ABSTRACT
Brachiopod setae occur in both larvae and juvenile/adult individuals of all Recent brachiopod subgroups, except thecideidines. Larval brachiopod setae, which are shed during later development, can be distinguished from adult brachiopod setae by several morphological characteristics, such as lack of an enamel layer, or (apart from the chaetoblast) association with only one additional epidermal cell rather than a row of follicle cells. The hatching stage of Discinisca cf. tenuis is lecithotrophic and is equipped with a pair of setal bundles – so-called embryonic setae. Ultrastructural investigations show that these embryonic setae resemble larval setae of craniid and articulate brachiopods except that the single additional epidermal cell, which is folded around each discinid embryonic seta like a cuff, secretes setal material and is involved in the formation of setal spines. In all other brachiopods setal spines, if present, are built exclusively by the chaetoblast. Setae of discinid embryos are therefore equivalent to larval setae, due to overall morphological similarity with those found in lecithotrophic larvae of craniid and articulate brachiopods. Thus, craniids, discinids and articulate brachiopods all have lecithotrophic larvae with setae of the ‘larval type’. Assuming brachiopod monophyly, the most parsimonious hypothesis of an ancestral brachiopod life cyle would be a lecithotrophic pelagic larva, equipped with larval setae, and followed by a planktotrophic sessile adult. As a consequence, direct development in lingulids is interpreted as a secondary loss of a ‘true’ larval stage.
