By Dr. Rick Hochberg, University of Massachusetts - Lowell
Phylum Gastrotricha contains over 700 species of unsegmented, microscopic worms present in marine and freshwater environments. There are approximately 400 described marine species, all of which are entirely meiobenthic and inhabit sediments from the littoral margin to the deep sea. Gastrotrichs are regularly present alongside other meiofaunal worms (e.g., annelids, gnathostomulids, flatworms), and while they are externally similar, gastrotrichs are easily distinguished from these meiofauna based on a suite of characteristics: a terminal mouth and straight gut, a well-defined cuticle that may be highly ornamented, adhesive tubes along the length of the body (in anterior, lateral, ventral, dorsal and posterior positions), and ventral locomotory cilia only. Most gastrotrich are less than 50 mm wide and 1 mm long, though some are as small as 100 mm or less in length. Their bodies are mostly transparent, making them difficult to see but, from a taxonomic standpoint, relatively easy to distinguish from other meiofauna.
The phylum is divided into two orders, Chaetonotida and Macrodasyida, and their respective species are easily identified based on characteristics of general body shape and the structure of the cuticle. In macrodasyidans, tentacles and/or antennae may be present on the head, while ventral adhesive tubes are positioned just posterior of the mouth. The main trunk of the body is often elongate and adorned with adhesive tubes in dorsal, lateral and/or ventral positions, with a round or bifid caudal end bearing additional adhesive tubes. Refractile epidermal glands are present along the length of the body, and the cuticle may be smooth or elaborated into spines and/or scales. Chaetonotidans are generally ten-pin shaped and in many cases, extremely small (< 300 mm). Their cuticle is often ornamented with scales and/or spines, and adhesive tubes are restricted to the bifid caudal end. The single exception is Neodasys, an aberrant genus with a long vermiform body, smooth cuticle, and ill-defined adhesive tubes along the trunk. All marine gastrotrichs are hermaphrodites with few exceptions, and may display a tendency toward protandry, protogyny, or simultaneous hermaphroditism; all have direct development.