SECT. 2] MICROTOPOGRAPHY 459 



of growth. The crinoids, however, are basically axially symmetric, but can 

 move their arms so as to form a fan perpendicular to the current. Crinoids have 

 been observed behaving in this way from the bathyscaph (Peres and Picard, 

 1955) and a preferred orientation has been noted in fields of crinoids in under- 

 water photographs. The orientation of many kinds of marine fauna has been 

 studied by Bromhall in the waters near Hong Kong and he has noted the 

 relationshij) between orientation and the funnelling of currents through a gap 

 in the rocks. 



A more obvious indication of current by marine fauna is the bending to the 

 current of seaweed, sea-pens and other long and flexible organisms. By a study 

 of both the slowly and quickly responding fauna appearing in photographs, it 

 may be possible to obtain both the mean current direction and also the in- 

 stantaneous current. 



C. Benthic Fauna 



The photographic method has been used to study the benthic fauna both 

 qualitatively and quantitatively, especially in the littoral zone, since it is the 

 most direct way of assessing the population and studying its environment and 

 behaviour (Vevers, 1952; Posgay, 1958). Underwater television has also 

 been used widely for this jiurpose (Barnes, 1955). However, much of the photo- 

 graphic information about the abyssal benthic fauna has been derived from 

 photographs taken more for geological than zoological study. On account of 

 the difficulties of deep-sea photography, it is important that the greatest use 

 should be made of the collections of photographs from all asj)ects, and that they 

 be made as widely available as possible. 



It is not the intention to discuss here the general problems of the distriljution 

 of benthic fauna, but to mention only some of the principal fauna observed 

 and their reaction with the sediment surface. 



In the deep basins where there is sediment only, the most commonly observed 

 fauna are the burrowing worms, sticking vertically through the sediment and 

 sometimes showing fans of ciliated tentacles. Brittle stars (ophiuroids), sponges, 

 scaphopods and holothurians are also frequently seen, or else their existence is 

 deduced from their disturbance of the sediment surface. Mounds, burrows and 

 tracks are almost universally observed wherever the sediment is coherent 

 enough to preserve them. 



Coming up out of the sediment basins onto the rocky slopes of seamounts, 

 the benthic population changes and increases. Rock faces become obscured by 

 matted bryozoa and sponges. Many species of coelenterates are common, 

 including Hydrozoa, corals, gorgonians, pennatulids and sea anemones (Actin- 

 iaria). Echinoderm are often seen, especially sea-urchins and sand-dollars 

 (echinoids), asteroids, ophiuroids and commatulid crinoids, the latter some- 

 times occurring in colonies covering large areas. Occasionally fish, deep-sea 

 decapods and copepods and other nekton can be seen, but only occasionally 

 can they be identified. Shallow waters on the tops of seamounts within the 

 euphotic zone abound in benthos, with the same diversity of species as is found 



