Comparison of gastropod mollusc (Apogastropoda: Hydrobiidae) habitats in two crater lakes in Nicaragua.

The aquatic gastropod mollusc, Pyrgophorus coronatus, may perform an important role in the transmission of an emergent ocular pathology among fishes in Lake Apoyo, Nicaragua. This disease emerged after an introduction of tilapia (Oreochromis niloticus) and the subsequent loss of Chara sp. beds in the lake. We compared the mollusc population densities in three habitats (sandy/muddy substrates, rocks, and Chara vegetation) at varying depths (1.5, 10, 20, and 30 m) in two volcanic crater lakes in Nicaragua: Lake Apoyo and Lake Xiloa. where lower numbers of affected fishes were found and tilapia has not been introduced. Duplicate samples at 1.5 m depth were taken in each habitat monthly for a year, and triplicate samples for bathymetric analysis of snail populations were performed during August, 2005. Samples of fixed surface area were filtered in a 0.4 cm size screen and live snails were counted from each sample. The preferred snail habitat in both lakes, Chara beds, was vastly reduced in Lake Apoyo via consumption by introduced Nile tilapia (Oreochromis niloticus). Structureless sandy substrates (mean +/- standard error 3.1+/-1.3 ind/m2) had lower population densities than other habitats in Lake Xiloa (rocks 590.9+/-185.3 ind/m2; vegetation 3 686.5+/-698.2 ind/m2; ANOVA 1, p<0.01 in both cases) but this difference was attenuated in Lake Apoyo (sand 384.4+/-111.1 ind/m2; rocks 1 480.4+/-384.8 ind/m2: 0.0 1<p<0.05), where the snails on sandy bottoms were mostly embedded in filamentous algae coating this substrate. Snail population densities in Chara were lower at 20 m than more shallow depths in both lakes. Chara was not found at 1.5 m depth in Lake Apoyo, and was not present at 30 m depth in either lake. Shell lengths in rocky substrate in Lake Apoyo (3.62+/-0.12 mm) were greater than in Lake Xiloá (2.39+/-0.12 mm), suggesting differential snail predation levels in the lakes (p<0.005). Snail populations increased by 67 % inside exclusion cages in sandy substrates in Lake Apoyo (paired t-test, p<0.05), demonstrating the importance of foraging pressure on snail populations. Snail populations in open cages, on the other hand, were reduced by 23 % (p<0.05), suggesting the importance of subacuatic structure in feeding areas for molluscivorous fishes. We found a significant preference toward Chara by the snails (p<0.05) and significant foraging pressure on them (p<0.05) in sandy/muddy substrates. The disruption of Chara by introduced tilapias in Lake Apoyo may play a role in the emergence of ocular pathologies in the lake.