Optimizing Sex Ratios of Hyalella azteca to Reduce Variability in Reproduction and Improve Reproductive Toxicity Test Methods.

Standard toxicological test methods with Hyalella azteca incorporate both lethal and sublethal (growth, reproduction) endpoints, though lethal endpoints are often favoured in routine testing. However, sublethal endpoints are important to consider as they are ecologically relevant and are often more sensitive than lethality. It is difficult to achieve robust data for reproduction in H. azteca because a high biological variability associated with reproductive yield, likely due to complex interactions in mate selection which can add to the variability in brood sizes that female produce. In addition, effects on reproduction often co-occur with effects on growth, making it difficult to differentiate effects of growth from those on reproduction in standard tests initiated with juveniles. This study characterized the reproductive capacity of H. azteca by investigating the role of sex ratios in reproductive yield. Experiments were initiated in the absence of toxicants with sexually mature (6-8-week-old) individuals that were placed in different female to male ratios (1:1, 2:3, 3:2 and 7:3). Reproduction was monitored during weekly static renewals for seven weeks. Results indicated that a higher female:male ratio (7 females to 3 males) improved reproductive success and lowered biological variability in reproduction. In addition, the body lengths and weights of newly mature amphipods were used to create a size distribution to aid in the identification of small female amphipods. Based on the results of this study, a novel reproductive toxicity test method is proposed that begins with sexually mature amphipods in a ratio of seven females to three males to minimize reproductive variability in amphipods and to enable a more effective assessment of contaminants using reproduction as a highly sensitive endpoint.