Experimental infection of the echinoid Strongylocentrotus droebachiensis with Paramoeba invadens: quantitative changes in the coelomic fluid

Abstract

Paramoebiasis, due to Paramoeba invadens, was transmitted from diseased to healthy echinoids Strongylocentrotus droebachiensis in the waterborne infection apparatus of Scheibl~ng & Stephenson (1984). Control echinoids were exposed to healthy S. droebachiensis in a parallel apparatus. Groups of 8 individuals from the infected treatment were sacrificed in the pre-symptomatic period and at 24 and 72 h after development of symptoms of the disease, as evidenced by loss of attachment (LOA) to aquanum surfaces. Groups of 8 individuals from the control treatment were removed and sampled at the same time. Coelom~c fluid (CF) was drawn into EGTA as an anticoagulant, and the concentration of coelomocytes determined dlrectly and after density-gradient fractionation into 3 different cell bands. The concentration of total coelomocytes was significantly lower in infected echinoids than in controls, due to smaller numbers of cells in the band containing white spherule and vibratile cells and that containing red spherule cells. The concentration of phagocytes did not differ significantly between the infected and control groups. Protein concentration in cell-free CF of infected echinoids at 72 h post-LOA (mean +SE = 446.25 k 51.69 L L g ml-l) was about twice that of control individuals (238.56 +-16.60 pg ml-l). Terminally sick echinoids yielded CF with large numbers of bacterial contaminants. However, in vltro bactericidal activity of the fluid towards a reference marine pseudomonad did not change significantly during progression of paramoebiasis. Membrane-filtered coelomic fluid with elevated protein values from moribund, infected individuals was injected into healthy echinoids for toxin detection, but no disease symptoms appeared. Subsequent exposure of these individuals to waterborne P. invadens resulted in the usual course of dsease. While pathogenesis of pararnoebiasis remains obscure, possible mechanisms include activation of autolysis in the echinoid or production of degradative enzymes by the amoeba.