Coral Reefs: The Elkhorn Coral (Acropora palmata)

Coral Reefs are actually communities made up of thousands of coral polyps, tiny animals that grow in sunlit shallow waters, they build up over time as corals attach themselves on top of the skeleton of dead animals.

Great Barrier Reef of Australia

The basis of the Coral Reefs are Corals (Anthazoa) which are marine invertebrates that typically live in colonies encompassing of many individual other polyp (called a Coral "Head"). A type of reef-building coral is the Elkhorn Coral(Acropora palmata).

Elkhorn Coral 

Current Conservation Status: Critically Endangered

Estimate population size: unknown(according to NOAA they know that it's declining since 1980 estimated 90-95% population of Elkhorn has been lost!)

Habitat: The Elkhorn corals usually live in shallow waters (3ft-16 ft deep) throughout the Florida reef tract and the Caribbeans. Designated areas that Elkhorn corals are known to especially inhabit are Puerto Rico, Florida, St.John/St.Thomas, and St.Croix. Once found they are in continuous aggregated thickets that extend mostly to the front side of the coral reefs (since 2008 scientists have only seen few, isolated colonies supporting few species which is what caused them to classify it as endangered).

Approximate distributions of Elkhorn Corals

Basic Structure:

Basic Structure of Coral

Actual stinging cells of coral

 Corals are usually only a few centimeters in length, they have a set of tentacles that surround their  mouth opening (huge) and open to their stomach cavity. They have an exoskeleton that excretes near the base ("underneath" the coral) and lacks many structures that other animals need for survival; such as intestines, anus, a brain, a heart, blood supply, nervous system, and eyes. Without intestines/an anus the waste thus is regurgitated from the mouth(or pushed back out). Within the epidermic layer of the tentacles there are special "stinging" cells that protect the coral and can be used to catch prey. Each "stinging" cell fires a poisonous barb that's attached to a thread which remains connected to the cell as it "ties" or entangles its prey. After being completely tangled, the prey is dragged in through the central mouth opening and into the stomach cavity where it is digested.

How the Coral captures its prey

     

Mutualistic relationship between algae and the coral

Eating Habits: 
The reef-building coral usually forms a mutualistic relationship with the zooxanthellae (which are tiny algae).  The zooxanthellae (living within the corals) make glucose through photosynthesis which provides much needed energy to the corals. In turn, the corals provide the zooxanthellae carbon dioxide and a home(which protects them). Corals also feed by utilizing their "stinging" cells which completely stretch out at night (each "stinging" cell fires a poisonous barb that's attached to a thread which remains connected to the cell as it "ties" or entangles its prey. After being completely tangled, the prey is dragged in through the central mouth opening and into the stomach cavity where it is digested).

Coral eating a jellyfish

Reproductive Behavior:

Close up photo of spawning(sexual reproduction)

Coral can reproduce both sexually and asexually but Elkhorn corals reproduce asexually primarily. In  asexual reproduction , new corals(polyps) split of from "parent" corals when the parent corals reach a certain size and divide or they can reproduce by fragmentation (when a piece or portion of the colony is separated/detached and happens to fall in suitable conditions). This process occurs throughout its lifetime forming what you would call an "ever-expanding colony." While in sexual reproduction, corals can also reproduce in different ways particularly by spawning in which they release massive amounts of gametes(egg and sperm) which then fuse out in the open(in the water to be precise) and lead to the production of a colony. Different species of  polyps reproduce in different ways (some are hermaphrodites able to produce both female and male gametes while a few can only produce egg or sperm; while a few reproduce by budding).

Overall, the coral doesn't have an average rate of reproduction since it occurs whenever conditions are suitable and the "number" produced depends on the type of reproduction/conditions.

Major Predators:

The elkhorn coral retracts its "body" to hide from predators which include many species of damselfish (suck coral out of its body), fireworms, and corraliophilid snails (graze on coral colonies). These major predators though aren't really contributing to its "Critically Endangered" title it's the global climate change which is causing diseases like white band disease and white pox disease and bleaching that's killing all the Elkhorn corals.

Corraliophilid snails preying(by grazing) on Elkhorn Corals

Major Threats:

According to NOAA since 1980, it's been diseases (such as white band disease, black band disease, and white pox disease;caused by global climate change or global warming), hurricanes, predation, bleaching, overgrowing of algae (algal blooms as a result of water pollution), sedimentation, temperature/salinity variations of water, and the low genetic diversity (as a result of many extinctions). Fragmentation allows rapid recovery of the Elkhorn coral population from physical disturbances like hurricanes but makes it more susceptible to bleaching and diseases since they are all genetically identical.

White band disease in Elkhorn Corals

White pox disease which only affects Elkhorn Corals caused by bacterium
(cause elkhorn to lose up to 0.9 sq.in of tissue a day)