Muscular systems in gymnolaemate bryozoan larvae (Bryozoa: Gymnolaemata)

摘要

Gymnolaemate bryozoan species exhibit larval forms that can either be planktotrophic cyphonautes or one of the several types of lecithotrophic larvae. Due to the lack of both a phylogenetic system of the Bryozoa and detailed information about larval morphology for many species, it is difficult to elaborate hypotheses about the evolution of larval forms in this group at the moment. In this study, phalloidin staining and confocal laser scanning microscopy are used to provide detailed morphological descriptions of the three-dimensional structure of muscular systems in the larvae of Flustrellidra hispida, Alcyonidium gelatinosum, Membranipora membranacea, Bugula fulva and Bowerbankia gracilis. A comparison to the available data from the literature is added. The results show muscular systems to be much more complex than previously recognized. In all but one species, an anterior median muscle inserts with one end at the basal side of the apical organ and with its other end at the papilla of the pyriform organ. This muscle can be paired or unpaired and was referred to as “neuromuscular strand” in earlier descriptions. All species except Bugula fulva exhibit a coronal ring muscle running concentric to the larva’s main locomotory ciliary band. Lateral muscle strands that connect the pallial epithelium to either the corona or oral epithelium are found mainly in the cyphonautes and pseudocyphonautes larvae and, in modified shape though, also in some coronate larvae. Transversal or shell adductor muscles can be shown to co-occur with lateral muscles and possibly originate from the same source. The findings are in accordance with the idea of a multiple origin of lecithotrophic forms within gymnolaemate Bryozoa.