618 THE BIOLOGY OF MARINE ANIMALS 



a drainage area where it obtains maximal light and safety from possible 

 desiccation. Positive phototaxis and negative geotaxis cause the animals 

 to migrate to the surface during the day. Mechanical stimulation, i.e. 

 vibrations, will release positive geotaxis, and cause the animals to migrate 

 downwards, and it is this factor which is operative in causing the animals 

 to burrow into the sand when the tide reaches them. C. convoluta, which 

 lives on weed at low-water mark, migrates up and down with spring and 

 neap tides, and thus secures optimal illumination without danger of 

 desiccation (29). 



Functional Relations between the Associates 



It is generally believed that in symbiotic relationships between plants 

 and animals the alga benefits by deriving protection and securing carbon 

 dioxide and nitrogenous waste products from the host, while the animal 

 obtains oxygen and nourishment and secures the elimination of waste 

 substances. 



Life is probably more secure for an alga within an animal than in the 

 surrounding medium, unless the animal eventually devours its symbionts 

 as Convoluta roscojfensis does. It has been demonstrated experimentally 

 that zooxanthellae utilize the C0 2 produced by various reef-building corals, 

 but this is probably of minor importance to the alga since this gas is norm- 

 ally sufficiently abundant to satisfy the physiological requirements of 

 plants in the sea. Nutrient salts containing nitrogen and phosphorus, how- 

 ever, are limiting factors for plant growth, and algae living in animals are 

 favourably situated to acquire these compounds at their source. Experi- 

 mental work with actinian Aiptasia has shown that its zooxanthellae take 

 up ammonia, whereas in the turbellarian Convoluta roscojfensis the green 

 cells utilize uric acid or urea better than nitrates (56, 76). The algal cells 

 also utilize phosphorus excreted by the animal and, in addition, may take 

 up phosphates from the medium (Fig. 14.23). Experiments have involved 

 Psammocora and other reef-building corals, the scyphomedusan Cassio- 

 peia, Anemonia sulcata, and the reef-inhabiting bivalve Tridacna crocea 

 (96,97, 107, 112). 



When the reverse relationship is considered it is discovered that oxygen 

 in abundance is produced by the algal symbionts when illuminated. The 

 oxygen tension is normally adequate for aquatic animals under most 

 conditions, but oxygen may become depleted in the heated shallow waters 

 over reefs and in tidal pools during ebb. Under these conditions the 

 oxygen produced by algae may have survival value (106). Smith (97) 

 suggests that Anemonia can better withstand conditions in poorly oxygen- 

 ated pools during tidal ebb because of the oxygen produced by the algal 

 associates. 



There is evidence that many animals obtain nutriment from their im- 

 prisoned algae. A transference of foodstuffs has been demonstrated in 

 Convoluta convoluta and in C. roscojfensis. Orbitolites utilizes the stored 

 oil and starch grains which are released when its enclosed algal symbionts 



