Musculature of the sessile rotifer Stephanoceros fimbriatus (Rotifera: Gnesiotrocha: Collothecaceae) with details on larval metamorphosis and development of the infundibulum

The sessile rotifer, Stephanoceros fimbriatus, is a member of the clade Collothecaceae (Monogononta: Gnesiotrocha) that includes species with indirect development and adults that lack a typical rotiferan corona. The short-lived larva undergoes a dramatic metamorphosis upon settlement, replacing the larval corona with the adult infundibulum, a highly unusual cup-shaped head that appears to develop from the anterior foregut. The infundibulum of S. fimbriatus bears five tentacles that function in the capture of small prey. We studied larval metamorphosis in this species and applied a fluorescent f-actin stain and confocal laser scanning microscopy to follow the development of the musculature from larva to adult. We determined that the larva contains a mostly orthogonal grid of somatic muscles, with 12 circular rings encompassing four thick pairs of longitudinal bands. Additional segmental muscles and a complex splanchnic component are also present. Interestingly, the larval infundibulum is an inverted pouch that itself is not muscular but does possess a thick muscular ring (infundibular muscle) at the anterior end of the larva, which will become the future base of the adult infundibulum. The infundibular muscle contributes to the muscle fibers that supply the five developing infundibular tentacles. During metamorphosis, the infundibular muscle slowly becomes reoriented from a slightly ventroanterior position to a terminal position at the top of the larva. The infundibulum then slowly everts through the presumed larval mouth, which has shifted to the top of the corona. The tentacles arise first, and contain a complex muscular network that is assumed to have a mostly supportive function since the tentacles themselves are rarely active. Despite this eversion and other dramatic changes to the larval body plan after metamorphosis - an elongating foot and thickening trunk - the muscle patterns of the adult remain extremely similar to the larva. The major exceptions to this otherwise conservative muscular body plan appear to include a repositioning of the somatic muscles as the infundibulum assumes the adult orientation, and the development of the tentacular muscular network.