Metamorphosis

Metamorphosis is a special and noticeable change that happens in an animal's body during its growth. Many different animals, like insects, fish, amphibians, and mollusks, go through this change. It often includes a shift in how they get food or act.

Some animals change a lot between stages of their life, called complete metamorphosis. Others change only a little, called incomplete metamorphosis. During these changes, young animals called larvae turn into adults. This process helps animals fit into different parts of nature and survive better.

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Hormonal control

In insects, growth and change in body shape are managed by special chemicals called hormones. These are made by endocrine glands near the front of the body. A hormone from the insect's brain tells other glands to release another hormone, ecdysone, which causes the insect to shed its outer shell, known as ecdysis. Another hormone, juvenile hormone, helps control whether the insect looks more like a baby or an adult after it sheds its shell.

Experiments with firebugs show how these hormones can change how many stages an insect goes through before becoming an adult. In animals like fish and amphibians, changes in body shape during growth are influenced by special chemicals related to iodine.

Insects

Insects show three types of metamorphosis: no change, incomplete change, and complete change. With no change, called ametaboly, the young look almost the same as the adults. With incomplete change, called hemimetaboly, the young, known as nymphs, look similar to adults but are smaller and lack wings and other adult features. These nymphs grow through stages called instars, molting (shedding their skin) between each stage.

With complete change, called holometaboly, the young, known as larvae, look very different from adults. They go through a resting stage called a pupa (or chrysalis in butterflies) before emerging as adults. For example, a butterfly starts as a caterpillar (larva), forms a chrysalis (pupa), and then becomes a beautiful butterfly (adult). Temperature can affect how fast these changes happen, and recent studies show that insects can remember things learned in earlier stages and that special cell processes help during metamorphosis.

Chordata

Amphioxus

In cephalochordata, change in body form is caused by a certain substance and might be an ancient feature of all chordates.

Fish

Some fish, like bony fish (Osteichthyes) and jawless fish (Agnatha), go through a change in body form. This change is often controlled by a hormone from the thyroid.

Examples include the lamprey. The salmon changes from living in freshwater to saltwater. Many flatfish start with an eye on each side of their body, but as they grow, one eye moves to join the other on top of the head.

The European eel goes through several changes in form, from its early stage to the glass eel, then to the elver, and finally to the adult migrating phase.

Most other bony fish start from an egg to small, immovable larvae, then to movable larvae that must find their own food, and finally to the juvenile stage where they start to look like adults.

Amphibians

In amphibians, eggs are laid in water, and the young are adapted to water. Frogs, toads, and newts hatch as larvae with gills, but later develop lungs.

Metamorphosis in amphibians is controlled by certain hormones in the blood. Tadpoles have special features like tooth ridges and fins that are later absorbed when they change into adults.

Frogs and toads

In frogs and toads, the young hatch with external gills that are quickly covered, and lungs form soon after. Front legs develop under the gill sac, and hind legs appear a few days later. Tadpoles eat plants and have long, spiral-shaped guts, but as they change, their guts shorten to help them eat insects.

Quick changes happen when their lifestyle shifts completely. The spiral-shaped mouth and gut are absorbed, the animal develops a big jaw, and the gills disappear. Eyes and legs grow fast, and a tongue forms. All these changes can happen in about a day. The tail is absorbed a few days later.

Salamanders

Salamander development varies; some change dramatically from water larvae to land adults, while others, like the axolotl, keep their larval features and never become land adults.

Newts

In newts, the change in body form happens because of a change in where they live, not because of food. Newts' gills are not covered by a sac and are absorbed just before they leave the water. Adults can move faster on land than in water. Newts often live in water in spring and summer and on land in winter. Hormones help them adapt to water or land living.

Caecilians

Some caecilians, like Ichthyophis, go through a change where water larvae become burrowing adults, losing certain features. Other caecilians stay burrowing throughout their lives and do not go through a big change like frogs.