Egg masses on the move: corixid oviposition on terrapin shells

In a very dry year, when aquatic habitats were scarce, we observed exceptionally massive corixid oviposition on terrapin shells in Donana National Park, southwestern Spain. Most terrapins inhabiting a large pond devoid of aquatic vegetation exhibited a dense cover of corixid eggs, whilst terrapins from vegetated ponds had no egg cover. Corixid oviposition had also been observed on terrapin shells in previous periods of severe drought. Considering that terrapins frequently alternate between aquatic and terrestrial environments, we experimentally assessed the viability of corixid eggs in varying degrees of dry-exposure. A significant proportion of corixid eggs did not hatch when they were occasionally dried under sun exposure, but egg mortality was lower when they were dried in the shade. The massive oviposition on terrapin shells was probably a consequence of the limited availability of adequate oviposition sites, and it might increase the odds of egg survival when terrapins migrate to other ponds.