Hydractinia laevispina Fraser, 1922

Common name(s): Snail fur hydroid

Synonyms: Hydractinia laevispina
Phylum Cnidaria 
Class Hydrozoa 
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 )
Description: Hydractinia, an athecatehydroid, grows as an interconnected mat of stolons (covered with perisarc) that form a dense carpetlike or matlike structure on the substrate. Individual polyps arise from the mat, and there is no branching above the mat itself. The polyps have no sheath of perisarc around even the base. The colony contains several types of polyps: feeding gastrozooids and reproductive gonozooids, as well as fingerlike dactylozooids. Most of the gastrozooids of this species have about 8 tentacles, though some have more.  Mat of stolons has many short spines which are smooth and slightly curved (photo). Gastrozooids are pink and up to 2.5 mm tall.  Usually has 4 gonophores in a single whorl.  Bears one egg per gonophore.

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.

Hydractinia colonies are complex and consist of 4 types of polyps.  The colonies are either male or female, and shed gametes into the water.  After fertilization, a planula larva develops.  The planula settles on a hermit crab shell, crawls around, and metamorphoses into a polyp.  This first polyp (a gastrozooid, shaped like a typical polyp with tentacles and a gastrovascular cavity) begans to sprout ropelike stolons from its base.  The stolons are hollow and continuous with the gastrovascular cavity, ectoderm, and gastrodermis.  These stolons spread across the shell, gripping the shell surfacee, and begin to grow up periodically into other polyps (also gastrozooids).  The stolon network becomes more and more interconnected, then the stolons begin to widen into a flattened mat.  This mat connects all the polyps and is innervated in its upper layer.  After the stolon mat has covered the entire gastropod shell the hermit crab is living in, several new types of polyps begin to grow.  Reproductive gonozooids arise from the mat.  These polyps have gonophores (where gametes are made) sticking out from their column, and don't have well-developed tentaclesDactylozooids are specialized, usually smaller finger-like polyps which only grow along the aperture of the shell the hermit crab is living in.  The dactylozooids seem to specialize in capturing hermit crab eggs.  Tentaculozooids are quite long and tentacle-like, and about as large as an entire gastrozooid polyp.  They grow in various areas of the colony and are used for defense (information from Cartwright, 2003)



 

References:

Dichotomous Keys:
  Carlton, 2007
  Kozloff, 1987, 1996

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

Scientific Articles:
  Cartwright, Paulyn, 2003.  Developmental insights into the origin of complex colonial hydrozoans.  Integrative and Comparative Biology 43: pp 82-86

Web sites:


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


Underside
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.


Polyps
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.


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



Authors and Editors of Page:
Dave Cowles (2010):  Created original page
CSS coding for page developed by Jonathan Cowles (2007)

Rosario Invertebrates web site provided courtesy of Walla Walla University