Ultrastructure of extracorporeal secretions of four sessile species of Rotifera (Gnesiotrocha), with observations on the chemistry of the gelatinous tube

Abstract

Secretion of an extracorporeal hydrogel is a common defensive strategy employed by many aquatic invertebrates including several species of sessile rotifers. Here, we provide a comparative study of the ultrastructure of the gelatinous secretions of four species of Superorder Gnesiotrocha (Monogononta): Conochilus unicornis, Collotheca ferox, Stephanoceros fimbriatus, and Stephanoceros millsii. Additionally, we use differential staining and enzyme degradation assays to explore the chemistry of the gelatinous tube of S. fimbriatus. At least three types of secretions are produced by these four species: an external gelatinous tube, a thick glycocalyx, and an adhesive peduncle. These three different secretions all have a fibrous appearance, but each has different electron densities, fiber thicknesses, and glandular origins. The gel tube, which is likely secreted by pedal glands, has a highly hydrated framework of mesh‐like fibers with thick internal and external laminae that differ slightly among species. Chemical staining suggests that the gel tube is composed of glycoproteins with a high content of sulfonic groups and limited or no obvious lipids or cationic proteins. This tube is resistant to chemical and enzymatic digestion. The proteinaceous glycocalyx is secreted by the syncytial integument and has various densities and thicknesses among species. The peduncle is an adhesive attachment that connects a rotifer's foot to a substratum, or, in some species, to conspecifics as part of a colony. The secretion is highly electron dense and contains membrane‐bound vesicles; it appears to be derived from a different set of pedal glands than the hydrogel tube. The different chemistries and anatomical origins of the three gelatinous secretions suggest an independent evolutionary history for each.

Fine structure of the retrocerebral organ in the rotifer Trichocerca similis (Monogononta)

“…Adhesive secretions tend to have an abundance of small proteins that form crosslinks, whereas secretions that function in locomotion tend to be larger glycoproteins with few crosslinks (Smith, 2002). A comparison of the secretions in T. similis to the pedal (adhesive) secretions in S. fimbriatus reveals the latter to be electron dense and much different (Yang et al, 2021; Figure 4) from those inside the reservoir in T. similis . Given the small size of T. similis (length ~150 μm) and the difficulty of isolating the extremely small volume of secretion, traditional biochemical methods of characterizing glycoproteins (e.g., SDS‐PAGE [Smith et al, 1999; Techawongstien et al, 2022]) seem an unlikely avenue for their characterization.…”

“…Given the small size of T. similis (length ~150 μm) and the difficulty of isolating the extremely small volume of secretion, traditional biochemical methods of characterizing glycoproteins (e.g., SDS‐PAGE [Smith et al, 1999; Techawongstien et al, 2022]) seem an unlikely avenue for their characterization. More specialized stains for ultrathin sections may be more revealing (Yang et al, 2021), but this will still need to be supported by behavioral evidence to determine whether the rotifer can adhere to surfaces or engages in any special form of movement along a substrate.…”

“…Adhesive secretions tend to have an abundance of small proteins that form crosslinks, whereas secretions that function in locomotion tend to be larger glycoproteins with few crosslinks (Smith, 2002). A comparison of the secretions in T. similis to the pedal (adhesive) secretions in S. fimbriatus reveals the latter to be electron dense and much different (Yang et al, 2021; Figure 4) from those inside the reservoir in T. similis.…”

“…And although all of these gland types are syncytial, their secretion granules are much different at the ultrastructural level. The granules tend to be either electron lucent or contain a mesh work that is distinctly different in appearance from those in T. similis: See figure 5 in Yang et al (2021).…”

“…Such networks are known to consist of proteins with negatively charged polysaccharide chains to form proteoglycan molecules that are standard in vertebrate and invertebrate gelatins such as mucus (Smith, 2002). These secretions are similar to the hydrogel secretions of rotifers-such as Conochilus unicornis ROUSSELET 1892, Collotheca ferox (PENARD 1914), and Stephanoceros fimbriatus (GOLDFUß 1820)that produce protective gelatinous housings (Yang et al, 2021). In these species, the gelatins contain an abundance of mesh-like filaments that likely function to enclose water and thus form the hydrogel house.…”