Visual versus non-visual selection of shell colour in an Israeli freshwater snail

Area of Science:

• Ecology
• Evolutionary Biology
• Malacology

Background:

• Theodoxus jordani, an Israeli freshwater snail, displays significant variation in shell color and pattern.
• In most Israeli locations, over 80% of snails are black, irrespective of their environment.
• Lake Kinneret shows a correlation between dark shell patterns and basalt backgrounds.

Purpose of the Study:

• To investigate the selective pressures influencing shell color and pattern variation in Theodoxus jordani.
• To understand the role of predators and environmental factors, such as UV radiation, in shaping snail shell phenotypes.
• To compare the factors driving shell variation within Lake Kinneret versus external localities.

Main Methods:

• Observational studies on snail populations in various Israeli habitats.
• Analysis of the correlation between snail shell patterns, background substrates, and predator distribution.
• Assessment of environmental factors, including UV radiation levels and water conditions, in different locations.

Main Results:

• A positive correlation exists between cryptic snail variants and the distribution of predators like Barbus longiceps and Blennius fluviatilis within Lake Kinneret.
• Outside Lake Kinneret, snails predominantly exhibit black shells, likely due to selection against harmful ultraviolet (UV) radiation.
• Within Lake Kinneret, reduced UV penetration (due to storms and algal blooms) lessens the need for UV protection, allowing crypsis to be a stronger selective force.

Conclusions:

• Selective forces for crypsis are relaxed outside Lake Kinneret, overridden by UV radiation protection.
• Within Lake Kinneret, reduced UV radiation allows crypsis to be a dominant selective pressure, influenced by predators and background matching.
• The study highlights the dynamic interplay between predation, UV radiation, and environmental conditions in driving shell phenotype evolution in freshwater snails.