Observation by electron microscopy of a gregarine parasite of Antarctic krill: its histological aspects and ecological explanations

Antarctic krill are parasitized by gregarines (Phylum Apicomplexa, Class Sporozoea, Order Eugregarinida), which were observed in this study by light, scanning, and transmission electron microscopy. Eighty seven percentage of the krill examined (n = 93) were infected with the parasites in the cephalin stage, which were regularly found in the hind-gut epithelium of their host. Cephalins were found attached to the epithelium, and then were liberated into the intestinal lumen. The gamont stage, which follows the cephalin stage, was found in the intestinal lumen, as well as the diverticulum of mid-gut gland. However, gamonts found in the mid-gut gland were considerably larger and elongated in comparison with those in the intestinal lumen. Gamonts in the diverticulum appear to damage microvilli, which are involved in the uptake of digested nutrients and secretion of various enzymes. Therefore, elevated infestation of this parasite in the mid-gut gland may have a significant impact on the nutritional state of the Antarctic krill host.