Oceanic islands are typically lacking in native amphibians, because very few species have any tolerance to salt water as a result of their sensitive skin, which is often used as a respiratory surface. The Seychelles is the only island group with an endemic family of frogs, i.e. one where all of the member species occur in the same island group and nowhere else on earth. This family is called the Sooglossidae (or sooglossids), and is commonly referred to as the “Seychelles frogs”. Although it is thought that amphibians may occasionally make the perilous journey across seas and oceans via rafting on mats of vegetation, being carried by strong air currents, or human introductions, the Seychelles frogs were transported to their current location by continental drift over millions of years. Around 200 million years ago at the beginning of the Jurassic period, the Seychelles islands were joined to the eastern part of the ancient southern supercontinent Gondwana, which went on to split apart into Australia, Antarctica, India, Madagascar and the Seychelles over millenia of movement of the earth’s plates. The sooglossids survived the geological breakup of India and the Seychelles around 65 million years ago, which is around the same time as the dinosaurs became extinct, and the closest known relatives of the Seychelles frogs are today found in India.
The sooglossids include some of the world’s smallest frogs, with adult Gardiner’s Seychelles frogs measuring just 10-11 mm in total length. Their closest living relative is the purple frog (EDGE rank 4) in the Western Ghats, India, which was only formally discovered in 2003 because it spends most of the year buried up to 4 metres underground. These lineages of amphibians may have been more diverse on Indo-Madagascar in the Cretaceous period, but now they are reduced to two miniscule ancient families that are consequently highly evolutionarily distinct. It is thought that the two lineages diverged around 131 million years ago in the Early Cretaceous period, which is over 30 million years before humans and elephants shared a common ancestor. The slow migration and fragmentation of the Indo-Madagascan fragment of Gondwana subsequently brought each landmass to its current position, separating the Seychelles frogs and the purple frog by about 2,500 km of Indian Ocean.
The Seychelles comprise eight main granitic islands and over 80 tiny islands and coral atolls. The group of islands is 1,000 km across at its widest point and the total land area is around 455 km sq., with the highest point being Morne Seychellois on the granitic island of Mahé, which 914 metres above sea level. The distribution of amphibian species across the Seychelles has probably been impacted by changing sea levels over the millennia. The central granite Seychelles islands were once a continuous landmass of around 130,000 km sq. called the Seychelles Bank, which has been reduced over time by erosion and rising sea levels. It was reduced to its present condition of scattered islands about 10,000 years ago. As a result, there is little endemism between islands since they were all connected relatively recently.
Today, the Seychelles frogs are only present on the larger granitic islands of Mahé and Silhouette. In total, the Seychelles contains 13 species of amphibians – 6 caecilians and 7 frogs, all of which are endemic apart from the Mascarene grassland frog (Ptychadena mascareniensis), which is widespread across sub-Saharan Africa, the Mascarene Islands and Madagascar. All the amphibian species are confined to the granitic islands of the Seychelles, with the larger islands having more species than the smaller islands. They are all ground dwelling, reproducing independently of water. They exhibit a high degree of parental care, with egg guarding occurring in all four species, and parents carrying tadpoles on their back until metamorphosis in one species, the Seychelles frog. Despite being very similar in external appearance, the Seychelles frogs may be identified by their advertisement calls, which are all highly distinctive.
Justin is the Scientific Co-ordinator of Nature Protection Trust of Seychelles
James is an EDGE Fellow working on sooglossid frogs in the Seychelles.
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