DefenseMechanisms

Defense Mechanisms

The Importance of Defense Mechanisms

Corals are sessile, colonial organisms, forever fixed in a certain position by attaching as young polyps to a substrate such as a rock or existing coral. This makes the ocean a very dangerous place for these immobile animals. However, in an effort to combat their immobility, many corals have developed different types of defense mechanisms to protect themselves from the ocean's looming dangers--leading to the production of some of the most lethal toxins found in nature. In this sense, chemical defense is vital to the life of the coral, whose life depends upon its ability to protect itself from predators and invasive species. In fact, some 73% of all coral is toxic to fish, and most of the other species of coral maintain some physical type of defense mechanism to protect against fish predation.

Chemical Defense Mechanisms

Toxins

Many corals posess toxic defense mechanisms for protection. Toxicity levels of different corals was naturally selected for, which has resulted in a direct relationship between how toxic a particular type of coral is and how much nutrition it provides to those who prey upon it. For instance, fish began preying more often upon those corals that had more nutritional benefits, so those corals had to increase their levels of toxicity in order to protect themselves and survive. Corals that had lower nutritional values were less susceptible to predation, so they did not adapt and increase their toxicity levels the way other types had to.

Fire Coral

Blade Fire Cora l

Despite their toxic defense mechanisms, most corals are relatively harmless to humans, with one exception: fire coral. The particular type of proteinaceous toxin in fire coral affects humans, but only mildly — most reactions just involve stinging pain and inflammatory effects, and the most severe, but rare, side effect is nausea or vomiting. The most common toxins are neurotoxins, and there are three main types. Saxitoxins block sodium channels in the body of species it comes in contact with, causing paralysis and respiratory failure. Palytoxins act on the antiporters that control cell membrane activity, thus disrupting the proper functioning of kidneys and red blood cells and leading to kidney, respiratory and heart failure. The third common type of toxin, the lophototoxin, causes muscle contractions and potential paralysis or respiratory failure by blocking the synapses where nerves connect with muscles.

Symbiotic Relationships

Symbiotic relationships, or close relationships between two species that benefits one or both organisms involved, also greatly benefit corals that are not able to produce toxins on their own. The Coral Probiotic hypothesis, for example, posits that a dynamic relationship exists between coral and the large array of bacteria on their surface, so that when environmental conditions change in the oceans, coral can change which microbial partners they are currently maintaining a relationship with in order to more quickly adapt to those changing conditions. Symbiotic relationships are also maintained between some types of coral and small organisms. The trapeziid crab-stony coral relationship is an example of a small organism that lives on a coral reef and benefits the reef by removing sediment that falls on the reef. Sedimentation on reefs has been steadily increasing worldwide, but it is detrimental in that it inhibits growth of the coral and accelerates tissue bleaching. In a field experiment conducted at UC-Santa Barbara, all corals outplanted with crabs survived, while 45-80 percent of those outplanted without crabs died within a month.

In a cross between a chemical and physical defense mechanisms, most corals also have nematocytes for protection — stinging cells on the end of coral tentacles that are used to sting, capture and kill off small prey and neighboring corals in a continuous battle for space. The nematocytes look like double-walled structures that each contain a coiled, venomous thread with a barb at the end, so that when the nematocyte is stimulated either physically or chemically, the thread releases, penetrates its victim's skin and releases poison.

Physical Defense Mechanisms

Cnidocyte

A cnidocyte is an explosive cell containing one giant secretory organelle or cnida (plural cnidae) that defines the phylum Cnidaria (corals, sea anemones, hydrae, jellyfish, etc.). Cnidae are used for prey capture and defense from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on fish and crustaceans. A cnidocyte fires a structure that contains the toxin, from a characteristic sub-cellular organelle called a nematocyst. This is responsible for the stings delivered by jellyfish.

Nematocysts

Nematocysts are very efficient weapons. An undischarged nematocyst contains a nematocyte. These small, sub-cellular organelles discharge by firing a barb into a potential victim, leaving a hollow fillament through which poisons are injected to immobilize prey. A single nematocyst has been shown to suffice in paralyzing a small arthropod .

Aggregating sea anemones may have the lowest sting intensity to humans, perhaps due to the inability of the nematocysts to penetrate the skin, providing only a feeling of that similar to touching sticky candies to human fingers. Besides feeding and defense, sea anemone and coral colonies use nematocytes to sting one another in order to defend or win space.

Notes

comments

good job!

we like the specificity of your topic
well organized
citations could be greatly improved - this is the formula  
pictures would be good for visualizing defense
also maybe a section on threats or what corals would be defending themselves from?