Life Cycle - Natural Environment (5A)
Pre Lab 

  • Reviewing systems of classification.
  • Exploring coral species. 
  • classification
  • corals
  • phylum
  • species
  • Internet

Students use the internet to research corals. 


Corals belong to the phylum that includes hydroids, jellyfish, and sea anemones. All Cnidaria  possess the following: radial symmetry, sac-like body with a central body cavity; body wall containing 3 basic cell layers; single opening that acts as mouth and anus and surrounded by food capturing tentacles; possess stinging cells (called nematocysts) situated on the tentacles. 

Corals usually take on one or two forms, a polyp or a medusa (see figures). If you take your hands, palms down and open and close your fingers, that represents  a medusa.  If you place your hands  palms up and repeat the same motion, this represents a polyp.  The medusa can move freely, like a jellyfish, but the polyp remains stationary (i.e. corals).  This is a good way to demonstrate the relationships between corals and other cnidarians.

Corals belong to the Class Anthozoa, along with the sea anenomes, sea fans, sea pansies, and many others.  Since this group can't run out and get a hamburger, they have to wait for the food to come to the polyp.  The polyp has its tentacles out waiting for debris (goodies that fall in the water column).  This is called suspension feeding.  When a tidbit hits its tentacles the tenacle brings the food toward the stomach where it is digested (raptorial feeding).  Because the corals are stationary, they have developed different protection strategies using their skeletons.

Corals can be solitary or colonial (more than one).  Massive colonial corals have individual polyps that range in size from 1-3 mm in diameter.  The solitary corals can reach up to 25 cm in diameter.  Most people think of corals as big massive structures, but few realize that these structures are really a colony of the same species who have built a common skeleton.  Because calcium carbonate is more abundant in warm water, corals tend to build larger structures in warm water.  Corals in colder water tend to be smaller and solitary.  The Pacific coast has small solitary corals that are bright orange which are commonly  mistaken for sea anenomes.

The shape of the skeleton varies from encrusting, spherical masses, to upright and branching growth forms.  There are basically three taxonomic types of living corals:  the cerianthipathanians (including black and thorny corals), hexacorallians (including stony corals), and octocorallians (including pipe and blue corals).

The cerianthipatharians have simple unbranched tentacles.  Many of these corals have the fleshy part of the organism on the outside of the skeleton.  Included in this group is the black coral, used in jewelry making.
 The hexacorallians are a diverse group with over 5300 living species.  The group grows septa (supporting "bars" where the organisms "sits") in multiples of six, hence the derivation of "hexa."  Included in this group are sea anemones and the scleractinia or stony corals.

The stony corals are very common reef builders because they make a robust and white skeleton that tends to be massive.  Also included are the brain, lettuce, and mushroom corals.  The skeleton is composed of calcium carbonate and is secreted at the base of the polyp.  The base contains radiating calcareous septa that help give the polyp support.  The skeleton provides a substrate on which the polyp attaches with its fellow corals and protection against fish which like the tender corals for a snack.  The octocorallian corals possess 8 tentacles that are pennate (looking like a feather).  The skeleton is made of calcium carbonate and/or a horny (organic) material.  The calcium carbonate however is not massive like in the stony coral and therefore not as robust.  The octocorallian include the sea pens, sea fans, red coral, sea pansies, and pipe corals.  This group tends to be more colorful in shades of red, blue, yellow, and brown.  

  1. Discuss with students the concept of classification.  There are currently six kingdoms that are recognized: Archaea, Bacteria,  Protista, Fungi, Animalia, and Plantae.  Each of the kingdoms is divided into phyla which are groupings of organisms that have similar characteristics.  Each of the phyla are broken into families, families are broken into genera, and genera are divided into species.  Classification is a useful tool that people use for naming organisms, and for getting a general relationship of how the organisms are grouped.  The way biologists and paleontologists classify organisms has to do with key characteristics  of a group, whether it is the way an organism eats, moves, or its physical structure.  The higher the taxonomic classification the more likely it will change as more information is obtained by scientists. 
  2. Since corals may be new to students, discuss what corals are. If you have a few samples show students the range of structures that corals have.
  3. Use the Internet search engines to find more information on corals. 

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