Economic Importance for Humans: Negative
The stings of some hydrozoans (most famously the Portuguese man-o-war, Physalis physalis), are dangerous to humans. Also, the fire corals (Milleporidae) get their name from the painful sensation divers get if they touch them.
Hydrozoans are common members of "fouling communities" -- the benthic organisms that naturally attach to hard substrates, and so grow on the hulls of ships and on submerged water pipes, interfering with their function.
Negative Impacts: injures humans (bites or stings, venomous )
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