Unique reproductive characteristics of Lake Tahoe’s Capnia lacustra (Plecoptera:Capniidae), a stonefly in decline

Capnia lacustra Jewett 1965, a wingless stonefly found in the deep benthic environment of Lake Tahoe, has declined in density and distribution since the 1960s. We investigated its life-history characteristics, including its pattern of voltinism. We also compared C. lacustra’s current density and distribution in plant beds to that in the 1960s. Size-frequency analysis showed the development of 2 asynchronous reproductive cohorts, each with a life-cycle duration of ̃1 y. The presence of eggs within mature females in May and November confirmed that spring and autumn cohorts develop simultaneously. We found eggs throughout the abdomen, thorax, and head of C. lacustra females. Females from the autumn cohort were viviparous, whereas the spring cohort was oviparous. This is the first time facultative viviparity has been described for Plecoptera. Capnia lacustra’s unique life history probably is driven by relatively stable temperatures and food availability in Chara beds in the lake. Capnia lacustra currently is known to occur only in 3 areas of Lake Tahoe, whereas it was widespread in the 1960s. It has maintained high densities at 1 of the sites, Camp Richardson, which has an extensive and dense bed of the macroalga Chara. Capnia lacustra’s dependence on Chara, which is declining in geographic and depth distribution in the lake, may have led to the observed declines in density and distribution of this remarkable stonefly.