Annelid chaetae

Abstract

With approximately 21,000 described species, Annelida represent one of the largest and best studied group of marine invertebrates. Their anatomy is characterized by a metameric organization, which covers potential for an extraordinarily diverse morphology and, therefore, the adaptation to basically every habitat and ecological niche. Each segment of their body bears equivalent structures, which comprise a pair of coelomic cavities, a pair of metanephridia and two pairs of parapodia. These parapodia are equipped with the most representative morphological structure of annelids, the chaetae. <br /> Chaetae are chitinous, extracellular structures, which are formed within a follicle of invaginated epidermal cells. The basalmost cell of this follicle, the chaetoblast, secretes chitin, which accumulates between the cell’s microvilli brush border. With ongoing secretion, the chitinous mass is pushed apically through the follicle until it eventually pierces the cuticle, thus forming the chaeta. Hollow canals are left inside the chaetae, where the microvilli of the chaetoblast were during chaetal formation. That way, a typical transverse-sectional honey-comb like pattern is generated, which guarantees mechanical stability and flexibility at the same time. The highly dynamic nature of the chaetoblast’s microvilli pattern during the chaetogenesis facilitates the formation of numerous different shapes of chaetae. Chaetae often fulfil essential functions for the lifestyle of the respective species, which eminently influences their exact shape. Several chaetal follicles form a chaetal sac, in most cases one per parapodial ramus. Within a chaetal sac, new chaetae are formed at distinct formative sites. The number and positions of these formative sites determine the eventual arrangement pattern of the chaetae. Formation, diversity and arrangement of chaetae represent relevant characters for identification and description of species as well as phylogenetic and evolutionary analyses. <br /> The vast morphological diversity of annelids always made it difficult to properly resolve their inner systematics. During the last decade, however, phylotranscriptomic studies revealed an updated and stable phylogenetic tree. This “new” annelid phylogeny demands a re-evaluation of morphological characters in order to understand their evolution and test established homology-hypotheses. In the course of three studies presented in this thesis, I investigated chaetal characteristics of several, predominantly basally branching annelid taxa. The first two studies examined the formation and arrangement of chaetae in Magelonidae and Oweniidae, together constituting the sister group to all remaining annelids. The third study focussed on the understudied family of Euphrosinidae, the sister group to Amphinomidae, which are notorious for their calcified, defensive stinging bristles. Additional unpublished results covered insights into chaetal properties of the two largest annelid groups including data on Sternaspidae (Sedentaria) and Nephtyidae as well as Glyceridae (both Errantia). <br /> All presented results support the hypothesis that chaetae are primarily arranged in one transverse row per parapodial ramus with a single formative site, which is located ventrally in the notopodium and dorsally in the neuropodium. More complex arrangement patterns like staggered rows (Magelonidae), bundles (Oweniidae, Euphrosinidae) or huge patches (Oweniidae) of chaetae are the result of a shift or multiplication of formative sites or constitute a modified, spirally curled up row. Moreover, comparisons of several internal, supportive chaetae in different annelid groups substantiate that this type of chaetae, often referred to as “aciculae”, evolved several times independently. Aciculae of Errantia and Amphinomida differ regarding their shape, arrangement and composition and the arcuate chaetae in abdominal segments of Magelonidae actually constitute internal hooded hooks. Furthermore, it could be shown that chaetae of Euphrosinidae feature incorporated calcium compounds just like those of Amphinomidae. Combined with the findings concerning ultrastructure and arrangement, this third study sheds light on the chaetal properties found in the ground pattern of Amphinomida. <br /> Altogether, the here presented results demonstrate the vast variability of chaetae, support the phylogenetic significance of their arrangement and help understanding the evolution of these characteristic structures.