ephippium

An ephippium (plural ephippia) is a protective resting-case produced by certain freshwater crustaceans, most notably some cladocerans such as Daphnia, to enclose diapause eggs. The ephippium is a specialized capsule that typically contains one or two dormant eggs and is released into the water when the female ceases asexual reproduction due to unfavorable conditions.

Structure and formation: The case is usually chitinous, sometimes pigmented, and may be saddle-shaped or elongated depending on species. It protects the enclosed eggs from desiccation, ultraviolet radiation, and temperature fluctuations. The ephippium forms in the brood chamber and is expelled as a unit into the surrounding water.

Hatching and ecology: Resting eggs within ephippia remain viable for extended periods, often years, in sediments, allowing populations to persist through droughts or winter. When conditions become favorable, the eggs hatch, producing juvenile individuals that can reestablish the population.

Significance: Ephippia are valuable in ecological and paleolimnological research because their shells can preserve well in lake sediments and provide information about past species composition, population dynamics, and environmental conditions. Species identification often relies on ephippial morphology, since adult individuals may be difficult to observe.