Actinidae Family of True Sea Anemones

Phylogeny: The True Sea Anemones or Actinids are members of the Actiniidae Family, the largest Family of Sea Anemones with three hundred and forty species of which twelve are found along Mexico’s Pacific Coast. Like Jellyfish, and Hydroids, Acroporid Corals belong to the Phylum Cnidaria. They are in the Class Hexacorallia and the Order Actiniaria.

Distribution: True Sea Anemones They are found worldwide, in tropical and temperate waters. They range in depth from the intertidal zone to depths exceeding 1,000 m (3,280 feet) generally found attached to hard substrate, though their attachment is not permanent or fixed in one location. They can crawl slowly, while remaining attached. Also, many species can detach from the substrate and drift or roll, to escape predation.

Morphology:  True Sea Anemones have radial symmetry, a hollow digestive cavity, and specialized stinging cells within the tentacles that surround their mouth. Their polyps have a flower-like appearance. These solitary hexacorals lack a calcareous skeleton. Their column has a base that is either rounded, or forms an adherent disc and may be smooth or covered with verrucae (warts). These verrucae may be adhesive, often with gravel attached, or non-adhesive. Adults have at least 36 tentacles. There may be one, or more than one, tentacle between mesenteries (sheets of tissue that divide the internal body cavity).  Anemones in this Order are often brightly colored. True Sea Anemones may reach up to 36 cm (14 inches) in diameter.

Ecosystems Role: True Sea Anemones are predators, using their stinging tentacles to capture crabs, fish, mollusks, sea cucumbers, and other invertebrates. The stinging cells paralyze the prey and the tentacles then pass the prey to the mouth. The stinging tentacles provide defense from some predators, and the Anemone can roll its tentacles and disc inward forming a stumpy blob that is difficult for predators to attack. Even with these defense mechanisms, True Sea Anemones are preyed upon fish, gastropods, nudibranchs,  sea turtles and starfish. True Sea Anemone species may also be zooxanthellate, indicative that they have a symbiotic relationship with single-cell dinoflagellates, the zooxanthellae. Zooxanthellae live within certain coral polyps, jellyfish, nudibranchs and sea anemones. The zooxanthellae produce energy, during daylight, by means of photosynthesis with the energy being passed along to their hosts, sometimes providing up to 90% of the host’s total energy needs. In return, the host provides nutrients, carbon dioxide, and a secure, sunlit, platform. With very few exceptions, True Sea Anemones do not form relationships with fish, such as Clownfish or Anemonefish, as other anemones do in the Indo-Pacific.

Reproduction:  True Sea Anemones reproduce sexually, or asexually, by budding or fission.