Fish
Sargassum Fish
Histrio histrio · also called sargassum fish, sargassum anglerfish, sargassum frogfish
The sargassum fish lives its entire life inside a raft of floating seaweed, and it has become that seaweed. Its weed-coloured skin is drawn out into fronds and filaments, its outline is a jumble, and it clambers through the mats on fins that end in little grasping hands. It is one of the best-camouflaged animals in the sea — and one of the greediest, swallowing prey nearly its own size in a single gulp.
Overview
The sargassum fish is found in the warm surface waters of every ocean, but only ever within floating rafts of Sargassum seaweed. It is 10–20 cm long, stout and big-headed, with loose weed-coloured skin in flaps and threads and paired fins that end in hand-like lobes.
Disguised as its habitat
Its mottled olive, brown, and yellow colouring and skin appendages mimic the fronds and floats of Sargassum so closely it is nearly impossible to see, and it can shift colour over days to match a new patch of weed.
It walks
It moves by gripping fronds with its arm-like fins and pulling itself along, almost never entering open water. When prey comes near, it expands its mouth explosively, pulling the victim in whole in a few milliseconds.
Along for the ride
Because it never leaves the weed, the fish is carried wherever the rafts drift, which is how a weak-swimming animal ended up across the warm oceans. The male tends a floating raft of eggs among the weed.
Status
The sargassum fish is Least Concern, but it depends entirely on healthy floating Sargassum, which is declining in some regions and forming vast harmful blooms in others.
Sargassum Fish: common questions
Does the sargassum fish really live only in seaweed?
Yes, more strictly than almost any other fish is tied to a single habitat. It spends its entire life within floating mats of Sargassum, a brown seaweed that drifts at the ocean surface rather than growing attached to the bottom. The fish feeds, hides, breeds, and travels within the weed, and it is so poorly built for open-water swimming that being separated from a raft, by a storm or a passing ship, is dangerous for it. Where the Sargassum goes, the fish goes, which is how a small, weak-swimming animal has ended up distributed across the warm parts of every ocean.
How good is its camouflage?
Exceptional. The sargassum fish combines several tricks. Its base colour is the exact olive-brown and yellow of Sargassum, its body is covered in flaps and threads of skin that mimic the seaweed's branching fronds and its round gas-filled floats, and its outline is so broken up that there is no clear fish shape for an eye to lock onto. On top of that it can slowly change colour over days if it moves to a lighter or darker patch of weed. Divers and researchers who know exactly where a sargassum fish is can still lose it the moment they look away.
How does it catch prey so fast?
Like other members of the anglerfish and frogfish group, the sargassum fish is a suction feeder with an extreme gape. It sits motionless, sometimes twitching a tiny lure on its snout, and when a fish or shrimp comes within reach it opens its mouth explosively while dropping the floor of the mouth cavity. This creates a sudden strong inflow of water that sweeps the prey inside before it can react, and the whole strike takes only a few thousandths of a second. The mouth can expand enough to take prey nearly as large as the sargassum fish itself.
Is the sargassum fish affected by the huge seaweed blooms in the news?
The relationship is complicated. In parts of its range, particularly the western Atlantic, Sargassum has been declining, which reduces the fish's habitat. Elsewhere, especially across the tropical Atlantic since around 2011, enormous blooms of Sargassum have been forming and washing ashore in rotting masses that smother beaches and coastal ecosystems. Whether these blooms help or harm the sargassum fish is not fully clear; more weed at sea could mean more habitat, but blooms that decay, sink, or pile up on land take that habitat, and the animals in it, out of circulation.