Large filter feeders (some whales, bivalve molluscs, and barnacles for example) are well known, but the smaller ones are often obscure. Life between grains of sand or mud implies tiny size, which means that these animals are also unfamiliar – rarely seen without the aid of a microscope.
Rotifers, Micrognathozoa, and Gnathostomulids
- The 100 or so species in the phylum Gnathostomulida are small marine animals. Most are less than 1 mm long, and they scrape food off sand or mud grains, especially in places where there is little oxygen. Gnathostomulid mouthparts suggest that they are related to the rotifers.
- Micrognathozoa have their own phylum – even though only one species is known so far. Limnognathia maerski was discovered in Greenland in 1994, living in spring water. The animal is minute, less than one tenth of a millimeter long, and therefore one of the smallest known. Very little is known about this phylum, but it is thought to be close to the Rotifera.
- Rotifers (Phylum Rotifera) are common in fresh water. There are over 2,000 described species of rotifer, and they were discovered early in the dawn of microscopy (by John Harris in 1696). They are able to survive long periods dehydrated, rather like the tardigrades, and this explains how they can get from pond to pond – as bits of ‘dust’ blown in on the wind. Most are less than a millimeter long, and they feed by filtering small particles (fish waste, dead bacteria, algae etc.) at the prodigious rate of 100,000 times their own volume per hour. They are ecologically important for this reason – helping to keep fresh water clean.
(Look at a ‘Gallery of Rotifer Images’).
Gastrotrichs and Kinorhynchs
- Gastrotrichs are very small and short-lived (most only living for a few days). They typically live between sand grains as part of the ‘meiofauna’. There are about 700 species in this phylum, and it is not at all clear which other phyla are closest to them – body form suggests one set of possibilities, genetic studies another.
- The 150 or so kinorhynchs, or ‘Mud Dragons’, also live in mud and sand, where they eat diatoms. Like the gastrotrichs it is unclear how this phylum fits in, in the evolutionary sense.
Brachiopods, Bryozoans, and Xenoturbellids
- Brachiopods are often large animals with two shell valves, much like bivalve molluscs, but they are not closely related to them (or any of the Phylum Mollusca). 99% of all brachiopods are extinct, and those that remain are sessile filter feeders. They usually attach to the substrate by means of a long stalk.
- Bryozoans (or ectoprocts), with over 8,000 living species, usually built tough calcareous casings for their colonies. This group of small individuals is often called a ‘sea mat’, and it is permanently attached to a rock or large plant. The individuals filter seawater. It is thought that the bryozoans are most closely related to the brachiopods.
- The phylum Xenoturbellida consists of two known species. They are worm-like and were once thought to be related to the molluscs (because molecular studies indicated the presence of molluscan DNA), but it is now thought that this DNA gets into the animals because they either eat, or parasitise, molluscs. They have a very simple body plan, and very little is known about them.
The animals in seven of these phyla are too small for most people to notice, and the brachiopods (the eighth phylum) will normally be encountered as fossils. As fossils the brachiopods can easily be mistaken for bivalve molluscs.