Predation on zebra mussels (Dreissena polymorpha) by captive-reared map turtles (Graptemys geographica)

The suitability of the Eurasian zebra mussel, Dreissena polymorpha, as prey for the common map turtle, Graptemys geographica, was tested under laboratory conditions. The turtles, which were reared in captivity without contact with molluscs, readily consumed zebra mussels in every feeding trial. Mussels were ingested whole. Repeated exposure to zebra mussels in successive trials did not increase consumption rates (ca. 11 mussels/turtle per day), suggesting that the turtles required little time to recognize and efficiently utilize zebra mussels as prey. The turtles were offered zebra mussels ranging from 4 to 34 mm in length and consumed mussels as large as 32 mm, but mussels larger than 25 mm were consumed at lower rates. Turtles consumed lower numbers of zebra mussels in the presence of an alternative prey, the prosobranch snail Bithynia tentaculata, which is common in map turtle habitats in the Great Lakes – St. Lawrence River system. Populations of map turtles and zebra mussels are sympatric in the upper St. Lawrence River, Lake Champlain, the lower Great Lakes, and the upper Mississippi River basin. Our results suggest that map turtles may forage on zebra mussels in nature, but zebra mussels will be important prey only when preferred or more profitable prey are scarce.