Diet and ecomorphological aspects of the sandpaper skate, Bathyraja kincaidii, from the eastern north Pacific

Chris Rinewalt
crinewalt@mlml.calstate.edu

Skates are common demersal fishes in nearshore and deep-sea habitats around the world. Frequently skates are taken as bycatch in important fisheries such as hake, whiting, cod, monkfish and shrimp as well as in research trawls. As with other elasmobranchs, skates can be top predators in their habitats and can not only compete with these targeted fishery species by consuming the same prey, such as shrimp, but they can also consume members of the targeted species. Because of these varied trophic interactions, studies on the diets of these chondrichthyans are important to uncover the role they play in the food web.

The sandpaper skate (at right), Bathyraja kincaidii, is a small deep-sea batoid that most commonly occurs from depths of 200 to 500m from the Gulf of Alaska to Baja California. Like most skates of the northeast Pacific, few studies have been done on the life history aspects of this species. Because this species is commonly collected off central California, samples were readily available to study its dietary habits. Over 500 stomachs were analyzed from skates collected from northern Washington to southern California by the Northwest Fisheries Science Center summer trawls in 2003 and monthly trawls around the Monterey Bay by the Southwest Fisheries Science Center Santa Cruz Lab from 2002 to 2005. The dietary description for B. kincaidii reveals that like most other skates, this species appears to have a generalized diet, feeding on many different groups of prey. They feed predominately on invertebrates such as Euphausids (krill- also a staple in whale diets), small shrimps such as Sergestes similis, Pasiphaea pacifica, Spirontocaris spp. & Crangon spp., cephalopods including the market squid (Loligo opalescens) & red octopus (Octopus rubescens) and benthic worms (Polychaetes). In addition, lanternfish (Myctophidae) and rockfish (Sebastes spp.), crabs (Paguridae and Axiidae) and mysids play minor roles. Future work will test for differences in the diets of male and female skates, as well as among size ranges and geographical areas.

In addition to the diet analysis, a second portion of the study examines how the animal’s form interacts with its environment, ecomorphology. Skates, like most other elasmobranchs, show sexual dimorphism- a physical distinction between males and females. In skates, this is manifested mainly by alar and malar thorns on the dorsal portion of the disc. In addition, certain batoids appear to have a dental component to their differences, with mature males having more pronounced cusps than females. This study will use the results from the comparison of the diets of these skates to determine if any observed differences can be correlated to morphology. If it turns out there is no difference in the diet between the sexes, it may well be that the differences in dentition and protrusion are primarily for use in life history aspects other than feeding. Preliminary qualitative analysis of the data seems to support the idea that there is no overall difference in diet between the sexes, so the purpose of these morphological differences is a mystery; other than feeding, what role could the mouth play?

In certain groups of elasmobranchs, the sharks and rays, sexually dimorphic differences have been attributed to mating. Previous studies have noted changes in the dentition of male Atlantic stingrays, Dasyatis sabina, during the breeding season and examined differences in the thickness of the skin of female blue sharks, Prionace glauca. One aspect of morphology that has not yet been examined for a role in mating is jaw protrusion, the ability to extend the upper jaw away from the body. Preliminary analysis reveals that like the stingray, male Bathyraja kincadii teeth have more pointed cusps than females but also that for any given size, males have a larger amount of protrusion than females. At right are pictures showing the upper jaws (palatoquadrate) of two mature sandpaper skates, female (center) and male (bottom); both skates are approximately the same size (530 and 537 mm TL respectively). However, other aspects of the mouth morphology, mouth width and the length of the snout to the upper jaw, do not seem to differ between the sexes. It would appear that there is some factor behind this difference in jaw protrusion that has been perpetuated over time without a negative effect on survival and without resultant differences in the diet. Thus it may be that upper jaw protrusion and dentition play more of a role in mating than they do in feeding for this species.