6 U. S. BUREAU OF FISHERIES. 



The food of these water fleas is mainly the lesser green algse and 

 diatoms, and their great importance in aquatic economy is largely 

 due to the facts that they are herbivores and form the principal food 

 of most young and small fishes, and that they reproduce at an ex- 

 ceedingly rapid rate. 



The reproduction of Cladocera is remarkable. At intervals of only 

 a few days, during the summer, successive broods of eggs appear in 

 the space on the back inclosed by the shell of the animal. These eggs 

 develop without being fertilized, and produce females only. The 

 eggs, which vary in number from 2 to 20 or more, according to the 

 species, are deposited in a cavity, bounded by the top part of the 

 valves and the upper side of the body, the so-called brood case. 



Here they develop and hatch in a form 

 quite like the parent and are well grown 

 before they are set free. Sooner or later 

 true females and males are hatched from 

 the eggs. These females produce only 

 one or two eggs, which must be fertilized 

 before they develop. The rapidly de- 

 FiG. 2.— Daphnia ; " water flea." veloped youug thcmsclves soou reproduce. 



Duphnia nitlex, variety, male. rri. ^ ^ i • i» 



Much enlarged. Ihere are many genera and species of 



cladocerans, but probably the best Iniown 

 and the most widely distributed are species of Daphnia, which 

 occur in lakes, ponds, and pools in every region and are the forms 

 which have been most commonly raised as food for young fishes 

 and small aquarium fishes. The largest species is Daphnia mag>m, 

 attaining a maximum length of from 2 mm. in males to 5 mm. in 

 females (0.078 plus to 0.19 plus inch). The commonest species is 

 probably Daphnia pulex, found everywhere. Its greatest size is 2.5 

 mm. (about 0.1 inch). These little animals have a great economic 

 value. Together with the copepods, they constitute the chief agency 

 for converting the smaller algse of fresh water into forms edible by 

 the carnivorous aquatic animals. They are the prey of insect larvae, 

 which are in turn an important item in the bill of fare of the larger 

 fishes. The Cladocera themselves are of great importance as food for 

 young fishes and, as previously stated, there is scarcely a fresh-water 

 fish which does not subsist almost exclusively upon Entomostraca at 

 some stage of its existence. 



Mr. Atkins (1894) observed that, besides copepods, several species 

 of daphnids w^ere present in pools of Craigs Brook in the spring, 

 even before the disappearance of ice. 



Referring to observations on the occurrence of aquatic animal life 

 during each day of the year ending October 31, 1893, in the northern 

 part of Germany, Atkins stated that at the biological station at Lake 

 Plon during every month of the year the presence, in abundance, of 

 several species of Entomostraca was disclosed. From January to 

 April, inclusive, there was the greatest scarcity, yet no less than 

 seven species of Entomostraca were abundant in mid- January, and 

 with the exception of the period from March 20 to April 10, there 

 was no part of the entire year when some species was not found in 

 abundance. 



Mr. Atkins then went on to say that in the year 1873 he had occa- 

 sion to note at Bucksport, Me., a phenomenon of like character with 



