei Whale (Balaenoptera borealis)
Bryde’s Whale (Balaenoptera edeni)
Blue Whale (Balaenoptera musculus)
Fin Whale (Balaenoptera physalus)
Gervais’ Beaked Whale (Mesoplodon europaeus)
Arnoux’s Beaked Whale (Berardius arnuxii)
Cuvier’s Beaked Whale (Ziphius cavirostris)
Sperm Whale (Physeter macrocephalus)
Harbor Porpoise (Phocoena phocoena)
Spotted Dolphin (Stenella attenuata)
Spinner Dolphin (Stenella longirostris)
Clymene Dolphin (Stenella clymene)
Rough-Toothed Dolphin (Steno bredanensis)
Bottlenose Dolphin (Tursiops truncatus)
Short-Finned Pilot Whale (Globicephala macrorhynchus)
Melon-Headed Whale (Peponocephala electra)
Human (Homo sapiens)