SYSTEMATICS, DISTRIBUTION AND ECOLOGY OF BALEA (=TRISTANIA) (PULMONATA: CLAUSILIIDAE) IN THE ISLANDS OF THE TRISTAN-GOUGH GROUP

Abstract

The genus Tristania Boettger, supposedly endemic to islands in the Tristan da Cunha-Gough group in the South Atlantic, is shown to be congeneric with Balea from the Palaearctic. Consequently, the subfamily Tristaniinae, recently erected to accommodate Tristania, becomes invalid and should be synonymized with the Baleinae. Austrobalea, the only member of the Clausiliidae known from southern Africa, exhibits similarities in shell form to Balea, but is shown to belong to the Mentissoideinae. Following a review by Odhner (1960), six species of Tristania (now Balea) had previously been described from the four islands in the Tristan-Gough group. We disagree with some of Odhner's conclusions. For example, we believe that the type of ventricosa from Tristan is specifically distinct from the form with a large, broad shell and pronounced spiral sculpture that occurs on Inaccessible and possibly Nightingale, which we here describe as a new species (Balea holdgatei). An additional undescribed species (B. swalesi) with a ribbed shell and distinctly flattened whorls is also described from Inaccessible. The status of B. costigera, known only from juvenile shells with strong ribbing, requires clarification, as does that of B. levior, previously described as a form of ventricosa. Anatomical descriptions are provided for seven of the nine species of Balea now known from these islands. Details are given of their occurrence on each island, together with their habitat preferences. Apart from B. swalesi, and the poorly understood B. costigera and B. levior, all the species occur on more than one island, where it is common for up to three (rarely four) species to occur microsympatrically in any one habitat. Balea tristensis is the most ubiquitous and variable species, occurring in every major habitat on each of the four islands. On Gough it was the only species to extend upwards into the montane moorland, whereas on Inaccessible it even lived in dried holdfasts attached to the upper part of the boulder beach, a novel habitat for any clausiliid. Balea costellata also has broad preferences, but is absent from montane moorland and coastal tussock grassland. Other species showed more definite habitat preferences: B. ventricosa on branches and under loose bark of Phylica arborea; B. goughensis and B. flavida in Spartina arundinacea tussock; B. holdgatei and B. swalesi in fern-bush.