Do Fish Have Bones, Hearts and Blood? What Is Inside a Fish

Do fish have bones? Most do. A bass, a catfish or a trout has a skull, a backbone, ribs and a row of small bones hidden in its muscle. Sharks, rays and chimaeras are built from cartilage instead, the bendy stuff in your ear and the tip of your nose. Two strange groups, hagfish and lampreys, have no jaws at all. Below: what a fish is made of, how its heart and blood work, and what that means when you fillet or bleed the one you keep.

Sixteen kinds of Antarctic icefish have no red blood. They lost the genes for hemoglobin about 8.5 million years ago, and their plasma carries about a tenth of the oxygen a red-blooded cousin moves. They get by on bigger hearts and more blood (Sidell and O’Brien 2006).

Do fish have bones? Three kinds of fish

The Australian Museum sorts all fishes into three groups. The bony fishes are the huge majority of the world’s 33,000-plus species: every bass, trout, perch, catfish and carp you will ever catch. Their skeleton is true bone, the same material as yours.

The cartilaginous fishes are the sharks, rays, skates and chimaeras. Their skeleton is cartilage from nose to tail, lighter and bendier than bone.

The jawless fishes are the odd ones out: hagfish and lampreys. A lamprey has a round sucker mouth with rasping teeth and no jaw.

Why fish have so many bones

A fish skeleton starts like yours: skull, backbone, ribs, plus the bones that hold up each fin. Then come the ones every angler meets at the fillet board: the pin bones.

Pin bones are thin bones that sit inside the muscle. They are not part of the backbone; they float in the meat and brace it while the fish swims by bending its whole body. Run a fingertip along a trout or salmon fillet from the head end and you feel them as a row of tiny points. In a northern pike they are forked, so anglers call them Y-bones. That row runs along the upper half of the fillet, above the lateral line, and a pike fillet is not boneless until it is out.

This is why big fish give cleaner fillets. A 30-inch (76 cm) pike has the same bones as a 16-inch (41 cm) one, only larger. Big pin bones are easy to feel and pull with pliers. A thick fillet leaves room to run the knife over the Y-bones and lift the meat off them. In a small fish the same bones are hair-thin, with barely any meat around them. That is why small trout are cooked whole and eaten around the bones. A catfish fillet, by contrast, comes off with almost no pin bones, one reason it is the first fish so many kids eat.

Do fish have hearts?

Yes. A fish heart has two chambers, one atrium that collects the blood and one ventricle that pumps it (OpenStax Biology). It sits low in the body, just behind the gills, under the throat. Yours has four chambers, and the difference is in the plumbing. Your blood makes two loops, heart to lungs and back, then heart to body and back. A fish’s blood makes one loop. The heart pumps it to the gills, where it picks up oxygen, and from there it flows on around the body. One push per trip. A resting fish slows that heart and its gill beats together, one sign of a sleeping fish (do fish sleep? has the rest).

That single loop is why a gill-hooked fish bleeds so hard. Every drop of blood in the fish crosses the gills right after the heart, spread through thin red filaments that sit against the water. A hook that tears those filaments opens the main line, and the heart keeps pumping through the tear. A fish bleeding freely from the gills rarely survives release. If it is legal to keep, that is the one for the cooler.

Do fish have blood?

Yes, and it is red for the same reason yours is: hemoglobin, the iron-rich protein that carries oxygen.

The exception is the family in the pull quote. Sixteen species of Antarctic icefish have no hemoglobin at all. Sidell and O’Brien showed they lost the genes for it about 8.5 million years ago. Their plasma carries about 10% of the oxygen a red-blooded relative moves. They survive because they live slowly in near-freezing water and grew bigger hearts and more blood to push the little oxygen they have around. It helps that cold water is rich in oxygen. Fresh water at 32 °F (0 °C) holds 14.6 mg of oxygen per liter, twice the 7.3 mg/L at 90 °F (32 °C) (USGS).

Do fish have brains? Yes, a full brain, about one-fifteenth the weight of a bird’s or mammal’s of the same body size (Brain, Behavior and Evolution 2021). Do fish have brains? covers memory and how a caught fish learns.

How to bleed a fish you keep

Cooks and commercial fishermen bleed fish because blood left in the meat darkens the fillet and gives it a stronger taste. The single-loop heart does the work for you: open the gills and it pumps the blood out. The steps:

1. Decide first. A bled fish is a kept fish. Check that it is in season and over the size limit before the knife comes out.
2. Do it at once, while the heart is beating. The heart is the pump. Wait until the fish is dead and most of the blood stays in the meat.
3. Lift the gill cover and cut the red gill arches. One clean cut across the arches on each side with the knife tip.
4. Put the fish head-down in a bucket of water. The blood runs out instead of clotting in the gills. A few minutes is enough; the flow stops on its own.
5. Straight onto ice. A cooler with ice or an ice-and-water slush, never the warm livewell.
6. Fillet, then find the pin bones. Run a fingertip along the fillet and pull each bone with clean pliers, or cut the row out in one strip.

More questions with answers in the fish facts collection, and the build of each game fish, from jaw to tail, in the species guides.