Hydractinia echinata forms pinkish-brown (when alive) or plain brown (when dead) patches on gastropod shells that are occupied by a hermit crab, often near the aperture of the shell.

The horny mat or hydrorhiza is about three millimetres thick.
It consists of thick jagged spines that reach three millimetres in height.
Amongst these there are three types of polyps.
The club- like feeding polyps grow up to thirteen millimetres in length, and have two rows of eight tentacles, the lower set being shorter than the upper ones.
There are also male and female reproductive polyps (gonozoids) that have a few short terminal tentacles.
Scattered among these are specialized defensive stinging polyps (dactylozooids) that are long, coiled threads.
The gonozoids liberate crawling planula larvae that search out moving gastropod shells
Hydractinia echinata forms pinkish-brown (when alive) or plain brown (when dead) patches on gastropod shells that are occupied by a hermit crab, often near the aperture of the shell.
The horny mat or hydrorhiza is about three millimetres thick. It consists of thick jagged spines that reach three millimetres in height. Amongst these there are three types of polyps. The club- like feeding polyps grow up to thirteen millimetres in length, and have two rows of eight tentacles, the lower set being shorter than the upper ones. There are also male and female reproductive polyps (gonozoids) that have a few short terminal tentacles. Scattered among these are specialized defensive stinging polyps (dactylozooids) that are long, coiled threads. The gonozoids liberate crawling planula larvae that search out moving gastropod shells
Hydractinia echinata