32 Marine Invertebrates 



Organisms living on debris, or those which may be fed small pieces of 

 fish or shellfish, may be kept in the laboratory tanks for long periods. 

 Greater difficulty is encountered in keeping alive the plankton-feeding 

 forms. Seawater in laboratory circulation contains but a small number 

 of microscopic algae and may therefore be lacking in essential food ele- 

 ments. On the other hand, it still may contain copepods and other 

 crustaceans which may directly or indirectly be destructive to delicate 

 larvae or other organisms under cultivation. It is therefore advisable to 

 use filtered seawater and to provide food by adding diatoms or other 

 algae. This method requires a constant supply of these forms which 

 must be grown in the laboratory. 



Caswell Grave (1902) originated the method of rearing marine larvae 

 by putting them in a balanced aquarium in which the diatoms growing 

 on the bottom furnished an abundant supply of natural food and kept 

 the water pure. This method consists of putting a liter or more of sand 

 dredged from the ocean bottom in an aquarium of seawater and allowing 

 it to stand several days before a window, but protected from direct sun- 

 light. Under these conditions a film of diatoms develops in several 

 days. The larvae, from 12 to 24 hours after fertilization, are placed in 

 an aquarium of fresh water to which a dozen or more pipettefuls of the 

 diatom-stocked surface sand are added. The aquarium is then covered 

 and set before a window. Using this method Grave succeeded in rearing 

 a number of spatangoids and sand-dollars until they had completed their 

 metamorphosis, and in keeping them in a healthy and growing condition 

 for three months thereafter. [See p. 557.] The capacity of the aqua- 

 rium was 1 liter and the water was changed only twice during this period. 



An abundant supply of various diatoms may be raised by using the 

 following method developed by Just (1928). Mud and scrapings from 

 eel grass are placed together with animals and plants in jars containing 

 an equal amount of seawater. The jars are covered and set aside in a 

 subdued light. After a period of putrefaction the culture purifies itself 

 and an abundant growth of diatoms ensues. From this stock culture 

 the diatoms are removed, suspended in filtered seawater and strained 

 through bolting silk. Only the diatoms that have passed through the 

 silk are used for feeding. It is advisable to start several cultures at 

 from 5 to 10 day intervals. 



In spite of the fact that a number of marine larvae were successfully 

 reared on diatoms growing in mud or sand cultures both methods suffer 

 from a certain degree of uncertainty. It is impossible to predict what 

 species of diatom will develop and whether similar cultures will always 

 be available. For a more critical work on the physiology of feeding 

 and food requirements, pure cultures should be used. At present cultures 

 of a single species of diatom may be carried on indefinitely under con- 



