Abrasive Action of Medium 57 



Sessile animals, of which there are many representatives in the tidal 

 zone, sometimes show a difference in growth form according to 

 whether the individuals are living in an exposed or a sheltered loca- 

 tion. Sponges and tunicates, for example, grow with long pendulous 

 processes in quiet water, but, if exposed to strong currents and wave 

 action, individuals of the same species grow closely appressed to the 

 rocks (Wilson, 1951). 



Certain physiological adaptations also exist for life where currents 

 are strong. Many motile animals exhibit a rheotoxis, that is, an 

 orientation of their locomotion with respect to the direction of the 

 current. The rheotactic reactions of most stream fish is such that 

 they swim against the current and thus maintain their position in the 

 stream. Interestingly enough, in some species at least, this reaction 

 is mediated through the eyes. The fish are stimulated to turn and 

 swim until the image of the stream bottom no longer moves across 

 the retina. Other fish are stimulated by differential pressure or 

 touch. Many stream animals react in such a way as to move out of 

 the current and thus reach quiet water or protected eddies. 



Other reactions to the current are even more elaborate. Some 

 caddis-fly larvae, for example, alter the shape of their cases and the 

 materials of which they are built according to the strength of the 



Modified from Dodds and Hisaw, 1925 



Fig. 2.15. A scries of caddis-fly larvae and their cases from quiet and swift 



waters. 



