Marine iguanas have become an acquired taste for mosquitoes
A tiny mosquito is threatening giant tortoises and other iconic reptiles living on the Galapagos Islands.
The mosquito has acquired a particular taste for reptilian blood, scientists have discovered.
They fear it could act as a vector, transmitting new infectious diseases such as West Nile virus to the reptiles.
The scientists have published their findings in the Proceedings of the National Academy of Sciences.
The black salt-marsh mosquito (Aedes taeniorhynchus) is the only mosquito that is widely distributed across the Galapagos archipelago.
It was first recorded on the islands in the late 1880s, but it was not known whether human explorers carried it there, or whether it naturally colonised the Galapagos.
Previous studies have shown that the mosquito regularly spreads the dog heart worm parasite across Central and South America and is capable of spreading other diseases, including St Louis encephalitis virus and West Nile virus.
So Arnaud Bataille of the University of Leeds, UK and colleagues from the Zoological Society of London and the Galapagos National Park studied the mitochondrial DNA of the mosquito to determine where it had come from.
The mosquito actually colonised the archipelago around 200,000 years ago, they found.
Since then, the mosquito has adapted to its new environment, being able to breed up to 20km inshore and at an altitude of up to 700m. On the mainland, the same species of mosquito is normally confined to mangroves and salt marshes along the coast, never moving more than 6km inshore.
More alarmingly, while mainland salt-marsh mosquitoes feed on the blood of mammals and the odd species of bird, the Galapagos salt-marsh mosquito has developed a taste for reptile blood.
Studies of the mitochondrial DNA found in the mosquito's guts showed that 58% of mosquitoes sampled had fed on reptile blood, with 47% biting marine iguanas and 11% biting Galapagos tortoises.
The Giant Galapagos tortoise is something to feast on
It is the first time that the salt-marsh mosquito has been shown to feed on reptiles.
No mammals were present on the Galapagos until brought there by people around 500 years ago. So the mosquito had to adapt to feeding on reptiles when it arrived, say the researchers.
They now fear it could pose a significant threat to the future of the tortoise and iguana, species that are found nowhere else.
The salt-marsh mosquito is already widespread and known to carry pathogens such as West Nile virus, which if nothing is done to prevent it, is predicted to reach the Galapagos in a few years time, says the team.
"With tourism growing so rapidly, the chance of a disease-carrying mosquito hitching a ride from the mainland on a plane is increasing," says Andrew Cunningham of the Zoological Society of London, one of the research team.
"If a new disease arrives via this route, the fear is that the Galapagos' own mosquitoes would pick it up and spread it throughout the archipelago."
Other remote islands such as Hawaii have mountainous regions, which are too high and cold for mosquitoes to colonise. These provide a refuge of sorts for animals susceptible to mosquito-borne diseases.
The Galapagos archipelago has no such refuge, say the researchers, increasing the risk to its native animals.