Declines in Both Redundant and Trace Species Characterize the Latitudinal Diversity Gradient in Tintinnid Ciliates of the Microzooplankton

Lower species richness with increasing latitude is a well-known biogeographic pattern but rarely considered is how lower species richness may be reflected in the characteristics of species assemblages. Fewer species may equal fewer distinct ecological types, or declines in redundancy (species functionally similar to one another), thus impacting potential adaptability of an assemblage. We focused on tintinnid ciliates of the microzooplankton in which the ciliate cell is housed inside a species-specific lorica or shell. The size of lorica oral aperture, the lorica oral diameter (LOD), is correlated with a preferred prey size and maximum growth rate. Consequently, species of a distinct LOD are distinct in key ecologic characteristics while those of a similar LOD are functionally similar or redundant species. We sampled from East Sea/Sea of Japan to the High Arctic Sea. We determined abundance distributions of biological species and also ecological types by grouping species in LOD size-classes, sets of ecologically similar species. Across the 5 assemblages, the overall size range of LODs was unchanged but there were declines in both the number of LOD size-classes occupied within the range, the portion of LOD size-classes containing multiple species, and numbers of trace species, those encountered as single individuals. In lower latitudes there are more size-classes and dominant species are accompanied by many apparently ecologically similar species, presumably able to replace the dominant species, at least with regard to the size of prey exploited. Such redundancy appears to decline markedly with latitude in assemblages of tintinnid ciliates.