Sargassum Fish

Image: Izuzuki
Histrio histrio

It's the tiger of the seaweed. The monkey of the floating garden. The... fish of the sargassum.

More to the point, the frogfish of the sargassum.

Image: Bill & Mark Bell
This hellish abomination's head is on the right, you can just about see an eye

Your usual frogfish is a chunky, perpetually grumpy, shallow-water anglerfish who sits around on the sea floor waiting to be fed. Or at least waiting for prey to swim close enough that they can snap it up without too much effort.

They have a few tools to help them out in this endeavour. One is that your usual frogfish is absurdly colourful and often covered in little blobs and circles. It makes them look just like a sponge growing on the seabed. Sponges don't usually have enormous mouths that engulf passing fish in the blink of an eye, so fish feel perfectly safe getting close to... an enormous mouth, as it turns out.

Image: NOAA

But the frogfish doesn't leave it all to chance. They have a clever trick up their sleeves.

Actually no, not up their sleeves. That would be silly. No-one would see it. And what's the point of a lure if no-one can see it? No. They have a clever trick sticking out of their forehead, where it's clearly visible.

Video: Wainwright Lab
NNNNNOOOOOMMMMM

It's a special fin spine that has become wholly detached from the rest of the dorsal fin. On the end of it is a little blob which is sometimes shaped like a worm or a shrimp, other times just an amorphous thingamajig. In any case, it looks... fascinating. Other fish swim by and are... fascinated. They wonder if they've just found their next meal. They get closer... closer... too close and suddenly what looked like a sponge opens an enormous mouth and engulfs the fish in the blink of an eye.

Frogfish seldom swim anywhere, which is good because they're rubbish at swimming. Their fins are adapted to walking on the sea floor. For them, the closest they'll ever get to the water's surface is when they decide to climb up a particularly tall sponge and perch there, enjoying the view. At least, I think they're enjoying it. Frogfish don't really smile.

This brings us finally to the frogfish of the sargassum.

Video: Glenn Madden

They have much in common with other frogfish. The weird, walking fins are all present. The incredible colours of camouflage. The sneaky lure to attract fish and shrimp to their doom. And of course, most important of all, an enormous mouth attached to an enormous belly. Frogfish can eat fish that are about as big as themselves! In the case of the Sargassum Fish, this can be up to 20 cm (8 in) long.

One thing, though. While the Sargassum Fish has all the frogfish necessaries, they put their own twist on it. They have to because they don't live on the seabed, they live in sargassum.

Image: David Remsen

Sargassum is a genus containing a couple hundred species of seaweed. Most of them are quite normal but two, S. natans and S. fluitans are perpetual floaters. They don't live attached to the sea floor, instead they spend their entire life cycle floating on the current.

From afar, sargassum looks like a big splodge of green and brown on the water's surface. It's as if someone dumped a load of hay out of a boat.

Image: Sean Nash

Look more closely, however,  and you can see the complicated, leafy branches and gas-filled bladders. This is the Sargassum Fish's domain.

Thus, the Sargassum Fish doesn't have the bright, bold colours of a sponge. They have the light and dark browns that befit their complex environment of light and shade, like tiger stripes in long grass. They also have a whole bunch of sticky-out protuberances all over the body. This is the finishing touch to their camouflage because sargassum is seldom smooth, it's covered in tiny hydroids, tube worms and other bits and bobs. And blobs.

Video: Our Oceans

Then there are the fins. Other frogfish need to walk. The Sargassum Fish needs to climb. Their pectoral fins are at the end of stalks and are strong enough to actually grip onto the branches around them. Mr. Men hands! Now the Sargassum Fish can prowl the seaweed in search of prey.

And there's a lot of prey about because fish just love stuff and things floating in the sea. The surface of the sea is a bit of a desert so anything floating there - seaweed, a plastic bottle, the deadly tentacles of a Portuguese Man o' War - anything will attract a milling community of fish. They probably hope to nibble on a few hydroids and crustaceans. They didn't expect a bit of seaweed to open an enormous mouth and engulf them in the blink of an eye.

Image: Tom Weilenmann

Settled in their floating gardens, Sargassum Fish drift with their seaweed. They're found in tropical seas all over the world. All except the eastern Pacific, for some reason.

Males and females have to meet up at some point. Get some actual work done. They swim up to the surface together, the female lays her eggs and the male fertilizes them. The eggs are embedded in a gelatinous mass so they float on the water's surface and stick onto the seaweed.

They soon hatch into tiny babies. All they have to do now is climb into that complicated seaweed garden and grow into the tiger-monkey-fish of the sargassum.