The simplicity of males: dwarf males of four species of Osedax (Siboglinidae; Annelida) investigated by confocal laser scanning microscopy.

Dwarf males of the bone-eating worms Osedax (Siboglinidae, Annelida) have been proposed to develop from larvae that settle on females rather than on bone. The apparent arrest in somatic development and resemblance of the males to trochophore larvae has been posited as an example of paedomorphosis. Here, we present the first investigation of the entire muscle and nervous system in dwarf males of Osedax frankpressi, O. roseus, O. rubiplumus, and O. "spiral" analyzed by multistaining and confocal laser scanning microscopy. Sperm shape and spermiogenesis, the sperm duct and internal and external ciliary patterns were likewise visualized. The males of all four species possess morphological traits typical of newly settled siboglinid larvae: a prostomium, a peristomium with a prototroch, one elongate segment and a second shorter segment. Each segment has a ring of eight long-handled hooked chaetae. The longitudinal muscles are distributed as evenly spaced strands forming a grid with the thin outer circular muscles. Oblique protractor and retractor muscles are associated with each of the chaetal sacs. The nervous system comprises a cerebral ganglion, a prototroch nerve ring, paired dorsolateral longitudinal nerves, five ventral longitudinal nerves with paired, posterior ganglia and a terminal commissure, as well as a net of fine peripheral transverse plexuses surrounding the first segment. Internal ciliation occurs as paired ventrolateral bands along the first segment. The bands appear to lead the free mature sperm to a ciliated duct and seminal vesicle lying just behind the prototroch region. A duct then runs from the seminal vesicle into the dorsal part of the prostomium. The similarity of Osedax males to the larvae of Osedax and other siboglinid annelids as well as similarities shown here to the neuromuscular organization seen in other annelid larvae supports the hypothesis of paedomorphosis in males of Osedax.