Ultrastructure of the extracorporeal tube and 'cement glands' in the sessile rotifer Limnias melicerta (Rotifera: Gnesiotrocha)

摘要

Rotifers are common aquatic microscopic invertebrates. Most rotifers are planktonic but several gnesiotrochan species are sessile and produce tubular sheaths around their bodies. These tubes have a variable morphology and may be produced by different glands. Here, we applied scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to study the ultrastructure of the tube and its potential origin of secretion in Limnias melicerta. Results from SEM confirm earlier observations that juvenile rotifers first secrete a segment-less tube and then add segments as they grow. Tubes consist of two distinct secretions: an inner mucus-like layer that extends from the base to the foot region of the adult, and an external layer that is secreted by "cement cells" sensu Wright. The external layer consists of a series of thickened rings and elongated girdles, both of which are somewhat fibrous in appearance and occasionally show differences in electron density. The ultrastructure of the "cement cells" indicates that these secretory regions are not cellular but rather a modified region of the syncytial integument that forms a belt-like gland around the animal. This gland is highly papillated due to localized folding of the intracytoplasmic lamina of the integument. The ultrastructure of the gland shows a voluminous swelling of the syncytium with abundant endoplasmic reticulum and secretion vesicles. At least three types of membrane-bound secretion vesicles are present based on electron density. We hypothesize that the gland is constitutively active but secretions are only released when a threshold level is reached.