| 1 | Ornate reef snake |
Maximum length: 115cm.
The ornate reef snake (Hydrophis ornatus) is one of the more flexible sea snakes, as it can inhabit muddy estuaries and shallow ocean floors all the same. Coral reefs are another of their habitats, where they hide in order to ambush catfish, cardinalfish and goatfish swimming freely in the open water nearby.
The ornate sea snake isn’t endangered in the slightest, appearing in over 20 countries, from Oman and Thailand to Australia and Fiji. This species is at home in shallow waters, and during low tides, can sometimes be spotted resting on the tops of exposed coral reefs. They particularly like to hunt in sandy underwater areas directly adjacent to coral reefs.
Scuba divers beware – the ornate sea snake has a strong venom, and at least one human death is known from its bite. This reef snake is active during both day and night, so there’s a decent chance of encountering them.
Ornate reef snakes vary from 85 to 115cm. They always have thick black patterns on their bodies, but the colour beneath varies from white to olive-yellow.
| 2 | Olive sea snake |
Maximum length: 2 metres.
The undisputed ruler of Great Barrier Reef. The olive sea snake (Aipysurus laevis) is a common Australian species, which appears all over the northern coast, reaching as far south as Brisbane (not quite reaching Sydney). Further afield, they’re found in New Guinea and the French territory of New Caledonia.
At an average length of 1.5 metres, and with a curious personality, the olive sea snake might be the easiest coral reef species to encounter. They’re relaxed around human beings, and have a weird tendency to chase exploring scuba divers and wrap harmlessly around their legs. While their venom has an LD50 rating of 0.22mg, they rarely bite.
Olive sea snakes are extremely patient hunters, and choose the vertical walls of coral reefs to explore for fish. They investigate each crevice thoroughly, before moving on to the next crevice 2 metres away, and then the next, repeating this laboriously for many hours.
Within Australia, Ashmore Reef, the Timor Coast and Shark Bay are also hotspots for this very common snake. The olive sea snake is docile, but never touch them, as this is the one thing that unleashes an aggression spiral.
Olive sea snakes live in coral reefs, but lack a particular base, as one study scooped them up and released them in new reefs a few miles away. The olive sea snakes easily adjusted to their new homes, making no attempt to swim back.
| 3 | Ijima’s sea snake |
Maximum length: 85cm.
One of the most northerly sea snakes, and perhaps the most northerly snake which is mainly a reef dweller. Ijima’s sea snake (Emydocephalus ijimae) is a relatively short coral reef snake, whose body patterns consist of repeatedly alternating black and pale yellow stripes. Its territory reaches as far north as Japan, orbiting the Ryukyu Islands such as Okinawa and Ishigaki Island, as well as Taiwan and southeast China.
This species has one of the narrowest niches in the entire snake kingdom. Ijima’s sea snake lives solely off fish eggs, allowing neither gobies nor eels into its diet.
To accomplish this, they have evolved many unique characteristics. Their nose has a large spike, which they move from side to side to dislodge eggs from the reef bed. Their jawbone is unusually structured, to allow them to suck eggs free. Their fangs are atrophied, as they no longer need them.
Ijima’s sea snakes produce few young, with just 2 newborns per litter (live young rather than eggs). Their entire world consists of the colourful chasms and twisting channels of coral reefs. This species is still relatively unresearched, although it’s believed that they fulfil an important ecological role, most likely keeping certain fish species under control, like with Alaskan brown bears and salmon.
| 4 | Stoke’s sea snake |
Maximum length: 179cm.
A widespread sea snake, which ranges from Mumbai on the west Indian coast to Sydney, even reaching as far south as mild Victoria. Stoke’s sea snake (Hydrophis stokesii) is a very common underwater serpent, with flexible habitats. You can find them on ocean floors and shallow sandy flats, but also coral reefs, including Great Barrier Reef and Ashmore Reef.
This is an extreme snake in many ways, beginning with powerful swimming skills, which allow them to seize slow-moving fish. One of their favourite prey is the banded frogfish, which they hunt down in tight coral reef crevices.
This species also looks unusual, with a massive transformation from birth to adulthood. Younglings look sleeker, and have alternating patterns of black and creamy yellow. As they age, they mutate into a thick and leathery creature with only faint patterns remaining. This species belongs to the large Hydrophis genus (46 members), but is weird enough that some scientists place it in its own unique genus of Astrotia.
Stoke’s sea snake has never caused a death, but has the longest fangs of any sea snake, long enough to pierce a scuba diver’s black wetsuit. They can be randomly aggressive to beach-goers and divers, for no reason at all. Danger lurks around every corner in coral reefs.
| 5 | Dubois’ sea snake |
Maximum length: 148cm.
A much darker coral reef snake, with purplish-brown scales, each bordered by faint white lines. Dubois’ sea snake (Aipysurus duboisii) is mainly found in Australian waters, with a few straying to the French territory of New Caledonia. This is a species of shallow coral reefs, surrounded by white sand and seagrass.
You can find Dubois’ sea snake in a variety of spots, including Great Barrier Reef, Seringapatam Reef, Scott Reef, and Ashmore Reef, although they haven’t been seen in the latter since 1994. Coral reefs are important for their hunting, as this species prefers to lurk patiently in the colourful trenches, before cornering fish in crevices. They sometimes venture out and crawl along the ocean floor as well, and can descend to 50 metres underwater when breeding.
Dubois’ sea snake is also known as the reef shallows snake. Their venom has a strong LD50 score of 0.044mg, and is extremely lethal against mice in a lab (the only time the species ever meet).
This species isn’t notoriously aggressive, but will bite if cornered, even if you weren’t intentionally cornering them, just swimming along and minding your own business. At up to nearly 1.5 metres long, Dubois’ sea snake easily has the leverage to get in your face.
| 6 | Belcher’s sea snake |
Maximum length: 94cm.
A sea snake with one simple claim to fame: being the deadliest. In fact, Belcher’s sea snake (Hydrophis belcheri) was once estimated to have the deadliest snake venom in the world, except that different testing systems (intramuscular, intravenous, etc) were incorrectly lumped together. The LD50 toxicity score was actually 0.24mg, versus 0.025mg for an inland taipan. Nevertheless, this sea snake still generates monstrously powerful neurotoxins, which can totally deactivate human brain signals.
Belcher’s sea snake is relatively widespread, ranging from New Guinea to the Philippines, appearing in the Java Sea and Gulf of Thailand alike. They mainly skip Australia, with the exception of Ashmore Reef in the Arafura sea.
As for their other characteristics, Belcher’s sea snakes are fairly normal. They have a flattened tail for swishing water out of the way, cutting through the ocean surface like a speedboat. From a distance, they have the standard black bands of most sea snakes, overlaid on white or pale olive. However, they have yellowish markings around the eye which other sea snakes lack.
Belcher’s sea snake remains a mysterious species in many ways. In 2000, they were discovered off the Vietnamese coast for the first time. Belcher’s sea snake has a very timid temperament, and will only bite if severely pressured.
| 7 | Blue-lipped sea krait |
Maximum length: 136cm.
A neat and tidy snake, with blue and black stripes contrasting sharply at regular intervals. The blue-lipped sea krait (Laticauda laticauda) is one of the most common coral reef snakes, occupying a vast range from New Caledonia in the south to Japan in the north. This species was an early discovery, first being recorded by western scientists in 1758.
Blue-lipped sea kraits typically measure 1 metre, and are a classic prowler, moving along coral reef walls diligently in order to find eels to flush out. These eels often fight back, tearing the snake’s skin with a nasty bite. Many blue-lipped sea kraits are covered with old scars from these eel battles.
Like other Laticauda sea kraits, this species spends 50% of its time on land, including for digesting food. Then they return to their underwater battle arena, the coral reef, where they mostly hunt under cover of darkness, but sometimes in daylight.
The blue-lipped krait lays eggs rather than live young, and stashes them in cracks in the coral rock. Eels are overwhelmingly their main prey, and they hunt them using a venomous bite. The LD50 rating of this venom is 0.16mg, but the yield is unknown, so be careful.
| 8 | Cogger’s sea snake |
Maximum length: 136.4cm.
A snake of Australian waters, plus Fiji, Vanuatu, and New Caledonia. Cogger’s sea snakes (Hydrophis coggeri) can inhabit ocean floors at medium depths, but are well at home in coral reefs. They’re also found in seagrass beds directly adjacent to coral reefs.
This species is an eel-eater, specialising in conger eels and snake eels, which it snatches from their dark burrows. To help them rummage around, Cogger’s sea snakes have an unusually small head, no thicker than their neck, which sharpens into a point.
This reef snake is found at lower depths than some, often 45 metres below the surface. It’s a venomous snake, but not as potent as Dubois’ sea snake, scoring 0.1-0.2mg on the LD50 scale.
You won’t find Cogger’s sea snake in Great Barrier Reef, but they’re common on the reefs of western Australia’s Sahul shelf, including Ashmore Reef, Scott Reef, and Hibernia Reef. They almost certainly have undiscovered colonies, as one in the French-controlled Loyalty Islands was only discovered in 2002.
As an adult, this species has olive-grey scales with only faint patterns. They have a large flat tail for paddling, swishing water from side to side in order to power themselves forward.
| 9 | Short-nosed sea snake |
Maximum length: 108cm.
100 miles north of mainland Australia lie Ashmore Island and the neighbouring Cartier Island. These isolated islets seem to be ripped straight from everyone’s summer fantasies, with sandy shores surrounded by unbroken blue sky and sparkling oceans everywhere.
In the 1990s, the nearby Ashmore Reef was home to the short-nosed sea snake (Aipysurus apraefrontalis). This was one of the shiest coral reef species, tending to appear beneath coral overhangs at 1-2 metres deep. They were different in that they barely strayed from their reefs at all; even one swimming up to 50 metres away was rare. Like other reef snakes, they prowled through colourful coral channels and flushed fish out from their hidey holes, before biting and injecting a powerful venom. Their diet consisted of goby fish and eels.
Why are we using the past tense? Because the short-nosed sea snake is at severe risk of extinction. It once made up 6-14% of sea snake sightings at Ashmore Reef, but completely vanished from 1998 onwards, with not a single sighting. That year, a severe coral bleaching event happened at Ashmore. The reef was believed to have been the species’ sole breeding grounds.
What followed was 17 years of fear and pessimism, as all searches came up empty-handed. But in 2015, hope emerged, as the short-nosed sea snake was spotted hundreds of miles away on Ningaloo Reef, on Australia’s west coast.
Scientists initially controlled their enthusiasm, debating whether it was a different species, but in 2021, a single short-nosed sea snake was spotted on Ashmore Reef, using remotely operated underwater vehicles (ROVs). Scientists dampened the hype once again, speculating that a member of the Ningaloo colony had been blown off course, but this coral reef snake may be on course for a miraculous comeback.
| 10 | Leaf-scaled sea snake |
Maximum length: 90cm.
An extremely similar story happened with the leaf-scaled sea snake (Aipysurus foliosquama). It was originally Ashmore Reef’s signature snake, with a population estimate of 6600. It stuck to shallow areas of the reef, rarely descending to below 10 metres. Leaf-scaled sea snakes were such a part of the reef ecosystem that barnacles would grow on their scales.
The leaf-scaled sea snake was a fish eater, particularly favouring the threespot wrasse, which it flushed out of dark holes by poking its head all the way in (with a small head to make this easier). As the images show, the two Aipysurus members were very different species, despite their similar habitats and shared Aipysurus genus.
Then the dark turn happened, as Aipysurus foliosquama disappeared from Ashmore Reef at almost the same time as its neighbour. Despite the new searches being 5 times more intensive than those of the 1990s, none were spotted from 2001 onwards.
Leaf-scaled sea snakes still haven’t reappeared at Ashmore Reef, but in 2015, an entirely new population was found in the seagrass beds of Shark Bay, western Australia, 1100 miles to the south. In 2010, a single leaf-scaled sea snake was grabbed near Barrow Island, in Australia’s far north. Yet another lone individual turned up in Port Hedland.
The big cause for optimism is that sea snakes are one of the most poorly researched groups of all, except the most common ones like banded sea kraits. There’s so many minor, less famous reefs and coasts where the leaf-scaled sea snake could still be lurking. Either way, the last two on this list are the rarest coral reef snakes in the world.