Infected lagoon sand shrimps (Gammarus insensibilis) at the surface of the sea, infected with parasitic flatworms. This amphipod crustacean, which reaches lengths of 20 millimetres, is found in coastal lagoons. The life cycle of the parasitic trematode (flatworm) Microphallus papillorobustus includes this shrimp as an intermediate host. When infected, the shrimp swims upwards, making it more vulnerable to predation by seagulls and other seabirds. A non-infected shrimp (bottom centre) swims further below the surface, avoiding predation. The flatworm's eggs are transmitted in the faeces of the seabirds, completing the life cycle.