Researcher seeks hard facts about hard clams 



Photo by Kathy Hart 



Charles Peterson may look like a 

 man after his own dinner as he sifts the 

 sands of Bogue Sound for clams. But 

 he's not. Peterson, a biologist at the 

 UNC Institute of Marine Sciences at 

 Morehead City, is after some hard 

 facts about hard clams. 



As part of a UNC Sea Grant project, 

 Peterson is looking into hard clam 

 biology as well as the effects of dif- 

 ferent harvesting methods on clam 

 populations and on the estuarine envi- 

 ronment. The information he collects 

 is being funneled into the Division of 

 Marine Fisheries for use in future 

 clam-management plans. 



One of Peterson's early findings un- 

 covers more information about one of 

 the hard clam's predators, the whelk, 

 often referred to as a conch in North 

 Carolina. Peterson knew the hard clam 

 was one of the whelk's favorite meals, 

 but he wasn't sure how factors like sea- 

 grass cover and clam size and density 

 affected the whelk's appetite. 



The whelk and the clam seem like 

 unlikely enemies since they're both en- 

 compassed by a hard shell. But Peter- 

 son says, "The whelk grabs the clam 

 with his foot and rubs the sharp edge 

 of his shell against the clam, chipping 

 away at the margin. Eventually the 

 whelk chips away enough shell to get 

 his lip in and pry open the shell." 

 Peterson says the whelk leaves the evi- 

 dence behind — an empty, rasped shell. 



To see how grass cover affects whelk 

 predation, Peterson and a group of 

 graduate students, headed by Hal 

 Summerson, set out for Bogue Sound 

 to set up sample plots. Some of the 

 one-meter-square plots were denuded 

 of their grass, while others were left 

 natural. Peterson then set out clams, 

 marked by a dot of paint, in each plot 

 and left them for several months. 



In the first experiment (October- 

 May), Peterson found that along 

 sandy bottoms with no grass coverage, 

 54 percent of the hard clams were 

 rasped and eaten by whelks. In a 

 second experiment (July-November), 

 84 percent of the clams were eaten by 

 whelks. Predation rates were higher for 

 the second experiment because of in- 

 creased whelk activity during warmer 

 months, Peterson says. Meanwhile, 

 the clams tucked away in grass beds 

 suffered little predation. 



Peterson also learned that density or 



To tell a clam 's age, Peterson halves 



the number of clams per area did not 

 affect the rate the clams were eaten by 

 whelks. But the size of the clam did 

 play a role in whelk predation along 

 sandy bottoms. Whelks tend to choose 

 larger over smaller clams to munch on. 

 "It may be the whelk is looking for 

 more return for his effort when he 

 chooses the larger clams to eat," Peter- 

 son says. "We don't know for sure. But 

 the data show clams can't outgrow 

 whelk predation the way they can with 

 blue crabs." 



Peterson feels that his findings again 

 point to the importance of seagrass 

 beds, this time as a refuge for clams. 

 He theorizes that seagrass roots and 

 rhizomes compact the sediment 

 around the clam, making it harder for 

 the whelk to dig out his prey. Or the 

 root material itself may deter the 

 whelk, Peterson says. 



the shell with this tool 



As another part of his study, Peter- 

 son has been learning how to deter- 

 mine a clam's life history by reading 

 the lines in his shell. The clam, like a 

 tree, lays down an annual line that 

 reveals its age. Scientists knew clams 

 in northern waters added a growth line 

 during winter months when they ex- 

 hibited little or no growth. But Peter- 

 son knew North Carolina winters 

 weren't cold enough to halt clam 

 growth. Maybe southern clams didn't 

 tell their age so easily. 



But they did. Peterson found that 

 most North Carolina clams add a 

 growth line during the late summer or 

 early fall. The clams growth rate slows 

 50 percent, causing the clam to add a 

 growth line, Peterson says. He 

 suspects the line is added during a 

 period of physiological difference that 

 may have some connection with the re- 



