Fine structure of a septate gregarine trophozoite in the black tiger shrimp Penaeus monodon.

Gregarines are parasitic protozoa that occasionally parasitize the gut lumen of penaeid shrimp and other crustaceans. Here we describe the morphology of gregarine trophozoites found in the black tiger shrimp Penaeus monodon using light microscopy (LM) and scanning (SEM) and transmission (TEM) electron microscopy. Using LM with fresh preparations and with paraffin sectioning followed by hematoxylin-eosin staining, several trophozoites were discovered in the foregut and midgut, and gametocysts were found in the hindgut. Trophozoites existed both as solitary individuals and in association and exhibited either caudofrontal or lateral patterns. They were cylindrical in shape, 60 to 300 microm long by 10 to 60 microm wide, and consisted of 2 parts: a small anterior portion known as a protomerite, which was separated by a septum from a larger posterior portion known as a deutomerite. Under SEM, both the protomerite and deutomerite were found to be entirely covered with longitudinal, parallel pellicular folds 0.1 microm thick. Under TEM, the deutomerite was seen to contain a nucleus with a single eccentric nucleolus and several areas of peripherally located chromatin. The cytoplasm of both the protomerite and deutomerite contained abundant vesicles and granules of different sizes. The pellicle consisted of a double layer of electron-dense membranes separated by an electron-lucent area 30 nm wide and containing a microfilament. A few microfilaments were also observed in the cytoplasm underneath the pellicle, possibly serving as locomotive apparatus for the parasite. Based on its morphology, this gregarine appears similar to those of the genus Nematopsis.