Abstract
Krill are bolo-planktonic crustaceans that are wide-spread across the ocean basins of the world. The 86 known species fulfil an important role in pelagic food webs. The first basin-wide investigation of krill zoogeography was conducted during the International Indian Ocean Expedition (1962 - 1965). Together with subsequent plankton and micro-nekton studies conducted over the past five decades, all available krill distribution information for the 57 krill species occurring throughout the Indian Ocean has been collated to produce spatially-explicit data on species richness and taxonomic distinctness. The number of krill species was highest throughout the tropical and subtropical areas, and within the south-ward flowing Agulhas and Leeuwin Currents, each of which, had 34 species of krill. Average taxonomic distinctness (Δ+) is a measure of biodiversity that be applied to presence/absence data and is robust to differences in sampling effort, which allows historical data sets to be included. Most areas of the Indian Ocean were comparable in Δ+ values, which reflects the high connectivity across the basin and, in turn, holo-planktonic krill. Areas with lower Δ+ included the Red Sea and the north-west coast of India, whilst, somewhat surprisingly, areas of higher Δ+ were found in the middle of the Bay of Bengal. Although only 10 species were recorded from this region, these species were spread across six different genera, including Bentheuphausia amblyops, from the monotypic family Bentheuphausiidae, and Pseudeuphausia latifrons, which is the only species of this genus found in the Indian Ocean. All values of fell within the 95% probability limits of the expected mean for A+, indicating that, for each 20 x 30 grid spanning the Indian Ocean, the krill species sampled were representative Of the Indian Ocean species pool. Basin-scale environmental data sets and ocean reveal patterns in environmental drivers such as temperature, salinity, dissolved oxygen and chlorophyll a across decadal time series. Using generalised additive models, these environmental factors have been correlated with krill zoogeography to establish basin-wide relationships across the Indian Ocean.