The Juan Fernández Diving Beetle, Anisomeria Bistriata (Brullé 1835): Habitats, Communities and Conservation

Abstract

The Juan Fernández archipelago is situated in the Pacific Ocean, about 600 km of the coast of Chile. It has been suggested that if environmental degradation is not reversed, the biota of these islands will soon be severely threatened.

Only three species of diving beetles (Dytiscidae) are known from Juan Fernández: Anisomeria bistriata (Brullé, 1835), the most remarkable species given its particular morphology, Rhantus signatus kuscheli Guignot, 1952, both endemic to Robinson Crusoe island, and Lancetes backstromi Zimmermann, 1924, endemic to Alejandro Selkirk island.

In order to study the abundance and habitats of A. bistriata and R. s. kuscheli, we visited Robinson Crusoe Island. The samplings involved small creeks and some artificial containers. Anisomeria bistriata was collected in Bahía Cumberland and Plazoleta El Yunque, at altitudes 80-235 m asl, mainly in artificial containers, maybe refl ecting certain affi nity for lentic habitats. By late summer, about 95% of the larvae were third instars, which would argue in favour of a marked seasonality in the life cycle of the species.

Rhantus signatus kuscheli was collected in Bahía Cumberland and Puerto Francés, at altitudes 34-99 m asl. Contrary to A. bistriata, it was more frequent in creeks. The larvae (most of them fi rst instars) were captured in the same creek as the adults, in the most vegetated portions. Despite the signifi cant environmental degradation than Robinson Crusoe island has supported, both species do not seem severely threatened.