Ultrastructural and elemental characterization of the extracorporeal tube of the sessile rotifer Floscularia conifera (Rotifera: Gnesiotrocha)

Abstract

Rotifers are aquatic microinvertebrates that live in the plankton or in the benthos, which may include a variety of macrophytes. Among these periphytic forms of rotifers, some have taken up a sessile existence and secrete protective tubes around their bodies. One type of tube common to species of Floscularia is made of small round pellets. To date, the building process and some fine structural details are known for Floscularia ringens, but many questions about the composition of the tube and its ultrastructure still remain unanswered. Here, we use transmission electron microscopy and scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDS) to study the ultrastructure of the pellets and their elemental composition, respectively, in the putative sister species, Floscularia conifera. We revealed several new details that add important information about the physiology of tube-making in species of Floscularia. First, we note an inner secretory membrane that is thin, electron lucent, and supports the external pellets. The pellets are relatively consistent in size and have a small depression on their inner surface. All pellets are individually wrapped in a secretory membrane that completely encapsulates suspended materials collected from the surrounding water. Elemental signatures of pellets reveal they consist mostly of carbon (C), nitrogen (N), and oxygen (O), with some silicon (Si) content that is likely the result of diatom shells. Other trace elements such as iron (Fe) and sodium (Na) are also present and likely the result of incorporated bacteria and suspended materials. When larval rotifers are cultured in filtered pond water, the pellets consist mostly of C and O, with little N and no Si; Fe is present in smaller amounts. These new discoveries provide a better understanding of the physiology of rotifer tube construction and tube composition, and their future utility in understanding if and how changes in freshwater environments might impact these factors.