By Martin Sorensen, Niels Bohr Institute & Biological Institute, University of Copenhagen
Gnathostomulida is a phylum of microscopic, interstitial, marine worms. They are characterized by a simple epithelium with monociliated cells, preoral sensory cilia, and a ventral mouth opening that leads to a blind gut, through a muscular pharynx with a complex jaw apparatus. All gnathostomulids are hermaphrodites and development is direct.
Gnathostomulids are mostly found in medium or fine-grained sand mixed with detritus, and are often very common in sand from seagrass beds. At present the group comprises 94 species in 25 genera, and is probably found worldwide, even though the best-studied areas are Western Europe, the Mediterranean Sea, the US coastline, the Caribbean and a few localities in the Pacific Ocean. Gnathostomulids do not have any commercial importance, but as they often occur in relatively high numbers, they may be of ecological importance as grazers of attached bacterial flora in sulfide systems.
The Gnathostomulida were first considered a free-living Platyhelminthes. Later, however, they were given phylum rank, and are currently considered part of Gnathifera, closely related with Rotifera and Micrognathozoa. The most comprehensive systematic revision of the Gnathostomulida was done by Sterrer (1972), who for the first time established a gnathostomulid classification. More recent re-evaluations of gnathostomulid systematics based on new detailed observations on the pharyngeal hard parts, on morphology alone, and on a combined approach of morphological and molecular data have largely confirmed the classification proposed by Sterrer (1972).