A phylum of microscopic marine worms related to Rotifera and Micrognathozoa, mainly characterized by complex cuticular structures in the pharynx and a monociliated skin epithelium. Discovered in the 1920s on the German coast but not described until 1956, Gnathostomulida have since been reported from many sheltered sandy shores around the world. With fewer than 100 known species, this is one of the smallest animal phyla.
Gnathostomulids range from 0.01 to 0.14 in. (0.3 to 3.5 mm) in length. They are worm-shaped, cylindrical, and semitransparent (or bright red), and sometimes have the external divisions of head and tail (see illustration). The skin is a one-layered epithelium that is completely monociliated; each of the polygonal epidermal cells bears only one cilium which can be up to 20 micrometers long. Parenchyma is poorly developed, and excretory organs may be present as three paired groups of protonephridia. The nervous system is largely basiepithelial, consisting of an unpaired buccal and unpaired frontal ganglion (brain) from which one to three pairs of longitudinal nerves originate. The sensory system usually consists of one or two pairs of simple bristles and three or four pairs of compound bristles (frontally and laterally) and a row of stiff cilia (dorsally on the head). In some genera, ciliary pits and spiral ciliary organs of unknown function are present in the anterior head region.
Great diversity exists in the foregut area. The mouth, located ventrally in the head (in Haplognathia far from the anterior end), is equipped with complex cuticularized structures: sometimes with a jugum, a cartilaginous structure, in the upper lip, mostly with paired jaws and an unpaired basal plate in the lower lip area. The basal plate occurs in many different forms throughout the species but always bears lamellae or teeth in its center. The large mouth cavity is surrounded by a complex muscle apparatus (consisting of a dozen pairs of individual muscles) which becomes progressively more compact from Haplognathia to Gnathostomula types. The same is true for the jaws which vary from simple forceps types (Haplognathia) to complicated lamellar snap jaws with three rows of up to 60 teeth (see illustration). The midgut is simple; large cells surround a gut cavity; a permanent anus is lacking.
All gnathostomulids are hermaphrodites. The reproductive system consists of an unpaired dorsal ovarium, and paired (in most genera) or unpaired (in Austrognathia) caudolateral testes in the same specimen. In the majority of the genera, the male apparatus includes a simple tubular copulatory stylet, composed of a concentrically arranged bunch of cuticular rods and accompanied by glands. This type of male organ is always associated with a female organ consisting of a dorsal bursa (for sperm storage) with a lamellar wall on whose anterior extension a cuticularized opening (“mouthpiece”) is found. Haplognathia, on the other hand, has no bursa at all, whereas Austrognathia often has a vagina and a bursa, the latter lacking cuticular structures. Sperm are very diverse, ranging from tiny aflagellar droplets (in the majority of genera) or large cones (in Austrognathia) to motile threads propelled by a single flagellum (as in Haplognathia).
Fertilization is internal; after injection through the skin or the vagina and subsequent storage in the bursa or in the gut wall, the sperms one by one reach the mature egg. Oviposition (egg deposition or laying) is effected by rupture of the dorsal body epithelium, whereupon the egg becomes spherical and sticks to the substratum. Development is direct, with spiral cleavage and gastrulation as is typical for mesolecithal eggs (that is, eggs containing an intermediate amount of yolk). The hatching juvenile measures some 100 μm in length; of its pharynx apparatus, only the central part of the basal plate is developed. Despite its impressive armature, the sophisticated pharyngeal apparatus simply serves to scrape the molecular film of organic material [together with cyanobacteria (blue-green algae), bacteria, and fungal hyphae] off the sand grains. Body fragmentation is a phenomenon that cannot be explained as yet, whereas the formation of a mucous cyst, as observed in some Austrognathia, might help to overcome deterioration of the environment.
Ecology and distribution
The late discovery of the phylum can be attributed to at least three facts: (1) Sand rich in organic detritus [which is the typical biotope (an area of uniform environmental conditions and biota)] did not receive much attention in classical sand-fauna research. (2) Many gnathostomulids prefer a deeper (and mostly anaerobic) sand layer, especially the boundary zone between the oxidized surface and the reduced “black” layer, where steep surface chemical gradients occur. (3) Methods of extraction, mostly based on the fact that in a sediment sample the animals migrate to the surface, were such that H2S-producing samples were discarded prior to the rather late appearance of gnathostomulids. The aspect of the biotope suggests that at least part of the metabolism (and obviously also development) takes place under anaerobic conditions. Investigations have shown that whenever the typical gnathostomulid biotope is encountered, it regularly produces several genera (up to six) and species (up to 15 so far), often in hundreds of specimens per liter of sand. Biotopes range in depth from the upper tidal to 1300 ft (400 m). The distribution of gnathostomulids is worldwide, the majority of localities being known from European coasts and the east coast of North America, with a growing number of reports from the Pacific Ocean.
The gnathostomulids belong to the general grouping of “lower worms.” First considered to be aberrant Turbellaria, they are now thought to be a separate phylum related to Rotifera and Micrognathozoa. A relationship with the enigmatic fossil conodonts could not be substantiated. See also: Conodont; Rotifera