External morphology of the cycliophoran dwarf male: a comparative study of Symbion pandora and S. americanus

Abstract

Cycliophora is a recently described phylum to which only two species have been assigned so far, Symbion pandora and S. americanus. The cycliophoran life cycle is complex and alternates between asexual and sexual stages. Although not recognized as an entirely independent free-swimming stage when the phylum was first described, the dwarf male has a remarkably complex bodyplan albeit its very small size (approx. 30-40 mu m in length). Aiming to increase the knowledge on the gross morphology of the cycliophoran dwarf male, specimens from S. pandora and S. americanus were analyzed by scanning electron microscopy. In both species, anterior and ventral ciliated fields, as well as paired lateral sensorial organs, were identified, thus confirming previous observations. However, new details are described herein such as the penial pouch that encloses the penis. We compare our findings on both Symbion species with the data currently available on other metazoan dwarf males.