Ectoprocts are tiny, sessile marine and freshwater invertebrates that live in colonies. They are moderately successful, having about 500 species worldwide and they are abundant,m found attached to hard substrates such as rocks, shells and wood in the benthos from shallow waters to 6000 m deep. They are ‘lophophorate coelomates’, which means they have a ring of tentacles on the head (a lophophore) and a true body cavity (coelom). Other members of this group are the Phoronids and the Brachiopods.
The lophophore can be circular or horseshoe shaped and is used to capture food, much like the tentacles in other filter feeders. As filter feeders, ectoprocts feed on tiny plants and animals plus detritus that floats down through the water. The lophophore is operated by a coelomic hydraulic system that can inflate and extrude it or deflate and pull it in for protection. The mouth is within the lophophore and leads to a U-shaped gut that leads from pharynx to esophagus to stomach and then an intestine which curves back up to the anus which is located just outside the lophophore (hence the name ectoproct).
There is no circulatory system and the coelomic fluids seem adequate for moving nutrients around. There are also no separate excretory organs like nephridia. Respiration takes place across body surfaces, especially the lophophore. The individuals in a colony (zooids) each secrete a protective covering which is a part of the body wall and can either be gelatinous or hard and encrusting. There are retractor muscles that can pull the animal and its lophophore down to protect it from predators.
Ectoprocts can reproduce asexually, using fission to divide and increase the size of the colony. They also have simple gonads and can reproduce sexually, leading to the formation of ciliated free-swimming larval stages, which disperse in the plankton and settle to form new colonies. In most species, the adults are hermaphroditic. Some species release the eggs directly into the water while others brood the young before releasing them. When the young larva settles, it glues itself to the substrate which it has found by vibrations and then metamorphoses into the first zooid of the new colony. Once settled, it divides into daughter zooids that are budded off in whatever pattern is characteristic of that species. Patterns are complex and are used by taxonomists to differentiate between species.
Ectoprocts were once classified together with Entoprocts as Bryozoa or Polyzoa but there are fundamental differences between the two even though they are superficially similar. Ectoprocts have a true coelom and entoprocts do not. Also the anus in the entoprocts is located within the ring of tentacles whereas, as mentioned above, it is on the outside of the lophophore in the ectoprocts. Ectoprocts are divided into two classes. One is predominately marine and has a circular lophophore (the Gymnolaemata) and the other lives in fresh-water and has a horseshoe-shaped lophophore (the Phylactolaemata). Interestingly, the gymnolaemata are more common and older in the fossil record but the phylactolaemata appear to be more primitive in modern-day forms. Some of the Phylactolaemata have a curious way of moving where whole colonies creep along using muscles on the bottoms of the body walls of individual zooids.
Freshwater ectoprocts can be a problem by growing in pipes and irrigation tubes, causing blockages that prevent the flow of water. Otherwise most people do not know they exist.
References: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press. http://animals.jrank.org/pages/1930/Freshwater-Marine-Ectoprocts-or-Bryozoans-Ectoprocta.html