During the study of idmidroniform bryozoans collected in different parts of the Antarctic in the course of the 34th and 36th Soviet (Russian) Antarctic Expeditions, many various kinds of the ooeciostome morphology and position were encountered. More than 550 fertile colonies and fragments with ooeciostomes belonging to seven species (Exidmonea arcuata Ostrovsky et Taylor, Idmidronea hula Borg, I. antarctica Borg, I. pseudocrisina Borg, I. obtecta Borg, I. pellucida Ostrovsky et Taylor, and I. fraudulenta Ostrovsky et Taylor) were studied. All characters examined (size and shape of ooeciostome and ooeciopore, ooeciostome position, orientation of ooeciopore) exhibit variability. Although every species is characterized by a distinct shape and position of ooeciostome that allows its use (togethere with other features) as dependable character in taxonomy, deviations from ooeciostome “normal” morphology are too common and considerable to be ignored. Therefore, they may be characterized as useful rather than diagnostic taxonomic characters.
Recent idmidroniform bryozoans are arranged in a morphoseries reflecting some possible stages in the evolution of the ooeciostome. Possible trends include loss of terminal position, length decrease, increased curvature and flattening. A reason supposed for the modification might be connected with the “passive defence” of the ooeciostome from breakage and obstruction.