Gnathostomulida (Unsegmented Marine Worms)

Abstract First described in 1956, Gnathostomulida is a phylum of microscopic, free‐living, unsegmented and hermaphroditic marine worms. Uniquely distinguished by a monociliated epidermis (that is, a body surface where each cell carries only one cilium) and possession of complex cuticular mouthparts consisting of paired jaws and an unpaired basal plate, they are most likely related to Rotifera, Acanthocephala and Micrognathozoa. There are two orders, Filospermoidea (with a pointed anterior devoid of compound sensory bristles; without a bursa copulatrix; and filiform sperm, with one flagellum) and Bursovaginoidea (with a rounded anterior with compound sensory bristles; with a bursa; and rounded or cone‐shaped sperm, aflagellate). Approximately 100 species have been recorded to date, of which many are with global or circumtropical distribution. Gnathostomulids may numerically dominate the invertebrate meiobenthos in detritus‐rich, sulfide‐rich and oxygen‐poor shallow sandy bottoms (0–25 m, rarely to 400 m) where they typically occur, seemingly feeding on microbes that they scrape off sand grains. Key Concepts: Gnathostomulida is a small phylum of microscopic, free‐living, unsegmented and hermaphroditic marine worms. Gnathostomulids are unique among animals in having an entirely monociliated epidermis. Gnathostomulids have complex cuticular mouthparts consisting of paired jaws and an unpaired basal plate. Gnathostomulids are most likely related to Rotifera, Acanthocephala and Micrognathozoa, with which they can be united in the superphylum Gnathozoa. Gnathostomulids typically occur interstitially in detritus‐ and sulfide‐rich but oxygen‐poor shallow sandy bottoms.