The effect of temperature on maturation threshold body-length in Daphnia magna

Aquatic invertebrates are usually larger at maturity when water temperatures are lower. For the freshwater cladoceran Daphnia, it has been suggested that a threshold size must be attained to initiate maturation, which results two instars later in the deposition of eggs into the brood chamber. This threshold size is believed to be independent of food. Here, we examine the effect of temperature on maturation threshold body-length in Daphnia magna. Daphnids were raised from birth to maturity under three constant-temperature regimes (12 degrees C, 16 degrees C, 22 degrees C), and two food-level conditions. Animals were measured daily, and a body-length based maturation threshold determined for each individual. We demonstrate that mean maturation threshold length is negatively correlated with ambient water temperature. Further, daphnids with a larger threshold length tended to be larger at maturity. A maturation threshold linked to body length suggests that reduced Variation in size at maturity is adaptive, even at the cost of additional variation in instar number or age at maturity.