Ultrastructure of the rotifer integument: peculiarities of Sinantherina socialis (Monogononta: Gnesiotrocha)

Abstract

The rotifer integument is a well-described syncytium that contains an apical intracytoplasmic lamina (ICL) that functions for both skeletal support and muscle insertion. To date, there is limited information on the structure of the integument in species of Gnesiotrocha, a diverse subclade of Monogononta that consists of solitary, colonial, sessile, and planktonic species. In this study, we examined the ultrastructure of the integument in the colonial rotifer Sinantherina socialis to determine how it corresponds to that of other monogononts. The integument of S. socialis was broadly similar to that of other rotifers, consisting of a thickened glycocalyx, multilaminate ICL, and syncytial epidermis. However, it was different in several regards. The ICL consisted of three distinct layers from apical to basal: layer 1 consisted of at least two electron-dense laminae; layer 2 was a thickened matrix of amorphous, electron-dense material or was fibrous; and layer 3 was an electrondense lamina of varying thickness that covered the underlying syncytium. Significantly, layers 1 and 2 formed a ridge-and-groove like system of finger-like projections across the trunk surface that has not been observed in other rotifers. A voluminous syncytial cytoplasm (up to 3 lm thick) was present beneath the ICL and was mostly electron lucent and with few organelles. Bundles of potential microtubules were scattered throughout the syncytium. We hypothesize that the voluminous cytoplasm with microtubules serves as skeletal support for the rotifer's sessile lifestyle, while the external ridges may function as a texturebased deterrent to predators, or serves to trap secretions from the species' defensive glands. Basally, the epidermis was highly folded and bordered by a thin basal lamina that separated the plasmalemma from the blastocoel. Membrane-bound vesicles were present throughout the integument's cytoplasm and are hypothesized to function in the secretion of extracellular matrix and in the maintenance of the ICL.

The ultrastructure of the integument and proventriculus in the raptorial rotifer Cupelopagis vorax (Monogononta: Collothecaceae: Atrochidae)

“…These tubercles are products of the intracytoplasmic lamina (ICL), like similar protuberances in another gnesiotrochan examined with TEM, Sinantherina socialis ( Linnaeus 1858) (Hochberg et al. ). In the latter species, a ridge‐and‐groove like system characterizes the rotifer's integument and is a result of unique projections of the ICL, as opposed to folds in the integument (Hochberg et al.…”

“…The ICL itself may have two distinct layers as revealed in S. socialis (Hochberg et al. ), but in C. vorax , the ICL consists of a single relatively homogeneous layer. This layer is always electron dense, but the density may vary slightly as evidenced in some sections (see Fig.…”

“…Moreover, while many rotifers are known to possess some uncommon body wall elaborations (Hochberg et al. ), stellate tubercles are unknown from other species. In terms of predation behavior, members of C. vorax are also unique in how they survey their surroundings for vibrations from prey prior to a rapid ambush attack on organisms such as protists, crustaceans, gastrotrichs, nematodes, and other rotifers (Bevington et al.…”

“…Hochberg et al. () have shown that light microscopical observations of rotifers provide an incomplete view of the body wall; EM research should also provide additional details that might be relevant for understanding both the functional biology of a species and its evolutionary history (Hochberg et al. ).…”

“…() have shown that light microscopical observations of rotifers provide an incomplete view of the body wall; EM research should also provide additional details that might be relevant for understanding both the functional biology of a species and its evolutionary history (Hochberg et al. ). Thus, we posit that our studies of the proventriculus will shed light on its evolutionary origins, since such a region is unknown from rotifers outside Order Collothecaceae, which otherwise have a standard foregut.…”