Ectoprocta

Abstract

First considered to be plantlike (moss) animals similar to cnidarians, the evolutionary origin and affinities of the ectoprocts or bryozoans have been enigmatic subjects of research since the sixteenth century. The term Bryozoa originally encompassed both the entoprocts (or kamptozoans, Chap. 6) and the ectoprocts; however, these animal groups were later separated and eventually organized into different phyla. Ectoprocts are aquatic invertebrates that can form elaborate and occasionally large colonies (1 m) composed of numerous individual zooids, each typically no more than a millimeter in length. Zooids in a colony may be one of several different polymorphic forms specialized for various functions such as feeding, reproduction, or defense. Current estimates of ectoproct diversity range from 4,0–00 to 8,0–00 extant species, many of which are broadly distributed throughout freshwater, brackish, and marine environments. More than 1–5,0–00 fossil species that trace their origins back to the Ordovician period approximately 4–83 million years ago have been described. This period of origin is much later than that of many other animal phyla that arose during or before the Cambrian. Although one Cambrian ectoproct fossil has been described, this morphotype has been reinterpreted as a type of octocoral. Whether the relatively late geologic origin of the ectoprocts is correct or merely the result of preservational bias against some as–013;yet–013;unknown soft–013;bodied form remains an open question, but all extant morphological grades of ectoprocts with and without mineralized zooids were clearly present by the Jurassic.