Epitoky is a process that occurs in many species of

polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...

marine worms wherein a sexually immature worm (the atoke) is modified or transformed into a sexually mature worm (the epitoke). Epitokes are

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...

morphs capable of

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

. Unlike the immature form, which is typically

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

(lives on the bottom), epitokes are specialized for swimming as well as reproducing. The primary benefit to epitoky is increased chances of finding other members of the same species for reproduction. There are two methods in which epitoky can occur: schizogamy and epigamy.

Schizogamy

Many species go through schizogamy, where the atoke uses

asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...

to produce buds from its posterior end. Each bud develops into an epitoke and, once fully formed, will then break off from the atoke and become free-swimming. Many genetically identical epitokes are formed in this way, thus allowing a higher chance of finding a mate of the same species and subsequent passing of genes to the next generation. Atokes may then live through another season to form more epitokes.

Epigamy

Epigamy is another common way to form epitokes. For species that use this method, the atoke undergoes physiological and morphological modifications as it transforms into the epitoke. Typically, male worms undergo a more pronounced transformation from atoke to epitoke. Modifications may include an increase in size of

parapodia In invertebrates, the term parapodium ( Gr. ''para'', beyond or beside + ''podia'', feet; plural: parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found in annelids, where they are paired, unjointed ...

and the development of paddle-like

chaeta A chaeta or cheta (from Greek χαίτη “crest, mane, flowing hair"; plural: chaetae) is a chitinous bristle or seta found in annelid worms, (although the term is also frequently used to describe similar structures in other invertebrates suc ...

e for enhanced swimming ability,

atrophy Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply t ...

of the gut, filling of the body cavity with

gametes A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...

(

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...

), the development of large eyes, and the musculature may even change to perform swimming movements instead of feeding movements. The majority of species that undergo epigamy are unable to revert to the atoke form and die after reproducing. Male and female epitokes are produced and swim to the water's surface only at certain times of the year and are often synchronized with moon cycles in a behavior called swarming. Swarming brings individuals of the same species together so that there is an increased rate of fertilization. Some polychaete species have been found to use

bioluminescence Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some b ...

, presumably to compact and maintain swarms. Both schizogamous and epigamous epitokes are non-feeding individuals that die once

have been released into the water. In the past, epitokes were thought to be a separate group of

marine worms, because epitokes may look very different than atokes. For instance, the atokes of ''

Platynereis dumerilii ''Platynereis dumerilii'' is a species of annelid polychaete worm. It was originally placed into the genus ''Nereis'' and later reassigned to the genus '' Platynereis''. ''Platynereis dumerilii'' lives in coastal marine waters from temperate t ...

'' are yellowish-brown, while the female epitokes are yellow because of the eggs they contain, and the male epitokes are white in the front part due to sperm and red in the hind part due to blood vessels{{cite journal, last1=Fischer, first1=Antje HL, last2=Henrich, first2=Thorsten, last3=Arendt, first3=Detlev, title=The normal development of Platynereis dumerilii (Nereididae, Annelida), journal=Frontiers in Zoology, date=2010, volume=7, issue=1, pages=31, doi=10.1186/1742-9994-7-31, pmc=3027123, pmid=21192805 (see pictures).

References

Reproduction in animals Asexual reproduction