Thursday 24 September 2015

Stick Insects of the Mascarene Islands.

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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