Dwarf male of symbion pandora (cycliophora)

Abstract

This study clarifies the identity and development of the male in the life cycle of Symbion pandora. The male is not produced directly by the feeding stage, as previously thought, but arises as a distinct individual from budding cells inside an intermediate stage named the Prometheus larva. The morphology and the development of the two distinct stages are described with light and electron microscopy. Furthermore, the following terminology is suggested to clearly distinguish between the different individuals: 1) the Prometheus larva, which is the free-swimming individual being produced inside the feeding stage; 2) the attached Prometheus larva on the feeding stage, which mostly degenerates following settlement, except for the internal budding cells; and 3) the dwarf male, which is the ciliated, sexually mature stage. The budding cells inside the attached Prometheus larva usually develop two internal dwarf males. Each dwarf male is heavily ciliated and has a well-developed nervous system with a relatively large brain, numerous gland and muscle cells, testis with bundles of sperm, and one penial structure. The male lacks a gut, as in the other free stages in the life cycle of Symbion pandora. This study also indicates that the dwarf male is freed from the attached Prometheus larva. Copulation, which has not been observed yet, probably takes place between a free-swimming male and the female, either while the female is released or afterwards.

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

“…1, 2), including the anterior and ventral ciliated fields, the paired lateral sensory organ, and the ventro-posterior penis (see Obst and Funch 2003;Obst et al 2006). The penis is located inside a pouch-like structure, which is laterally constricted and forms an elongated slit at the cuticle level (Figs.…”

“…The Prometheus larva then produces 1-3 dwarf males inside its body. The dwarf male is regarded as a free stage and is morphologically characterized by a heavy ciliated body and a ventro-posterior penis, among other features (Obst and Funch 2003). The fertilized female is thought to settle on the mouthparts of the same host where it encysts (Funch 1996).…”

“…The cycliophoran dwarf male has already been described using light and transmission electron microscopy, and at the cytochemical level using confocal laser scanning microscopy (Funch and Kristensen 1997;Obst and Funch 2003;Neves et al 2009aNeves et al , b, 2010. However, only one male of Symbion pandora has so far been described by scanning electron microscopy, and its external morphology was apparently altered during the freeze-drying technique used to prepare specimens (cf.…”

“…However, only one male of Symbion pandora has so far been described by scanning electron microscopy, and its external morphology was apparently altered during the freeze-drying technique used to prepare specimens (cf. Obst and Funch 2003). Also, the high loss of specimens during preparation, due to the minute size of the males, limited the success of this approach.…”

Biological Systematics in the Evo-Devo era

“…Think of cirripeds like Balanus da Costa, 1778 and Lepas linnaeus, 1758, which begin the post-embryonic phase of development as typical crustacean larvae, but later change into very unusual adults whose real affinities were discovered only when their metamorphosis was first observed by Thompson (1830Thompson ( , 1835. This contrasts with organisms represented along their entire life cycle by oddly shaped stages such as the minuscule cycliophorans that live on the appendages of the norwegian lobster: to describe their developmental stages, zoologists were forced to introduce new terms such as the Pandora larva and the Prometheus larva, because the terminology available for other animals did not offer adequate labels for these unique forms (Obst & Funch 2003), whose phylogenetic affinities still remain problematic (Sørensen et al 2000;Giribet et al 2004;Neves et al 2012).…”