In life there is nothing more certain than the uncertainty of change. For most creatures time brings change, but for some it brings total transformation.
Most species undergo their most dramatic transformation as they develop as embryos before birth or hatching. However some species undergo a sudden, conspicuous change after birth or hatching and this process is called metamorphosis.
Insects – many species of insects reach maturity through developmental stages, for some species these stages are described as incomplete metamorphosis and for other species the metamorphosis is complete.
Incomplete insect metamorphosis has three stages egg, nymph, and adult. Immature nymphs change and grow by moulting in stages known as instars, after the final moult a fully mature adult emerges. In some species the nymphs closely resemble adults while in others such as dragonflies and cicadas the appearance and life of the nymph differs markedly from the adult form.
Complete insect metamorphosis has four stages egg, larva, pupa and adult. The larva is an eating organism which simply eats and avoids being eaten, once full capacity is reached the larva forms itself into a chrysalis or pupa within which it is transformed into an adult insect. The most familiar complete insect metamorphosis is the life cycle of the butterfly.
Marine animals – the lifecycle of many marine species including fish is as yet unknown. The huge number of zooplankton species includes many larval forms. Eels are remarkable both in terms of metamorphoses and life change. Huge aggregations of eels spawn in the ocean at great depth, the egg is transformed into a leaf-shaped larval stage known as leptocephalus, which develops and drifts in the ocean as plankton for several years. Eventually the leptocephalus approaches the coast and changes into a juvenile elver, which resembles a small transparent adult eel. The elver swims to fresh water until it reaches a place to stop at this point it develops into an adult eel. Eventually the time comes for the eel to spawn; it becomes bright silvery and descends downstream towards the sea, seeking salt water. Once the sea Perigee full Moonis reached, the eel descends to great depth, swims to its birthplace, where it joins huge aggregations, spawns, then dies.
Amphibians - such as frogs and toads metamorphosise – they hatch as tadpoles with gills, tails, vegetarian digestion and mouthparts, over time they transform into frogs which are carnivores, with lungs, jaws and tongue for hunting. The tadpole's redundant organs are reabsorbed and their nervous system rewired for vision and movement as an adult frog in a different environment.
Overwhelmingly the process of metamorphosis and maturity over time is a one way street, however there does appear to be an exception. Turritopsis nutricula is a species of jelly fish which is apparently capable of reversing its life cycle by reverting from a mature to an immature polyp stage by transforming its cells, thereby achieving biological immortality.