82 REPORT OF THE COMMISSIONER OF FISHERIES. 



were artificially pro])a^ated and reared in confinement. Studies and 

 experiments indicate tliat the natm-al food of fresh-water mussels is 

 made up principally of detritus, which is decayed or decaying animal 

 and vegetable matter, and that vegetable matter is preferred to ani- 

 mal. Investigations completed just at the beginning Af the fiscal 

 year have shown also that fresh-water mussels have the power of 

 absorbing nutriment in the form of fats (olive oil) and protein (egg 

 albumen) directly from solution in the water and through the cells 

 of the smiace of the body (gills, mouth, palps, and foot). It was 

 possible to determine that the fats, so taken up by the cells of the 

 outer body waUs, were transported through the circulatory system 

 to the various parts of the body. 



Considerable attention has been given to the matter of securing 

 for the mussels proper protective legislation on the part of the several 

 States. Under present conditions, the efi^orts of the Bm"eau to propa- 

 gate fresh-water mussels are not supplemented as they should be by 

 the extension of a reasonable measure of protection to the young 

 mussels. 



A study of the causes of pearl formation in fresh-water mussels 

 has been brought to a stage of reporting. There has also been 

 obtained during the year a. valuable fund of information regarding 

 the pearl fisheries and pearl culture in the Far East to which it is 

 hoped to give publication within a reasonable time. 



BIOLOGY OF THE BLUE CRAB, 



A new investigation of the life history and habits of the blue crab, 

 although begmi only at the beginning of the fiscal year, has already 

 made such progress as to supply the information most needed as a 

 basis for the regulation of the fishery and the conservation of the 

 blue crab, particularly in Chesapeake Bay, the headquarters of the 

 world's greatest crab fishery. 



The blue crab may spawn more than once. Crabs under observation 

 have spawned twice in the same summer. The female crabs which 

 are dredged during the winter are prospective spawners whether or 

 not they have spawned during the preceding season. The life history 

 of this species in Chesapeake Bay is, in brief, as follows: 



Nearly all the young are hatched in the lower bay from the last 

 of Jmie to about the first of September. The great majority of the 

 young begin a migration northward up the bay, settling on the bot- 

 tom when cold weather comes and ceasing to feed or to shed. The 

 next spring they resume development and their northerly migration. 

 They reach maturity in Maryland waters, where mating occurs, prin- 

 cipally during the last of July and August. Mating occurs only once 

 during the Ufetime of the female, but sufficient sperm is received and 

 carried to fertiUze two or more successive batches of eggs. The 

 females then mi^ate southward to the lower part of the bay, while 

 the males generally stay behind, spending the winter in deep water or 

 in creeks and rivers. About 80 per cent of the adult crabs taken in 

 the upper waters of the bay are males, and, correspondingly, about 

 80 per cent of the adult crabs taken in the waters near the mouth of 

 the bay are females. 



