Sargassum is weird stuff – and it gets weirder

May 16, 2017 by Charles Clover


For those of us whose only previous experience of Sargassum seaweed was in the pages of Samuel Taylor Coleridge’s Rime of the Ancient Mariner, the experience of finding the real thing is strangely different.  Five miles off Bermuda on a fine day in a boat looking for Sargassum with Dr Robbie Smith, curator of the Natural History Museum at the Bermuda Aquarium, Museum and Zoo, I kept repeating to myself the stanza which tells of the long days when the Ancient Mariner is becalmed surrounded by his dead fellows:


The very deep did rot, O Christ!

That ever this should be!

Yea, slimy things did crawl with legs

Upon the slimy sea.


Coleridge, I was taught, was describing what he had read about in seafarers’ accounts, though there is a strange veracity in the “slimy things that did crawl with legs” description which we were keen to see in real life when we were out there last month.  First find your Sargassum, though.  For one of a series of weird things about this spiky form of algae that freely floats in the ocean between Africa and North America, is how seldom it is actually there.  There was hardly any Sargassum around Bermuda for nearly two years from 2015 and well into 2016.  Even though we had heard that mats of it had returned, it took a few hours of motoring about, seeing nothing but seabirds and Portuguese man o’ war jellyfish gently dipping their sails to keep them wet, before I spotted a small patch of yellow, the size of a dinner plate floating on the deep blue sea.


Portuguese man o’war off Bermuda
First sargassum patch sighted, alone in a wide wide sea









As far as we could see, it was alone.  Robbie enthusiastically netted the patch and popped it in a bucket of sea water.  Cue the second weird thing waiting to be discovered about Sargassum: even the smallest patches are like tiny cities filled with life.  Robbie held up barnacles about half the size of your little finger-nail, tiny red shrimps, a crab and even a nudibranch, a sea-slug with Mickey Mouse ears, all perfectly camouflaged against the yellow-brown weed.  Separated from the Sargassum the shrimps attacked the crab in the bottom of the bucket.  These creatures seem to need the Sargassum for peaceful co-existence.  The Sargassum is a form of brown algae, not a vascular plant, another weird thing if you ask me.  It may not have vessels that carry nutrients around but it does have air bladders that make it float.  Unlike other seaweeds it floats without any connection with the ocean floor, making it a true wonder: how all the forms of life within it – several of which are endemic to Sargassum, and some to particular forms of Sargassum – actually manage to find their host mat in a wide, wide sea.  Both Sargassum and the life within it are miracles of evolution.


Endemic sargassum shrimp having a go at a sargassum crab
Endemic sargassum shrimp having a go at a sargassum crab
Sargassum nudibranch










We motored on and then, after a while, we began to see something like the wind-rows of Sargassum that I had been told to expect: several plate-sized lumps in a wind lane.  Robbie duly scooped these up and we looked hard for the one thing we had yet to see, a Sargassum fish.  These amazing little orange and yellow things spend their time hanging about in Sargassum, so their fins have evolved to grasp like hands and they use these to pull themselves about in the dense mats of weed which can form.  Coleridge was right: they really do crawl with legs, or rather arms.  Sadly, we didn’t find any big mats of Sargassum that day, though we saw some off Bermuda’s shores later in the week, decorated with the inevitable plastic debris.  Hence we didn’t find any Sargassum fish.  Luckily, a wonderful photo of Sargassum fish adorned Jessica Riederer’s article for Bernews, the Bermuda news agency, a week after our trip when the Sargassum had really begun to roll in to Bermuda’s shores.  Click on it when you have finished here – –  for I haven’t quite exhausted the full weirdness of Sargassum yet.


Dr Robbie Smith and Dr Dan Laffoley off Bermuda
Beasties and a bit of plastic









Sargassum was studied off and on through the 20th century and into this.  Two species generally predominate: Sargassum natans at more northerly latitudes, such as Bermuda, and Sargassum fluitans, more common closer to the Caribbean.  So what is the weirdest thing of all about Sargassum?  For me it was what I discovered when we were back in Robbie’s lab at the aquarium.  It is that the seaweed we found was a variant of Sargassum fluitans, –  first recorded in the 1930s. Not only that, but as Robbie explained, both this version of fluitans and a variant of Sargassum natans  developed off west Africa and lower latitudes of the eastern Atlantic in 2015 and then bloomed spectacularly in the Caribbean in 2015 and 2016.  They only began to show up in Bermuda late last year.  Both are aberrant forms previously unknown to Robbie and present day scientists.  Even more peculiarly, Sargassum natans is now exceptionally rare.  The majority of animals that live in the Sargassum probably don’t care which form it is, but there are some which are specific to one form or another and they might expect to have had their life cycles interrupted  if it does not re-appear.  “Isn’t science exciting?” remarked Robbie.  “You think you’ve got it all figured out and then you have to go and do something else because something unexpected has occurred. The plants have changed which is quite remarkable and completely unpredictable.”



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