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Dispersal of the Genus Phelsuma in the Mascarenes

 

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Introduction

All Mascarene species are closely related, indicating they must have had a common ancestor. On the other hand are the islands to young to have generated such a diverse group, with Phelsuma gigas on one end and Phelsuma inexpectata on the other. Most likely the source pool of this group was situated away from the fast speciation process and taxon cycle on Madagascar. 20.000 Years ago, Saya de Malha and Nazareth (incl. Cargados Carajos) formed large islands about 700 km north-east of Madagascar. The first of these islands (Saya de Malha) was formed about 35 Ma and was probably never submerged until the last ice age 18.000 years ago.

Materials and Methods

In order to compare the status, distribution, and other aspects related to the biogeography of the genus Phelsuma, a thorough literature review was done. A map of the region has been generated with the sea levels present 20.000 years ago based on the bathymetry and today's sub-sea level topography.  Several specimen where examined, both alive and preserved. Comparisons between the different mascarene species were conducted.

Results

Geography -The Mascarene Plateau now extends approximately 2,000 km between Seychelles and Mauritius and is one of the few submerged features clearly visible from space. It covers an area of over 115,000 km² of shallow water with depths ranging from 8 m to 150 m on the plateau, plunging to abyssal depths of 4000 m at its edges. It is granite at its core, with a mantle of basalt and limestone.

Some of the granite remains still above sea level, forming the granitic islands of the Seychelles, located in the northern tip of the Mascarene Plateau. While the plateau drifted northwards over the Réunion Hotspot a series of now submerged islands were formed. This process started 64 Ma, after the separation of the Mascarene Plateau and India, about 35 Ma the Saya de Malha bank was formed, later the Nazareth bank and the Cargados Carajos Shoals (Saint Brandon). Cargados Carajos today still counts 22 small islands. About 7 Ma the island of Mauritius was formed. Reunion only reached the surface about 2 million years ago, and is the youngest island originating from the Réunion Hotspot.

Map of the Western Indian Ocean 20.000 years agoThe limestone banks found on the plateau were once coral reefs, indicating that the Mascarene Plateau once formed an archipelago of islands much bigger then Mauritius or Réunion today. Erosion, subsidence and sea level fluctuations made the islands disappear.

Over time there has been a continuous change in sea levels due to periods of glaciation near the poles (ice ages) that determined the climate and sea levels on earth. The last of the ice ages reached its maximum roughly 18,000 years ago, and then gave way to warming. Sea level rose rapidly between 18,000 and 6,000 years ago (about 130 m) , drowning almost the entire Mascarene Plateau.

Today only a few areas remain un-submerged forming the islands of Saint Brandon, Albatross Island and some offshore islets near Mauritius (incl. Round Island). The rest is submerged, large areas by only about 10-40 meters below current sea level.

Plate tectonic processes like subsidence, subduction and sea-floor spreading are not to be neglected even if they occur on much longer time scales than glacial cycles. If the subsidence rate of the plateau is only 0.5 mm per year then the plateau never has been submerged during earlier sea-level fluctuations.

Biology -All Mascarene species are closely related to each other as similar morphological features are present. The location of the nasal cavity in between the nostril and the first super labial clearly distinguishes all Mascarene species, including Phelsuma guentheri and Phelsuma edwardnewtoni, from the other members of the genus.

The vertical pupil and the colouration of Phelsuma guentheri  indicates that it evolved from an ancestor that colonized the islands in a very early stage. Probably during the time the genus just became diurnal on Madagascar. Especially the presence of another semi-nocturnal species, Phelsuma gigas (now extinct), on the very young island of Rodrigues (1.5 Million years) indicates that this species has reached the island recently from a much older source pool.

Discussion

Most likely the now submerged islands played a very important role in the evolution of the genus Phelsuma as stepping stones to colonize the current islands of the Mascarene Archipelago, Mauritius, Réunion and Rodrigues. But also Agalega was probably colonized from these now submerged islands.

Even during the ice ages, average temperatures in the area were only 1° to 3° C lower than today. It is thus very likely that some, now submerged, islands of the plateau were inhabited by the genus Phelsuma. The morphology of the current Mascarene species indicates an early dispersal from Madagascar. The young age of Mauritius does not suspect this, meaning that the ancestor of the Mascarene forms probably used the islands of the Nazareth bank and the Cargados Carajos Shoals as stepping stones to colonize in a later stage Mauritius.

The Mascarenes were probably colonized from the more northern Nazareth bank (now submerged) and Cargados Carajos Islands. There, the early colonists, arriving from Madagascar, evolved at a slower paste due to the limited topographic features. Speciation occurred with little radiation (anagenesis). When Mauritius, Rodrigues and later Réunion where colonized with forms that slowly evolved here, adaptive radiation became more frequent and the speciation process was supported by the many topographic features and the ecological niches on the islands.  

 

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The Mascarene plateau

 

 

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