Saturday, 23 January 2021

SEA ANEMONES: CNIDARIA

These colourful beauties are sea anemones. They are familiar inhabitants of rocky shores and coral reefs around the world — though many of their species can be found at very low depths in our oceans. They are one of the wonderful examples of the diversity that radiated out of the Cambrian Explosion.

At first glance, they look like beautiful and delicate marine flowers. If you've discovered them in tidepools, you'll know that they retract or pull into themselves with the lightest touch. These would-be flowers are predatory marine animals of the order Actiniaria that have graced our oceans for over half a billion years. 

They are named after anemones — Anemonastrum, a genus of flowering plants in the family Ranunculaceae — because of their colourful flower-like appearance. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Hexacorallia. 

As cnidarians, sea anemones are related to corals, jellyfish, tube-dwelling anemones, and Hydra. Jellyfish have a complex life cycle which includes both sexual and asexual phases, with the medusa being the sexual stage in most instances. 

A typical sea anemone is a single polyp attached to a hard surface by its base, but some species live in soft sediment and a few float near the surface of the water. The polyp has a columnar trunk topped by an oral disc with a ring of tentacles and a central mouth. 

The tentacles can be retracted or pulled back inside the body cavity or stretched out and expanded to catch passing prey. They are armed with cnidocytes or stinging cells. In many species, additional nourishment comes from a symbiotic relationship with single-celled dinoflagellates, zooxanthellae or with green algae, zoochlorellae, that live within the cells. Some species of sea anemone live in association with hermit crabs, small fish or other animals to their mutual benefit.

Most actinarians are sessile; that is, they live attached to rocks or other substrates and do not move, or move only very slowly by contractions of the pedal disk. A number of anemones burrow into sand, and a few can even swim short distances, by bending the column back and forth or by "flapping" their tentacles. In all, there are about 1000 species of sea anemone in the world's oceans.

Sea anemones breed by liberating sperm and eggs through their mouth into the sea. The fertilized eggs develop into planula larvae which, after being planktonic for a while, settle on the seabed and develop directly into juvenile polyps. Sea anemones can also breed asexually, by breaking in half or into smaller pieces which regenerate into polyps.

They are sometimes kept in reef aquariums; the global trade in marine ornamentals is expanding and threatens sea anemone populations in some localities, as the trade depends on collection from the wild. 

Most Actiniaria do not form hard parts that can be recognized as fossils, but a few fossils of sea anemones have been found. The bag-like — almost sea cucumber-like — Mackenzia, from the Middle Cambrian, Stephen Formation in the Canadian Rockies of British Columbia and Alberta, is the oldest fossil identified as a sea anemone. These ancient sea anemones attached themselves to hard surfaces, such as brachiopod shells. 

Mackenzia costalis, Walcott 1911
Fourteen specimens of Mackenzia costalis are known from the Greater Phyllopod bed, where they make up less than <0.1% of the fossil community. Mackenzia was originally described by Charles Walcott in 1911 — but as a holothurian echinoderm, which was a reasonable assumption at the time. Once additional specimens had been found and studied, Mackenzia costalis was reclassified as a cnidarian and the great grandparent of our modern sea anemones.

Some fossil sea anemones have also been found from the Lower Cambrian of China. The new find lends support to genetic data that suggests anthozoans — anemones, corals, octocorals and their kin — were one the first Cnidarian groups to diversify.

Photo: Charles Doolittle Walcott - Charles D. Walcott: Middle Cambrian Holothurians and Medusae. Smithsonian Miscellaneous Collections Volume 57, Number 3 (Publication 2011). City of Washington. Published by the Smithsonian Institution. June 13, 1911. 

References:  

Caron, Jean-Bernard; Jackson, Donald A. (October 2006). "Taphonomy of the Greater Phyllopod Bed community, Burgess Shale". PALAIOS. 21 (5): 451–65. doi:10.2110/palo.2003.P05-070R. JSTOR 20173022.

 Durham, J. W. (1974). "Systematic Position of Eldonia ludwigi Walcott". Journal of Paleontology. 48 (4): 750–755. JSTOR 1303225.

Conway Morris, S. (1993). "Ediacaran-like fossils in Cambrian Burgess Shale–type faunas of North America". Palaeontology. 36 (31–0239): 593–635.