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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