Biogeography of the tropical seagrasses in the western Pacific

Abstract

Seagrass species and their distributions were recorded from 18 localities in the tropical and subtropical Pacific. These field observations were collated with a review of the recent literature, enabling the present distribution patterns of seagrass species in the western Pacific to be described and the origin of those species to be discussed in relation to the major oceanic currents of the region. The major tropical seagrasses of the western Pacific are Thalassia hemprichii, Enhalus acoroides, Cymodocea rotundata, C. serrulata, Halodule uninervis, Syringodium isoetifolium and Thalassodendron ciliatum, with T. hemprichii and E. acoroides being the two most widely distributed species. Species are found in coastal waters along the Equatorial Countercurrent and the Kuroshio and East Australian Currents. The highest number of seagrass species occurs in the coastal waters of Malesia enclosed by Indonesia, Borneo, Papua New Guinea and the Torres Strait (northern Australia). This area is considered to be the source of all of the seagrass species of the western Pacific, i.e. the centre of their origin. A relationship between the distance from this centre of origin along the above three major ocean currents and the diversity of seagrass species was found: the further from the origin, the poorer the seagrass diversity. These observations support the centre-of-origin theory that was first proposed by den Hartog to describe the processes responsible for the present distribution patterns of seagrasses in the western Pacific.