FURTHER EVIDENCE OF THE DESTRUCTION OF BIVALVE LARVAE BY BACTERIA

¹ The Oyster Institute of North America, Inc.

1. Twelve strains of bacteria were isolated from a moribund Venus mercenaria larva in a laboratory culture. These, ten other clones, and mixed bacteria from sea water were assayed by adding broth culture yielding 10⁶-10⁷ cells/ml. to beaker cultures of healthy clam larvae. Only the mixed bacterial culture from the moribund larva and two of the 12 strains isolated from it caused extensive mortality. One of the virulent clones (6-) is a species of Vibrio,the other (13-1) is a Pseudomonas species.

2. Larvae exposed to virulent bacteria and simultaneously treated with antibiotics were as healthy as controls, showing that active bacteria were necessary to destroy larvae and that metabolites in the bacterial inoculum were not harmful to larvae.

3. Larvae were grown free of contaminating micro-organisms by allowing washed eggs to develop in antibiotic solutions and then isolating straight-hinge larvae by pipette. Either virulent clone (10⁶-10⁷/ml.) destroyed 10-100% of such larvae. However, exposing the animals to large amounts of glass-filtered or heated broth in which bacteria had been grown (corresponding to ca. 10⁹ bacteria/ml.) caused no mortality, but retarded growth.

4. Mortality caused by clones 6-1 and 13-1 in groups of clams kept at 20°, 25°, and 30° C. did not vary significantly. However, both virulent clones grow well at 30° C. and higher; thus high temperatures in laboratory larval cultures favor these strains.