Gnathostomulids

Phylum Gnathostomulida Number of families 12

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Marine group of microscopic free-living worms characterized by an entirely monociliated epidermis and complex cuticular mouthparts

Photo: A few species of Filospermoidea, such as this Haplognathia ruberrima, are bright red, while all other gnathostomulids are colorless-opaque. (Photo by Wolfgang Sterrer. Reproduced by per-

Evolution and systematics

There is no fossil record for this group. Described in 1956 as aberrant Turbellaria (flatworms), the gnathostomulids are now considered related to the rotifers (Rotifera) and Mi-crognathozoa. The phylum (and single class) encompasses two orders, Filospermoidea and Bursovaginoidea, the latter with two suborders, Conophoralia and Scleroperalia. The 12 families contain 25 genera, with fewer than 100 valid species.

Physical characteristics

These thread-shaped worms range from 0.01 in (0.3 mm) to more than 0.1 in (3 mm) in length. Most species are colorless or transparent, but a few are bright red. The anterior end of Filospermoidea is pointed, while that of Bursovaginoidea appears as a rounded head. The posterior end is rounded or extends into a tail. The epidermis is completely monociliated, i.e., each cell carries a single, long locomotory cilium. Some cilia, singly or in paired groups, may have sensory functions. The nervous system, which is largely situated at the basis of the epidermis, consists of an unpaired frontal ganglion (brain) and an unpaired buccal ganglion from which paired nerves originate. The musculature is simple and rather weak, except for a complex pharynx. Circulatory and respiratory organs are lacking. The digestive tract provides the greatest number of distinguishing characters. The mouth is located ventrally, behind the anterior end, and there is no permanent anus. In the majority of species the complex, muscular, bilaterally symmet ric pharynx contains cuticular, hard mouthparts consisting of an unpaired basal plate in the lower lip, and paired jaws. The basal plate may be flat and dorsally set with ridges or teeth, as in the family Haplognathiidae; transverse rod shaped, as in Pterognathiidae; or consist of wings and set with rows of teeth, as in the Gnathostomulidae. The jaws may be solid and forcepslike, as in the order Filospermoidea, or hollow like a pair of forward-pointing funnels, with muscles inserting from behind, as in most Bursovaginoidea. In most species, the inner, anterior parts of the jaw are set with groups or rows of teeth.

Distribution

Gnathostomulids are distributed worldwide, with most species known from the North Atlantic and South Pacific Oceans.

Habitat

Gnathostomulids occur exclusively, sometimes in large numbers, in detritus-rich marine sand as typically found on sheltered beaches, near sea grasses and mangroves, and between coral reefs. Such habitats are often characterized by low oxygen but high hydrogen sulfide concentrations (that create a rotten egg smell), which gnathostomulids seem to tolerate. Most species have been found in the intertidal and shallow subtidal zones, with occasional finds at 1,310 ft (400 m).

Light micrograph of Gnathostomula armata. (Photo by Matthew Hooge. Reproduced by permission.)

Behavior

Gnathostomulids glide through the interstices between sand grains, propelled slowly by their sparse ciliation, and contracting when disturbed. Some species spin a mucous cocoon which may let them survive a deteriorating environment.

Feeding ecology and diet

Belying their fearsome jaws, gnathostomulids are not predators but seem to graze on the microflora (bacteria and fungal threads) attached to sand grains. Scientists do not yet know what role the basal plate and jaws play in feeding.

Reproductive biology

All gnathostomulids are hermaphrodites. The male organs are located in the posterior part of the body. They consist of either an unpaired or paired testes, and a copulatory organ (penis) which, in Scleroperalia, contains a tubelike penis stylet. The sperm is diverse. In Filospermoidea, it is threadlike, with a single ciliary tail of 9+2 microtubules. In Bursovaginoidea, it is aflagellate and droplet shaped, in Conophoralia, aflagel-late and cone shaped. Sperm is transferred by copulation, and stored either freely between gut and epidermis, or in a storage pouch (the bursa copulatrix). In all species, the single, unpaired ovary is located dorsally, behind the mouth. Only one large egg matures at a time, and is presumably fertilized by sperm from the bursa before being laid via rupture of the dorsal body wall. Development is direct, and cleavage probably follows the spiral pattern.

Conservation status

No species of gnathostomulid is listed by the IUCN.

Significance to humans

None known.

Hidradenitis Illustration
1. Gnathostomula paradoxa; 2. Austrognathia australiensis; 3. Red haplognathia (Haplognathia ruberrima). (Illustration by Dan Erickson)
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