Co-infesting symbionts on a threatened marine host: evaluating correlations between an introduced parasitic isopod and a native symbiotic clam

Abstract

In marine ecosystems, increased global-scale transportation creates opportunities for rapid introduction of invasive parasitic species that, in some cases, result in dramatic shifts within the native communities. A lack of detailed knowledge regarding the ecology of invasive marine parasites hinders our ability to develop effective conservation strategies and avoid unforeseen ecological consequences. We examined co-infestation patterns of a highly pathogenic, introduced parasitic isopod (Orthione griffenis) and a native symbiotic clam (Neaeromya rugifera) on the North American native blue mud shrimp Upogebia pugettensis. Our comparisons included infestations of O. griffenis and N. rugifera among 447 U. pugettensis hosts over 3 study years and were designed to statistically assess whether the 2 symbionts exhibited significant associations with one another. Our results indicate that infestations by the 2 symbiont species are positively correlated, such that the presence of one symbiont is a strong, positive predictor for the presence of the other. For both symbionts, host size is an important factor that drives the observed correlation. Host sex is also influential for O. griffenis. Interestingly, even after accounting for these host attributes, the infestations by the 2 symbionts continue to correlate positively, particularly among older (second-year and beyond) symbionts, highlighting the likely influence of additional host and environmental factors in driving the symbiont correlation post-settlement. We consider potential mechanisms, including differential energetic reserves and longevities between infested and co-infested hosts, in detail. These results offer insights into the ecological drivers of symbiont co-infestation, which have important implications for understanding host-parasite interactions and future conservation measures.