Wednesday 17 December 2014

The origin of the Canary Islands Trapdoor Spider.

The Canary Islands are a group of volcanic islands in the northeast Atlantic, approximately 110 km off the coast of Morocco. They are true oceanic islands, never having been connected to a continent, and started to form in the Miocene about 22 million years ago, with volcanism starting on what became the island of Fuerteventura and subsequently moving to the west. The Eastern Canary Islands are now considered to be erosional in nature (i.e. the islands are now shrinking due to erosion rather than growing due to volcanism), and have an arid climate, though volcanic activity is still occasionally recorded even on Fuerteventura. The Western Canary islands are higher with volcanism being the main geological process occurring, and have a moist climate.

Like all oceanic islands the Canaries have a distinct flora and fauna, with 50% of invertebrates and 27% of vascular plants being endemic to the islands (i.e. found nowhere else). Unusually for such islands, the Canaries have their own species of Mygalomorph Spider, the Canary Islands Trapdoor Spider, Titanidiops canariensis, which is found on the East Canary Islands of Fuerteventura, Lanzarote and La Graciosa. Mygalomorph Spiders are unusual on oceanic islands, as they do not disperse easily across water. They tend to be large (the group includes Tarantulas and Baboon Spiders), ground dwelling and not prone to extensive dispersal. Several species of Tarantula are known from the Caribbean, but all belong to the genus Ummidia, the young of which are known to practice ballooning (i.e. the young Spiders let out long strings of silk which they use to catch the wind and drift to new locations, common in many Spider groups but very unusual in Mygalomorphs) and a few species are known from islands in the South Pacific.

In a paper published in the journal PLoS One on 10 December 2014, Vera Opatova and Miquel Arnedo of the Institut de Recerca de la Biodiversitat & Departamentde Biologia Animal at the Universitat de Barcelona publish the results of a genetic analysis intended to determine when the Canary Island Trapdoor Spider diverged from the related Moroccan Trapdoor Spider, Titanidiops maroccanus, and the subsequent dispersal of the Spiders within the Canaries.

Specimen of Titanidiops canariensis from Betancuria on Fuerteventura.

Opatova and Arnedo compared mutation rates in four nuclear and four mitochondrial genes in order to attempt to develop a timescale for population divergences in Titanidiops  canariensis (nuclear genes are located on the chromosomes in the cell nucleus and are subject to sexual recombination with each generation; mitochondrial genes are found in the cells mitochondria, and are always inherited from the maternal side without sexual recombination).

They found that Titanidiops canariensis probably shared a last common ancestor with species on the mainland about 12 million years ago, and that this mainland population later split into the ancestors of Titanidiops maroccanus and Idiops syriacus (a species found in Israel) about 11 million years ago. On the Canary Islands Titanidiops canariensis split into two clades (populations with a common ancestry) about 8.08 million years ago, and one of these clades again split in two about 6.98 million years ago. However, while they are confident about the relationships between the different groups, discrepancies between the nuclear and mitochondrial DNA results mean that they are not completely confident about the timings of the splits.Titanidiops maroccanus also appears to have a number of deep evolutionary splits, though this was not the main subject of the study.

This suggests that the genera Titanidiopsand Idiopsare polyphyletic, with Titanidiops maroccanus and Idiops syriacus more closely related to one-another than they are to Titanidiops canariensis, and all three species more closely related to one-another than to other members of the genus Idiops. It also suggests that there are probably at least two cryptic species of Trapdoor Spider in the Canaries classified under the name Titanidiops canariensis (cryptic species are species which appear identical, but which are nevertheless reproductively isolated; this is very common in morphologically conservative groups with low dispersal rates, such as Mygalomorph Spiders), and that Titanidiops maroccanusis also probably a cluster of cryptic species.

The genus Titanidiops appears to have only reached the Canaries on a single occasion, unsurprising given the difficulty of large Mygalomorph Spiders crossing stretches of open ocean. They note that members of the genus are found around the River Sous in Morocco, and that in the past this river is thought to have produced seasonal torrential currants as a result of a much wetter climate. This could have resulted in large rafts of vegetation being carried out to sea, some of which may have reached the Canaries. Such rafts may have carried Trapdoor Spiders, as well members of other groups which do not readily cross oceans but which are found in the Canary Islands, such as other ground Spiders, Beetles, Skinks, Geckos and Rodents.

See also…

Mygalomorph Spiders (Tarantulas and related species) are considered to be one of the most ancient groups of Spiders. They have two pairs of book lungs (many other Spiders have lost a pair) and downward pointing, rather than opposable fangs, again considered to be a primitive state in Spiders. Many species of Mygalomorph attain large sizes, all have flattened, disk-shaped bodies (rather than the more globular bodies of most other Spiders), and most are ambush predators.

Tarantulas belonging to the Subfamily Aviculariinae are tree-dwelling Tarantulas from Central and South America and the islands of the Caribbean. They are popular in the pet trade due to their docile natures; they are generally...

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