Note: Every now and again I post an invited blog post by a member of the community. This time around it is the turn of Victoria Protheroe, a prolific author from the currently-dormant blog Bioweb. Thanks in advance to James Nankivell for the use of this photos within!

This article will briefly note the four groups of marine reptiles, followed by a discussion with James Nankivell, Herpetologist and PhD candidate at the University of Adelaide, who focuses on sea snake evolution, discussing the complicated situation of the once thought to be extinct Short-nosed Sea Snake (Aipysurus apraefrontails). Marine reptiles are reptiles that have become adapted to life in marine environments by exhibiting an aquatic or semi aquatic lifestyle.

 Millions of years ago, this group was dominating the oceans and seas, but today most reptiles are adapted to land. Of over 8,000 species of living reptiles, only around 100 are categorised as marine species, this includes sea turtles, marine iguanas, sea snakes and crocodiles. They have a tropical and subtropical distribution, with a few exceptions of sea turtles adapted to temperate to cold waters.

Features

Marine reptiles are ectotherms or ‘cold blooded’ meaning that they do not regulate their own temperature, but instead, they require external heat to warm their body. These organisms are extremely tolerant of lower body temperatures in comparison to other groups of marine vertebrates. They possess scales made of keratin and breathe through their lungs. One of their main adaptions is having a slow metabolic rate because it enables them to consume much less oxygen than mammals in order to spend prolonged periods of time inside the water without having to return to the surface to breathe.

 The general features of marine reptiles depend on the group of organisms as each one of them are adapted to different habitats and play different roles in the marine environment.

Marine reptiles are divided into 4 groups :

Chelonids

Chelonids, also known as sea turtles, are migratory marine reptiles that are generally found in tropical and subtropical ecosystems. There are several living species of sea turtles – green, leatherback, loggerhead, olive ridley, hawksbill, flatback and Kemp’s ridley turtles – that are distributed in all oceans except for Arctic and Antarctic.  Sea turtles spend most of their lives in the open ocean living solitary lives but form large groups to get close to the coastline to nest. Apart from their strong shell that brings protection, they possess salt glands to release the excess salt obtained from the water. This salt is excreted through a lachrymal gland located in the orbital cavity, which is why sea turtles are commonly seen ‘crying’ in land.

Iguanids

The group Iguanids, also known as marine iguanas, are restricted to the Galápagos Islands in Ecuador. Therefore, they are exclusively tropical and are adapted to a semi aquatic life in the marine environment. They spend most of their life in land, where they sunbathe to thermoregulate, but enter the ocean to dive and feed on red and green algae. Depending on their habitat, the size of marine iguanas ranges from 11 to 55 cm in length (without the tail) and, depending on the species, they possess a tail that ranges from 17 to 84 cm. Their body is characterised by a row of spines and bony plates, which are longer in males.

Crocodilids

Within this group, there is only one marine species that is commonly known as the saltwater crocodile (Crocodylus porosus) and is the largest extant reptile. These marine reptiles can be found from India and Southeast Asia to Australia and Micronesia.   The male saltwater crocodile can reach a size of about 6 metres long and a weight of 1,000kg, while the female can reach 3 metres long. It is characterised by its wide and long snout and the possession of oval scales and triangular scutes. They are carnivorous and prey on fish, amphibians, molluscs, crustaceans, and small to medium sized mammals.

Hydrophiids

Hydrophiids refers to a group of venomous marine reptiles known as sea snakes. There are approximately 70 species and all of them spend the vast majority of their lives in the ocean. Their laterally compressed body is fully adapted to the aquatic environment, preventing them from occupying a habitat in land. The size of a sea snake ranges between 120 to 300cm in length and they possess a paddle-like tail that makes their swimming more efficient. These marine reptiles have dorsal nostrils and almost no ventral scales in comparison to other reptiles. They live in both pelagic and costal zones and most species are highly venomous, but bites to humans are very rare.

 Sadly, there are major threats facing marine reptiles, such as polluted habitats, deforestation, habitat loss and a host of threats by man.  The majority of marine reptiles are sea snakes. There are 9 species of sea snake which are under significant threat:

1. Leaf-scaled Sea Snake (Aipysurus foliosquama) A venomous snake found on the islands of Ashmore and Cartier in Australia.

2. Short-nosed Sea Snake (Aipysurus apraefrontails) A venomous species found in the reefs of Western Australia.

3. Timor Reef Snake (Aipysurus fuscus) which is also often called the Dusky sea snake. This snake is native to the Timor Sea. 

4. Large-headed Sea Snake (Hydrophis pacificus) this snake is at risk of going into complete extinction. It is a native species to the eastern Arafura Sea.

5. Rennell Island Sea Krait (Laticauda crockeri) this species can only be found in the Solomon Islands.

6. Yellow-lipped Sea Snake (Laticauda frontalis) found in the coral reefs of Vanuatu.

7. Laticauda guineai, this endangered sea snake can only be found in the Southern parts of Papua New Guinea. Not a lot of information is available for this sea snake due to its extreme rarity (Deepoceanfacts).

8. Flat-tail Sea Snake (Laticauda schistorhynchus), this venomous snake is a native species to the Pacific Island nation of Niue.

9. Chinese Sea Snake (Laticauda semifasciata), the Chinese sea snake lives in the Pacific Ocean since they prefer waters with warm temperature. It also often inhabit the coral reefs.

Research

James Nankivell works with sea snakes as a group from an evolutionary context, and described a new species of Turtle-headed Sea Snake in 2020, although, he notes that, thankfully, it is unlikely to be threatened. “Much of what I do involves finding snakes, running genetic analysis and comparing preserved museum specimens. I do a bit of work with the prawn trawling industry although in WA where I work the industry is small and only affects a small portion of the species’ ranges.”

James also details that the biggest conservation concern in Australia is from Ashmore Reef. “This offshore Reef used to have some of the highest densities of sea snakes in the world. However in the early 2000s the snakes essentially vanished, snakes were nearly impossible to find in the shallow reef where they were once abundant. Pretty much all but 1 species from the area disappeared, and two of those were at the time only known from Ashmore Reef. Luckily we’ve found that those species are reasonably common in coastal Western Australia, but the region was essentially unsampled for sea snakes until recently.”

Sea snakes are the most diverse group of marine reptiles in the world, but they are poorly understood, less is known about their ecology than that of other groups of reptiles. Populations of Sea Snakes have been declining rapidly for the last 20 years worldwide, as a result of climate change, pollution, fishing and so forth. They are especially sensitive to underwater vibrations making them particularly vulnerable to boat engines and other noises that are generated by humans. (Lockwood, 2019)

The critically endangered, venomous Short-Nosed Sea Snake (Aipysurus apraefrontalis) occupies areas in the Ashmore Reefs and Hibernia Reefs off the coast of northern Western Australia. It grows to just over a metre in length and is usually a brownish-purple colour with light banding across its body.

Family – Elapidae: Hydrophiinae

Risk Factor – Highly venomous: postsynaptic neurotoxins, and possibly also mycotoxins, with secondary nephrotoxicity and cardiotoxicity

Distribution- Timor Sea: Western Australia (Ashmore and Hibernia reefs, possibly Western Australia coastline)

Elevation- Sea-level to 33 ft (10 m) bsl

Habitat- Shallow coral reef flats and coral sand

Diet- Fish (eels)

Reproduction- Viviparous, litter size unknown

Conservation Status- IUCN Critically Endangered

English herpetologist Malcolm Arthur Smith described the species in 1926 from a specimen collected on the Ashmore Reef. It had not been sighted at Ashmore Reef since 1998 and was feared extinct in the region. It was thought to only breed on Ashmore Reef, where it had not been recorded for fifteen years and hence was feared extinct. A 2015 study found that samples from the short-nosed sea snake, found in Exmouth Gulf, offshore from Roebourne Broome, and from the Arafura Sea indicated that these represented distinct breeding populations, and not vagrants from elsewhere. A courting pair was observed at the Ningaloo Reef in December 2015, suggesting that a breeding population may be extant there.

Ashmore Reef, which sits about 320 kilometres off the north-west coast of Australia, was once home to one of the most abundant assemblages of sea snakes on the planet, but began mysteriously disappearing. Australian Geographic notes that populations of short-nosed sea snakes are declining at rates of around 90% yet the decline has not been attributed to one specific cause. “Global warming has certainly affected the species’ habitat, with coral bleaching occurring in reefs due to the warming of the ocean. In addition, there is a possibility that the ocean temperature has exceeded the snake’s tolerance level, which is believed to be 36°C.”

It is believed that commercial prawn fishing has had an affect on the sea snake populations. The Northern Prawn Fishery operates in the vicinity of reefs that sea snakes inhabit, and accidental capture of sea snakes often results in death or injury to the species. The low reproduction rate adds to the species’ vulnerability. (Nadge, 2015) The reason for their decline are rather complex, and likely related to a decline in the whole ecosystem. Their reproduction is slow due to their small broods and high rate of juvenile mortality. 

Two of the closest relatives of Aipysurus apraefrontalis, the Leaf-scaled Seasnake (A. foliosquama) and Dusky Seasnake (A. fuscus), are also endemic to Ashmore and Hibernia Reefs, and are respectively listed as Critically Endangered and Endangered due to the sudden and inexplicable decline in seasnake numbers and diversity.

References

Andrews, C.W, ‘A Descriptive Catalogue of the Marine Reptiles of the Oxford Clay: Part 1’ Adamant Media Corporation (22 Nov. 2000).

Andrews, C.W, ‘A Descriptive Catalogue of the Marine Reptiles of the Oxford Clay: Part 2’ Adamant Media Corporation (22 Nov. 2000).

DeeperBlue (https://www.deeperblue.com).

Lockwood, D . ‘She Studies Sea Snakes by the Seafloor’, The New York Times, August 19, 2019.

Nadge, R. Short-nosed sea snake. www.australiangeographic 2015.

Somaweera, R. et al. ‘Pinpointing Drivers of Extirpation in Sea Snakes: A Synthesis of Evidence From Ashmore Reef’ Frontiers in Marine Science (2021).

With thanks to James Nankivell for his time and contribution – Twitter @NankivellJames

If you liked this post and enjoy reading this blog, please consider supporting me on Patreon where you will also gain access to exclusive content.