Bryozoa
Reproduction and life cycles

Zooids of all phylactolaemate species are simultaneous hermaphrodites. Although those of many marine species are protandric, in other words function first as males and then as females, their colonies contain a combination of zooids that are in their male and female stages. In all species the ovaries develop on the inside of the body wall, and the testes on the funiculus connecting the stomach to the body wall. Eggs and sperm are released into the coelom, and sperm exit into the water through pores in the tips of some of the tentacles, and then are captured by the feeding currents of zooids that are producing eggs. Some species' eggs are fertilized externally after being released through a pore between two tentacles, which in some cases is at the tip of a small projection called the "intertentacular organ" in the base of a pair of tentacles. Others' are fertilized internally, in the intertentacular organ or in the coelom.

All phylactolaemates and stenolaemates, and most gymnolaemates, exhibit placentation, and has therefore lecithotrophic (non-feeding) larvae. Except for Cyclostomata and the small gymnolaemate family Epistomiidae, which are viviparous, all are brooders. Phylactolaemata brood their embryos in an internal brood sac, but Gymnolaemata both external membranous sacs, skeletal chambers (ovicells) and internal brooding sacs exist. The developing embryo relies on egg's yolk, extraembryonic nutrition (matrotrophy) or both.

In ctenostomes the mother provides a brood chamber for the fertilized eggs, and her polypide disintegrates, providing nourishment to the embryo. Stenolaemates produce specialized zooids to serve as brood chambers, and their eggs divide within this to produce up to 100 identical embryos.

Planktotrophic (feeding) larvae are only found in class Gymnolaemata: In the cheilostomatan suborder Malacostegina they are found in the two families Membraniporidae and Electridae, and in the three ctenostome families Alcyonidiidae, Farrellidae, and Hislopiidae. In addition there are a few unconfirmed records, like the solitary form Aethozoid where larvae has never been observed, but which is assumed to have planktotrophic larvae.

The cleavage of bryozoan eggs is biradial, in other words the early stages are bilaterally symmetrical. It is unknown how the coelom forms, since the metamorphosis from larva to adult destroys all of the larva's internal tissues. In many animals the blastopore, an opening in the surface of the early embryo, tunnels through to form the gut. However, in bryozoans the blastopore closes, and a new opening develops to create the mouth.

Bryozoan larvae vary in form, but all have a band of cilia round the body which enables them to swim, a tuft of cilia at the top, and an adhesive sac that everts and anchors them when they settle on a surface. Some gymnolaemate species produce cyphonautes larvae which have little yolk but a well-developed mouth and gut, and live as plankton for a considerable time before settling. These larvae have triangular shells of chitin, with one corner at the top and the base open, forming a hood round the downward-facing mouth. In 2006 it was reported that the cilia of cyphonautes larvae use the same range of techniques as those of adults to capture food. Species that brood their embryos form larvae that are nourished by large yolks, have no gut and do not feed, and such larvae quickly settle on a surface. In all marine species the larvae produce cocoons in which they metamorphose completely after settling: the larva's epidermis becomes the lining of the coelom, and the internal tissues are converted to a food reserve that nourishes the developing zooid until it is ready to feed. The larvae of phylactolaemates produce multiple polypides, so that each new colony starts with several zooids. In all species the founder zooids then grow the new colonies by budding clones of themselves. In phylactolaemates, zooids die after producing several clones, so that living zooids are found only round the edges of a colony.

Phylactolaemates can also reproduce asexually by a method that enables a colony's lineage to survive the variable and uncertain conditions of freshwater environments. Throughout summer and autumn they produce disc-shaped statoblasts, masses of cells that function as "survival pods" rather like the gemmules of sponges. Statoblasts form on the funiculus connected to the parent's gut, which nourishes them. As they grow, statoblasts develop protective bivalve-like shells made of chitin. When they mature, some statoblasts stick to the parent colony, some fall to the bottom ("sessoblasts"), some contain air spaces that enable them to float ("floatoblasts"), and some remain in the parent's cystid to re-build the colony if it dies. Statoblasts can remain dormant for considerable periods, and while dormant can survive harsh conditions such as freezing and desiccation. They can be transported across long distances by animals, floating vegetation, currents and winds, and even in the guts of larger animals. When conditions improve, the valves of the shell separate and the cells inside develop into a zooid that tries to form a new colony. Plumatella emarginata produces both "sessoblasts", which enable the lineage to control a good territory even if hard times decimate the parent colonies, and "floatoblasts", which spread to new sites. New colonies of Plumatella repens produce mainly "sessoblasts" while mature ones switch to "floatoblasts". A study estimated that one group of colonies in a patch measuring 1 square meter (11 sq ft) produced 800,000 statoblasts.

Cupuladriid Bryozoa are capable of both sexual and asexual reproduction. The sexually reproducing colonies (aclonal) are the result of a larval cupuladriid growing into an adult stage whereas the asexual colonies(clonal) are a result of a fragment of a colony of cupuladriids growing into its own colony. The different forms of reproduction in cupuladriids are achieved through a variety of methods depending on the morphology and classification of the zooid.
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