Hydroids fool many people by looking like feathery plants or fireweed. The truth of the matter is that Hydroids are colonies of animals with strong stinging cells used to capture prey and to fight of predators. Some of the species can cause great pain with the stinging of these stinging cells. Divers take great caution about brushing against these hydroids. The two dangerous types of hydroids are white, fine feathery and the denser brown/yellow type. These hydroids may be found in fairly shallow coral reef areas and on wharfs.