BOUNDARY EFFECTS ON THE VERTICAL RANGES OF DEEP-SEA BENTHIC SPECIES

Since environmental gradients are more pronounced on the continental shelf and continental slope than in deeper waters, benthic deep-sea species occurring in shelf and slope regions should have smaller vertical (bathymetric) distributional ranges than species occurring in continental rise and abyssal regions. This paper explains a species' vertical range with a model that considers the existence of boundaries to species' vertical distribution (the deepest sea floor and the sea surface). The model assumes that a species' center of distribution corresponds to its mean depth of occurrence [(minimum depth of occurrence+maximum depth of occurrence)/2]. The model shows that the maximum Vertical range must be dependent on the mean depth of occurrence. This value peaks at intermediate depths and decreases as one approaches either shallow or abyssal depths; the interaction of the shallow and abyssal boundaries with the endpoints of species' range produces a constraint envelope that precludes large ranges for species close to the boundaries. Data from different taxa show a relationship between mean depth of occurrence and vertical range for several taxa. Vertical range is small at continental slope (200-1500 m) and upper slope depths, increases with depth to a maximum at upper continental rise depths (1500-3500 m), and then contracts at lower continental rise (3500-4500 m) and abyssal plain (4500-6000 m) depths. Results suggest that boundaries to distributions can have an important effect on the bathymetric gradient of species Vertical distributions.