Daphnia laevis ephippia cemetery in the sediment of a permanent tropical lake

Abstract

Aiming to evaluate the importance of egg banks from a tropical lake for the active Daphnia laevis dynamics, we investigated ephippia down to a 28-cm sediment depth, and used data obtained by traps to compare with the in situ production and hatching rates of fresh ephippia. All the ephippia from the sediment were opened for inspection of the presence of resting eggs, and those found were incubated to verify hatching. Ephippia density degradation rate was tested by putting isolated ephippia immersed in the sediment lake for 75 days, and found to exponentially decrease with increasing sediment depth. Higher densities were recorded in 2017 (maximum of 1.43 ephippia per cm2) than in 2016 (maximum of 0.8 ephippia per cm2). The empty ephippia rates were high in all sediment depths ranging from 83% to 100%. In contrast, the hatching rates in situ of fresh ephippia were high, suggesting that those that sink are mostly unviable or empty, forming an unusual egg bank in the sediment. In the laboratory, ephippia degradation signs were observed as early as the fifteenth day after contact with sediment. We conclude that the egg bank is not able to support an active population since most of the ephippia are empty and also rapidly degrade in sediment. Our results contribute to the current understanding of diapause in tropical cladocerans, demonstrating that the presence of Daphnia ephippia in the sediment of permanent lakes do not always correspond to an egg bank for temporal dispersion.