Overview

Comprehensive Description

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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Source: Invertebrates of the Salish Sea

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Hydractinia, an athecate hydroid, 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. The gastrozooids of this species are a uniform pink color and have a single whorl of about 12-20 tentacles near the oral end. The basal perisarc mat contains long, smooth spines. The female gonophores (gonozooids) contain only one egg. The polyps, when expanded, can extend as high as 5 mm tall though the polyps above are 1 mm or less.
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© Rosario Beach Marine Laboratory

Source: Invertebrates of the Salish Sea

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Distribution

Geographical Range: Vancouver Island, Canada to Monterey Bay, CA

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© Rosario Beach Marine Laboratory

Source: Invertebrates of the Salish Sea

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Physical Description

Look Alikes

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.
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© Rosario Beach Marine Laboratory

Source: Invertebrates of the Salish Sea

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Ecology

Habitat

Depth Range: Low intertidal and subtidal.

Habitat: Sides and undersides of rocks and on hermit crab shells

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© Rosario Beach Marine Laboratory

Source: Invertebrates of the Salish Sea

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