Hydroids are colonies of tiny stinging jellies, best
described as hundreds of inverted jellyfish
attached to a feather- or seaweed-like base, known as the organism's
polyp phase. Reproduction is achieved by releasing these fertile
medusae into the plankton to feed and spawn. They are abundant on most submerged surfaces
including seaweeds but are very small
and easily overlooked. Powerful stinging cells are a primary cause of skin irritation when swimmers contact the reef,
seaweed, pilings, floating docks, lines, or debris. Symptoms may
be delayed a day or more, appearing as intensely itchy welts and
blisters. Many species
have been unintentionally spread around the world as adults on ship
hulls or larvae in ballast water. Hydroids are eaten by some
species of sea slugs, filefishes,
puffers, and the Moorish
Idol. Fire Corals
are reef-building, photosynthetic hydroids.
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