Majestic marine creatures: the amazing size of the largest seadragon

By Eleonora Pilastro
sea dragons with black background

No, we are not talking about actual dragons: seadragons are a lesser-known, sea-riously adorable inhabitant of the sea. 

Almost everyone has seen a seahorse, with their iconic shape and vibrant colours, but the same can't be said about their close relatives. 

Although they are both members of the Syngnathidae family, these two different sea creatures sport notable differences: for example, seahorses lack the leafy appendices of the seadragons but can count on a rather useful prehensile tail.

True to their name, seadragons can flaunt majestic appearances and reach an impressive size... at least, compared to seahorses! 

In fact, the largest seadragon ever registered (a specimen of common seadragon) measured a total of 45 centimetres (18 inches). 

That's about the same size of a femur for an average person.

colorful yellow seadragon

The size reached by the common seadragon surpasses by 10 centimeters the largest species of seahorse ever registered: the Australia-native pot-bellied, or big-belly, seahorse (Hippocampus abdominalis) can only measure up to 35 centimetres (around 13 inches).

On the contrary, the record for the world's smallest species of seahorse goes to the Indonesian Satomi's pygmy seahorse, also known as Hippocampus satomiae. 

First discovered in 2008 by dive guide Satomi Onishi, it has an average length of only 13.8 mm (smaller than an average human fingernail).

Diver looking at seadragon

Easily findable in most global waters, seahorses are much more common than seadragons. 

Although you might find your average seahorse almost everywhere in the world, seadragons can only be encountered in the lush natural reefs and seagrass meadows off the shores of Australia. Here, seadragons used to be hunted down, dried and used as amulets thanks to their vivid shades and alleged esoteric properties. 

Thankfully, their minute size and leaf-like appendages along its back and fins provide excellent camouflage among the seaweed.

Perhaps, however, these tiny animals are most famous for their unique male pregnancy: a rare event in the natural world. 

Even though the seahorse holds the eggs in a proper pouch, the male seadragon carries the eggs on his tail for up to six weeks until they are ready to hatch.

Close-up of the eggs on a seadragon's tail

Majestic sea creatures

Thriving in the waters of Australia, the seadragons share the seagrass meadows and colourful corals with another record holder: the dugong (also known with its scientific name of Dugong dugon).

Famously providing the inspiration for ancient mermaid tales, and a member of the family of the Sirenia, these gentle giants hold the record as the most herbivorous marine mammal – as well as being the only exclusively marine mammal that is deemed entirely vegetarian. 

Grazing underwater grass, they can happily consume up to 40 kg (the equivalent of 88 lb) of seagrass in a single day: roughly the same weight as a sack of 200 potatoes. 

They are also apnea champions, and can stay underwater for up to six minutes. 

Immediate relatives to the sweet-water manatees, dugongs are shockingly closer to elephants than to other sea mammals such as whales or dolphins. 

 Dugong grazing seagrass

Often spotted alone, these majestic beasts roam the warm coastal waters in the Pacific and Indian oceans, but they can also be found in the Red Sea as well as in the coasts of Africa and Australia. 

They are considered a vulnerable species as pollution, global warming and relentless hunting pose a threat to their numbers. 

Most of all, dugongs are loners. They prefer peace and loneliness, sometimes travelling in pairs, but they can sometimes reunite in big herds with hundreds of members to feast on the delicious seagrass meadows.

They have rather poor eyesight and rely on sound and touch for communication. 

Eye-conic fish records

Speaking of eyesight, all eyes are on these species when it comes to incredible eyes and astounding eye-to-body ratio. 

Although not exactly an eye candy, the bigeye thresher shark is the owner of the impressive record for the largest eyes on a fish.

Immediately recognizable thanks to the long, scythe-like tail, the eyeballs of this shark (scientific name: Alopias superciliosus) can span up to an impressive 12.5 cm (4.9 in). 

Unusually, their sight organs are taller than they are wide, shaped more like an upside-down pear rather than being spherical.

Such an amazing measurement exceeds even the eyes of the largest fish to roam the waters, the whale shark, although it doesn't meet the incredible ratio of the colossal squid, owner of the largest eyes in the animal kingdom.

But the thresher shark is not the only specimen that adapted their eyes to their habitat. 

swordfish swimming

The hottest eyes ever registered on a fish belong to the immediately recognizable swordfish (Xiphias gladius)

Although these fish are mostly found in tropical and temperate areas of the Atlantic Ocean, they have incredibly sensitive retinas and are able to utilise special organs to heat up their eyes to temperatures as high as 28C. 

Such temperature allows the shark to spot any moving prey up to 12 times faster than it would be possible with cold eyes, and it gives them an edge over the other fish. 

Their unlucky (and unsuspecting) prey have no way of foreseeing the attack!

Plastic aliens of the abyss

The deeper we dive, the easier it gets to meet fantastic, alien-like creatures. 

Some of them have even learned to adapt to an environment contaminated by plastic, which still remains the first water pollutant overall and poses very real threat to the oceans.

Such is the case of the Eurythenes plasticus: discovered in 2014 and first described as late as 2020, the deep-sea amphipod is the first ever species contaminated with plastic. 

This alien-looking, tiny creature lives deep in the Mariana Trench, between 6,010 m (19,720 ft) and 6,949 m (22,799 ft) of depth, and owes its unfortunate name to the microplastic microfiber found inside its hindgut. 

Further confirming the terrifying effect of plastic pollution in our waters, 10 specimens of Hirondellea gigas amphipods collected in January 2017 displayed synthetic/semi-synthetic fibres in their digestive tracts. 

Such discovery gave this scavenger inhabitant of the abyssal zone the title of the deepest plastic-contaminated animal.

The contaminated amphipod was found at a depth of 10,890 m (35,728 ft) in the Mariana Trench, stressing the seriousness of the threat that microplastic poses as it reaches habitats far beyond human reach. 

Something as seemingly unimportant as a piece of plastic we fail to recycle in our homes might — in fact, it probably will — reach even the darkest and deepest zone of the ocean. 

There, via the abyss, plastic enters the food chain. 

Many other species feed on the plastic-contaminated amphipods, with implications for the entire marine ecosystem. 

The plastic issue goes deep, and it threatens everyone.

purple sea dragon black background

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