California Sea Grant College Program

To determine diet, the stomach contents of sandpaper skates, Bathyraja kincaidii (Garman, 1908), were examined from a limited depth and geographic area off central California and from a wider depth and area range along the eastern North Pacific (ENP). The overall diet was dominated by euphausiids and shrimps, but polychaetes, squids, and gammarid amphipods were important secondary prey. Shrimp-like crustaceans, polychaetes, and teleosts were of similar importance in both data sources, but small benthic crustaceans and crabs were comparatively more important in the diet of skates from the ENP whereas cephalopods were more important in central Californian samples. A three-factor MANOVA demonstrated significant differences in the importance of major prey categories by sex, maturity status, and oceanographic season in the central California data. These three main factors explained more variation in diet than interactions between the factors, and season explained the most variance overall. A detailed analysis of the seasonal variation among the prey categories indicated that environmental abundance changes in the most important prey, euphausiids, were coupled with changes in the importance of other prey. Differences in the diet by sex, maturity status, and geographic zone of capture occurred in the ENP. Geographic zone explained the most variance in the diet, though much less than that explained by the central California data. Information on prey availability for these samples was limited, but it appears that latitudinal variation in euphausiids, again the most important prey, may be correlated with changes in the importance of other prey categories.

An ecomorphological study of the oral and dental morphology of B. kincaidii was conducted to determine if the intra-specific differences in diet could be linked with associated differences in morphology. Many of the structures associated with feeding grew allometrically, both positively and negatively, and the growth relationships were often different between the sexes. The results of a three-factor MANCOVA revealed that there were frequent significant differences by all of the factors (sex, maturity status, and geographic zone) and their interactions among all of the measured variables. However, variation between the sexes explained approximately one third of all the variance in the measurements in both data sets. The sex*maturity interaction explained the second most amount of variance, indicating that relational differences in the morphology of male and female skates further changed as they matured. The differences among the factor levels in oral and dental morphology were compared with the differences among the same levels determined from the diet study and it was concluded that intra-specific variation in morphology did not correlate well with intra-specific differences in diet. Based on the lack of a relationship, I suggest that intra-specific differences in the morphology of skates, as with other batoids, are related more to mating. The increased mouth width, amount of palatoquadrate protrusion, shorter pre-oral length, and teeth with higher and longer cusps of mature males allows them to better capture and hold females during courtship but such differences do not satisfactorily account for differential exploitation of any prey category.