Look under a ledge on almost any of our reefs and you may find a pair of huge, dark eyes looking back at you. They belong to the porcupine fish, a heavy-bodied, slow-swimming cousin of the pufferfish that spends the day tucked into a hole and comes out at night to crush shells. It has a face that seems to be smiling, and for many of our divers it becomes a favorite on the first dive.

It is also famous for something we hope you never see: inflating into a spiky ball. That trick is a last-resort defense, and it costs the fish dearly. This guide covers how to recognize a porcupine fish, how to tell it from its smaller relative the balloonfish, and why the best porcupine fish encounter is a calm one.

Quick Facts

Name: porcupine fish (or porcupinefish), Diodon hystrix; family Diodontidae
Size: commonly around 40 cm (16 in); the largest recorded reach about 91 cm (3 ft), making it the biggest of the spiny puffers
Diet: hard-shelled invertebrates such as sea urchins, snails and hermit crabs, cracked with a fused, beak-like set of teeth
Activity: solitary and nocturnal; rests in caves, holes and under ledges by day
Depth: about 2 to 50 m, most often 3 to 20 m (10 to 65 ft)
Range: circumtropical, including the whole western Atlantic from Massachusetts and Bermuda to Brazil
Status: listed as Least Concern by the IUCN

How to Recognize a Porcupine Fish

At rest, a porcupine fish does not look very spiny. The spines are modified scales that lie flat against the body, pointing backward, so what you notice first is a broad, blunt head, large bulging eyes and a long, rounded body. The body is grayish tan with a white belly, and it is evenly sprinkled with small black spots that continue onto the fins.

The fish most often confused with it is the balloonfish (Diodon holocanthus), which lives on the same reefs. Three clues separate them:

Spots on the fins: the porcupine fish has small dark spots on its fins; the balloonfish does not.
Body pattern: the balloonfish has large dark blotches or saddles across the back and a dark bar above and below each eye; the porcupine fish has only small, even spots.
Size: the balloonfish is smaller, usually 20–35 cm (8–14 in) and rarely more than 50 cm. A big, heavy fish over 50 cm is a porcupine fish.

Neither should be confused with true pufferfishes, which have no long spines, or with the boxy trunkfish, which is armored with rigid plates instead.

Behavior: Shell Crushers of the Night Reef

By day, a porcupine fish usually holds still in the same kind of shelter: a cave, a gap under a coral head or a hole in a wreck. After dark it leaves to hunt over the reef and sand. Its teeth are fused into a powerful beak, strong enough to crunch through urchin tests, snail shells and the shells borrowed by hermit crabs. On a reef, that makes it one of the few predators that can deal with well-armored prey.

Reproduction happens in open water. The eggs drift with the current and hatch after a few days, and the young live in the open ocean, often sheltering under floating sargassum, until they are about 20 cm long and settle onto the reef. That drifting early life is part of why the species is found in every tropical ocean.

Why You Must Never Make One Inflate

When a porcupine fish feels threatened, it rapidly pumps water into a very stretchy stomach until its body becomes a ball and the spines stand straight out. It works well against a predator, but it is an emergency response, not a show:

It is exhausting. In a study of a small pufferfish relative, oxygen use while inflated rose to about five times resting levels, and the fish needed an average of 5.6 hours to fully recover afterward.
It leaves the fish exposed. A round, inflated fish can barely swim, so it cannot flee or feed until it deflates.
Air is worse than water. A fish lifted out of the water can gulp air instead, which is harder to get rid of. Never pick one up for a photo.
It is risky for you too. The spines are sharp, the beak can give a hard bite, and the Florida Museum notes that the skin secretes a toxic substance.

If you see a porcupine fish inflated, someone or something has frightened it. Back off and give it space to recover.

Where to See Porcupine Fish in Punta Cana

Porcupine fish are a fairly regular sight on our local reefs, but you have to look for them. At Cuevitas, at 10 m (33 ft), the swim-throughs and small caves offer the kind of shelter they like. The structure of Wreck Monica, at 15 m (50 ft), and the crevices of Rock City are also good places to check. Usually you will find a single fish, staying still and watching you. Balloonfish turn up on the same sites, often over sand and rubble.

You will find it on our species guide along with the other reef animals our guides point out most often.

Tips for Divers and Snorkelers

Use a light or your eyes to peek under ledges, but do not reach in. Let the fish stay in its shelter.
Keep a respectful distance and avoid blocking the entrance to its hiding spot.
Never touch, poke or chase one to make it puff up. If it starts to inflate, you are too close.
A slow approach at eye level gives the best photos. Those big eyes are the shot, so focus there.
Snorkelers occasionally see porcupine fish in shallow rocky areas, but they are much easier to find on a dive.

Frequently Asked Questions

Are porcupine fish dangerous?

Not if you leave them alone. They are shy and will retreat from divers. The risk comes from handling them, which can lead to spine punctures or a bite from that strong beak.

What is the difference between a porcupine fish and a pufferfish?

Both belong to the same group of inflating fishes. Porcupine fish have long spines covering the body and fused teeth forming a single beak in each jaw. Pufferfishes have no long spines and have two teeth in each jaw.

Can you eat porcupine fish?

It is generally considered poisonous, and FishBase lists it as poisonous to eat. It is rarely eaten, and we strongly suggest you leave it on the reef.