Funk, D.H., B.W. Sweeney, and J.K. Jackson. 2019. Freshwater Science 38(4): 725–741.
Under summer-like photoperiods (15∶9 L∶D) with abundant high-quality food, larval growth and development of the baetid mayflies (Ephemeroptera) Cloeon dipterum-IS1 and C. dipterum-CT1 were predictable and linear across a wide temperature range (14–30°C). However, some individuals exhibited larval quiescence with delayed adult emergence at a substantially larger size under certain combinations of photoperiod, temperature, and larval density. Results from a series of experiments indicate that photoperiod appears to be the primary factor underlying larval quiescence because at 20°C and short day length (10∶14 L∶D), cohorts split, with 1 group growing and developing quickly and the other entering a quiescence. The proportion of larvae exhibiting quiescence became progressively greater when temperatures declined, larval densities increased, and exposure to short day length occurred earlier in development. However, subsequent increases in day length terminated the quiescence, and larvae resumed normal (fast) growth and development. We show that larvae remain active and continue to feed, grow, and molt (albeit slowly) during quiescence. However, because development appears to be slowed proportionately more than growth at a given temperature, adult size and fecundity of larvae that enter quiescence is significantly greater. Cloeon dipterum-IS1 and C. dipterum-CT1 are multivoltine and, like many mayflies, overwinter as aquatic larvae. The species are ovoviviparous such that mated adult females must gestate their eggs for 2 wk or longer prior to oviposition, and we show that this process becomes less successful at progressively colder air temperatures. We propose that the adaptive significance of larval quiescence includes the avoidance of low reproductive success likely to be experienced by adults emerging in the fall. Thus, quiescence delays some adult emergence until the following spring when conditions for gestation may be more favorable. We also propose that larval quiescence induced by photoperiod may be widespread in mayflies and might contribute to the seasonal variability observed in adult size and fecundity for both univoltine and multivoltine species.