Life history of a lowland burrowing mayfly, Ephemera orientalis (Ephemeroptera: Ephemeridae), in a Korean stream

Life history aspects of Ephemera orientalis, a common lowland burrowing mayfly that resides in temperate East Asia, were studied in terms of voltinism, secondary production, and accumulated degree days for larval development. From March 1998 to June 1999, larvae were sampled monthly (weekly or biweekly during the emergence period) from a lower reach of the Gapyeong stream in Korea, a stream typical of temperate East Asia, using a Surber sampler (0.25 m ², mesh 0.25 mm). As a result, the mean density of E. orientalis was 47.21 ± 13.58 indiv. m^-2 during the study period. Very small larvae less than 2 mm in body length were sampled on three separate occasions, and emergence was observed between late April and early October, except during late May. Based on the larval body length distribution and emergence time, two different developmental groups could be distinguished: the slow developmental group (S-group) and the fast developmental group (F-group). The F-group completed its life cycle within 4 months, whereas the S-group had a one-year life cycle. The S-group could be divided into two subgroups, dubbed the S1 and S2-groups, based on larval development. The developmental groups alternate their life cycles in the order: S1→F→S2→S1. Estimated annual production of the larvae was 68.81 mg DW m^-2y^-1; mean biomass was 8.43 mg DW m^-2; the annual production to mean biomass ratio was 8.16. The annual mean water temperature of the study year was 14.76 ± 6.63°C. The total accumulated degree days for larval development was 1396°C for the F-group, 2,055°C for the S1-group, and 1,975°C for the S2-group. Two different adult body size groups were distinguished (P < 0.001 by t-test): larger adults, which belonged to the S-group, were present throughout the emergence period, whereas smaller adults, which belonged to the F-group, were mainly present later in the emergence period (August-October). The difference in the accumulated degree days between the developmental groups may explain the size differences of adults.