European populations of the native flat oyster, Ostrea edulis, have been heavily depleted by two protozoan parasites, Marteila refringens and Bonamia ostreae, with mortalities of up to 90% reported in na´ve populations. However, in studies carried out over a ten-year period, researching the parasite-host relationship of B. ostreae and O. edulis in several age cohorts within a na´ve O. edulis population from Loch Ryan (LR), Scotland, 1,364 specimens were challenged and only 64 (5%), across multiple testing protocols, screened positive for B. ostreae. This article presents a case for the development of S-strategy life traits in the LR population that coincide with enhanced immune function and survival. Oysters are considered typical r-strategists (small in size with fast development and high fecundity) while S-strategists, as outlined in Grime's (1977) C-S-R (competitor-stress tolerant-ruderal) triangle theory, are characterized by slow growth and investment in the durability of individuals. This study hypothesises that slower growth and reduced reproductive output in LR oysters has resulted in the investment of an enhanced immune function and reduced susceptibility to B. ostreae i.e. r-strategists with S-strategy life traits equates to protection from significant pathogens. The findings presented here within provide a strong case study for local adaptation of energy allocation and provides empirical support for the C-S-R triangle theory in a marine organism.