Hydractinia milleri Torrey, 1902
Common name(s): Miller hydractinia, Hedgehog hydroid, Snail fur
Suborder Athecata (Anthomedusae)
|Hydractinia milleri forming a "house" on the subtidal hermit crab Pagurus dalli. Total diameter of "house" is 2 cm. Specimen was captured at 20 m at Mukilteo.|
|(Photo by: Dave Cowles, Dec 2007)|
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. H. laevispina and an unnamed local Hydractinia species have 8 tentacles. H. laevispina has pink gastrozooids and many short, curved spines on the perisarc while the unnamed Hydractinia has few, long, uncurved spines in the mat and the gastrozooids are white except near the mouth. H. aggregata has 20-24 tentacles on the gastrozooids and females carry several eggs. Several of the Hydractinia species can be found growing on hermit crab shells.
Note: Some gastrozooid polyps of this colony seem to have only 8 tentacles, which would make it H. laevispina. Others have 12 or more tentacles, however. It could perhaps be a hybrid between H. laevispina and H milleri.
Geographical Range: Vancouver Island, Canada to Monterey Bay, CA
Depth Range: Low intertidal and subtidal.
Habitat: Sides and undersides of rocks and on hermit crab shells
Biology/Natural History:Hydractinia species are often found on the shells of hermit crabs, though I can find no report of Hydractinia living on this species of hermit crab (Pagurus dalli), which usually inhabits sponges. Colonies if this species bear only one sex of gonozooids--the above colony is bearing females. The gonozooids produce medusoids which are little more than gonads and are not released from the colony as free-living medusae. In related Hydractinia species, release of eggs and sperm is light-dependent and occurs in the morning. Newly settled invidividuals of H. milleri have found in August in Monterey Bay. The hermit crab seems to frequently rub the colony with the flagellum of its second antennae. In hermit crab symbioses with related species of Hydractinia, this action has been shown to result in the hermit crab's scraping off some of the larger plankton captured on the Hydractinia and provides a supplementary food source for the hermit crab. Thy hydroid consumes a variety of small planktonic species such as crustacean larvae, nematodes, and even small benthic animals. Predators of Hydractinia include nudibranchs such as Dendronotus and Cuthona spp. If two colonies of Hydractinia occur on the same shell they seem to remain 1-2 mm apart from one another.The presence of this hydroid on a hermit crab seems to at least partially deter predation by octopus. Octopus usually readily capture hermit crabs and other crustaceans. However, an octopus clearly thinks twice about attacking a hermit crab with Hydractinia on its shell. Click here for a movie showing how octopus deal with Hydractinia-covered hermit crabs.
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American Fisheries Society, 2002
Morris et al., 1980
Ricketts et al., 1985
Cartwright, Paulyn, 2003. Developmental insights into the origin of complex colonial hydrozoans. Integrative and Comparative Biology 43: pp 82-86
General Notes and
abundances, unusual behaviors:
I do not often see hermit crab houses made of Hydractinia, as in this case, and I have never found a Pagurus dalli hermit crab with Hydractinia. On this individual the greatest density of polyps is near the edges of the "house" close to the hermit crab, while most of the "house" distant from the hermit crab is composed of the stolon mat with a few scattered polyps. The most common hermit crab species that I have found in this area to have Hydractinia "houses" are deeper-living Labidochirus splendescens.
My thanks to Kirt Onthank for his sharp-eyed viewing during a dive which resulted in finding this symbiotic pair.
This closeup shows details of the colony morphology. The matlike meshwork of stolons covered with perisarc can be seen here and there between the polyps (the stippled color at the bottom left is the carapace of the hermit crab host that the hydroids are forming a "house" for). Individual polyps, not covered by perisarc, arise from the mat. The feeding gastrozooids are pink and have around 8 tentacles (some seem to have more). The female reproductive gonozooids (called gonophores when there are carrying eggs) carry a single egg which mostly fills the gonozooid.
This closeup of the edge of the colony shows the gastrozooids. A few tiny stalks of a red alga seem to have begun growth within the colony.