Ultrastructure and formation of hooded hooks in Capitella capitata (Annelida, Capitellida)

Abstract

 The hooded hooks of Capitella capitata are aligned in a transverse row inside each neuro- and notopodial rim of the last thoracic and all abdominal setigers. Each seta consists of a rostrum, a capitium, the spines of which surmount the rostrum, and a long, sigmoid shaft or manubrium, towards which rostrum and capitial spines are curved. A thin hood, complete except for a subapical opening and a short, subrostral cleft, encloses the apical portions of the seta. Generally, the tip of the rostrum extends beyond the hood. The hood consists of an outer and an inner lamella, between which is a compartment loosely filled with fibrillar material. Hooded hooks are generated at the dorsal edge of the neuropodial rim and at the ventral edge of the notopodial rim during the entire life of C. capitata. Chaetogenesis starts in a small compartment surrounded by the basal chaetoblast and four follicle cells. Initially a group of microvilli emanating from the chaetoblast preforms the rostrum. Next, stout microvilli appear adrostrally, each preforming a spine of the capitium. When both structures have been formed, the longitudinal axis of the anlage shifts, because the actin filaments inside the microvilli reorientate and initiate formation of the manubrium. During this initial phase of chaetogenesis the anlage sinks into the chaetoblast, until the latter finally enwraps the anlage, except the tip of the rostrum. The chaetoblast now generates microvilli that face the new setal structures and preform the hood. During further development the microvilli separate into two layers, an inner and an outer one. The inner layer of microvilli merges with the manubrium prior to the outer layer. Addition of setal material occurs between the bases of the microvilli and elongates the manubrium until it extends beyond the epidermal surface. The microvilli, which have continuously been withdrawn from the seta during chaetogenesis, remain in the basal section. Specific morphogenetic and structural correspondence between the hooked setae of species of Maldanomorpha, Psammodrilida and Oweniida, the uncini of species of the Sabellida, Terebellida and Pogonophora, and the hooded hooks of species of Capitellidae justify the hypothesis that all these setae are homologous. This hypothesis implies the existence of a monophyletic group consisting of all polychaetous Annelida with such setae.