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Reproduction

Most species occurring in the North Sea, live for between five and ten years and probably become sexually mature in their second or third year. Ova and sperm mature in different specimens and are released to the sea at various times of the year, controlled perhaps, in temperate regions, by sea temperature fluctuations. Fertilisation takes place outside the body cavity but fertilised eggs may be found in the brachial cavity. In some species, especially the warm water thecideaceans, a brooding mechanism exists to hold the developing larvae within the lophophore. Cleavage of the fertilised egg leads to gastrulation after which the external surface commonly becomes ciliated. Differentiation of the posterior lobe leads to the development of the pedicle and mantle rudiments. The free-swimming larva develops for several days providing the only normal means of dispersal for the species. At about the time of settlement to the substratum, the mantles of articulate species reverse their positions from enclosing the pedicle to extending anteriorly around the body. This brings the ciliated surface of the mantle to the inner side, freeing the new outer surface for the secretion of shell material. The larvae of inarticulates do not reverse their mantle and the pedicle develops at a later stage. The first formed shell, the protegulum, is only about 0.3 mm long, and appears to be similarly shaped in most articulates. Some internal organs are recognizable at this stage and the lophophore, with only a few filaments, is a simple ring extending forward from the mouth. It is not until the shells have grown to more distinctive shapes or developed characteristic ornamentation that species recognition becomes possible. Once the outside surface of the shell has been secreted it remains unmodified by the brachiopod itself and so provides a record of the specimen's life history. The external growth lines develop when shell growth is interrupted by the retraction of the marginal mantle epithelium. This retraction is caused by disturbances such as seasonal availability of food, spawning activity, or storms, or by the predation of other organisms. Predation or damage to the shell is recorded by localized areas of distortion of the valve.

The valves grow by peripheral shell accretion and thicken by the addition of shell material over their inner surfaces. At the same time the lophophore grows, passing through more or less complex stages of development, some of which resemble the adult stages found in other species. Inside the terebratulid lophophore the brachial loop develops, also altering shape as it grows. For these reasons it is important to recognise whether a specimen is juvenile or adult before identifying the species.

Most brachiopods remain attached to the substratum throughout life. There is some evidence that a few species, living outside the North Sea, may alter their positions, if their pedicle is "rooted" in coarse sediment, by withdrawing the pedicle from the sediment and re-rooting close by. Lingulids move up and down their burrows in silty sand, according to the state of the tide, but these are the only burrowing brachiopods (Lingula affinis).

Source: Brunton and Curry, 1979.