Northward expansion of a marine parasite: Testing the role of temperature adaptation

The known range of the eastern oyster (Crassostrea virginica) parasite, Perkinsus marinus, expanded into the northeastern United States in the early 1990s. We used both in vitro and in vivo data to test the hypothesis that the northward expansion was associated with a low-temperature adapted strain of the parasite. In vitro proliferation of nine P. marinus isolates from three geographic sites, Massachusetts and New Jersey in the new range, and South Carolina in the historic southern range, was measured at seven temperatures (5 to 35 degrees C) using a tetrazolium blue dye assay. We wanted to determine if there were between- and within-geographic location differences in the P. marinus proliferation rate, and if so, whether they were associated with temperature. We found no evidence of low-temperature adaptation based on the fact that net proliferation rates for isolates from all three geographic locations were similar at temperatures from 5 to 20 degrees C. On the other hand, at temperatures of 25 to 35 degrees C, the South Carolina isolates exhibited higher proliferation rates than the northern isolates suggesting possible high-temperature adaptation of parasite strains that are routinely exposed to higher temperatures. Although there was significant within-location variation among isolates, the data tended to group together by geographic location supporting the hypothesis that there is an important regional component to the proliferation rate of P. marinus isolates. A survey of published data showed that the temperature at which in vivo proliferation was first observed in oysters at sites from the Gulf of Mexico to Massachusetts was typically between 20 and 23 degrees C with no evidence of a geographic cline. These results lend support to the hypothesis that the recent warming trend in the northeastern US is the most likely explanation for the P. marinus range extension. Crown Copyright (c) 2006 Published by Elsevier B.V. All rights reserved.