Although like most people I’m not all that thrilled about the damage caused by hurricanes and tropical storms when they landfall, I’m guessing that unlike most folks I eagerly await the arrival of one of the by-products of these storms. Southeasterly winds (remember, winds are named for the direction they are coming from, not for where they’re going) carry floating debris from way out in the middle of the Atlantic across the ocean and onto the shores of the Eastern seaboard. I spent most of last week picking through the mats of algae drifting towards the coast, especially sargassum weed, looking for stranded pelagic creatures clinging to the flotsam.
Sargassum weed is the most exciting type of algae to arrive on shore since it inevitably began it’s journey nearly 3,000 miles away. South of Bermuda is a huge patch of ocean called the Sargasso Sea. The surface of this million-square-mile area is covered with a giant tangled mat of sargassum seaweed. And living in this algae are approximately seventy species of highly specialized animals that have evolved to live in this floating oasis.
One of the most common members of this community is the sargassum crab. This is a typical sargassum animal in that the coloration of the body perfectly matches the color scheme of the sargassum weed. It is also typical in that this species is nearly identical in every way but color to a closely related species that lives closer to shore. In this case it’s the blue crab.
Nearly every sizable clump of sargassum has one of these crabs on it. And when I say "sizable", I mean larger than your fist. As vast as the sargassum weed is out at sea, by the time it makes the journey across the ocean it has broken up, and then broken up again and again, into ever smaller masses. A clump larger than a basketball is rare. How any animal can cling to something this small for weeks or months at a time, and still find enough food to survive, is remarkable. This crab, however, is a very strong swimmer. If it loses hold of it’s algae patch it can swim back to it. Another member of this community can barely swim at all, but it makes the journey as well.
The sargassum sea slug can swim, and it does so gracefully. But it’s a very weak swimmer, and if it’s dislodged from the weeds it has no chance of getting back on. Still, this animal not only makes the journey, but many arrive in mid-spawn, laying long strands of tiny white eggs all over the sargassum. In the photo above, this one is trying to reach back up to the weeds after I dislodged it for the shot. You can see the white developing eggs through the center of the body. I won’t have luck keeping this specimen very long since they specialize in eating tiny hydroids and bryozoans that grow on the stems of the algae.
The gooseneck barnacle is generally pelagic, not specifically adapted to live in the sargasso sea. So it’s the one animal not weed-colored. Gooseneck barnacles attach themselves to pretty much anything that is floating in the open ocean. Drifting weeds, plastic soda bottles, drifting logs. They are opportunistic, and when the rare floating substrate makes itself available out in the open sea the barnacles can invade it completely. Sometimes so much so that when they wash ashore the colony is mistaken for a sea monster.
Ok, now we get to the prize. It took me a whole week of sorting through mats of sargassum weed to finally come up with a sargassum fish.
This one is fairly large at about three inches in length (full grown adults are only six inches.) Sargassumfish are very weak swimmers and rely more on their grasping pectoral fins to move them through the weeds. I did a separate essay on this fish a while back, and that one will give you all the details of its life history. But I do want to use this one to highlight an unusual method of movement employed by this animal. Because the fins are busy grasping the weeds, if it does need to move through open water it does so using jet propulsion. It gulps water into the mouth and directs the flow out of the tube-shaped gill openings. The movement produced isn’t very fast or impressive, but its pretty amazing to watch in action. Its sort of like those jet packs characters used in old sci-fi movies.
Watch the movie below. I separated the fish from its host so you can see it try to move back into it using jet propulsion. As you might guess, this fish is better off clinging to the weeds in the ocean since this process wouldn’t work very well in a wave-tossed sea. Also notice the feeding gooseneck barnacle attached to the left side of the algae.
Other diaries in this series can be found here.