Reptiles are the most diverse terrestrial vertebrates with about 12,000 described forms (9,350 recognized species, 3,000 subspecies).
Reptiles evolved 260 million years ago from the acquatic amphibians. During the Jurassic (150-200 Myr) modern reptiles had appeared.
Mesozoic plesiosaurs, mosasaurs, and ichthyosaurs are long extinct, now the most conspicuous and familiar marine reptiles are the sea turtles, consisting of seven species present in tropical and subtropical marine habitats in all of the world’s oceans.
Like the sea turtle, another groups of reptiles re-entered the oceans: we are speaking about sea snakes. This group is one of the best, among reptiles, adapted to marine environments.
Sea snakes are represented by eight species of semiaquatic oviparous (embryo released with the deposition of eggs) sea kraits (Laticauda spp.) and 60 species of fully marine viviparous (embryo in the mother’s body) “true” sea snakes.
Combining morfological and molecular studies, we cannot refute the hypothesis that sea snakes have a marine ancestor and cannot derive from a terrestrial one (that can eliminate the hypothesis that the reptile, and sea snakes returned in the sea after a period on land).
The majority of speciation events occurred during changes of sea level that created and dissolved barriers between marine basins during the last 2.5 million years. Therefore, the fully marine sea snakes represent a recent, rapid radiation.
The evolutionary origin and diversification of the viviparous sea snakes were in the Coral Triangle (Indo-Malayan Archipelago) and Australasia. Speciation within different lineages (including Laticauda) involved trophic associations with coral reefs and numerous islands, generally extending outward from the Coral Triangle to other coasts and water of the Pacific and Indian oceans.
Snakes inhabit also brekish and fresh waters. Effectively, some terrestrial and arboreal species, have learned to exploit marine resources. Furthermore, some freshwater species have adapted to the life in brackish water, sometimes enter the ocean, or live there permanently. This is a continue evolution of this group.
But the most typical feature of a sea snake is the vertically flattened paddle-like tail, which is not found in any other terrestrial or aquatic snakes.
Sea snakes occur in the tropical and subtropical waters of the Indian and Pacific Oceans from the east coast of Africa to the Gulf of Panama. However, two specimens of Pelamis platurus reported from Namibia indicate that the species may be extending its range into the Atlantic Ocean. In reality no species of marine snakes now inhabits the Atlantic ocean. In the past the Atlantic ocean had at least 20 species of fully acquacit snakes. These species are called Palaeopheids and lived during the Paleocene–Eocene Thermal Maximum, when global temperatures increased by 5-8°C and caused oceanic surface temperatures near the North Pole to increase from about 18°C to over 23°C. Afterwards, the extinction was correlated with the rapid cooling of the Earth at the end of the Eocene.
Sea snakes don’t appreciate cold water so they are absent from colder water such as the polar seas, areas of high salinity such as the Red Sea, and the entire Atlantic Ocean and Caribbean Sea. The Atlantic ocean and the Carribean Sea are an appropriate habitat for these species, so why sea snakes are absent here?
We can consider the example of a pelagic species present in the southern tip of Africa, Hydrophis platurus, from the Indian ocean. But why this species is present only in the east part and not in the west part of the african continent?
- Oceanic temperatures are relatively low and not especially favorable for reproduction.
- Presence of a stable trough of high pressure along the western coast of South Africa that extends northward along the coast of Namibia.
- The surface temperatures are also influenced by the upwelling (we have already spoken about this pheomena) thanks to the Benguela current (that exists from at least 5 million years). This current takes the cold water from the deep to the shallow water. At these temperature sea snakes can’t survive (under 18°C) and can die in 5-12 days.
- The average rainfall is less than 20 millimiters per year. Without precipitation, these species dehydrate.
This can explain why the sea snake doesn’t pass the african continent, but we have another way of income in the Atlantic: the Isthmus of Panama.
The Isthmus of Panama may have started to emerge as early as approximately 15 to 12 Ma ago thereby fully separating the Pacific and Atlantic Oceans no later than 4.5 Ma ago. This was much earlier than the speciation of H. platurus (divergence dating to about 6 Ma ago) so there was no dispersal corridor to the Atlantic Ocean when this pelagic species reached the Central American coastline.
Now, nevertheless, we have the Panama Canal that reconnects the Pacific Ocean and the Carribean Sea but the establishment of a breeding population originating from rare dispersal events would be highly improbable.
It is wonderful to observe how thin the thread that regulates existence is, and the boundaries of these magnificent specimens. Like all animals, they remind us of where we have started. Being confined to a world without borders. Fortunately, we have something that all other living beings lack, and I’m not talking about limbs. The mind, which allows us to leave without changing residence. We have the mind that allows us to know the sea serpent never seen on the other side of the world.
We started from time immemorial, who knows how we will be, who knows when we will arrive and if it will ever happen.
P.S. if you don’t want to meet sea snakes, avoid being exotic 😉 at least for the moment (million of years)
Harvey B. Lilliwhite et al, January 2018. BioScience / Vol. 68 No. 1.
Kanishka D. B. Ukuwela et al, 2016. Journal of Biogeography 43, 243–255.
Arne Redsted Rasmussen et al, 2011. Plos One 6(11): e27373.
Michael S.Y. Lee, 2005. Biol. Lett. 1, 227–230.