Hydractinia

Hydractinia is a genus of small colonial cnidarians in the family Hydractiniidae. Species occur in marine environments worldwide, often forming colonies on hard substrates such as shells inhabited by hermit crabs. The best-studied species is Hydractinia echinata, with others including Hydractinia symbiocarpia.

A Hydractinia colony is a connected network of hydrocauli and coenosarc that covers the surface of a shell. Among the exposed polyps are specialized types: gastrozooids, which feed with tentacles; dactylozooids, which defend the colony with stinging tentacles; and gonozooids, which bear the reproductive organs and release medusae or planulae. Growth occurs by budding from existing polyps, while sexual reproduction produces planula larvae that settle to form new colonies. Some species can also produce free-moving reproductive units.

Hydractinia has become a model organism in developmental and stem-cell biology. Its colonies contain abundant somatic stem cells that can differentiate into multiple cell types, enabling studies of regeneration and cell lineage. The species also exhibits allorecognition in which neighboring colonies can fuse or reject one another depending on genetic compatibility; this provides a simple system for investigating tissue compatibility and immune-like responses in invertebrates.

Habitat and distribution: found in shallow marine waters, notably on the shells of hermit crabs in temperate and tropical seas. The genus is not considered endangered, and laboratory colonies are maintained for research and education.