Scleractinians

Scleractinian/Stony Corals

Stony Corals are also known as Hard Corals. They are considered the reef builders of the ecosystem because of the calcium carbonate skeleton they secrete, which distinguishes them from soft corals. We will discuss the classification, evolutionary history, skeleton and colony formation, behavior and environment of stony corals, and then discuss some particular examples common to St. John and the US Virgin Islands.

Classification

Kingdom:Animalia

Stony Corals belong to the animal kingdom, which include eukaryotic multicellular organisms that i) form from blastula embryos ii) are loosely mobile and iii) gain nutrients through ingesting food.

Phylum: Cnidaria

Members of the phylum Cnidaria share common characteristics including i) a body open to the external environment ii) radial or biradial symmetry iii) a diploblast structure and iv) nematocysts. Intrinsic nematocysts are capsules of specialized cells that will uncoil and extend rapidly when stimulated, usually to either sting as a form of defense or to capture food. Other organisms other than stony corals that are classified as Cnidaria include jellyfish, sea anenomes, portuguese man of war, hydroids and freshwater hydra. This means that, despite popular belief, stony corals are more closely related to jellyfish than sponges.

Class: Anthozoa

The class anthozoa is classified by i) having no medusoid phase in the life cycle and ii) liveing in the form of polyps because their skeletons function for protection and not movement. and iii) existing solely in marine environments.

Subclass: Hexacorallia

The subclass hexacorallia to which sea anenomes, black corals and stony corals belong is characterized by tubular polyps that have tentacles in multiples of six.

Order: Scleractinia

Scleractinia comprise of stony corals. This order is distinct because of their hard skeleton attached to a firm substrate.

Evolutionary History

• first appeared in the mid-triassic period

Skeleton and Colony Formation

A coral may be either a solitary polyp (an individual coral animal) or a colony of thousands of polyps that share food, water and waste through a common gastrovascular system and are anchored to a substrate. Polyps generally range in diameter from 1-3 mm for colonial corals and can get up to 25 cm in some solitary corals

The entire coral skeleton is called the corallum and it is made up of individual coral units called corallites. The calcium carbonate (CaCO3) skeleton is secreted by ectodermal calcioblasts in the lower portion of the polyp. After landing on the substrate, the polyp begins by secreting a thin skeletal basal plate, from which septa (vertical plate-like partitions) radiate outwards; this is followed by formation of the theca (the skeletal wall). The inner-septa often fuse to form a columella (central axis of the corallite found beneath the polyp mouth). The polyp now sits in a cup-like depression called the calyx (defined by the theca and the basal plate). From the above description you can see that most of the polyp is impregnated with a calcareous (composed of calcite, or calcium carbonate) skeleton. Periodically a polyp may lift off its base to secrete a new basal plate above the old one, leading to the creation of a new chamber within the skeleton. Living coral deposits CaCO3 in this manner, adding partitions and elevating the corallum.

Behavior

Reproduction

• Sexual: allows for spread of coral to new places
• Asexual: allows for growth of corals
  • Budding
  • Splitting

Environment

• Stony Corals do well in tropical and subtropical areas which have warm and clear water
• Stony Corals have also been found in deep, dark water that is up to 6,500 feet deep

Class Photos from the US/British Virgin Islands in the Caribbean

• Brain
• Branching
• Encrusting, Mound, and Boulder
• Fleshy
• Flowering and Cup
• Leaf, Plate, and Sheet

Notes

1. MarineBio Conservation Society. (n.d.). Coral Reefs. Web. http://marinebio.org/oceans/coral-reefs.asp
2. Stanley, G. D. (1996). Paleobiology and biology of corals. Columbus, OH: Paleontological Society.
3. Dubinsky, Z., & Stambler, N. (Eds.). (2011). Coral Reefs: An Ecosystem in Transition. London: Springer.
4. National Ocean Service. (March 25,2008). Corals: How Do Corals Grow? What Forms Do They Take?. Web. http://oceanservice.noaa.gov/education/kits/corals/coral03_growth.html
5. Sheppard, C. R., Davy, S. K., & Pilling, G. M. (2009). The Main Reef Builders and Space Occupiers. The Biology of Coral Reefs. New York: Oxford University Press Inc. doi:10.1093/acprof:oso/9780198566359.001.0001