L Journal of Experimental Marine Biology and Ecology, 222 (1998) 93–111 Reduced genetic diversity in a meiobenthic copepod exposed to a xenobiotic a, b a b * G.T. Street , G.R. Lotufo , P.A. Montagna , J.W. Fleeger a University of Texas Marine Science Institute, 750 Channelview Drive, Port Aransas, TX 78373, USA b Louisiana State University, Department of Zoology and Physiology, Baton Rouge, LA 70803, USA Received 26 November 1996; received in revised form 7 April 1997; accepted 27 May 1997 Abstract Populations that encounter a novel environment can lose genetic diversity. Intense, directional selection can affect reproduction and lead to a gene pool dominated by relatively few genotypes. A loss of haplotype diversity (h) in the mitochondrial genome in populations of meiobenthic, harpacticoid copepods around offshore oil platforms has been linked to lethal and sublethal responses to factors proximal to offshore oil platforms. However, it was not possible to determine if the effects were related directly to increased concentrations of contaminants produced by drilling at the platforms. An experiment was designed to test whether populations of Nitocra lacustris Schmankevitsch (Copepoda: Harpacticoida) would lose haplotype diversity over 3 generations following exposure to sediment-associated hydrocarbons. Haplotype diversity was estimated by comparing restriction fragment length polymorphisms generated from mitochondrial DNA amplified from individual copepods. An additional component of the study was to determine the effects of population size on estimates of diversity. Haplotype diversity could be under- estimated by using too few individuals, because random genetic drift in very small populations could cause lower diversity even in the absence of strong selection. The relationship between sample size and h was determined by randomly sub-sampling, at different sample sizes, a large data set collected for two copepod species. Cultures treated with phenanthrene, a polycyclic aromatic hydrocarbon, experienced lower adult survival, fewer surviving offspring per female, and lower haplotype diversity relative to control cultures. After the first generation, mortality declined, but larval survival and haplotype diversity did not change in subsequent generations. Diversity was slightly underestimated due to assaying too small a number of individuals, but could not account for all of the differences between treatments. Despite mortality, populations in culture were not reduced to the point where random drift seemed to have a significant effect. These results support the hypothesis that loss of mtDNA diversity in naturally occurring populations of Harpacticoida may serve as a marker for negative effects of exposure to xenobiotic contaminants.  1998 Elsevier Science B.V. * Corresponding author. Belle Baruch Institute, University of South Carolina, Columbia, SC 29208. Tel.: 1 1 803 777-3941; e-mail: gstreet@vm.sc.edu 0022-0981 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved. PII S0022-0981(97)00140-8