Seven new species of funnel-web spiders have been discovered in caves in Israel, and they are all in various stages of losing their sight.
With Charles Darwin famous example of customizationa population of finches divided on different islands gradually drifted away from their common ancestral form to the point where they were distinct species.
This early observation recognized that isolation is a key factor for organisms to change enough to specialize.
But as this latest cave discovery shows, vast expanses of water are just one way to separate populations.
“In this study, we sought to understand the evolutionary relationships between normal-eyed funnel-web spiders found at the burrow entrance and those further down the burrow that are pigmentless, eye-reduced, and even completely blind.” explained Ecologist Shlomi Aharon from the Hebrew University of Jerusalem (HUJI).
Aharon and colleagues found types of Tegenaria Funnel Web Spiders in 30 Dens. Twenty-six of these were cave-loving species that still depended on some external conditions such as light (troglophiles), and they lived at the entrances and twilight zones of the caves. In the meantime, 14 other species have been identified as obligate cave dwellers (troglobites) that only inhabit the twilight or dark zones.
Seven of these troglobite species were new to science, five with reduced eyes and two completely blind.
“Of the spiders we found, five were unique to different caves, and the other two species were found in several caves in the Galilee and in caves in the Ofra karst field, which is now threatened due to building plans.” explained HUJI ecologist Efrat Gavish-Regev.
The researchers analyzed the spiders’ DNA to trace their history in the landscape.
Adapted to the unique cave conditions, these spiders can no longer thrive in the world beyond these dark, sheltered crevices. This means that there was very little, if any, gene flow between the cave systems, even when each population is relatively close.
“One of the surprising results of the study shows that the new species are evolutionarily closer to species from caves in Mediterranean areas in southern Europe than to species living in close proximity to them at cave entrances in Israel,” says Gavish Regev.
The genetic trace points to a single one Tegenaria Species swept through the area and eventually ventured deeper into the caves. By removing the need for functional eyes, the new environment shaped their characteristics and selected offspring that did not waste energy on unnecessary visual systems. This happened independently in the different cave systems – a process known as convergent evolution – resulting in different species all losing their eyes after going their separate ways.
Then the original species became locally extinct outside the caves before other related species moved in outside.
The team suspects that the extinction event was a result of climate change about 5 million years ago, at the beginning of the Pliocene, when seas rose to current levels.
“We are currently witnessing the impacts of climate change on many habitats, compelling us to consider, sustain and promote programs that include the preservation of subsurface habitats – many of which are imminently endangered.” closes Hawlena.
“We must protect Israel’s unique nature, preserve its underground systems for the future, and continue to explore the processes that created these systems in the country.”
This study was published in Molecular Phylogenetics and Evolution.
