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Semivoltinism, seasonal emergence, and adult size variation in a tropical stream mayfly (Euthyplocia hecuba)

1024 681 Stroud Water Research Center

Sweeney, B.W., J.K. Jackson, and D.H. Funk. 1995. Journal of the North American Benthological Society 14(1):131–146.

doi:10.2307/1467729

Abstract

Life history characteristics of the stream mayfly Euthyplocia hecuba (Hagen) (Polymitarcyidae:Euthyplociinae) were studied over a 4-yr period in two tropical streams (Río Tempisquito and Quebrada Marilin) draining primary evergreen forest in northern Costa Rica. Larvae burrow in the substratum of the stream, living under small to large cobbles that are firmly embedded in the stream bottom. Euthyplocia hecuba is sexually dimorphic and one of the largest mayflies of Central America, with maximum larval size of 149 mg (dry mass) for females and 35 mg for males. Mature female and male larvae are about 40% larger in Q. Marilin relative to R. Tempisquito. Subimagoes emerge about 1 h before dawn (∼0400 h); the molt to the imago (or true adult stage) occurs ∼20 min later. Adult emergence and reproduction occurs mainly between June and November. Females mate and then oviposit in riffle areas just before dawn, with the time period gradually changing from 0446 and 0507 h in June to 0508 and 0529 h in October. We found that size of adult males and females gradually declined during the emergence period. Fecundity averaged 984 in October (range 364-2851), with eggs being very large for a mayfly (0.46 × 0.4 mm; 0.017 mg dry mass). The proportion of adult biomass allocated to eggs (reproductive effort) averaged 0.51 but varied according to female size. Median egg development time was 113, 55, and 31 d at 15, 20, and 25°C. The hatch success of eggs was >90% at 15 and 20°C, only 68% at 25°C, and 0% at 10 and 30°C. First instar larvae are relatively large for a mayfly, having a head width of 0.11 mm and a body length of 1.30 mm. Seasonal changes in larval size distribution suggest that larval development takes ∼22 mo to complete and may involve a developmental quiescence or diapause during the first year. A 2-yr life history appears to be the most parsimonious hypothesis given a 2-mo egg development time, the occurrence of two size cohorts of larvae throughout most of the year, and the temporal pattern of maximum and average larval sizes over the course of the sampling period. Our data suggest that seasonality in the developmental dynamics of larvae underlies both the seasonal emergence and decline in dry mass of both adult males and females during the emergence period. The data provide the first evidence for semivoltinism in a tropical mayfly.