Rotifers are present in a wide range of freshwater habitats, including in so-called “container habitats”—pools of water which collect outdoors in birdbaths, buckets, and other containers—and in the thin layers of water surrounding mosses (Wallace et al., 2015). These minute critters made the headlines in 2021, when a group of scientists discovered that some 24,000-year-old rotifers, found preserved within Siberian permafrost, were still alive and capable of reproduction (Renault, 2021)!

Rotifers may be anywhere between 40-2000 μm in length, range in shape from cylindrical to sack-like, and may look similar to worms (Wallace et al., 2015). A hard outer shell—called a lorica—is present in some rotifers; for instance, species of Keratella, a very common genus, bear patterns of polygonal facets on the lorica surface (Tausz et al., 2019). One of rotifers’ most distinguishing features is the corona, a ciliated structure present at the anterior end, which is used to sweep food into the mouth (Wallace et al., 2015). The rhythmic beating of the cilia, which are often arranged in concentric rings, inspired the name rotifer, meaning “wheel-bearer” (Wallace et al., 2015). Adorably, rotifers are also nicknamed “wheel animalcules.” 

Gastrotrichs

Gastrotrichs can be found in a variety of freshwater habitats, dwelling within sediments and amongst vegetation. They are colorless, and freshwater species are about 50-800 μm in length (Strayer et al., 2010). Their shape is often similar to that of a bowling pin—with a head region, narrowed neck, and a bulbous trunk—and in many species the posterior end features paired extensions, or “feet” (Schmidt-Rhaesa & Kieneke, 2015). Within these “feet” are adhesive tubes which gastrotrichs employ to attach themselves to surfaces (Strayer et al., 2010). 

The name “gastrotrich” refers to the cilia present on the ventral surface, which enable locomotion by gliding; many gastrotrichs also bear spines and/or scales (Schmidt-Rhaesa & Kieneke, 2015).