Burrow density of the endangered Maltese Freshwater Crab Potamon fluviatile lanfrancoi at Lunzjata and Xlendi valleys, Gozo

Abstract

The density of burrows of the highly threatened and locally protected freshwater crab Potamon luviatile lanfrancoi at its only known locality of occurrence on Gozo, the Wied tal-Lunzjata / Wied tax-Xlendi valley system, was surveyed during the 2006-2007 wet season. Of a length of 851m of the main channel of Wied tal-Lunzjata surveyed, 665m had burrows, while of 303m of Wied tax-Xlendi surveyed, only 60m had burrows, giving a mean burrow density of 1.29 and 0.10 burrows per metre, respectively. The highest concentrations of burrows occurred in three areas within Wied tal-Lunzjata: its upper reaches, in the Ta’ Ghajn Tuta area, and in the Wied tas-Saqwi area. Only few burrows occurred in the lower reaches of Wied tax-Xlendi, the only accessible part of this valley that could be surveyed. There was no clear relationship between burrow density and either water depth or flow rate, although there were indications that high flow rates tended to favour high burrow densities. Burrows were invariably excavated in the soil or muddy sediment close to the water level and the availability of muddy sediment was a key factor determining the occurrence of burrows. Burrow occurrence and burrow density did not seem to be affected much by material dumped into the valley, or by moderately poor water quality, but there were clear indications that the crabs may be particularly susceptible to severe changes in water quality and in the hydrologic regime, particularly where this affects the availability of critical habitat.