Mass extinction on the islands of snail diversity: An invasive predatory snail has exterminated over 50 tree snail species on Tahiti and its neighboring islands in the past few decades. Using sophisticated technology, researchers have now solved the mystery of why a certain species was able to escape this fate. The prominent Partula hyalina snail can therefore tolerate more sunlight than its rather dark enemy due to the “cool” white of its house. Therefore, the native snail was able to survive in sunlit forest edge areas, report the biologists.
Humans were and are a catastrophe for many ecosystems in the world: The arrival of our species has had a particularly devastating effect on islands on which animal and plant species have been able to develop undisturbed for a long time. On the South Pacific Society Islands with their center in Tahiti, the Partula snails in particular were able to fan out into many species: They were therefore also known as the “Darwin’s finches of the snail world”. But this treasure trove of snail diversity was lost to evolutionary biologists through the influence of humans.
The sad story began with the deliberate introduction of the giant African land snail to the Society Islands. It was supposed to serve as a human source of food, but then the voracious mollusks began to attack crops and became pests. In 1974, agronomists again decided to import a snail – but this time it was a predator: the rosy wolf snail (Euglandina rosea) was supposed to bring the African land snails under control. This also succeeded – albeit with devastating collateral damage: The native tree snails of the Society Islands were also easy prey for the predatory molluscs. “The endemic species had never been confronted with such a predator before, and so the wolf slug was able to very quickly bring most of the populations to extinction,” says co-author Diarmaid Ó Foighi of the University of Michigan.
Why did the white snail survive the mass extinction?
From the once 61 species, only five are left today and their future fate is uncertain. The stocks of Partula hyalina have held up best. For the Polynesians this is luck in bad luck, because in their culture this species plays a particularly important role as a decorative object due to its shimmering white color. But why did this species survive? In their current study, Ó Foighi and his colleagues examined the question of whether the brilliant white might play a role. Because maybe it makes Partula hyalina more heat-resistant than the wolf snail.
To test their suspicion, they decided to study the typical daytime light exposure of P. hyalina and the wolf slugs. The researchers adapted the “Michigan Micro Mote”, known as the smallest computer in the world, for their purposes: In order to be able to measure the light intensity with the device, which is only a few millimeters in size, the researchers used data from the energy production by the tiny solar cell. The light intensity could be measured continuously by the scientists recording the speed at which the battery was charged. “So, with the computing system that was small enough to stick on a snail, we were able to generate data that no one could have obtained before,” says co-author David Blaauw of the University of Michigan.
White makes you less hot
The team then stuck the sensors for data acquisition directly onto the houses of the wolf snails. With the protected P. hyalina, however, they had to be more careful: With the help of magnets, the team placed the devices on both the upper and lower sides of leaves on which the snails were staying in their natural habitat. At the end of each day, the researchers could then wirelessly download the data from each of the small computers.
It was shown that the wolf snail typically only exposes itself to comparatively low levels of radiation. In contrast, P. hyalina is exposed to ten times more sunlight on average. Peak values of up to 71,000 lux occur. However, due to the reflective power of its white shell, this apparently does not lead to overheating in P. hyalina, the scientists explain. However, the high radiation values are presumably fatal for the wolf snail, as the light on its rather dark surface is intensively converted into heat. The researchers suspect that the predator does not even venture to the edge of the forest at night to catch P. hyalina because there is a risk that she will not be able to escape into the shade in time.
Thanks to the collaboration between biologists and engineers, it now seems clear why P. hyalina of all people was able to defy the rabid predatory snail: The light of the sun protects it in the loosely overgrown forest edge habitats. “The sensor computers can thus show how we can protect the endemic species on the islands,” says co-author Cindy Bick from the University of Michigan. “If we are able to map their remaining habitats and protect them through appropriate measures, we may be able to ensure the survival of the species.”
Source: University of Michigan, technical article: Communications Biology, doi: 10.1038 / s42003-021-02124-y