Actinozoa

Actinozoa, also known as Anthozoa, is a large class within the phylum Cnidaria. It encompasses exclusively polypoid marine animals, meaning they exist only in the polyp form, lacking the medusa stage found in other cnidarians like jellyfish. This class includes corals, sea anemones, sea pens, and sea fans, showcasing a wide array of forms and ecological roles within marine ecosystems.

Actinozoans are characterized by their flower-like appearance, a feature reflected in their name (from Greek anthos = flower, zoa = animals). Their bodies consist of a cylindrical column topped by an oral disc containing a mouth surrounded by tentacles. These tentacles are armed with stinging cells called nematocysts, which are used to capture prey.

Internally, the body cavity, or gastrovascular cavity, is partitioned by mesenteries (septa) that increase the surface area for digestion. These septa bear gonads in sexually mature individuals. Actinozoans exhibit both solitary and colonial lifestyles. Colonial forms, particularly reef-building corals, are vital for the formation of complex marine habitats.

Reproduction in Actinozoa occurs both sexually and asexually. Asexual reproduction can take place through budding, fission, or pedal laceration. Sexual reproduction involves the production of gametes, which are released into the water column for fertilization. The resulting larva, called a planula, settles and metamorphoses into a polyp.

Actinozoans are further divided into two subclasses: Alcyonaria (or Octocorallia) and Zoantharia (or Hexacorallia). Alcyonarians, such as soft corals, sea pens, and sea fans, have polyps with eight tentacles and eight mesenteries. Zoantharians, which include sea anemones, stony corals, and tube anemones, typically have polyps with tentacles and mesenteries arranged in multiples of six.

Actinozoans play crucial roles in marine ecosystems. Reef-building corals, for instance, create three-dimensional habitats that support a vast diversity of marine life. Sea anemones provide refuge for symbiotic organisms like clownfish. Many actinozoans are also important components of the food web, serving as prey for various marine animals. However, Actinozoans are increasingly threatened by factors such as ocean acidification, rising sea temperatures, and pollution, which can lead to coral bleaching and other detrimental effects.