Species Spotlight: Coral

Forming the backbone of the reefs all around the world are the rainbow-coloured, plant-like creatures known as coral. Coming in all shapes and sizes, with every imaginable colour in between, corals are essential to the continued existence of any reef, and these remarkable creatures, closely related to jellyfish and anemones, start their lives in an equally remarkable way.

In this video (BlueWorldTV, 2013), underwater photographer Jonathan Bird manages to record the systematic release of coral reproductive material at night. With military precision, huge batches of eggs and sperm are released all at once into the seawater. These massed releases are a part of a survival strategy, where safety in numbers increases the chances of one of the gamete bundles, as Bird calls them, will progenate the next generation of coral.

Once fertilised, the gamete bundles settle on the seafloor. Now called polyps, the individual cells form colonies, using limestone to create skeletons to support themselves or as a means of defense by shaping them into sharpened spines known as spicules. Colonies grow when their constituent polyps undergo cell division, and colonies that meet merge to form even larger colonies. Each polyp, though sometimes possessing its own brightly coloured tissue, can also take on colours from algae (known as zooxanthellae) that they host within their bodies. These algae provide nutrients for the polyp, although there are also coral species that feed on plankton and even small fish.

Spicules of various species of coral, photographed by scientists under a scanning electron microscope. (Wikipedia, 2012) (Click to Enlarge)
Spicules of various species of coral, photographed by scientists under a scanning electron microscope. (http://en.wikipedia.org/wiki/Sponge_spicule#mediaviewer/File:Demospongiae_spicule_diversity.png)

Coral are basically split into two distinct types: soft coral and hard coral. Soft corals are the fashioniastas of the reef, with vibrant colours of pinks and oranges. Besides their spicules, soft coral lack a proper skeleton and are thus gelatinous, swaying in the ocean currents while their polyps wave their tiny, feathery tentacles in the water. Given their soft texture, soft corals are easy targets for predators, but they will not go down without a fight. Certain species of soft corals secrete foul-tasting chemicals or poisons while others resort to spiny spicules as defense mechanisms.

Red Sea Fingers coral, Alcyonium glomeratum. (Kenmare Bay Diving, 2010) (Click to Enlarge)
Red Sea Fingers coral, Alcyonium glomeratum. (http://kenmarebaydiving.wordpress.com/2010/08/17/night-dives-bbqs-and-tompot-blenny-sightings/)

Hard coral, on the other hand, are slightly different. These types of coral create hardened limestone skeletons for themselves, eventually forming colonies of millions of individual polyps. Unlike their soft coral cousins, which can double or even triple the size of their colonies annually, hard coral generally manage a modest one to one-and-a-half centimetres of growth per year. They are the true building blocks of the reef and, given time, can reach truly massive sizes.

Photographer Dano Pendygrasse photographs healthyreefs.org's Ian Drysdale inspecting a bed of Staghorn hard corals. (Pendygrasse, n.d.) (Click to Enlarge)
Photographer Dano Pendygrasse photographs healthyreefs.org’s Ian Drysdale inspecting a bed of Staghorn hard corals. (http://www.danopendygrasse.com/Details/details/blog_files/tag-scuba-diving.php)

Both types of coral are fundamental to a substantial portion of the world’s marine biodiversity. Although covering less than one percent of the world’s seafloor, coral reefs support around one quarter of all the world’s marine species.