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Out in the middle of the Atlantic Ocean is a roughly million-square-mile area known as the Sargasso Sea. On the surface of this sea is an enormous mat of entangled, floating algae, kept in place by a combination of ocean currents and lack of winds.

The movement of the currents which surround this sea, including the Gulf Stream, the Equatorial, the Canary and the Caribbean, collude to entrap anything that floats into this area and keep it, mostly, from escaping.

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The dominant plant of the Sargasso Sea is sargassum weed, which grows attached to rocks in shallow waters around the world. Like many types of algae, sargassum is often torn from its substrate by waves or wind and ends up drifting in the water until it washes up on shore or dies. Unlike other types of algae, sargassum adapts to this free-floating existence and thrives in open water.  Many of these plants will drift towards the Sargasso Sea and add to the vegetation already growing there.  

Sargassum weed stays afloat using numerous gas bladders that grow from its stipe (stem) and fronds (leaves), keeping it at the surface where it may continue to photosynthesize.

Lying beneath this surface layer of floating seaweed is miles of desert. The surface layer of weeds form an oasis of life above this vast swath of nearly lifeless waters. And the animal life that live in this floating oasis have adapted themselves to what is essentially an ecosystem of organisms living among this mat of vegetation. One of the most spectacular of these animals is the sargasumfish.

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The sargassumfish (Histrio histrio) is a type of anglerfish. Anglers belong to a family of fishes known as Antennariidae and have a fleshy fold of skin on their foreheads which is used to attract prey. This filament, called an "illicium", is actually a detached and modified spine of the dorsal fin. Some deep-water anglers (and the one in the movie "Finding Nemo") actually have this filament tipped with a bioluminescent organ. On other species this "lure" is shaped like a wriggling worm. This modified tip of the angler's illicium is known as an "esca". In the sargassumfish's case, the esca is simply a fleshy lump of tissue that can be waved in front of its mouth in the hope that the movement will attract a curious shrimp or fish.

This fish is highly specialized to live in the Sargasso Sea. Everything about its body is adapted to living among these floating weeds, from its camouflaged coloration to the hand-like pectoral fins. These fins are incredible in that rather than being used for swimming, they have evolved into grasping organs used to literally "climb" through the sargassum forest.

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Because the Sargasso Sea is so isolated, this fish's choices of food are limited to other members of the sargassum community. Dozens of species of animals, all of which have close relatives living in coastal zones, have adapted to life in this oasis and all have one thing in common: They mimic the weeds in both shape and coloration. Sargassumfish do so to such an extent that fleshy folds protruding from the body even resemble the fronds and air bladders of the algae.

This fish, although only six inches long, is the top predator of the sargassum community. Its mouth is huge and its stomach and skin is highly elastic, enabling it to feed on animals twice as large as itself. This is important because prey is often very scarce and it could be days or even weeks before the next potential meal presents itself. This ability causes problems when the fish is kept in captivity, however. If fed too much at one time, the food will begin to decompose before it is fully digested, killing the fish.

Being a lying-in-wait predator, the sargassumfish does not actively hunt for its food, but patiently waits, camouflaged, for another animal to come close enough (with the help of its lure) to be snapped up unsuspectingly.  

Sargassumfish are highly cannibalistic and tend to stake out a territory among the weed clumps and protect it against others of its kind. The only time one fish will tolerate another is during mating, and in its reproductive behavior this amazing animal does not disappoint. When a female's ovaries begin to mature she searches for a male. Once found, the pair will swim together for several days as her body bloats to double its size with developing eggs. Once the eggs completely mature, she shoots to the surface, followed by the male, and ejects an enormous mass of eggs embedded in a gelatinous ribbon-like structure known as an "egg veil". This floating mass of eggs is then fertilized by the male. While he's busy doing the fertilizing, she makes an escape back to her home base to avoid being devoured by her mate.

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I've actually caught these fish in New England, although they are extremely rare. They are so dependent on the sargassum that if a clump of weeds is pushed out of the Sargasso Sea and into the Gulf Stream Current by tropical storms, the fish will remain with its cover for weeks or months at a time, giving it a chance to flow all the way to our shores. I've found some still clinging to weed remnants barely larger than itself. In fact, if the weed is washed ashore, the fish will die on the beach with its host rather than abandon it.

Fun Fact: If attacked by a predator, the sargassumfish will leap through the weeds and out of the water, landing on top of the floating algae, sometimes remaining out of water for up to a half hour.

Other diaries in this series can be found here.

Support me for the DFA scholarship so I can go to Netroots Nation this year.

Originally posted to Mark H on Fri May 22, 2009 at 06:13 PM PDT.

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