Distribution of external parasites indicates boundaries to dispersal of sillaginid fishes in the Indo-West Pacific

Abstract

The distribution of sillaginid-specific ectoparasites permits the distinction of two provinces with high parasite diversity: one on the continental shelf of Australia, and one on the shelf of Asia. The Australian province has 15 endemics (five monogeneans, one leech and nine copepods), and the Asian province has 14 endemics (two monogeneans and 12 copepods). These provinces are separated by a region with coastlines that descend very steeply to the ocean floor and by deep ocean waters that largely inhibit sillaginid movements. Some sillaginids must have dispersed across eastern Indonesia, however, leading to the occurrence of four parasites in both provinces. Three Australian parasites also appear to be presently encroaching onto the southern periphery of the Asian shelf. At least one less-recent invasion of Australian waters by Asian sillaginids would account for the occurrence of six pairs of copepod congeners that have one member in each province. The most widespread sillaginid, Sillago sihama, seems to have dispersed to African shores from the Arabian Sea as planktonic larvae only (no Asian parasites were present in samples of 29 hosts) and relatively recently (only one locally endemic parasite appears to have been acquired).