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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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© Rosario Beach Marine Laboratory

Source: Invertebrates of the Salish Sea

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