470 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



of sediment sizes did not correspond with the pattern found in natural 

 clam beds. Then the studies of one of our colleagues in the U.S. Fish 

 and Wildlife Service gave us a clue. 



Dr. Osgood Smith who was working in Newburyport, Mass., at- 

 tempted to determine the rate of settlement of clams by placmg trays 

 of sand in the flats for short periods of time and then screening out 

 and counting the newly settled baby clams. In the course of his ex- 

 periments he replaced the newly settled clams in certain trays to de- 

 termine how fast the population would build up. It so happened that 

 the numbers varied from time to time, sometimes increasing and some- 

 times diminishing. He suspected predation and attempted to protect 

 an area of flat by staking down a piece of plastic fly screen. After a 

 few weeks. Dr. Smith noticed that the upper surface of the screen be- 

 came coated with tiny clams with their shells stuck in the openings 

 in such a way that it was clear that they were attempting to burrow 

 down from above. This could only mean one thing. The newly settled 

 clams were not secure in their burrows as had always been supposed, 

 but were continually being washed about by currents and w^ave action, 

 becoming tangled in any suitable material such as experimental 

 screens, clumps of roots or fibrous seaweed. The migrating clams 

 were so small at this stage and generally so few in numbers that they 

 had escaped observation up to that time. Our resurfaced plots con- 

 tained a considerable quantity of marsh-grass root fibers wliich pro- 

 vided ideal places for the entanglement of these migrating clams. 

 Here they became concentrated in the root masses, where they grew big 

 enough to take up their final sessile existence. 



The entanglement hypothesis provided a beautiful explanation for 

 the accumulation of clams in the experimental resurfaced areas, but 

 failed to show how dense populations of clams arose under natural 

 conditions. A fortunate circumstance occurred during the month of 

 August one sunmier, when we were making a survey of the soft-clam 

 resources of Boston Harbor. We came across an area along the 

 Quincy shore where several acres of low-lying flat were so heavily 

 populated with tiny soft clams that they took up just about all the 

 available space on the surface. They were all approximately one- 

 quarter inch long and numbered in tens of thousands per square foot. 

 We surveyed the area carefully and visited it at frequent intervals. 

 By November of the same year the populated area had moved nearer 

 to the shore and the density was reduced to one or more thousands of 

 individuals per square foot. In August of the following year when 

 the clams were a year old and 1 inch long, the population w^as halfway 

 up the beach and numbered about 400 per square foot. Three yeare 

 later, when the clams were ready for harvesting, the population oc- 

 cupied essentially the same area but the numbers were reduced to a 

 hundred or so per square foot. 



