An ultrastructural study of the cuticle in the marine annelid Heterodrilus (Tubificidae, Clitellata)

Abstract

The ultrastructure of the cuticle in four species of the marine Heterodrilus (H. paucifascis, H. pentcheffi, H. flexuosus, H. minisetosus) is investigated with transmission electron microscopy. The noncellular cuticle consists of several parts; closest to the epidermis is a thick zone of collagen fibers embedded in a matrix. The matrix continues outside the fiber zone, forming a layered epicuticle. The external surface of the epicuticle is covered by evenly distributed, membrane-bound bodies, termed epicuticular projections. The epicuticular projections have their longitudinal axis perpendicular to the surface of the cuticle and are attached to the surface by either the surrounding membrane itself or by short pedestals. Microvilli, extensions from the epidermal cells, penetrate and sometimes pass completely through the cuticle. There is interspecific variation in the morphology of the cuticle. The four studied species differ in the arrangement of the collagen fibers, from irregularly distributed fibril bundles to orthogonally arranged fiber layers, as well as in the number and density of layers in the epicuticle. One of the studied species, H. paucifascis, shows intraspecific variation, which is associated with sample locality. The Bahamian specimens of H. paucifascis have four layers in the epicuticle, club-shaped epicuticular projections, and collagen fibers forming a less defined orthogonal grid, while the Belizean specimens have three layers in the epicuticle, epicuticular projections with a bulging part at midlevel, and a distinct orthogonal grid. Based on these findings the variation in the morphology of the cuticle appears to be dependent on both phylogenetic constraints, and functional and environmental factors.

Ultrastructure of the body wall of three species of Grania (Annelida: Clitellata: Enchytraeidae)

“…The epicuticle of naidids has also recently been shown to vary significantly in the number of layers (Sjölin and Gustavsson 2008). In Grania , however, no variation is evident, neither in structure nor in thickness.…”

“…The cuticle seems to host little variation within this family (Hess and Vena 1974; Richards 1984; Jamieson 1992), as opposed to the cuticle of Naididae ( sensu Erséus et al. 2008) which has been shown to vary both in the structure of the epicuticle (Sjölin and Gustavsson 2008) and the fiber zone (Gustavsson and Erséus 2000; Gustavsson 2001). Within Grania , as in other enchytraeids, the fiber zone is arranged in an orthogonal grid, similar to the pattern seen in Crassiclitellata (Richards 1974).…”