Macrofauna of Laufuti Stream, Tau, American Samoa, and the Role of Physiography in Its Zonation

Laufuti Stream, on the island of Tau, American Samoa, is a complex interrupted perennial stream, consisting of three accessible sections, lower Laufuti (perennial), middle Laufuti (intermittent), and upper Laufuti (perennial), and the inaccessible falls zone, a series offour sheer, intermittent waterfalls separating lower Laufuti from middle Laufuti. The macrofauna consists primarily of amphidromous species that are relatively common and widespread in the tropical Pacific. However, in comparison with stream communities on Tutuila, Laufuti is unusual. Its shrimp community is more diverse and abundant, dominated by Macrobrachium latimanus, a species neither widespread nor abundant on Tutuila. It also supports a relatively diverse, alien-free freshwater fish community of six species representing three families, Gobiidae, Eleotridae, and Anguillidae, including Anguilla megastoma, a species of limited occurrence on Tutuila. The fish community of Laufuti is similar to that of other tropical Pacific high-island streams in terms of dominant families, but zonation of macrofauna differs. There are no euryhaline fish species, and only Anguilla megastoma occurs above the falls zone. There are seven species of shrimps in lower Laufuti, but only Macrobrachium lar and M. latimanus occur above the falls zone. The severe dispersal barrier represented by the falls zone plus the absence of estuarine conditions, both products of the islands' geologic history, have produced a pattern of species distributions unlike that of most other tropical Pacific high islands