Cyst ornamentation in aquatic invertebrates: a defence against egg-predation

Eggs, including encysted embryos (cysts) of aquatic invertebrates may not only be thick-walled, but also provided with various external ornamentations such as spines and honeycombings. We argue that these provide protection, additional to that of the walls themselves, against invertebrate predators, and test this idea by offering intact (honeycombed) and decapsulated cysts of Chirocephalus diaphanus to the carnivorous flatworm Mesostoma sp. Except at the lowest density of cysts, when intact as well as decapsulated cysts seem to be poorly detected by the (tactile?) searching system of the flatworms, intact cysts were relatively immune from predation. Similar causes elicit similar responses: the defences that occur in eggs are essentially similar to those present in adults of rotifers and various branchiopod microcrustaceans, copepods (marine species), and ostracods. Circumstantial evidence suggests that egg defences may vary in intensity with predation pressure. In some cases, they might be predator-inducible.