Morphology and ultrastructure of a protistan pathogen in the haemolymph of shrimp (Pandalusspp.) in the northeastern Pacific Ocean

Abstract

A eukaryotic parasite of uncertain taxonomic affiliation, with superficial similarity to parasitic dinoflagellates (large plasmodia and numerous trophonts) but a different mechanism of nuclear division and a lack of organelles characteristic of parasitic dinoflagellates, is described from spot prawns (Pandalus platyceros). Up to 20% of the spot prawns examined from Malaspina Strait, British Columbia, were infected. Infections in the majority of the prawns were cryptic (asymptomatic) but of sufficient duration to affect secondary sexual characteristics and castrate the host. Cryptic infections consisted of plasmodia containing numerous nuclei. The plasmodia were observed in the haemal sinuses of all tissues. In mature plasmodia the nuclei stopped dividing and showed a peripheral chromatin ring, an internal chromatin web, and up to three tiny nucleoli. Mature plasmodia divided into numerous uninucleate trophonts, resulting in symptoms of lethargy, orange discoloration, and milky haemolymph caused by a plethora of either spherical or discoid trophonts. Symptomatic infections of the prawns fished with traps rarely exceeded 2%. In 3 of the 156 symptomatic prawns examined, about 30% of the trophonts were in the process of binary fission. During mitosis the nuclear membrane was persistent, but openings (about 0.8 µm in width) at either pole accommodated emergent spindle-pole bodies to which the few chromosomes were attached by microtubules. Attempts to transmit the infection between prawns in the laboratory were unsuccessful.