Effects of juvenile non-indigenous Carcinus maenas on the growth and condition of juvenile Cancer irroratus

The Atlantic rock crab, Cancer irroratus, is a commercially fished species and a critical prey item for the American lobster, Homarus americanus, in Atlantic Canada. The recent invasion of European green crab, Carcinus maenas, may have significant effects on the growth and condition of native C. irroratus, because both species overlap spatially and temporally and have similar habitat and dietary requirements. To examine such potential effects, we measured the growth of juvenile C. irroratus in the presence of juvenile C. maenas over a period of 4 months (growing season), under the following species combinations: (1) one C. irroratus (10–25 mm CW); (2) two C. irroratus (10–25 mm CW); (3) one C. irroratus (10–25 mm CW) and one C. maenas (10–15 mm CW). Morphological measurements included pre- and post-molt carapace width, chela height, abdomen width (mm), weight (g), and estimates of molt increment (%) and intermolt duration (days). Analysis of the hepatopancreas for % lipid content at the end of the experiment provided an estimate of physiological condition. The effect of the presence of C. maenas on the growth of C. irroratus shifted from negative to positive, when C. irroratus reached CW of 19–22 mm and gained a presumably significant size advantage over C. maenas. The positive effect resulted from increased energy intake through crab consumption. In the absence of crab consumption, the presence of a second crab (conspecific or C. maenas) had no effect on growth. C. irroratus consumed crabs more frequently when the second individual was a green crab than a conspecific. Consumption of C. maenas had a pronounced effect on the growth rate of C. irroratus, resulting in shorter intermolt periods and larger percent molt increments than in the presence of a conspecific. Therefore, the presence of juvenile C. maenas does not appear to have a prolonged negative effect on the growth of C. irroratus; rather, it may provide an additional food item as rock crabs grow, as long as encounters between the two species occur at high enough rates.