The spatial distribution of Perkinsus marinus, a protozoan parasite, in relation to its oyster host (Crassostrea virginica) and an ectoparasitic gastropod (Boonea impressa).

The endoparasitic protozoan Perkinsus (=Dermocystidium) marinus is a major cause of oyster mortality in the Gulf of Mexico. The small-scale spatial distribution of P. marinus, its oyster host, and a second oyster parasite, the ectoparasitic snail Boonea impressa, was examined on two oyster reefs in Aransas Bay, Texas. Large oysters (>5 cm long) were infected 3 to 4 times as frequently by P. marinus as smaller oysters on both reefs. In both cases, infected oysters were less contagiously distributed (lower variance/ mean) than the entire oyster population, being most similar to the distribution of large oysters where most infections were found. In both cases the spatial distribution of infected oysters, when different, was more nearly random than the distributions of the oyster or snail populations. The distribution of large Boonea impressa explained the distribution of infected oysters better than any other parameter measured. B. impressa transmits P. marinus and feeding by B. impressa increases infection intensity in infected oysters. Hence the influence of snail parasitism on P. marinus prevalence and infection intensity, in large measure, determined the distribution of P. marinus on these reefs. Since P. marinus is estimated to be responsible for over half of all mortality in market-size oysters, the distribution of snail patches may play an important role in the distribution of mortality in some oyster populations.