570 Phylum Chordata 



organisms. The ascidians will suffer if the growth, especially on the 

 walls of the vessel, becomes too thick. This growth may be controlled 

 by limiting with shades the amount of light that reaches the vessel. A 

 slight circulation produced by the slow bubbling of air through the water 

 is an advantage, but is by no means essential. Under these conditions 

 it should be possible to rear most ascidians to a comparatively large size, 

 even to maturity. 



pH REQUIREMENTS 



It is most unlikely that the pH of the water will fall to so low a value 

 that development or growth becomes affected. Only when the pH falls 

 to 7.0 does development tend to become abnormal and hatching in Ciona 

 and Ascidia become inhibited (Berrill, 1929). In the other direction 

 normal development is possible up to a pH of 9.0. There is considerable 

 danger, however, that a luxuriant growth of algae may cause the pH to 

 rise to even higher values, especially if small culture vessels are used. 

 Tests should accordingly be made at intervals, and if the pH tends to 

 become too high the light should be cut off in order to inhibit further 

 photosynthesis. 



TEMPERATURE REQUIREMENTS 



As in the case of many other forms there is for ascidians a temperature 

 range of about 15 C. within which normal development is possible. 

 This range may be high or low, it may vary for the same species in 

 different localities, or it may vary for the same species in summer 

 and winter, e.g. Parechinus (Horstadius, 1926). Thus Ejigyra pilidaris 

 will develop at 18 C. at Woods Hole (the southern extremity of its 

 distribution range) but not above 12 ° C. if taken from the colder waters 

 of the Bay of Fundy (Berrill, 1931). 



In general, however, it is reasonably safe to assume that normal de- 

 velopment and growth is possible when there is little difference between 

 sea temperature and air temperature, whatever it may be, and that the 

 temperature of the water in which development is proceeding should 

 never exceed by more than 5 C. the temperature of the water from 

 which the parents were taken. This margin should be reduced where 

 sea temperatures exceed 25 C. Keeping culture vessels beneath a small 

 tent with the walls kept saturated with water has been found to reduce 

 the air temperature by several degrees. The distribution range of a 

 species relative to the locality where it is taken gives some indication of 

 its temperature tolerance. Species found near the southern end of their 

 range are not likely to tolerate temperatures higher than that of the 

 water in which they are found; species from the northern end of their 

 range will tolerate temperatures considerably higher. 



