Jaw growth and replacement in Ophryotrocha labronica (Polychaeta, Dorvilleidae)
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Ophryotrocha labronica, as typical for Eunicida, has a complex jaw apparatus consisting of ventral mandibles and dorsal maxillae. Mandibles are not replaced but are retained throughout life. Larval mandibles have adult-sized cutting plates but their proximal shafts lengthen and enlarge as the worm grows. The maxillary apparatus of O. labronica undergoes three moults or replacements. The initial, or larval maxillae, consisting of two paired basal plates and two paired free denticles, develop in the unreleased larvae. They are replaced in the 5-setiger juvenile by the P1-maxillae consisting of falcate forceps and six denticles. The second moult occurs in the 8- to 9-setiger juveniles and results in the P2-maxillae with bidentate forceps and seven denticles, and the third and final moult results in the K-maxillae and seven denticles. The K-maxillae develop in 9- to 12-setiger males and 13- to 15-setiger females and are not replaced but enlarge proximally. Thus the K-forceps can be traced back through the P2-forceps, P1-forceps, to the larval basal plates, indicating the apomorphic state of the K-forceps. Three pulp cavities, separated by darker fusion lines are visible in weakly sclerotised young K-forceps suggesting the fusion of three separate elements. It is concluded that the Ophryotrocha forceps are homologous to the superior and probably inferior basal plates of other dorvilleids. The internal structure of the Ophryotrocha forceps demonstrates that they are not homologous to the labidognath maxilla I as has been suggested.