Sea moths – what the hell are they?

Seahorses, sea cucumbers, sea hares… if you follow me along you’ll recognise them all… but sea moths, maybe that will stump you? It sure stumped my friend JK, a keen diver and frequent visitor to Perth’s Swan River, who messaged me recently saying he’d finally figured out what the hell this strange fish he kept seeing was.

It was a sea moth.

Photo: Klaus Steifel
Look. This is a sea moth. Gawk at it’s funny lil’ face. Photo: Flickr/Klaus Stiefel

So, what the hell are sea moths?

There are just five species of sea moths, strange fish collectively called pegasids – a name that alludes to their huge fanning fins, reminiscent of wings (think Pegasus).

Oddly enough, their wing-like fins aren’t even their strangest – they’ve also got two feet-like fins. These are small, hook shaped appendages on their front that they use to crawl along the sea floor. Only once thoroughly disturbed by a diver will they actually swim up into the water column and away.

See the small, curved fins closest to the sea moth’s mouth? These are their feet-like fins, used for crawling along the sea floor. Photo: Flickr/jdphotos2007

Let’s talk about their weird skin thing…

Like seahorses (who’re in a closely related sister family), sea moths don’t have scales but are covered in bony plates (a carapace) and thin skin. This skin [odd alert] is shed every week and all in one piece.

This means that every 1 – 5 days sea moths make jerking movements and strong tail beats to lift off a full translucent copy of their body, fins and all. Think of it like taking retainers out of your mouth, leaving you with a perfect, translucent copy of your teeth.

Why do they shed their skin?

Sea moths shed their skin so often because their slow-moving lifestyle means they’re quickly overgrown with algae and other organisms. Other fish leading similarly slow lives (such as stonefish and scorpion fish) also shed their skin for the same reason, but sea moths seem to be the only oddballs to shed their skin in one complete piece.

Together 4eva

Also like seahorses, sea moths are monogamous – with pairs forming strong bonds to increase their reproductive success. Males will closely follow behind their female and block other males from approaching her by constantly, and strategically, placing their own body in the way of the prowling competition.

Sticking together is thought to be the best option for sea moths because they occur in low densities, living few and far between, making it hard to find another mate. Therefore it works better to stay together and mate often instead.

A prickly pair of sea moths. Photo: Flickr/Silke Baron

How do sea moths do it?

Sea moths spawn – releasing hundreds of gametes (a.k.a sex cells: eggs from the female and sperm from the male) into the water column, hoping they’ll bump together and fertilise.

To do this both partners swim up from the sea floor and quickly press their bellies together, releasing the gametes. The belly touching gives the gametes a good chance to crash together and by swimming upwards, the couple reduce the immediate predation risk of their eggs from other animals lurking on the sea floor.

Despite this, heaps of their offspring still perish – only a tiny percentage will actually survive till adulthood. Therefore pairs mate frequently during the breeding season to create as many chances as possible to make new lil’ sea moth babes.

So, where can you find sea moths?

Sea moths are found in both tropical and temperate waters in the indo-pacific. They generally occur in shallow, coastal locations where there are calm waters, large grainy sand and/or sea grass.

They’re also common in bays and estuaries and, as my sources tell me (thanks JK!), they’re even common in Perth’s Swan River – so if you’re a local river goer have a look next time you’ve got your snorkel! Look carefully though, because their camouflage can be pretty dang good.

Please send me more stuff

And now… if you’ve ever come across some odd organisms in your own travels – let me know! Help a sister out with some funky blog post ideas or just entertain me with your colourful tales of freaky nature things in the comments below. I live for this shiz!

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Herold, D., & Clark, E. (1993). Monogamy, spawning and skin-shedding of the sea moth, Eurypegasus draconis (Pisces: Pegasidae). Environmental biology of fishes37(3), 219-236.



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