Coral Biology Connections

Ko’a, or stony corals, are ancient animals in the family Cnidarians that play a critical role in the coastal ecosystem. Corals are usually colonial organisms, meaning each coral is made up of many individual polyps connected by living tissue. Polyps are soft bodied organisms with a cup-like shape and a ring of tentacles around the central opening called the pharynx which acts as mouth and anus. Polyps sit on top of an external skeleton, called an exoskeleton, which is made up of calcium carbonate secretions from the polyp. This hard exoskeleton, sort of like a house made of concrete, builds up over millions of years to create the large structures that make up the coral reef. In fact, coral reefs are the oldest and largest living structures on the planet. Corals, like jellyfish and sea anemones, are cnidarians characterized by stinging cells, called nematocytes, which contain harpoon organelles, called nematocysts, which are triggered to release the stinging harpoon on touch. Nematocysts are used both to defend corals from predators and to ensnare prey. Many corals live in a symbiotic relationship with Zooxanthellae. Zooxanthellae are single-celled algae which live inside the corals where they use the sun’s energy to convert carbon dioxide into energy rich sugars and fats to feed the coral, produce oxygen for the coral, and provide the coral with its color. In return, zooxanthellae have a safe place to live and can feed on waste nutrients from the coral. There are two main types of corals–hard corals (stony corals, or scleractinians) and soft corals (gorgonians or octocorals)–which have very different skeletal structures. Hard corals, also known as reef-building corals, have rigid calcium carbonate skeletons whereas soft corals have flexible skeletons made of a protein called gorgonin and floating spicules of calcium carbonate.

Hawaii has very unique coral reefs. A quarter of the coral species are endemic to (only found in) Hawaii and the most commonly found stony corals are the Cauliflower Coral (Pocillopora meandrina), Lace Coral (Pocillopora damicornis), Rice Coral (Montipora capitata), Antler Coral (Pocillopora eydouxi), Lobe Coral (Porites lobata), Finger Coral (Porites compressa), Mushroom Coral (Fungia scutaria), and Cup Coral (Tubastraea coccinea). The coral reefs around Hawaii are vast, spanning 410, 000 acres, and are biologically, economically, and culturally critical. Coral reefs provide critical habitat for many other animals, including juvenile fish, invertebrates, and turtles. In fact, coral reefs are home to 25% of the fish in the sea, with the Hawaiian reefs home to over 5,000 different species. Given the amount of animals that depend on them, reefs are extremely important for providing food through the fishing industry, to the tourism industry, and for recreational sports like scuba diving, surfing, and snorkeling. Reefs also provide a critical coastal defense, dissipating wave energy offshore to prevent coastal erosion, and in Hawaii, the coral reefs are the only source of sand for beaches. Remarkably, the materials found in coral reefs are also used in healthcare, medicine, toiletries, food additives, and for cement building. For example, soft corals have anticancer and antiviral properties which make them useful as medications.

Below are the 8 most common types of coral in Hawaii. Some are easily distinguishable, such as Mushroom Coral, Lobe Coral, and Cup Coral. Others are a bit harder to tell apart, such as Lace Coral and Rice Coral. Make sure to look closely, as a matching quiz is coming up!

Threats to corals:

Corals around the world are increasingly threatened by human activities. Greenhouse gas emissions from humans are causing climate change and global average ocean temperatures to rise. The rising temperatures are also causing oceans to become more acidic. High ocean temperatures and acidity cause stress to corals, forcing them to eject their symbiotic food producers–the zooxanthellae–and slowly starve. This is called coral bleaching. In a stressed state, corals are also more susceptible to coral diseases such as Black-band disease. Human induced pollution, toxic sunscreen, excess sedimentation, and nutrient runoff also threaten corals and increase corals susceptibility to disease. Another threat to corals are invasive species which are disturbing the natural ecosystem on the reef. For example, the Roi is an invasive fish from French Polynesia which eats many herbivorous fish (fish that eat algae or limu). Without these herbivore fish on the reef, seaweed can grow out of control and smother coral so that it cannot survive. 

If we lose our coral reefs, we will lose the animals that depend on them, the shoreline protection they provide, the money they bring in terms of fishing, tourism, and outdoor activities (including surfing), the resources for medicines they provide, as well as the incredible animals themselves. Individuals can do their part to protect corals by limiting their carbon footprint, using reef safe sunscreen, disposing of waste properly, and snorkeling in a responsible manner.