Hydractinia laevispina Fraser, 1922 Common name(s): Snail fur hydroid
Synonyms:
Phylum Cnidaria  Class Hydrozoa  Order Hydroida Suborder Athecata (Anthomedusae)  Family Hydractiniidae
	Hydractinia laevispina growing as an extension to a small gastropod shell inhabited by the hermit crab Labidochirus splendescens.  Collected from 60-100 m depth in the San Juan Channel, July 2010.  The shell is to the bottom right and the extension which has grown beyond the shell is to the upper left.  Total colony width about 2 cm.
	(Photo by: Dave Cowles )

How to Distinguish from Similar Species:   All Hydractinia species grow in a similar mat of stolons covered with perisarc, with naked (no perisarc), unbranched polyps arising individually from the mat. Hydractinia sp has 8 tentacles but the hypostome of the gastrozooids is white and the mat has fewer, longer spines. Gastrozooids of Hydractinia milleri and H. aggregata have 12-24 tentacles.

Geographical Range:  In portions of the the Pacific and Arctic oceans.

Depth Range:

Habitat:  Grows on snail shells inhabited by hermit crabs.

Biology/Natural History: Predators on Hydractiniahydroids include the nudibranchs Dendronotus frondosus and Cuthona divae.

This colony was inhabited by a hermit crab, Labidochirus splendescens.  The crab's long legs extended far beyond the limits of the colony and could not be even partially drawn inside.  When the crab was presented to a hungry red octopus, Octopus rubescens, the octopus quickly pulled the hermit crab out of the shell, dropped the shell with the hydroid colony, and ate the crab.

References:

General References:
  American Fisheries Society, 2002
  Morris et al., 1980

Scientific Articles:

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:
 

This view shows the underside of the colony.  The original shell the colony encrusted is to the top right and the expansion which has grown beyond the shell, which covered the hermit crab's carapace, is to the left.

This closeup view shows partly expanded polyps, which are about 0.5 mm diameter.  Several of these polyps have more than 8 tentacles.  Note also the blunt, short, slightly curved spines arising from the stolon mat between the polyps.  A few of the smallest polyps may be dactylozooids.

This view of the edge of the colony shows how the stolons weave together to make a solid mat.

Rosario Invertebrates web site provided courtesy of Walla Walla University