100 UNDERWATER GUIDE TO MARINE LIFE 



tropical waters. One in particular, Cassiopeia, is common in the West Indies. 

 It has no true single mouth and cannot sting, but it possesses a much-divided 

 and lobed digestive system which contains symbiotic algae. This jellyfish is often 

 seen lying lazily upside down in shallow water, exposing the algae to sunlight. 

 The algae, therefore, produce some of the food for this jellyfish by photo- 

 synthesis. All rhizostomes have the many-lobed digestive system like Cassiofeia, 

 but most have no algae in them and therefore must live by catching plankton 

 or small fishes. 



The young of many fishes use jellyfishes for protection, escaping the tentacles 

 by alertness. Sometimes, however, they fall prey to their jellyfish protectors. 



Gorgonians, Anemones, and Corals: Class Anthozoa — Figure 27 



There is no medusa stage at all in this group. Instead, the polyp has become 

 complex and either secretes a home around itself and is colonial or is large in size 

 and solitary. There are over six thousand species of anthozoans found in all seas, 

 but they are far more common and larger in tropical seas than elsewhere. The 

 colors found in this group are often very beautiful and caused partially by 

 pigment and partially by the presence of organisms, zooxanthellae and zoo- 

 chlorellae, that live in the tissues of the polyp. Hard corals are most often 

 brownish, but they may be green, blue, or, in the case of the Mediterranean 

 precious coral. Cor allium, a gorgeous red. The sea pen is red and phosphorescent. 

 Gorgonians are brownish. The browns, yellows, and greens are the colors caused 

 by symbiotic animals and plants. Most coral skeletons are white like coral sand. 



All of the members of this group are carnivorous, but the size of the prey 

 varies greatly, and in many, the oxygen supply is augmented by the presence 

 of zoochlorellae or zooxanthellae. The small, colonial anthozoans eat planktonic 

 food, and the largest polyps, the anemones, catch fish or whatever else they 

 can trap in their stinging tentacles. All are sedentary, though anemones can 

 creep very slowly on their bases, and must wait for food to come their way. 

 They either feed by passively filtering sea water or by attracting fishes with their 

 flowerlike body form and colors. Some anemones even have little fishes living 

 symbiotically in the protection of their tentacles, and these may serve to attract 

 other fishes near enough to be caught. In any case, food is not acquired 

 actively by the tentacles as in the case of the octopus. A fish must bump into 

 a tentacle through its own carelessness to be captured. 



Many of the anthozoans, notably the corals, are nocturnal. With the coming 

 of dusk, the coral colony blooms as thousands of tiny polyps emerge from their 

 calcareous homes. This seems to be somewhat a contradiction of the fact that 

 light is needed by the zooxanthellae that produce oxygen for the coral, but 

 enough light is probably able to reach the retracted polyp to allow photosynthesis 

 to occur. 



Very few members of this group possess nematocysts large enough to do 

 damage to humans. Only one large anemone of the Indo-Pacific is really 

 dangerous. Others may produce a barely perceptible burning when touched. 

 Corals, however, may inflict cuts that are slow to heal and become easily infected. 

 This is known as "coral poisoning" and is not very prevalent in North America. 

 Ergophine ointment is said to be of use in alleviating the irritation of coral cuts. 



