Hydractiniida

Hydractiniida is an order of hydrozoans, a class of marine invertebrates belonging to the phylum Cnidaria. These colonial organisms are characterized by their polymorphic colonies, meaning individual polyps within a colony can have specialized structures and functions. A defining feature of Hydractiniida is the presence of a chitinous exoskeleton, or perisarc, that covers the stolons, which are the root-like structures connecting the individual polyps. This perisarc often encrusts hard substrates like shells, rocks, or even the backs of living animals, such as sea turtles and hermit crabs.

The colonies typically consist of feeding polyps (gastrozooids) and reproductive polyps (gonozooids). Some species also possess specialized defensive polyps called dactylozooids, which are equipped with stinging cells (nematocysts). Reproduction in Hydractiniida can occur both asexually through budding and sexually, with the release of gametes into the water column. The planula larvae that develop from fertilized eggs then settle and grow into new colonies. Hydractiniida are found in marine environments worldwide, from shallow coastal waters to the deep sea. Their ecological roles vary depending on the species, with some acting as epibionts, growing on and potentially influencing their hosts.