On the surface, there is little to distinguish small islands on continental fragments from oceanic islands that have no continental connections. Careful observation, however, can reveal some interesting differences. The Seychelles microcontinent in the Indian Ocean is an intriguing case in point. The islands sitting on this chunk of continental crust constitute a sovereign state, also called Seychelles. Seychelles is a small country of only 176 square miles (457 km2) spread out over 115 islands, fewer than a dozen of which are inhabited. Roughly 90 percent of its 118,000 inhabitants live on Mahé, the largest island in the archipelago, which is only 60.7 square miles in extent. A relatively prosperous country – with the highest per capita GDP in Africa, the continent to which it is conventionally appended – Seychelles relies heavily on tourism, much of which is up-scale.
I doubt that many tourists notice anything distinctive about the physical geography of Seychelles. To be sure, its mountainous main islands are nothing like the low coral atolls of the Maldives, another tourism-dependent Indian Ocean country. But they might seem reminiscent of the volcanic high islands of the Pacific, such as Tahiti or those of the Hawaiian archipelago. But a geologically knowledgeable traveler would note the exposed granite of the larger Seychelles islands, which are very different from the volcanic rocks found in elevated oceanic islands. Granite erodes in a different manner than volcanic rocks, often giving rise to distinctive landscapes (see the illustration below).
The Seychelles’ granite core is old, having formed during Precambrian times roughly 750 million years ago. The area in which they formed was then part of the supercontinent of Gondwana. A large segment of Gondwana, which included Australia, Antarctica, India, Madagascar, and the Seychelles, began to rift away from the landmass roughly 145 million years ago. Madagascar-Seychelles-India later separated from Australia-Antarctica, and, later still, Madagascar broke away from this landmass. The Seychelles micro-continent finally began to rift away from India around 66 million years ago, roughly the time the dinosaurs died out and the Earth transitioned from the Mesozoic to the Cenozoic era.
As an isolated microcontinent that separated from larger land masses many millions of years ago, Seychelles has no indigenous land mammals. It does, however, have several amphibians and reptiles whose ancestors were present before it split away from the larger lands. Of these, the most interesting to my mind are eight species of caecilians, all of which are endemic to the archipelago.*
Caecilians constitute one of the three living orders of amphibians, the Gymnophiona (the others are the frogs & toads [Anura] and salamanders [Urodela]). Caecilians are a relatively diverse lot, divided into ten extant families with 44 genera and 215 species. They can be quite abundant, with hundreds or even thousands of individuals living per hectare in undisturbed tropical rainforests. Although most caecilians are relatively small, Caecilia thompsoni can reach five pounds in weight and five feet in length.
But despite their abundance in certain habitats, caecilians are rarely encountered and have been relatively little studied. Although a few species are fully aquatic, most live underground in moist soil. Although they have vestigial eyes, burrowing caecilians are effectively blind and several species have a superficial resemblance to earthworms. But, like other amphibians, caecilians are vertebrates and are thus much more closely related to humans than to earthworms. Many have thick and hardened skulls, which they use to push through the soil as they hunt invertebrates. All species have teeth, which can be sharp and of fearsome appearance. Some people with whom I shared the photos posted below recoiled, viewing caecilians as the stuff of nightmares.
As the map posted below shows, caecilians live only in humid tropical zones. Their center of diversity is the Amazon Basin, with southwestern India forming a secondary center. Seychelles is also significant, with eight species and three genera. In the Americas, the range of caecilian does not extend north of southern Mexico, but individuals of the fully aquatic species Typhlonectes natans, commonly called the “rubber eel,” have recently been found in southern Florida. Presumable, they were released into the wild by people who had kept them in aquariums.
One of the most intriguing aspect of caecilians is the care that some species give to their young, a trait that is relatively unusual among amphibians. Some caecilians, belonging to at least two families, nourish their young through “maternal dermatophagy.” As explained by Wikipedia:
[They feed their] young by developing an outer layer of skin, high in fat and other nutrients, which the young peel off with modified teeth. This allows them to grow by up to 10 times their own weight in a week. The skin is consumed every three days, the time it takes for a new layer to grow, and the young have only been observed to eat it at night.
Scientists have recently discovered that some female caecilians produce a substance very similar to milk, which is also used to feed their young. Milk is produced only by mammals, although pigeons, flamingos, and male emperor penguins feed their young with a somewhat similar substance, known as “crop milk.” Oddly, the nourishing caecilian secretions seem to be more similar to those of mammals than of birds. The discovery of amphibian “nursing” is recounted in an All That’s Interesting article:
[Researchers] … noticed that ringed caecilian young kept themselves close to the mother’s cloacal opening, the rear channel that serves urinary, digestive, and reproductive purposes. “The babies’ heads were close to the female’s cloacal opening all the time,” Jared told Scientific American. “Some even put their heads inside and seemed very excited.” The team decided to compare the intestines, bladders, cloacae, and oviducts of female ringed caecilians with and without offspring. Within the mothers’ oviducts, they found large glands full of fatty acid and a sugar-rich milky substance. The makeup of this “milk” was very similar to that of mammals. Researchers noted that it likely supplemented the protein-rich skin that the babies fed on once a week. Additionally, researchers discovered that the offspring wriggled near the cloacal opening and emitted high-pitched sounds to encourage the mother to release her milk. This phenomenon could occur up to six times a day. Such begging behavior has never been observed in amphibians before.
Another other interesting but sad fact about caecilians must be mentioned. In the folklore of several regions of India, these secretive and innocuous amphibians are feared and despised, incorrectly regarded as venomous. As a result, they are often killed whenever encountered, reportedly by being doused with salt or kerosene. Ironically, in India cobras – which kill several thousand Indians every year – are generally respected and even revered, largely because of their association with Lord Shiva.
Finally, it may be instructive to compare the geographical ranges of caecilians and moles (mammals in the family Talpidae). These highly dissimilar animals occupy similar ecological niches, living underground and burrowing to pursue earthworms and other soil invertebrates. Significantly, their ranges do not seem to overlap. Moles are found in humid (or seasonally humid) temperate areas in North America, Europe, and Asia, and do not extend into the tropics. Is it possible that they have been excluded from such areas by competition with caecilians? One place where their ranges might overlap is the southeastern Himalayas. The chatbot Grok 3, however, claims that adaptation to different microhabitats prevents moles and caecilians from living in the same places within this larger region:
In northeastern India (e.g., Assam or Arunachal Pradesh), caecilians are found at lower elevations in wet, forested areas, while Talpidae moles may inhabit higher elevations in the same region’s montane zones. However, their microhabitats rarely intersect. Caecilians favor saturated, organic-rich soils or streamside environments where their moist skin and amphibian life cycle are supported. Talpidae moles, being mammals, prefer drier, looser soils for tunneling and are less dependent on constant moisture. Elevation plays a key role: caecilians are scarce above 2,000 meters due to colder temperatures, while moles thrive at these higher altitudes.