Micrognathozoa: a new class with complicated jaws like those of Rotifera and Gnathostomulida

A new microscopic aschelminth-like animal, Limnognathia maerski nov. gen. et sp., is described from a cold spring at Disko Island, West Greenland, and assigned to Micrognathozoa nov. class. It has a complex of jaws in its pharynx, and the ultrastructure of the main jaws is similar to that of the jaws of advanced scleroperalian gnathostomulids. However, other jaw elements appear also to have characteristics of the trophi of Rotifera. Jaw-like structures are found in other protostome taxa as well-for instance, in proboscises of kalyptorhynch platyhelminths, in dorvilleid polychaetes and aplacophoran mollusks-but studies of their ultrastructure show that none of these jaws is homologous with jaws found in Gnathostomulida, Rotifera, and Micrognathozoa. The latter three groups have recently been joined into the monophylum Gnathifera Ahlrichs, 1995, an interpretation supported by the presence of jaw elements with cuticular rods with osmiophilic cores in all three groups. Such tubular structures are found in the fulcrum of all Rotifera and in several cuticular sclerites of both Gnathostomulida and Micrognathozoa. The gross morphology of the pharyngeal apparatus is similar in the three groups. It consists of a ventral pharyngeal bulb and a dorsal pharyngeal lumen. The absence of pharyngeal ciliation cannot be used as an autapomorphy in the ground pattern of the Gnathifera because the Micrognathozoa has the plesiomorphic alternative with a ciliated pharyngeal epithelium. The body of Limnognathia maerski nov. gen. et sp. consists of a head, thorax, and abdomen. The dorsal and lateral epidermis have plates formed by an intracellular matrix, as in Rotifera and Acanthocephala; however, the epidermis is not syncytial. The ventral epidermis lacks internal plates, but has a cuticular oral plate without ciliary structures. Two ventral rows of multiciliated cells form a locomotory organ. These ciliated cells resemble the ciliophores present in some interstitial annelids. An adhesive ciliated pad is located ventrally close to a caudal plate. As in many marine interstitial animals-e.g., gnathostomulids, gastrotrichs, and polychaetes-a special form of tactile bristles or sensoria is found on the body. Two pairs of protonephridia with unicellular terminal cells are found in the trunk; this unicellular condition may be the plesiomorphic condition in Bilateria. Only specimens with the female reproductive system have been found, indicating that all adult animals are parthenogenetic females. We suggest that 1) jaws of Gnathostomulida, Rotifera, and the new taxon, Micrognathozoa, are homologous structures; 2) Rotifera (including Acanthocephala) and the new group might be sister groups, while Gnathostomulida could be the sister-group to this assemblage; and 3) the similarities to certain gastrotrichs and interstitial polychaetes are convergent.