The absorptive surfaces of Nectonema sp. (Nematomorpha: Nectonematoidea) from Pandalus montagui: histology, ultrastructure, and absorptive capabilities of the body wall and intestine

Abstract

The histology, ultrastructure, and absorptive capabilities of the body wall and intestine of the juvenile stages of Nectonema sp. (Nematomorpha: Nectonematoidea) that parasitize the shrimp Pandalus montagui in Passamaquoddy Bay, New Brunswick, were examined using histological, histochemical, ultrastructural, and in vitro labeling techniques. The body wall consists of a multilayered cuticle that rests on, and is produced by, a thin cellular hypodermis. The intestinal tract consists of a minute mouth, a cuticularized oesophagus, and a blind-ending intestine consisting of a lumen surrounded at different places by two, three, or four elongated cells. These cells consists of a maximum of two "absorptive" cells with microvillar luminal surfaces, and a maximum of two "secretory" cells, which contain numerous electron-dense granules. Acid and alkaline phosphatases and nonspecific esterases were detected in the outer layers of the body wall (cuticle and hypodermis) and in the intestinal cells. Such enzymes appear to be related to absorption of nutrient substances. Autoradiography experiments using [3H]leucine showed that following incubation in [3H]leucine-labeled seawater, leucine was concentrated in the hypodermis and the intestinal cells. Similar results were obtained when worms were incubated first in [3H]leucine-labeled seawater and then chased in nonlabeled L-leucine. Uptake of [3H]leucine was inhibited by L-leucine when worms were incubated in a seawater solution of [3H]leucine and excess L-leucine. In vitro absorption of [3H]leucine provides evidence that a carrier-mediated transport system operates across both the cuticle and the intestine of Nectonema sp.