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.

Chemical Defense Mechanisms

Toxins

Many corals posess toxic defense mechanisms.

• Toxicity was naturally selected for
  • Corals that were often preyed upon by fish now have higher toxicity levels, for their own protection
  • The low nutritional value of some corals made them less susceptible to predation, so the lower the nutritional value, the lower the toxicity level

• • Relatively harmless to humans
    • Exception: Fire coral – contain a cocktail of toxins than can cause pain, inflammatory effects
  • Most toxins are neurotoxins
    • Interfere with signal transmission in animals’ nervous systems
    • Three main types of toxins, all neurotoxic in origin
      • Saxitoxin – causes paralysis and respiratory failure
        • Produced by a dinoflagellate
        • Sodium channels are blocked, which quickly prevents sodium ions from channels where they need to be
      • Palytoxin - causes kidney, respiratory and heart failure
        • Produced by a dinoflagellate
        • Acts on the sodium/potassium antiporters that control cell membrane activity essential for proper functioning of the kidneys and red blood cells
        • So potent that a dose large enough to kill an adult human is invisible to the human eye
      • Lophototoxin – causes muscle contractions, possibly paralysis and respiratory failure
        • Blocks the connections, or synapses, where nerves connect with muscles
        • Disruption of these receptors causes muscle contractions

• Symbiotic Relationships
  • A close relationship between two species
  • Corals either have their own toxins, or live in symbiosis with bacteria and protists that produce toxins
    • The symbiotic relationships help protect coral that cannot produce the toxins themselves
    • Coral probiotic hypothesis
  • Some corals maintain symbiotic relationships with small animals
    • Trapeziid crabs and stony coral
      • Reefs worldwide are experiencing increasing sedimentation, or larger amounts of sediment deposited on coral reefs
      • The sediment 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.

===Nematocytes

• • Stinging cells on the coral's tentacles used to capture small prey and kill off neighboring corals in a continuous battle for space
  • Double-walled structures each containing a coiled, venomous thread with a barb at the end
  • Tiny sensors on the outside of each nematocyte, when stimulated physically or chemically, will release the threat, penetrate its victim's skin and release poison
  • Most corals possess these in addition to everything else

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. 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. The most deadly cnidocytes (to humans, at least) are found on the body of a box jellyfish. Aggregating sea anemones may have the lowest sting intensity, 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.