Life History Characteristics of Two Introduced Populations of Mysis Relicta

Introduced populations of the opossum shrimp Mysis relicta take 4 yr to reach reproductive age in Lake Tahoe, California—Nevada and 1—2 yr in Emerald Bay, an isolated (for Mysis) embayment of Lake Tahoe. Reproduction in both populations occurs in the winter and spring. The duration of embryonic stages in the Emerald Bay population based on field population statistics ranged from 50 d for the egg stage to 6 d for the last embryonic stage. The number of eggs per egg—bearing female in Emerald Bay increased significantly with the total length of the female. Over the entire brooding period, the average number of embryos per female decreased from 13.89 eggs per gravid female to 10.33 stage 5 embryos per female. This decline in brood size with age of the brood was not due to size—related mortality, but probably resulted from embryos falling out of the marsupium. A similar decline in brood size with age of the brood was not observed in Lake Tahoe. The average number of eggs per brood in Lake Tahoe was 10.73 which was significantly less than that observed in Emerald Bay. However, there was no difference in the number of stage 5 embryos per brood in both systems. Differences in generation time and total reproductive output in the Lake Tahoe and Emerald Bay populations were most closely related to differences in the overall production and mean depth of the two systems. In addition, these traits and whether reproduction is seasonal or continuous among other mysid populations are strongly correlated with the total production of the environment. These results suggest that differences in M. relicta life history traits among populations may represent a phenotypic and not an evolutionary life history adaptation.