Reproductive biology

Phoronids generally breed between spring and autumn over a fairly long period. These organisms are hermaphroditic or dioecious. Gametes are released through the nephridia, which serve as gonoducts. Sperm is disseminated by means of spermatophores formed in two lophophoral organs. Fertilization is internal, and cross-fertilization takes place in hermaphroditic species. Egg cleavage is total, equal, and typically radial.

Three types of developmental patterns occur. Species with small eggs undergo complete planktonic development, and species with larger eggs brood either in nidamental glands within the lophophore concavity or within the parental tube until the first larval stages and then undergo planktotrophic development. Embryonic development leads to a characteristic free-swimming larva, called the actinotroch, that bears an anterior preoral lobe on the top of which is located the nervous ganglion, a tentacular ridge, a pair of protonephridia, and, in the posterior, a ciliated ring around the anus. The larvae undergo planktotrophic development and settle after approximately 20 days. Metamorphosis is "catastrophic," occurring in less than 30 minutes and leading to a slender young phoronid, which burrows into the substratum.

Phoronids also reproduce asexually, usually by transverse fission around the middle of the trunk.

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