Influence of the Aquatic Substrata 67 



will characteristically support a rich growth of brown, green, and red 

 algae attached by "holdfasts" and a wide variety of snails, mussels, 

 sea anemones, starfish, and many other invertebrates secured by suck- 

 ing or cementing devices. Some of these animals indirectly derive 

 nourishment as well as attachment from the rock substratum. The 

 snails, for example, scrape off and eat the slime that forms on all 

 underwater surfaces as well as the bacteria and algae contained in the 

 slime. In shifting sand or gravel few species except rapidly burrow- 

 ing animals can maintain themselves; but on and in firm sand, espe- 

 cially when mixed with mud, a distinctive and rich population of 

 mollusks, worms, and crustaceans will be found, provided that other 

 environmental factors are favorable. On a mud bottom where the 

 water is quieter, rooted plants like the eel grass often grow in abun- 

 dance, and sea cucumbers, brittle stars, sea urchins, and a different 

 selection of worms are among the common inhabitants. However, if 

 oxygen has been depleted and hydrogen sulphide is formed in the 

 mud habitat, the benthic population will be greatly reduced. Further 



Photo made with Ezving undersea camera by D. M. Owen, Woods Hole Oceanographic Institution 



Fig. 3.4. Ripples on the sea floor at a depth of about 100 m on Georges Bank ofl^ 



Massachusetts. The numerous brittle stars and sand dollars visible in the 10 ni' 



area shown are able to maintain themselves on the shifting surface. 



