Testing microbial pathogens as a cause of early juvenile mortality in wild populations of benthic invertebrates

Microbial pathogens such as bacteria, viruses, fungi, and protozoa are potentially important causes of mortality during the critical first days of juvenile life in benthic invertebrates, but their effect on early juveniles in natural habitats remains unexplored. We therefore placed newly-settled mussels (Mytilus trossulus) and barnacles (Balanus glandula, Chthamalus dalli), and newly-hatched snails (Nucella ostrina) in screened cages submerged in the field, exposing them to ambient microbial organisms while isolating them from most non-microbial mortality factors. Mortality over a 5 d period was only 0-3% in M. trossulus and N. ostrina and 0-10% in B. glandula and C. dalli. Mortality was consistent among replicate trials in the same summer and between 2 different years, and was much lower than previously reported natural early juvenile mortality in these species. We then examined whether bacterial infection caused the few juvenile deaths in our field experiment. Dosage testing in the laboratory with 5 antibiotics resulted in an antibiotic cocktail providing broad-spectrum antibacterial protection without affecting the health of juveniles. In a second field experiment, early juvenile M. trossulus and N. ostrina exposed to the antibiotic cocktail 3 times daily did not experience reduced mortality relative to controls. Our findings suggest that, at least in some species, microbial pathogens are not a direct cause of juvenile mortality in the field, thus narrowing the list of potential causes of early juvenile mortality to the non-microbial factors excluded from our experiments.