A group of experts from the University of Barcelona (UB) has described a new endemic lineage of spider genus Loxosceles -- a group of arachnids widely distributed around the world which can cause quite severe bites -- in the Canary Islands (Spain). The scientific finding has been published in the Journal of Biogeography; the paper is authored by experts Carles Ribera and Enric Planas, from the Department of Animal Biology and the Biodiversity Research Institute of UB (IRBio).
More than one hundred species around the world
There are more than 107 species of the genus Loxosceles distributed all over the world, particularly in Africa and America. They are small and brown spiders, active at night, and they have three pairs of eyes distributed in a triangle form. Loxosceles species are medically significant: in some cases, their bite may cause different severe effects.
"Loxosceles spiders are naturally non-aggressive," points out Professor Carles Ribera, expert on systematics and animal phylogeny and member of the Research Group on Zoological Systematics and Evolution of UB. "This spider genus is mostly distributed across South and North America and Africa. Only in some extreme cases Loxosceles bite may cause serious loxoscelism conditions for its venom component toxicity, which has necrotic and haemolytic effects."
Loxosceles rufescens: the only species known to date in the Canary Islands
In the Canary Islands, the only representative of the genus presently recorded is L. rufescens, which was probably introduced by humans as its native distribution appears to be the Mediterranean region. Experts affirm that all of the islands, except La Palma, harbour at least one endemic lineage of Loxoceles. The new endemic Loxosceles group -- which shows great diversification -- extends the catalogue of arthropod biodiversity in this insular area.
The expert Enric Planas explains that "there are morphological, genetic and ecological differences between the new endemic group of spiders and L. Rufescens." "The endemic Loxosceles -- he points out -- has different reproductive organs, it can be found in natural and more preserved habitats (cracks, caves, stones, etc.) and we suppose they have naturally moved from one island to another." On the contrary, the human introduced species, L. rufescens, is normally found urban areas, closed to humans, and its colonization is recent.
UB experts have analysed Loxoceles diversification patterns and colonization pathways across the Canary Islands. By applying molecular phylogenetic techniques, they have determined the evolution of Loxosceles spiders in the Canary Islands, including the study of Loxosceles species in Tunisia, the Iberian Peninsula, Guinea, Madeira, Morocco and Namibia.
Roque Nublo eruption
How did endemic populations move from one island to another? The stepping-stone model of colonization has been postulated as the most common pattern of colonization in oceanic islands. It explains how the distribution of endemic populations in the Canary Islands took place: more than ten million years ago the colonization began with a dispersal event from the continent to the older islands (Fuerteventura and Lanzarote), and then to younger islands of the archipelago. The results of the study follow the general dynamic theory of oceanic island biogeography: the largest biological diversity is found in middle age islands (i.e. Tenerife and Gran Canaria).
Volcanism also plays a key role in shaping within-island diversification. Some hypotheses postulate that a Gran Canaria biota was nearly all extinct as a result of the Roque Nublo volcanic event. "The results of the study do not support this hypothesis," highlights Enric Planas. "To be exact, it appears reasonable to associate the Roque Nublo eruption with the vicariant origin of the two local Loxosceles species of Gran Canaria," he details.
Oceanic islands: discovering evolution mechanisms
Oceanic islands are excellent natural laboratories for revealing the evolutionary mechanisms involved in colonization and species diversity. "The Canary Islands harbour greatest fauna richness," emphasizes Professor Carles Ribera, who has headed many works on taxonomy, systematics and animal phylogeny. "Spiders have been shown to be a good model for the study of island biogeography and speciation in oceanic archipelagos around the world," he adds.
Biogeographical studies carried out by the research group in the Canary Islands can be used in other ecological systems. "These studies -- states Professor Ribera -- contribute to improve scientific knowledge on natural ecosystems and provide new perspectives to solve problems related to ecology, management, conservation biology and the impact of non-native species on local populations."
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