Aboral (pallial) epithelium in bryozoan larvae:a comparative morpho-functional analysis
The outer epithelium of free-swimming larvae of marine invertebrates is composed of several cell types and performs a variety of vital functions. In bryozoan larvae, early anatomical studies revealed a rather complex organization of this epithelium. The present contribution re-examines the available morphological descriptions, as well as investigates anatomically and ultrastructurally the epispheral (aboral) part of the outer pallial epithelium in contrasting larval types of six marine bryozoans from three orders - Cheilostomatida, Ctenostomatida, and Cyclostomatida. A total of 11 cell types were identified - three of them are typical for most of the studied larvae, the rest are unique to specific larval types. Based on the data obtained, we conducted a comparative analysis and assigned potential functions to particular types of cells during larval life. We proposed the hypothesis that immersion/invagination of the larval pallial epithelium (which was accompanied by cell enlargement in ciliary locomotory corona) occurred independently in different bryozoan lineages and was associated with acquisition of incubation. We also consider the presence of the cuticle above the episphere and the absence of cilia in most principal pallial epithelial cells as plesiomorphic traits of bryozoan larvae.