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.

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 tentacles.  Dactylozooids 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) 

As a side note, Hydractinia spp (not this species but especially H. echinata and H. symbiolongicarpus) have long been used as model organisms for animal development (Gahan et al., 2016). Hydractinia colonies have two components: anemone-like polyps and pipe-like stolons, which connect the different polyps and attach to the substrate (photo). The first, settled animal is a single polyp which begins to grow stolons which asexually bud new polyps from them. Its tissues contain cells called "interstitial cells" (called "i-cells for short), especially in the epidermis. I-cells are small cells which reside in the interstitial spaces among regular epithelial cells. They have a large nucleus but not much cytoplasm and divide frequently, as well as migrate. The suite of genes they contain is similar to that  seen in stem cells and germ cells of more complex organisms. I-cells are in both polyps and stolons, but in polyps they mostly cluster near the base of the column while in stolons they are scattered more randomly. If a polyp is injured the i-cells rapidly collect there and seem to be the main agents in regeneration. They do the same in stolons but more slowly. It is currently not clear whether the i-cells are multipotent (or even totipotent), or whether they consist of morphologically indistinguishable lines of cells which together regenerate the different cell types of the colony. I-cells express several genes characteristic of stem cells and several others characteristic of specific cell lines, such as neural cells. Proliferation of the cells, formation of the oral end of the polyp, and commitment to neural cell lineage is associated with Wnt signaling. Wnt signaling needs to be inhibited in order for cells to the stolon, and if Wnt expression is inhibited, the polyps become stolons. Hydractinia can regenerate any lost part, and, like the frewhwater Hydra, do not appear to have age-related decline (increase in mortality or lowered reproductive potential). In other words, Hydractinia seem to enjoy immortal youth!

References:

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

Gahan, James M., Brian Bradshaw, Hakima Flici, and Uri Frank, 2016. The interstitial stem cells in Hydractinia and their role in regeneration. Current Opinion in Genetics & Development 40: pp65-73. doi 2443/10/j.gde.2016.06.006

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