Description: Hydroid genus Hydractinia is called 'snail fur' because the hydroids form a colonial mat on the surface of snails, especially living symbiotically with hermit crabs who have taken up residence in the dead snail shells. Colonies include feeding polyps (gastrozooids), reproductive polyps (gonozooids), and spinelike protective polyps (dactylozooids). Neither the entire hydranths (polyps) nor their basal portions are covered with a hydrotheca, though the polyps grow up out of a mat-like maze of perisarc-covered stolons. The polyps arise individually, without branching, from the stolons. The gastrozoooid polyps are colored white and most have just 8 filiform tentacles in 1 whorl, while the perisarc-covered mat is usually golden yellow. Spinelike dactylozooids, also colored yellow, also arise from the mat. The female gonozooids have only 1 egg in each gonophore, but may contain more than one gonophore. Gastrozooids can extend up to a maximum of about 2.5 mm.
is a genus of hydrozoan polyps
that does not have a medusa
stage. The polyps release
planula larvae which settle and grow into polyps.
It characteristically occurs on snail shells in which hermit crabs are
living. As the hermit crab grows, the hydroid
colony forms a mat which extends farther and farther out from the aperture
of the shell, effectively enlarging the shell and making it unnecessary
for the hermit crab to discard the shell and find a new one. This no doubt
benefits both the hydroid
and the crab.
Kozloff, 1987, 1996
General Notes and Observations: Locations, abundances, unusual behaviors:
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