Spiny but Subitaneous Eggs: Egg Morphology and Hatching in Acartia Copepods in the Tropics

Egg surface ornamentation, egg production rates, and hatching success of two calanoid copepod species (Acartia pacifica and Acartia cf. erythraea) were examined in a tropical coral reef of Tioman Island, Malaysia, near the equator (2°N). All observed females of A. pacifica and A. cf. erythraea produced eggs with spines on the surface, either short or long, which are common in resting eggs in temperate ecosystems that hatch when environmental conditions are favorable or after a certain refractory period. However, all of these spiny eggs in the present study hatched within 48 hours when exposed to ambient seawater temperature, suggesting that they were subitaneous eggs. Such spine ornamentation has been thought to be an adaptive feature only for resting eggs to survive during egg dormancy, but the presence of spines on subitaneous eggs suggests further possible functions of these spines such as reducing the risks of ingestion by predators.