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 freeswimming stage when the phylum was first described, the dwarf male has a remarkably complex bodyplan albeit its very small size (approx. 30-40 lm 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.

Morphology of the nervous system of monogonont rotiferEpiphanes sentawith focus on sexual dimorphism between feeding females and dwarfed males

“…Among Spiralia (to which rotifers belong(40-42)) it has been reported in e.g. Cycliophora(43-47), Orthonectida(48), which are now considered parasitic annelids(49, 50), some octopi(51) and in several annelid clades including some dinophilids(52-54), Osedax (55-57), Spionidae(58, 59) and bonellid echiurans(60). There are two proposed mechanisms responsible for the origin of dwarfism: progenesis or reduction(61, 62), which are reflected in the morphology of the dwarfed forms, including their nervous system and musculature(52).…”

Cycliophora

“…5.2A ), encysts, and generates internally one to three mature dwarf males with a cuticular penile structure ( Fig. 5.2D ; Neves et al 2010c ). Electron microscopy studies showed that younger males are larger and possess more nucleated somatic cells (aproximately 200) than mature individuals, who only have around 50 nucleated cells and nuclei-free muscles and epidermis (Neves and Reichert 2015 ).…”

Are copepods secondary hosts of Cycliophora?

“…The sexual part of the life cycle involves initially a Prometheus larva and a female, which are also generated by inner budding inside the feeding stage. Once freed, the Prometheus larva settles on the trunk of a feeding individual and generates dwarf males inside its body (Obst and Funch 2003;Neves et al 2010). The female is impregnated by the dwarf male and the embryo develops into a chordoid larva (Funch 1996;Neves et al 2012).…”