Hydractiniarian

Hydractiniarian is a common colonial hydroid found on the shells of marine snails. These small, sessile invertebrates belong to the order Hydractiniida. A hydractiniarian colony is composed of genetically identical zooids, which are specialized individuals performing different functions within the colony. Typical zooids include gastrozooids for feeding, gonozooids for reproduction, and dactylozooids for defense. The colony typically grows on gastropod shells, often those that have been abandoned by their original snail inhabitant. This specific arrangement provides both a substrate for attachment and some protection for the hydroid. In some cases, hydractiniarians can also be found on the submerged portions of buoys, piers, and even the hulls of ships. A notable characteristic of many hydractiniarian species is their association with hermit crabs. The hydroid colony can grow on the hermit crab's shell, providing camouflage and potentially deterring predators of the crab. In return, the hermit crab's movement disperses the hydroid colony to new locations. The feeding process involves the gastrozooids extending tentacles armed with stinging cells called nematocysts to capture small planktonic organisms. Reproduction can occur both asexually through budding and sexually, with gonozooids releasing gametes into the water column.