Hydractinia laevispina Fraser, 1922
Common name(s): Snail fur hydroid
Suborder Athecata (Anthomedusae)
|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.
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
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
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
(where gametes are made) sticking out from their column, and don't have
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
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American Fisheries Society, 2002
Morris et al., 1980
Cartwright, Paulyn, 2003. Developmental insights into the origin of complex colonial hydrozoans. Integrative and Comparative Biology 43: pp 82-86
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