Marine Science 2016 Edition

Marine Invertebrates (1) Page 283 Figure 283.1 Corals feeding (Illustration Bob Moffatt) Figure 283.2 Coral colonies secrete a limestone skeleton as they grow by extracting calcium and carbonate ions from seawater. Ca ++ + C0 3 — —> CaC0 3 (Illustration Bob Moffatt) Corals feeding Most corals feed at night, and at this time their polyps are fully extended in search of food as shown in Figure 283.1. During the daytime they are retracted within their corallites. This maybe because the zooxanthellae produce enough food for the coral during the day, or they retract to avoid predators. The night feeding pattern of corals probably occurs because zooplankton reefs are most active at night. Corals feed by extending their tentacles armed with nematocysts and catching microscopic plankton that pass by (Figure 283.1). When the tentacle encounters an animal such as a shrimp, the barbed darts of the stinging cell are fired into the flesh of the prey, killing it instantly. The prey is then forced into the animal's mouth and into the coelenteron where it is digested. Once the food has been broken down and absorbed into the polyp’s tissue, some of the nutrients are passed to other polyps by the interconnections that join with other polyps. The limestone base is added slowly as a result of the polyp absorbing calcium carbonate from the seawater as shown in Figure 283.2. Algae and corals Reefs are not made entirely of coral. Algae play a very important role in the production of hard calcareous (calcium type) materials, which helps cement sand, coral pieces as well as living and dead material together to form the reef. Algae also play another important role in promoting coral growth. Corals are carnivorous animals, but they also obtain some nourishment from the plant cells embedded in their tissues. The success of corals as reef builders is due in part to the remarkable association with these tiny single-celled algae called zooxanthellae as shown in Figure 283.1. Symbiosis - mutualism + + (see also page 406) The relationship is one of mutual benefit or symbiosis. The plant cells gain a suitable place to live, (like other plants they harness energy from sunlight to manufacture materials necessary for their own nutrition and reproduction). They also use the waste products of the coral polyp for nutrition and growth. The polyp gains oxygen, and between 70-90% of body sugars from the zooxanthellae. During the day, reef-building corals usually have their polyps retracted into their limy skeletons. This allows the microscopic plants to absorb as much light as possible.