ASSOCIATIONS 



617 



waters. Zooplanktonic species containing zooxanthellae tend to remain in 

 surface waters during day-time, when other forms descend into deeper 

 waters. Behaviour of this kind has been observed in Radiolaria, larvae of 

 the zoantharians Zoanthella and Zoanthina, and the ephyrae of Mastigias 

 (91). 



The habits of littoral and sedentary forms are also strongly influenced 

 by illumination. The scyphomedusan Cassiopeia, found in shallow tropical 

 waters, is partially sedentary in habit and reposes on the bottom with the 

 sub-umbrella surface uppermost. In this position the zooxanthellae, 

 which are concentrated especially about the mouths, are exposed to the 

 light. The planula larvae of different species of reef-building corals show 

 positive phototaxis which appears to be associated with their algal content 



Fig. 14.22. Structural Specialization in Tridacnid Bivalves 



a. Diagram of an unspecialized lamellibranch for comparison with Tridacna. The 

 arrows indicate the rotation of the umbo («), exhalent and inhalent siphons (e, i), 

 posterior adductor (a), and pedal retractor (r), involved in the morphological trans- 

 formation of the Tridacnidae. 



b. Lateral view of Tridacna showing the result of rotation. Byssus (b) and gills (g) 

 remain in the same relative position. The extent of the large kidneys (k) is indicated by 

 the broken lines. Enlarged mantle lobes above are shown in solid black. (From 

 Yonge (109).) 



(53). In contrast, the planulae of Dendrophyllia, which is a deep-water 

 coral lacking zooxanthellae, settle in darkness. Reef-building corals are 

 typical inhabitants of shallow water, and display maximal development 

 in depths of less than 30 m. This partiality for shallow waters is probably 

 related to symbiosis with zooxanthellae. The effect of light on the growth 

 of corals has been reviewed by Yonge (108), who concludes that reef- 

 building corals are influenced in manner, speed and solidity of growth by 

 light, whereas the growth of deep-water corals is not so affected. Photo- 

 tropism is of great importance in the formation of reefs; in the absence of 

 light zooxanthellae are sparse or absent, and growth and metabolism of 

 the coral are probably depressed since the algae are not available to provide 

 nourishment. 



The turbellarian Convoluta roscoffensis, intensively studied by Gamble 

 and Keeble, lives on sandy shores at the level of high-water neaps within 



M.A.— 20* 



