Animals That Go Through Metamorphosis

Metamorphosis is a type of animal development from one stage to another. However, unlike other types of development, the change is relatively fast and dramatic. The animal's body structure is significantly different from how it was in the previous stage. Metamorphosis is controlled by hormone release in the cells of the body. Specifically, the hormone released to control the process is known as idothyronine.

It is important to differentiate metamorphosis with other types of growth. In mammals, moving through adolescence is not a metamorphosis as the development is gradual and the adult organism resembles its juvenile form. This is why there are no mammals which go through metamorphosis. Birds cannot metamorphose either, but insects, amphibians and fish.

There are different types of metamorphosis in animals, with three main distinctions: complete metamorphosis, partial metamorphosis or no metamorphosis. There is still much we do not know about this fascinating type of development. One aspect we have learned is that butterflies can remember life as a caterpillar. A study from 2008 showed that butterflies could recall pain stimulus then received in their butterfly form^[1].

Insects are a metamorphic group par excellence. They are the animal group which undergoes this change most commonly and, often, the most dramatically. They are oviparous animals which means they are born from eggs. Their development requires the shedding of their skin as this outer covering prevents it from developing in size, unlike other animals. Insects are hexapod invertebrates, meaning they have six legs in their adult stage.

Not all insects will metamorphose in the same way. For example, hexapods in the order Diplura are considered ametabolic, meaning they do not undergo significant morphological change after the nymph stage. They are primarily apterous insects, which means they do not have wings. After the embryonic stage, they only go through small changes such as:

1. Progressive development of genitals
2. Increase in biomass (i.e. animal weight)
3. Slight variations in size of parts which are relatively proportioned

In pterygotic insects (those which do have wings), there are several types of metamorphosis. These types are categorized into three categories, dictated by the amout and type of change the insect undergoes. They are:

• Hemimetaboly (incomplete metamorphosis): when insect emerges from the egg, it is known as a nymph. The nymph resembles the adult insect, but will not yet have wings. They are insects which do not have a pupal stage. Instead, they go through different molts and then go directly into adulthood. Some examples include mayflies, dragonflies, grasshoppers, bed bugs, termites, etc.
• Holometaboly (complete metamorphosis): larva are born from the egg in a form which is very different from the adult stage. At a certain point, the larva will become a pupa or chrysalis which will hatch into the adult individual. This is the metamorphosis which occurs in the majority of insects such as butterflies, cockroaches, ants, bees, wasps, crickets, beetles, etc.
• Hypermetaboly (hypermetamorphosis): hypermetamorphic insects are those which have a very long larval development. The larvae look different from each other as they grow due to being in different habitats. Nymphs do not have developed wings until they reach adulthood. Not very common in insects in general, but does occur in certain parasitoid insects.

The biological reason for metamorphosis, other than the fact it needs to change skin is to separate the offspring from parents. This is partly due to avoid competition for the same resources. Typically, the larvae will live in different places to the adult insects. For example, the larvae may stay in an aquatic environment, while the adults stay away from water. They feed differently as larvae may be herbivorous while adults feed on other insects (or vice versa).

Amphibian animals also present metamorphosis, with some being more or less subtle than others. The main purpose of amphibian metamorphosis is to eliminate the gills they have in younger stages and allow for lung development which allows them to breathe on land. There are some exceptions, such as the Mexican axolotl (Ambystoma mexicanum). In its adult stage, the axolotl still has gills meaning it is considered an evolutionary neoteny (the maintenance of juvenile features in the adult stage).

Amphibians are also oviparous animals. From the egg emerges a small larva which can be very similar to the adult, as in the case with salamanders and newts, or very different, as occurs in frogs or toads. In fact, the frog is a very common example used to explain amphibian metamorphosis.

At birth, salamanders already have legs and tails like their adult stages. However, they also have gills. After metamorphosis, the gills disappear and lungs develop. This process may be delayed for several months, depending on the individual species.

In anuran animals (tailless amphibians) such as frogs and toads, the metamorphosis is much more complex. When the eggs hatch, small larvae with gills and tails emerge. They do not have legs and the mouth is only half developed. This is known as a tadpole. After some time, a layer of skin begins to grow over the gills and small teeth appear in the mouth.

After this point, the hind legs develop. Two bumps appear at the front where eventually front limbs will develop. In this stage, the tadpole will still have a tail, but it will also be able to breathe air. The tail will slowly diminish until it disappears completely, giving rise to an adult frog or toad.

It is not only amphibians and insects which go through the process of metamorphosis. Many other animals belonging to different taxonomic groups also undergo these dramatic changes, for example:

• Cnidarians which include jellyfish and sea anemones, among others.
• Crustaceans, such as lobsters, crabs or prawns.
• Tunicates, or more specifically, ascidians. After the metamorphosis process and the establishment as an adult individual they become sessile (or immobile) They also have a ganglion which shrinks in size, although they don't quite 'eat their brain' as some have claimed.
• Echinoderms like starfish, sea urchins or holothurians (sea cucumbers).

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