Eat or Be Eaten: Hunting Strategies Underwater

The worm that wasn’t

Picture a small fish nosing along a reef. A thin filament sways above the seabed like a worm, and the fish draws closer. Behind it, what looks like a piece of sponge or rock is a frogfish. Its mouth opens so fast that the rush of water sweeps the small fish inside. The frogfish has barely moved. The stalk that carries its lure is a modified dorsal fin ray. It can draw prey within range of a mouth that opens wide enough to engulf a surprisingly large fish. To see why the strike works at such close quarters, follow what happens to the water.

When the mouth draws water in

A frogfish’s irregular outline, skin projections and colouring can make it hard to pick out from the reef. Some resemble sponges, algae or pieces of rock. Above the head, a stalk called the illicium carries a lure, the esca. Its shape varies among species. Where the lure resembles a small animal, a frogfish can move it in front of its mouth while keeping the rest of its body almost still.

When a fish comes close enough, coordinated movements of the jaws and skull rapidly expand the mouth cavity. The pressure inside drops and water rushes in, carrying the prey with it. The capture can take a few milliseconds. A frogfish can stretch its mouth wide enough to swallow fish surprisingly large for its size, but suction is effective only at close range. The current weakens quickly away from the mouth. That is where the lure earns its keep: it brings the prey within reach. Many other fish use suction, including species that pursue their prey.

A predator can also wait for a fish to come close without offering it anything to investigate.

The geography cone changes the odds

On a tropical sandy seabed, a cone snail moves slowly. Its patterned shell gives little away about its diet. The geography cone, Conus geographus, hunts fish. Fish-eating cone snails can deliver venom through a modified radular tooth. Their venom contains a mixture of peptides, some of which act on ion channels and receptors involved in nerve and muscle function. These effects can make escape harder.

The geography cone can also spread its mouthparts around small fish and engulf them. Its chemistry helps explain how a slow-moving snail can catch such quick prey.

In 2015, researchers showed that Conus geographus and Conus tulipa produce particular forms of insulin, different from those normally used by these molluscs to regulate their own metabolism. These venomous insulins are more similar to those of fish. They can cause a sharp drop in the prey’s blood glucose. The researchers suggested that their release into the water could contribute to the reduction of fish activity, facilitating their capture. In 2024, another study in the geography cone revealed a peptide imitating the action of somatostatin, a hormone that is involved in regulating metabolism. In experiments, this substance could reduce glucagon release, a hormone that normally helps to increase blood glucose levels. Together, these molecules may disrupt the fish’s glucose balance for longer. The cone, therefore, does not simply paralyse its victims with substances acting on their nerves. It also has molecules that are able to interfere with their hormonal regulation. This mechanism is an unusual example of venom evolution.

A different kind of strike comes from the long bill of a sailfish.

A grouper and a moray hunt together

In the Red Sea, a fish fleeing a grouper disappears into a crack in the reef. The grouper can go no farther. Its body is too large to fit deep inside the crevice. The prey seems to have found shelter. The grouper can then do something surprising. It swims to a giant moray, Gymnothorax javanicus, and shakes its head rapidly in front of it. Biologists have observed and studied this behaviour in the Red Sea. In 2006, Redouan Bshary and colleagues reported that some coral groupers, Plectropomus pessuliferus, use visual signals to recruit giant morays. In a substantial proportion of the observed encounters, the morays responded by leaving their shelters.

The predators could then move across the reef together. Their bodies suit different hunting methods. The grouper can quickly chase prey in open water. The moray’s long, flexible body lets it search the cracks where fish hide. The prey faces danger on both sides. Inside the crack, the moray can reach it. In open water, it risks encountering the grouper. Researchers also saw groupers make distinctive head and body movements near a hiding place containing prey. These signals could draw other predators’ attention to that spot. Later studies examined the gestures more closely. Some have features of referential communication because they direct a partner’s attention to a specific location. The observations show that two predator species can coordinate their behaviour. They leave open whether the animals consciously plan the hunt. Whichever predator catches the fish usually swallows it whole. Both predators gain more opportunities to catch prey without having to share a meal. If hunters can combine their abilities in this way, how far can they also take advantage of the physical properties of water itself?

The moment the hunt turns

The frogfish’s outline blends into the reef, its stillness limits the disturbances it sends through the water, and its lure draws a fish close. Timing still matters. If the mouth opens too early, the prey can stay beyond the strongest part of the flow. Wait too long and the fish may leave. The same constraint takes a different form for each hunter. A sailfish must get its bill among the sardines. A cone snail must expose the fish to its venom. A moray has to reach into the crevice, and a snapping shrimp’s pressure pulse has to reach its prey.

The prey can change the outcome too. A fish can sense a disturbance, turn sharply, join a school or take refuge. Some prey have physical or chemical defences. Those responses and the ways predators get past them have helped shape one another over evolutionary time.