372 APPLIED BIOLOGY 



covered externally by colonies of hydroids (Fig. 101). These 

 eat the small particles of food which float in the water when 

 the crab is feeding. The crabs are believed to be benefited by 

 the presence of the animals on its shell because the hydroids 

 bear stinging organs, which, if not painful, are at least very 

 disagreeable to the delicate mouths of the fishes and other 

 enemies that attempt to steal away the food of the crab. 

 Such cases where two species of animals are associated to- 

 gether for mutual advantage are examples of commensalism, 

 a term meaning mess-mates. 



315. Economic Importance of Decapods. The most 

 valuable of these larger Crustacea is the North American 

 lobster, which lives on the ocean bottom within fifty miles 

 of the shore along our North Atlantic coast, and the Nor- 

 way species, which is supplied to European markets. The 

 large size and the peculiar qualities of lobster flesh have led 

 to such a market demand that there has been excessive catch- 

 ing in recent years. The market value of the lobsters caught 

 each year is estimated to be many millions of dollars. In 

 recent years it has become evident that steps must be taken 

 to prevent extinction of the American lobster. Accordingly, 

 the United States Bureau of Fisheries has engaged exten- 

 sively in hatching lobster eggs, and several states are enforc- 

 ing laws which prohibit taking short lobsters (under six 

 inches long in some states, nine in others) and females with 

 eggs attached to the abdominal appendages. The advan- 

 tage in hatching lobster eggs artificially is due to the fact 

 that fishes and other enemies usually destroy a very large 

 number of eggs or young larvae, but in the hatching troughs 

 they can be protected until several days old and better able 

 to care for themselves. The eggs are collected by brushing 

 them from the abdominal appendages of the females. A 

 female a foot long may have over ten thousand eggs, and 

 giant specimens over eighteen inches long have been found 

 with more than 150,000 eggs. 



