Bacterial clearance by the sea urchin, Strongylocentrotus droebachiensis

Abstract

I n t r o d u c t i o n The survival of echinoderms in the microbe-rich marine environment is dependent on their ability to defend themselves against microbial invasion. An external protective test limits the entry of potentially invasive organisms, but certain areas of the body are vulnerable to injury and could serve as a route of entry for potential pathogens. Therefore, diseases occur in both natural and captive populations. Relatively little work has been done on the diseases of echinoderms. Gilles and Pearse (1) have shown that Vibrio and Aeromonas are among the probable etiological agents that cause lesions in sea urchins. They have isolated 14 different bacterial strains from these lesions of naturally diseased purple urchins, Strongylocentrotus purpuratus. Of these isolates, only Vibrio anguillarum and Aeromonas salmonicida were infectious and caused the formation of characteristic lesions when transferred onto abraded surfaces of healthy urchins. Both bacterial species are wellknown fish pathogens and are frequently found in sediments and marine waters, even in relatively unpolluted areas (2). The lesions induced in the laboratory during Gilles and Pearse's investigations (1) resembled those observed during naturally occurring spontaneous infections. However , all exper imenta l urchins healed during the experimental period, possibly because the dosage was too small or the bacteria were not highly virulent. Healing also implies that sea urchin internal defenses to potentially infectious agents are efficient. The characteristics and mechanisms of bacterial clearance in echinoderms have been derived from only a few studies. Yui and Bayne (3) reported efficient bacterial clearance by purple sea urchins injected with three different bacteria, including A. salmonic ida. Secondary clearance was not significantly different from primary clearance for any of the three bacteria, implying a probable absence of immunological memory. In the present study, we wished to analyze whether the sea urchin S. droebachiensis, common along the Norwegian coast, can exhibit an effective bacterial clearance after injecting it in vivo with its potential pathogen, V. anguillarum. In this laboratory a method for obtaining and culturing in vitro of sea urchin phagocy tes [ a m o e b o c y t e s , acco rd ing to Smith's classification (4)] has been developed (5) and used for their characterization (6). We have now investigated the bactericidal activities of pure amoebocytes cocultured in vitro with V. anguillarum.