life cycle of Hydractina exigua 

(Anthomedusae, Hydractiniidae)

The colonial hydroid stage of Hydractinia exigua grows on marine gastropod shells. The individual polyps are about 5 mm high. They multiply by asexual budding from the stolonal plate (not visible). Note the grape-like meduse buds on the hydranths. They will be liberated as young medusae

Free medusa of Hydractinia exigua (diameter about 2 mm). This is the sexual stage of this species. The eggs are clearly visible on the manubrium. They will be shed into the free water and fertilised. The embryo develops into a planula larva that will metamorphose into a polyp stage, thereby closing the life cycle.

Early development of Hydractinia exigua, egg size about 0.2 mm

one cell stage

unipolar cleavage

two cell stage

four cell stage

eight cell stage

~12 cell stage

blastula stage

 blastula elongation

 3 day old planula