Sharks

Every muscle contraction and every ion gradient across a living membrane leaks a small electric field into seawater, which conducts well. Sharks read those fields through the ampullae of Lorenzini — jelly-filled pores across the snout — and the demonstration is unusually clean: a shark will attack bare electrodes reproducing a prey field, with no animal, no movement and no smell present, and will choose them over a source of fish odour.

The range is centimetres. This is the sense that lands the strike, not the one that finds the meal, and it is also why sharks sometimes bite undersea cables and metal fittings — a cost of the sense rather than aggression.

The claim fails on transport before it fails on sensitivity. Molecules have to reach the animal, and in water they travel at the speed of the current. A shark upstream of a wound detects nothing at all, whatever its threshold, and a plume takes hours to cover a mile.

What smell does do is more interesting. Leopard sharks taken nine kilometres offshore and released found their way back to the coast; those with olfaction temporarily blocked also got back, by much more wandering routes. Smell is one input to a navigation system, and the sharks without it were impaired rather than lost.

Set alongside the whole sensory relay, the single-sense version of a shark falls apart. Blocking any one channel leaves an animal that still hunts, slightly worse.

Sharks cannot be aged the way other fish are. There are no otoliths and no bones laying down annual rings, and vertebral banding proved unreliable in slow-growing deep-water species. The Greenland shark was known to be long-lived and nobody could say how long.

The answer came from finding the one tissue in the body that never turns over. The nucleus of the eye lens forms before birth and is metabolically inert thereafter, so its carbon atoms are as old as the animal. Radiocarbon in lens nuclei, calibrated against the atmospheric spike from 1950s nuclear testing, put the largest females at several hundred years.

The conservation implication is severe and worth stating without softening. A species that does not reproduce until roughly 150 years old cannot absorb fishing mortality on any timescale that means anything to a fishery, a policy cycle or a human life.

A shark’s skeleton is cartilage rather than bone — the same material as the tip of a human nose, stiffened with mineral in the jaws and vertebrae where load demands it. It is lighter than bone, which matters for an animal with no swim bladder, and more flexible, which matters for an animal that turns by bending.

Not having a swim bladder is the constraint that shapes much of the rest. A bony fish adjusts a gas-filled sac to hold depth without effort; a shark instead uses an enormous oil-rich liver, which can be a quarter of its body mass, and generates lift from its pectoral fins by moving forward. This is why many sharks sink when they stop, and why "sharks must keep swimming" is half true.

• Skin covered in dermal denticles — tooth-like scales that reduce drag and feel like sandpaper one way and smooth the other.
• Teeth in continuously replaced rows, so a lost tooth is a routine event rather than an injury.
• No tongue in the mammalian sense; a basihyal, a small cartilage plate, sits in its place.
• Ectothermic, with a handful of exceptions — mako, porbeagle and white sharks keep parts of their bodies above water temperature.

The dermal denticles are worth a moment. They are structurally teeth — enamel, dentine, a pulp cavity — covering the entire animal, and the tooth-like scales of early fish are where vertebrate teeth came from. A shark’s skin and its jaws are made of the same thing.

Juvenile lemon sharks paired with a shark that already knew a foraging task learned it faster than those paired with an equally ignorant partner, and kept performing it alone afterwards. The naive-partner control is what makes this social learning rather than company: a shark simply following another around would look identical.

Separately, blacktip reef sharks tracked over three years turned out to have stable preferred companions. Sharks using the same reef will inevitably be seen together, so the analysis explicitly subtracts shared space use — and structured communities remain after it does.

The scope needs holding firmly. Two species, one in a tank on a contrived task. Sharks span solitary deep-sea species to schooling reef species, and nothing here describes the group. What it does do is rule out the idea that sharks are behaviourally simple by default.

A bamboo shark embryo develops inside a translucent egg case, flushing water through it with a rhythmic tail beat. Expose it to a weak electric field of the kind an approaching predator produces and it stops — gills still, tail still — until the field passes.

An unborn animal recognising a predator by its electric field, and knowing that stillness is the answer, settles two things at once. The sense is functional before any experience could have shaped it. And electroreception is not purely a hunting sense: here it is defensive, in an animal that has never hunted anything.

1. 1678

   First observation

   Lorenzini describes the pores

   Stefano Lorenzini documented the jelly-filled pores covering the snouts of sharks and rays. They were named after him and their function remained entirely unknown.

2. 1938

   Reinterpretation

   Proposed as mechanical or temperature sensors

   For most of the twentieth century the ampullae were assumed to detect pressure, touch or temperature — reasonable guesses given that no vertebrate sense of electricity was known to exist.

   Changes how the 1678 result reads

   Each proposal accounted for the anatomy — a canal running from a surface pore to a sensory base is consistent with pressure or temperature — and none was ever confirmed behaviourally, which left the organ described but unexplained for 260 years.

3. 1971

   Landmark experiment

   Kalmijn shows sharks attack bare electrodes

   Sharks attacked flatfish buried in agar that blocked odour but passed electric fields, attacked bare electrodes reproducing a prey field with no animal present, and preferred those electrodes to a source of fish odour.

   Changes how the 1938 result reads

   The mechanical and thermal hypotheses could not survive an animal attacking two wires that smelled of nothing and did not move. Ampullae of Lorenzini became electroreceptors, and a sense no land vertebrate has was added to the list.

   The electric sense of sharks and rays

4. 2002

   Modern discovery

   The hammerhead question answered by morphology

   Hammerheads and sandbar sharks detected fields at comparable thresholds, but the hammerhead’s wide head swept a far larger area — the cephalofoil is a wider sweep, not a more sensitive antenna.

   Electroreception in juvenile scalloped hammerhead and sandbar sharks

5. 2013

   Modern discovery

   Embryos found to use the sense defensively

   Shark embryos still inside their egg cases froze in response to simulated predator fields, showing the sense is functional before hatching and is not solely for hunting.

   Survival of the stillest: predator avoidance in shark embryos

6. 2014

   Reinterpretation

   Electroreception placed within a sensory relay

   Blocking senses selectively across three species showed each dominates a different phase of a hunt, with electroreception guiding the final strike and other senses substituting when it is removed.

   Changes how the 1971 result reads

   Kalmijn’s result invited the reading that electroreception is how sharks find prey. The blocking experiments show it is the last stage of a relay that smell and the lateral line began — decisive at centimetres, irrelevant at a hundred metres.

   Multisensory integration and behavioral plasticity in sharks from different ecological niches

The decline estimate — roughly 71% for oceanic species since 1970 — is modelled from fisheries data and should be read with a wide interval rather than as a measurement. The direction and the cause are not in dispute: relative fishing pressure rose about eighteen-fold over the same period.

• Do sharks use the electric sense to detect the Earth’s magnetic field, and by what mechanism?

  Why it matters: It would unify two separate literatures — electroreception and magnetic navigation — and explain how sharks cross oceans.

• Which chemical gradients support long-distance navigation?

  Why it matters: Olfaction is demonstrated to contribute without anyone knowing what it follows, which leaves the mechanism a black box.

• How quickly can slow-reproducing oceanic species recover if fishing pressure falls?

  Why it matters: Recovery time determines whether protection now is sufficient or already too late for some species.

• How is information from four senses combined in the shark brain?

  Why it matters: The behavioural relay is well described and its neural basis is not, which limits what can be predicted about a shark with one sense impaired.