Endocrine systems of microcrustaceans, such as Daphnia, are poorly investigated. With regard to the molt-inducing ecdysteroids, the focus of current research is on exposure-effects of exogenously supplied environmental xenobiotics acting as endocrine disrupters in D. magna. Ecdysteroids are important for various developmental processes in all arthropods and therefore such endocrine disrupters might affect the population dynamics of Daphnia. As ecdysteroids are synthesized from cholesterol, which cannot be synthesized de novo, the availability of dietary sterols might also affect ecdysteroid levels in Daphnia.
In a preliminary study, we used a highly sensitive radioimmunoassay (RIA) to determine free and conjugated ecdysteroid levels in whole-body extracts of adult D. magna during a complete molt cycle. We found that ecdysteroid levels were predominated by free ecdysteroids. Starting from basal levels in the postmolt stage the concentration of free ecdysteroids increased sharply in the early premolt stage, followed by a sharp decline back to basal levels just prior to ecdysis. Polar and apolar ecdysteroid conjugates were found in rather low amounts with little variation during the molt cycle. Only small amounts of ecdysteroids were found in newly deposited eggs of D. magna which suggest a sparing investment of maternal ecdysteroids into the eggs for early embryogenesis. As in whole-body extracts, free ecdysteroids were the predominant ecdysteroids found in eggs of D. magna, together with small amounts of polar and intermediary amounts of apolar conjugates. Hence, the pathways leading to polar and apolar ecdysteroid conjugates appear to be of minor importance in D. magna. Analyses of the immunodetectable peak in free ecdysteroids by liquid chromatography-mass spectrometry (LC-MS) revealed that molting is induced most probably by an increased level of 20-hydroxyecdysone (Martin-Creuzburg et al. 2007).
In continuative studies, we will investigate if these ecdysteroid levels are affected by the availability of dietary sterols, in order to assess possible ecological implications.