The Liquid Earth

Coral and Zooxanthallae

Coral (in the class Antrozoa) is a vital part of many aquatic ecosystems, with it’s ability to form large calcium carbonate structures over time which act as a habitat for many different organisms, it plays an important role in it’s ecosystem. A coral reef will consist of many small polyps which surround themselves in a calcium carbonate skeleton in order to protect themselves from predators which will visit the reef. The polyps typically emerge overnight to feed.

Coral polyps feed by catching zooplankton and other microscopic organisms with their tentacles and directing them towards their mouth parts where the zooplankton will be digested in the stomach. They can occasionally also eat slightly larger prey by incapacitating or killing it with their nematocysts before it enters the mouth part. This however is only able to provide around 50% of each coral polyp’s daily requirements the rest it must source from elsewhere.

To ensure their full daily requirements are met coral polyps form a symbiotic relationship with called a group of single celled protozoa called zooxanthallae. The polyp provides the zooxanthallae with a suitable habitat within its soft tissues where it is protected from harsh abiotic factors and filter feeding organisms that would eat the zooxanthallae and the coral itself benefits from oxygen and nutrients expelled by the zooxanthallae.

What is Bleaching?

Bleaching a the process by which coral polyps remove the zooxanthallae when abiotic factors around the polyps are no longer capable of maintaining the populations of zooxanthalae causing the populations to decrease. In response to this the coral polyps expel the zooxanthallae from their soft tissues and into the water column. This is known as bleaching because the individual coral polyps appear white in colour after this as pigments present in their soft tissues where specific to the zooxanthallae and therefore left with the zooxanthallae. The loss of zooxanthallae is detrimental to the health of the coral as it now lacks the means to produce enough nutrients to sustain itself; after a relatively short period of time if the environmental stresses which lead to the expulsion of zooxanthallae do not cease the coral will die.

Why Does Bleaching Occur?

The reason why coral undergoes bleaching when subjected to environmental stresses is not yet discovered, however it was suggested to be a defence mechanism by R. Buddemeier and D. Fautin in 1993 who hypothesised that when coral undergoes bleaching it will enter a state of reduced activity and wait for a new species of zooxanthallae to arrive which is better adapted to live in the new conditions. When the new species of zooxanthallae is found it will be introduced to the soft tissue of the individual polyps and they will continue to function as usual, the benfit of this system to the coral is in future it will be more resistant if conditions were to change further.

Zooxanthallae visible on coral polyps

Picture of a manta ray showing the big triangular pectoral fins credit: Shiyam ElkCloner

The manta ray (Manta birostris) is one of the largest rays to belong to the Myliobatidae family, growing up to 7 metres in length. Manta rays are classed as Elasmobranchii, along with sharks, but they don’t share the same family as manta rays are part of the Myliobatidae family.

The manta ray is much larger in comparison to most other rays as they have two large triangular pectoral fins allowing the manta ray to move through water by the wing like movements from the pectoral fins, causing water to be driven backwards (Deakos, Mark R. 2010). Manta rays also have two cephalic fins located on the head of the ray in front of both eyes, the cephalic fins are horn shaped in nature and can be rolled up, when the ray is swimming, and can be rolled down, when the ray is searching for food (http://www.mantaray-world.com/manta-ray-anatomy/).

Despite the giant size of the manta ray, the primary source of food for the ray is tiny planktonic organisms. The manta ray is able to feed off tiny planktonic organisms due to the ray constantly filtering water through its mouth. The planktonic organisms are then filtered from the water passing into the mouth using gill rakers, sponge like tissue around the gills capable of filtering water passing through the gills. Whilst the manta ray is searching for food the cephalic fins unravel and are used to direct more water into the mouth whilst searching for food. Manta rays often swim along a sea bed whilst searching for food often scooping up plankton from along the sea floor. Manta rays only have a handful of predators including large sharks and orca (http://www.mantatrust.org/) (http://www.elasmo-research.org/index.html).

The mating process in manta rays is initiated through the formation of a “mating train”; several male manta rays following a single female competing for the right to mate, this courtship can last several days. One of the male manta rays then grabs the tip of the female’s pectoral fin with its teeth and the pair mate belly to belly. Fertilized eggs then remain in the female rays for up to 12 months. The eggs then hatch internally so the female ray ends up giving birth to live young. Generally the manta ray only gives birth to one or 2 pups at a time. These new born manta rays are an impressive 1.2-1.5 metres across. The manta ray doesn’t reproduce quickly with births taking place only once every 2-5 years. (http://www.flmnh.ufl.edu/fish/Gallery/Descript/MantaRay/MantaRay.html) (http://www.mantatrust.org/) (http://www.elasmo-research.org/index.html).

Manta rays are generally found in tropical, sub-tropical and temperate waters in the major oceans of the world and has been sighted as far north as new jersey and as far south as South Africa. Generally the manta ray is found between the latitudes of 31 degrees North and 36 degrees South. The manta ray is believed to travel vast distances across the ocean and is a seasonal visitor to various coastlines and offshore sites, this shows the manta ray to be more of an oceanic species than other rays such as the reef manta ray. (http://www.iucnredlist.org/) (Marshall, A.D., Compagno, L.J.V. and Bennett, M.B. (2009) Redescription of the genus Manta with resurrection of Manta alfredi (Krefft, 1868) (Chondrichthyes; Myliobatoidei; Mobulidae). Zootaxa, 2301: 1-28.)