Cuticular ultrastructure in some marine oligochaetes (Tubificidae)

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, Oluvius vacuus, and Heterodrilus puucifascis, 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.

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

“…Otherwise the epicuticular projections have a constant shape within a species. This is generally the case for other oligochaetes, with the exception of Smithsonidrilus minusculus (Gustavsson and Erséus, 2000), where the projections vary, from round to oval or even club-shaped. The function of the epicuticular projections is still unknown.…”

“…There have also been suggestions that the arrangement of fibers is correlated to the size of the worm-a reduction of a more ordered arrangement is associated with the small size of the worm (Goodman and Parrish, 1971;Westheide and Rieger, 1978;Richards, 1984). However, Gustavsson and Erséus (2000) concluded that within the Tubificidae, there is no strict correlation between the size of a worm and the fiber structure of its cuticle, and that the arrangement of the fibers is more likely to reflect a phylogenetic constraint.…”

“…A layered epicuticle has been reported for lumbricids (Richards, 1974), for some Brazilian earthworms (Rodriguez Alves et al, 1984), and for several groups of tubificids (Gustavsson and Erséus, 2000;Gustavsson, 2001), and the substructure of the epicuticle has been unique for each studied species. However, in this study, specimens of Heterodrilus paucifascis from different localities deviate in the morphology of the epicuticle.…”

“…Lack of an orthogonal grid was also reported by Giere (1983) for two tubificids, Thalassodrilus prostatus and Aktedrilus monospermathecus, and by Jamieson (1992) in the naidine Chaetogaster limnaei. The findings by Gustavsson and Erséus (2000) and Gustavsson (2001) show that the Tubificidae encompass a larger variation in the morphology of the cuticle than previously known; these authors concluded that the variation could be associated with phylogenetic relationships. However, thus far the cuticle has only been described at the species level, and the amount of interspecific variation of the morphology of the cuticle is not known.…”

“…In recent years, however, several morphological investigations on systems other than the reproductive have been performed on different groups of annelids. Sensory organs in sedentary polychaetes were studied by Purschke and Hausen (2007), the ultrastructure of phaosomous photoreceptors was studied in Stylaria lacustris (Purschke, 2003), and cuticular morphology in oligochaetes was studied by Gustavsson and Erséus (2000) and Gustavsson (2001).…”