Cuticular ultrastructure in some marine oligochaetes (Tubificidae)

Abstract

Abstract. The ultrastructure of the thin, non‐cellular cuticle is described for 6 marine oligochaetes, representing 3 of the subfamilies (Phallodrilinae, Limnodriloidinae, and Rhyacodrilinae) of the Tubificidae. The main components of the cuticle in these 6 species, as in most other oligochaetes examined, are: (1) a fiber zone closest to the epidermis, consisting of collagen fibers embedded in a matrix, (2) an epicuticle, which is a continuation of the matrix outside the fiber zone, and (3) epicuticular projections, which are membrane‐bound bodies covering the outer surface of the epicuticle. The projections are probably formed by the microvilli that penetrate the cuticle from the epidermal cells below, but this was confirmed only in the studied limnodriloidines. Three of the species examined, Duridrilus turdus, Olavius vacuus, and Heterodrilus paucifascis, lack microvilli. The morphology of the components in the cuticle differs between the studied species. The collagen fibers may form an “orthogonal grid” (i.e., layers of parallel fibers perpendicular to the layers immediately above and below), or they may form parallel layers, or be irregularly scattered. The number of dense layers in the epicuticle, as well as the shape and internal structure of the epicuticular projections, also vary. All these characters might be useful in future phylogenetic analyses to achieve better hypotheses of relationships within oligochaetes as well as to other groups.