Coral: Plant or Animal?

Corals are actually not plants, they are animals that belong to phylum Cnidaria, the same phylum as jellyfish and sea anemones.

However, the living portion of coral is only a thin superficial layer, while the subsurface is a calcareous skeleton. It is what is on the inside of the coral that actually gives it its color, as it would otherwise just appear white.

Two Types of Corals

Hermatypic - 700 Species

These corals are the more well known corals, as these are the ones that build coral reefs. However, these are restricted to the tropics. We will be focusing on hermatypic corals, since these are the ones that build reefs.

Ahermatypic

These types of corals are distributed worldwide, and do not build coral reefs. These corals also do not contain zooxanthellae algae, a very important organism that we will get into shortly...

Coral Feeding

Corals, even though they receive most of their nutrition from Zooxanthellae, are also suspension feeders, and receive ~10% of their nutrition this way. Corals use specialized cells on their tentacles to capture planktonic prey. On the coral's tentacles are cnidocytes, specialized cells that contain the stinging nematocyst. When a small organism, like plankton, brushes against the "trigger" of these cnidocytes, the nematocyst is discharged, paralyzing the prey.

Most corals feed at night, as there are more zooplankton active at this time. It also reduces predation on the tentacles, UV exposure, and self-shading.

Coral reefs are typically found in shallow water (0'-90') as this guarantees the most possible sunlight for fastest growth.

Coral reefs prefer temperatures between 23-25°C (73-77°F). However, they can grow between 16-36°C (61-97°F).

Coral reefs have been reduced or even absent from the west coasts of continents due to upwelling of cold water.

Coral reefs thrive in salinity (salt) levels between 25-40 ppt (parts per thousand).

Corals are not present near river deltas because of freshwater, nutrients, and poor visibility.

In South Florida (Monroe, Dade, Broward, and Palm each counties) alone, coral reefs provide $7.6 billion in annual revenue, as well as providing more than 70,000 jobs.

Commercial Fishing

Fishing methods and gear may cause severe damage to coral reefs, as well as surrounding life. Activities such as trawling, the act of pulling large fishing nets behind boats, may greatly damage reefs and the substrate while being dragged across the ocean floor. This may also decrease the biodiversity of these reefs as bycatch becomes a huge issue with this method of fishing.

Overfishing as well can alter the food web structure, depleting the amount of fish who may clean the coral of algal overgrowth. Without these fish, algae can overtake the corals, causing them to die off.

Fishing gear, such as fishing line, may be improperly discarded into the ocean where it will remain for 500 years. This line may get caught around corals, damaging them and hindering their growth.

Divers

While scuba diving is a popular way to enjoy the beauty coral reefs have to offer, some divers who lack improper training or are careless may damage and even kill corals on their dives. By kicking corals with fins, these corals may have a hard time recovering from the damage. Touching or taking the corals can also leave lasting impacts, as removing corals from their natural environment may cause some species to be over-harvested, destroying the reef habitat and decreasing biodiversity.

Toxic Run-off

Human-made toxins, such as pesticides, herbicides, sunscreens, urban/agriculture runoff, mining activities, and landfill runoff all contain harmful materials that end up in our oceans and on our coral reefs. Metals, such as lead and mercury, and organic chemicals, such as polychlorobiphenyls (PCBs), oxybenzone, and dioxin, affect coral reproduction, growth, feeding, and defensive responses.

Many herbicides used in agriculture or in everyday use contain harmful chemicals that affect the symbiotic algae in the corals. When the coral becomes stressed, the algae leaves the corals behind, resulting in the coral to turn white, as it has lost a main food source, a process known as coral bleaching. This can destruct and kill entire coral reefs.

Ocean Acidification

Ocean acidification refers to the change in ocean chemistry as a response to the uptake of carbon dioxide by the atmosphere.

The ocean and the atmosphere are at equilibrium, so when atmospheric concentrations increase, so do oceanic concentrations. When carbon dioxide enters the seawater, carbonic acid is produced, increasing the acidity of the ocean. This acidity increase actually reduces the availability of salts and ions needed by corals to create a calcium carbonate structure. In turn, this causes coral growth to slow or even stop, potentially even causing some corals to dissolve altogether. This phenomenon can also be caused by increased nutrient runoff that also causes oceans acidity levels to rise.

Plastic Pollution

Trash, specifically plastics, may make their way into oceans from improper disposal methods and snag onto coral reefs. This may cause blockage of the sunlight needed by Zooxanthellae to complete photosynthesis, or entanglement that can break off or damage corals. Degraded plastics may be consumed by corals or other marine organisms, blocking their digestive track or further introducing toxins to their bodies.

Global Warming

As the world, including the oceans, continue to increase in temperature from factors such as increased greenhouse gases and the polar ice caps melting (are unable to reflect sunlight back into space, so heat is absorbed by ocean), the corals are really taking a toll. Coral bleaching, caused by toxic run-off, as stated before, is more severely impacted by ocean warming than anything else.

As the water increases in temperature, this adds major stress onto the corals and the Zooxanthellae, causing these dinoflagellates to expel from the coral. Since corals receive ~90% of their nutrition from the Zooxanthellae, once they leave, the corals are left to starve and then die, ending entire reef systems.