Ultrastructural study of the jaw structures in two species of Ampharetidae (Annelida: Polychaeta)

Two species of jaw bearing Ampharetidae (Adercodon pleijeli (Mackie 1994) and Ampharete sp. B) were investigated in order to describe the microanatomy of the mouth parts and especially jaws of these enigmatic polychaetes. The animals of both studied species have 14–18 mouth tentacles that are about 30 µm in diameter each. In both species, the ventral pharyngeal organ is well developed and situated on the ventral side of the buccal cavity. It is composed of a ventral muscle bulb and investing muscles. The bulb consists of posterior and anterior parts separated by a deep median transversal groove. In both species, the triangular teeth or denticles are arranged in a single transversal row on the surface of the posterior part of the ventral bulb just in front of its posterior edge. There are 36 denticles in Adercodon pleijeli and 50 in Ampharete sp. B. The height of the denticles (6–12 µm) is similar in both species. Each tooth is composed of two main layers. The outer one (dental) is the electron‐dense sclerotized layer that covers the tooth. The inner one consists of long microvilli with a collagen matrix between them. The thickness of the dental layer ranges from 0.95 to 0.6 µm. The jaws of the studied worms may play a certain role in scraping off microfouling. The fine structure of the jaws in Ampharetidae is very similar to that of the mandibles of Dorvilleidae, the mandibles and the maxillae of Lumbrineridae, Eunicidae and Onuphidae, and the jaws of other Aciculata. This type of jaw is characterized by unlimited growth and the absence of replacement. The occurrence of jaws in a few smaller Ampharetidae is considered as an apomorphic state.