Hydractinia laevispina Fraser 1922

Grows on snail shells inhabited by hermit crabs.

Geographical Range: In portions of the the Pacific and Arctic oceans.

Hydractinia, an athecatehydroid, grows as an interconnected mat of stolons 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. Most of the gastrozooids of this species have about 8 tentacles, though some have more. Mat of stolons has many short spines which are smooth and slightly curved. Gastrozooids are pink and up to 2.5 mm tall. Usually has 4 gonophores in a single whorl. Bears one egg per gonophore.

How to Distinguish from Similar Species: All Hydractinia species grow in a similar mat of stolons covered with perisarc, with naked, 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.

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 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 have gonophores 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