The Mascarene Islands,
Mauritius, Réunion and Rodrigues, lie in the western Indian Ocean,
to the east of Madagascar and to the south of the Seychelles. They
form part of a chain of biodiversity hotspots across the eastern
Indian Ocean, where many islands and island groups have developed
unique faunas and floras seen nowhere else. However, while Madagascar
and the Seychelles are continental fragments left over from the
breakup of Gondwana and started as islands with some unique fauna,
the Mascarene Islands are of volcanic origin, formed by hotspot
volcanism in a similar way to Hawaii or the Canary Islands, and all
of the flora and fauna there must have reached the islands since they
first rose above the waves, about 8-10 million years ago in the case
of Mauritius, the oldest and largest of the islands.
Much of the fauna of the
Mascarenes, shows a close affinity with that of Madagascar, the
largest landmass in the southeast Indian Ocean and only 700 km from
Mauritius. Mascarene Groups such as Day Geckos, Slit Eared Skinks,
Giant Tortoises, Land Snails and Orb Spiders have all been shown to
have originated from Madagascar. Other animals have been shown to be
of Indo-Pacific origin, having apparently descended from animals that
crossed the 5600 km of open-ocean from Australia or 7000 km from New
Guinea. This group includes the famous but extinct Dodos and
Solitaires, Skinks of the genus Leilopisma and Geckos of the
genus Nactus.
The Stick Insects,
Phasmatodea, of the Mascarenes present a unique problem to
biogeographers, as they include members of four different subfamilies
and one species of uncertain origin, apparently implying repeated
colonization of the islands by members of different Stick Insects
from different areas. Given the remoteness of the Mascarene Islands,
this would be implausible for a group of flying, wide-ranging
Insects, and is highly unlikely for Stick Insects, which have only
very weak flying abilities, being able to slow their descent when
falling from a tree, but not to make long, trans-oceanic migrations.
In a paper published in
the journal BMC Evolutionary Biology on 16 September 2015, Sven Bradler of the Johann-Friedrich-Blumenbach-Institute of Zoology andAnthropology at the Georg-August-University Göttingen, Nicolas
Cliquennois of the Collège français in Antsirabe, Madagascar and
Thomas Buckley of Landcare Research, the School of BiologicalSciences at The University of Auckland and the Allan Wilson Centre,
describe the results of a genetic study of the origin of Mascarene
Stick Insects, which included data from 120 species of Stick Insects
from around the globe, including seven species from Mauritius and
three from Réunion (the single described Stick Insect species from
Rodrigues, Xenomaches incommodus, is thought to be extinct,
and no material was available for inclusion in the study).
Mascarene stick
insects. (a) Couple of Apterograeffea reunionensis
(Platycraninae) from Réunion. (b) Male of Epicharmus
marchali (Xeroderinae) from Mauritius. (c) Female of
Rhaphiderus spiniger (Tropidoderinae) from
Réunion. Bradler et al. (2015).
Bradler et al. found
that, contrary to expectations, all of the Mascarene Stick Insects
belonged to a single lineage within the Lanceocercata, an
Australasian group, having split from their closest Australian
relatives about 27.15 million years ago, and with all the Mascarene
Island Stick Insects having shared a last common ancestor that lived
about 22.03 million years ago. The group has subsequently diversified
into a number of forms that closely resemble members of other groups
through convergent evolution, i.e. evolving the same traits to deal
with similar ecological problems. For example, the Mascarene genus
Apterograeffea has been considered to be a member of the
Platycraninae (Coconut Stick Insects), closely resembling members of
this group due to also having greatly enlarged cheeks, which support
extra muscles which support the mandibles when chewing on tough
foliage, while the Mauritian Epicharmus has been assigned to
the Xeroderinae, resembling several members of this group with which
it shares the habit of living on tree bark.
Chronogram of the
sampled stick insect specimens with taxa distributed across the
Indian Ocean highlighted in tones of red. Numbers at nodes indicate
bootstrap values (left) and clade posterior probabilities (right);
grey bars show 95 % highest probability density. Circled numbers
refer to fossil calibration points: (1) Renphasma,
(2) Eophasma, (3) fossil Malacomorpha,
(4) fossil Clonistria. Abbreviations: Pl,
Pliocene; Qu, Quaternary. Bradler et al. (2015).
This timescale is
slightly problematic for the colonisation of the Mascarene Islands,
as it requires the Stick Insects to have separated from their
Australian relatives and begun diversifying into newly available
niches considerably before the origin of the islands.
However, the Mascarene
islands are of volcanic origin, the latest in a chain of islands
formed by the movement of a volcanic hotspot, the Réunion Hot Spot,
moving southwards across the Indian Ocean. Volcanic hot spots are
formed where deep plumes of hot magma rising up from the Earth's
interior intersect with the tectonic plates on the surface of the
planet. Since these plumes originate deep within the mantle, their
movement at the surface is independent of the movement of the
tectonic plates. This means that while the currant Mascarene Islands,
Mauritius, Réunion and Rodrigues, are quite young, a chain of
previous islands, now sunken beneath the seas, extends northwards
from them along the line of the volcanic hotspot.
Bradler et al.
reason that the Mascarene Island Stick Insects could have originally
colonised one of these sunken islands, most probably Siant Brandon,
which lies 385 km to the northeast of Mauritius and which first
emerged from the sea about 31 million years ago, or Nazareth Bank,
slightly to the north of Saint Brandon, which first emerged about 35
million years ago, and subsequently colonised the current Mascarene
Islands from these now sunken 'stepping stones', possibly via the
smaller uprisings of Baissac Bank and Soudan Bank.
Dispersal
scenario of Mascarene stick insects superimposed on a map of the
Indian Ocean. (a) Current map of the Indian Ocean. (b) Enlarged view
of Mascarene plateau. Red arrows indicate postulated colonisation
events: The ancestral Mascarene stick insect arrived on currently
submerged islands located to the North of Mauritius followed by a
radiation and at least three (maximal six) independent dispersals to
Mauritius and another three dispersals to Réunion, most likely
facilitated by ocean currents. Bradler et
al. (2015).
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