Metamorphosis is a process by which animals undergo extreme, rapid physical changes sometime after birth. The result of metamorphosis may be a change to the organism’s entire body plan, such as a change in the animal’s number of legs, its means of eating, or its means of breathing.
In species that use metamorphosis, metamorphosis is also typically required for sexual maturity. Pre-metamorphic members of these species are typically unable to mate or reproduce.
Commonly known examples of metamorphosis include the process undergone by most insects and the transformation of tadpoles into frogs. The diagram below shows the stages of this change, wherein the small fish-like tadpoles transform into what seems a completely different animal:
Animals that you may not know undergo metamorphosis include fish, mollusks, and many other types of sea creatures that are related to insects, mollusks, or fish. Lobsters, for example, which is closely related to insects, do undergo metamorphosis as part of their life cycle.
Metamorphosis is a remarkable process. The speed and extent of cell growth and differentiation are astonishing. In most species, such rapid growth and such sweeping changes to cell type only happen during embryonic development. Indeed, some scientists believe that the process of metamorphosis involves a sort of re-activating of genes that allow animal cells to change from one cell type to another.
The changes leading to metamorphosis are triggered by hormones, which the animal’s body releases as the right conditions for the metamorphosis approach. In some animals, a hormone cascade follows, with the trigger hormone causing the release of several other hormones that act on different parts of the animal’s body.
The hormones cause drastic changes to the functioning of cells and even behavioral changes such as the caterpillar spinning its cocoon.
The effects of hormones on metamorphosis can be studied by artificially administering these hormones to pre-metamorphic animals. Tadpoles, for example, can be triggered to begin losing their tails and growing limbs early by the addition of thyroid hormones to their water supply. Unfortunately, this has a detrimental effect on the animal’s health.
Function of Metamorphosis
Scientists remain uncertain why metamorphosis evolved. For the animals of today, its purpose is obvious: if metamorphosis did not occur, tadpoles could not become frogs and larvae could not become full-grown adults capable of reproduction. Without reproductively mature members, these species would quickly die off.
But why would these species evolve to need this extra step in the first place? Why not just hatch full-grown butterflies or frogs from eggs?
At least some metamorphosing species did not start out that way: the earliest insects basically did hatch as full-grown adults. But a few hundred million years ago, some species stumbled upon the trick of metamorphosis. It was apparently wildly successful; it is thought that almost two-thirds of species alive today use metamorphosis to accomplish large changes between their adult and juvenile forms.
The benefit of metamorphosis may lie in its ability to reduce competition. Pre-metamorphic animals typically consume completely different resources from their adult forms. Tadpoles live in water, eating algae and plants. Frogs live on land, breathing air and eating insects. Caterpillars eat leaves; butterflies live off of nectar. Etc.
This effectively prevents older members of the species from competing with younger members. This may lead more members of the species to successfully reach sexual maturity, without the risk of being out-competed by older members of their species.
Types of Metamorphosis
In complete metamorphosis, a larva completely changes its body plan to become an adult. The most famous example is that of the butterfly, which starts out as a worm-like, leaf-eating caterpillar and transforms into a flying, nectar-drinking creature with an exoskeleton.
Organisms that undergo complete metamorphosis are called “holometabolous,” from the Greek words “holo” for “complete” or “whole,” “meta” for “change,” and the noun “bole” for “to throw.” “Holometabolous,” then, means “completely changing,” or “wholly changing.”
This transformation is so swift and complete that the caterpillar must spin a cocoon and lie dormant for weeks while its body undergoes these radical changes.
Other animals which transform from a worm-like larval stage into an animal that looks completely different include beetles, flies, moths, ants, and bees.
Some scientists believe that the larval stage of complete metamorphosis may have evolved from insects that hatched from their eggs without developing properly. Some of these embryos may have survived long enough to find food in the outside world; and this may have ended up giving them an advantage, as they would be able to feed longer and gain more strength than their peers before metamorphosing into the adult stage.
In incomplete metamorphosis, only some parts of the animal’s body change during metamorphosis. Animals that only partially change their bodies as they mature are called “hemimetabolous,” from the Greek words “Hemi” for “half,” “meta,” for “change,” and the verb “bole” for “to throw.”
“Hemimetabolous,” then, is a word meaning “half-changing.” Cockroaches, grasshoppers, and dragonflies, for example, hatch from eggs looking a lot like their adult selves. They do acquire wings and functioning reproductive organs as they grow, but they do not completely remake their bodies like their completely metamorphosing cousins do.
Examples of Metamorphosis
Many of us may have witnessed the process of metamorphosis firsthand, by raising caterpillars into butterflies in school. The idea of a worm-like caterpillar wrapping itself in a cocoon for weeks and then emerging as a beautiful butterfly is certainly strange. But the obvious changes of appearance, such as the growth of wings, don’t do justice to just how strange this process is.
In the cocoon, caterpillars don’t simply gain legs, wings, and an exoskeleton. They also grow new eyes, lose their leaf-eating mouthparts and replace them with nectar-sucking proboscises, and gain mature reproductive organs. To accomplish this drastic change, a metamorphosing caterpillar basically digests itself.
A great deal of energy and raw materials are required to turn a caterpillar into a butterfly. So to make it possible, caterpillars release enzymes that dissolve most of their bodies! Indeed, the hard shell of the cocoon is required not just to protect the metamorphosing insect from attack: it is required to keep its liquefying body bound together, lest it oozes away!
Not all of the caterpillar’s cells are dissolved by these enzymes. Special tissues called imaginal discs to survive – and they use the soup that used to be the rest of the caterpillar’s body for nutrition.
By consuming the proteins, vitamins, and minerals – everything you need to build a butterfly – these imaginal discs are able to grow incredibly quickly, developing into the butterfly’s mature body parts.
The new body has almost nothing in common with the old body. It has new legs, new sensory organs, a new exoskeleton, a new reproductive system. Even its digestive system does not work the same way, since it must now digest nectar instead of leaves. That’s all in addition to the beautiful wings.
This radical change allows butterflies to complete their life cycle very efficiently, with no competition between adult butterflies and caterpillars for food.
Many other insects pass through a similar process. They hatch as worm-like larva, eventually encase themselves in hard pupas, and emerge as adults with legs, exoskeletons, and other features that have little in common with the larva they once were. Bees, beetles, ants, and flies all use this strategy.
The metamorphosis of a tadpole into a frog is a little less violent than that of a caterpillar into a butterfly, but the processes share some important common features.
Tadpoles do not dissolve their bodies into mush, but they do “digest” them in a less spectacular way. Using the process of apoptosis – or “programmed cell death” – the tadpoles “order” the cells they don’t need anymore to shred their DNA and die. The dead cells are then cannibalized for energy and raw materials to make other cells.
The cells of their tails are broken down and used to make their developing legs; a similar process happens with the gills, which disappear as the tadpole begins to develop air-breathing lungs.
One interesting thing to note is that tadpole metamorphosis and insect metamorphosis likely developed separately; the common ancestor of insects and amphibians diverged long ago, and the ancestors of modern insects are not thought to have used metamorphosis. When the same phenomenon evolves twice in radically different organisms, that’s a sure sign that it is a useful adaptation!
Some species of fish undergo metamorphoses similar to those of the tadpole. Though those changes are not so dramatic, they can result in changes in the fish’s food source, its body plan, and where it’s able to live. Just like the more drastic forms of evolution, this may function to prevent adults from competing with juveniles for food.
Salmon, for example, is a freshwater fish in its juvenile form. After undergoing a partial metamorphosis, it becomes a saltwater fish.
When thinking about this process it is important to keep in mind that all organisms must regulate their salt/water balance. This is why humans can’t drink seawater without dying: the salt would overwhelm our cellular chemistry, and our cells would not function properly.
In just the same way, freshwater fish typically cannot live in saltwater. To become saltwater fish, then, salmon must develop new organs and cellular mechanisms to cope with the saltwater.
That’s why salmon must perform their annual migration upstream; adult salmon live in the ocean, but their eggs must hatch in fresh water in order for the juveniles to survive. That means that adult salmon must leave their homes in the ocean for freshwater rivers, and swim as far upstream as possible before laying their eggs!
Flounders, bizarrely, undergo a metamorphosis in which one of their eyes and nostrils move from one side of the head to the other. As juveniles, flounders look much like most fish: they swim vertical relative to the current, with one eye and one nostril on each side of their bladelike body. This body type allows them to swim fast like most other species of fish.
But in adulthood, flounders are flatfish which camouflage themselves by swimming on their bellies, pressed against the sea bed. To accomplish this lifestyle change, juvenile flounders essentially flip over on their sides and make one side of their body into their belly.
Through cellular changes, the eye and nostril from the belly side actually migrate to join the other eye and nostril on what is now the “top” side of the fish. Evolution sure has some creative ways of doing things!