Pattern of occurrence of supraneural coelomopores and intertentacular organs in Gymnolaemata (Bryozoa) and its evolutionary implications

The evolution of bryozoan female gonopores (the supraneural coelomopore (SNP) and the intertentacular organ (ITO)) is considered in the light of two alternative hypotheses. In the first hypothesis it is proposed that the ITO originated from the shortening and fusion of two tentacles possessing terminal pore(s), with further transformation into a simple pore. In the alternative hypothesis it is suggested that the ITO evolved from a coelomopore with a contribution from the basal parts of two disto-medial tentacles in an ancestor. Favouring the second hypothesis, in this paper we present a hypothetical scenario, according to which the earliest gymnolaemate bryozoans with uniserial growth and a broadcasting reproductive pattern possessed the supraneural coelomopore (SNP). This could serve both as a female gonopore and as a conduit for sperm entry. Evolution of large colonies of closely packed zooids led to development of the tube-like intertentacular organ (ITO) that is formed by epithelial proliferation of the basal parts of two dorso-medial tentacles. This prevented egg swallowing in the situation when water exchange was hampered within the large colony. The ITO independently evolved in both ctenostome and cheilostome gymnolaemates when multiserial colonies appeared. Evolution of brooding in species with colonies of closely packed zooids led to reduction of the ITO, except for the cheilostomes Tendra and Thalamoporella that acquired brooding independently. A rudimentary ITO also “survived” in two ctenostomes with the “mixed” type of brooding. An alternative, analogous organ—the ovipositor—has evolved in the cheilostome taxon Schizoporella.