Dale Fort Blog Number 5

25 04 2012

This article is about a large brown seaweed that is currently spreading up the coast of Wales.  I did a PhD on it many many many years ago.

Sargassum muticum in Britain

by Steve Morrell

Sargassum muticum (taken from the “Wanted” poster produced in 1975 to raise public awareness of the species)

Historical Background

 Sargassum muticum is a large brown alga native to Japan. In 1973 Bill Farnham of Portsmouth University found it growing at Bembridge on the Isle of Wight.  The Bembridge Ledges are a Site of Special Scientific Interest for seaweeds and the presence of a large non-native species was of both interest and concern.  Biologists were worried that it might replace native species.  Boat users, fishermen and recreational users of The Solent were also concerned.  They thought that it might disrupt their activities by blocking engine cooling systems, clogging fishing equipment and so on.

There was some evidence from the Pacific coast of the USA that the introduction of Sargassum there had resulted in the loss of native species as a result of shading and abrasion by Sargassum fronds.  This, together with the SSSI status of The Bembridge Ledges, convinced a meeting of 15 marine biologists (held at Portsmouth Polytechnic in 1973) that an attempt should be made to eradicate the alga.  It was conceded that it would be a somewhat optimistic undertaking but it was thought worth having a go.  If nothing else useful information about a poorly understood field would be gained.

The media were informed and delighted in the story of an alien species invading our native shores.  The local MP asked a Sargassum related question in the House of Commons.  As a result, for a few years Sargassum became quite famous (as seaweeds go).

A small army of volunteers was assembled from the local Naturalist Trusts and they set off to try to remove the alga from Bembridge.  It was a difficult shore to work on because the plants were growing in deepish lagoons for the most part.  The basal parts were attached very firmly to the substratum and if left could easily regenerate.  Bits of fronds that became detached would float off and help spread the species further.  It soon became clear that such efforts were doomed to failure and no serious attempt was made to eradicate the species after about 1979.  Clearance efforts on the south coast ceased entirely after about 1982.

Until 1975 its distribution was limited to The Isle of Wight and the Solent.  By 1976 it had been seen as far away as the estuary of the River Yealm in Devon and on the Cherbourg Peninsula in France.

The spread of Sargassum in Britain seemed to suffer a hiatus in the 1980s, as it seemed to be difficult for it to survive on the generally more exposed west coast. It took it a long time to round Lands End and become established in a suitable place.  There was no evidence of Sargassum beyond Land’s End up to 1986 when the author was employed to look for it.

In 1997 however, some drifting fronds were spotted at West Dale Bay, Pembrokeshire implying a resident population somewhere reasonably nearby. That year, a small, attached population was found at west Angle Bay, Milford Haven, Pembrokeshire, by a CCW (Countryside Council for Wales) survey team. These plants were removed before they became fertile but since some of the specimens were at least two years old and likely to have reproduced it may have been too late.

A new population was found established in a pool at nearby Broadhaven (North) in 2000, this was also removed by the CCW.  Despite this, I found cast off Sargassum fronds that same year on Broadhaven Beach.  They were fertile fronds bearing mature receptacles, so it looked like there was an as yet undiscovered population or populations somewhere else in the locality.  An attached population of Sargassum was found at Black Rock, Dale in Pembrokeshire in August 2001

The population appeared to be well established, 93 plants were counted mostly well developed with primary and secondary laterals, air bladders and receptacles (not hairy yet, i.e. no developing germlings).

Figure 1.  Sargassum in a pool at Black Rock

Figure 2.  Close-up of part of a frond

Figure 1 shows a pool at Black Rock containing a thriving population of Sargassum.

Figure 2 shows part of a frond from one plant. Cigar-shaped receptacles (fruiting bodies) and air vesicles (bladders) can be seen.


Feeling the basal parts seemed to reveal old lateral scars (evidence of last years growth). It was estimated that some of the bigger plants were up to three seasons old and may have settled about the same time that drift Sargassum was found at West Dale in 1997.

There were some large (70cm or so) plants that did not have scars so presumably were year one plants. Some of these may have settled from the drift material of June 2000 (lots of it, found by the author at Castle Beach Bay, roughly 2km East of the site) or offspring of the first settled plants. A few short plants of 10-15cm were found that had small holdfasts relative to primary laterals (like newly growing germlings). It was thought that these were probably late/slow developers rather than plants settled this year. In total, 9 rockpools in this area had Sargassum in them.

In February 2002 I discovered another attached population in Milford Haven at Musselwick near Dale. The site was visited on an exceptionally low tide and is accessible on foot only about twice a year.   We were able to obtain a limited number of quadrat samples in the time available and estimated a population density of about 2 individuals per square metre.  By about 2006 it had established itself at Castle Beach and Jetty Beach so Dale Fort was surrounded.  What it does not seem to have done (so far) is dominate the intertidal as it has done elsewhere.

Morphology and Reproduction

Sargassum muticum has a long-lived basal structure that looks a little like a small tree (up to about 10cm high), this bears lateral branches.  The main (primary) laterals can be anything up to 8 metres long and they bear secondary laterals.  When held up out of water you can see these secondary laterals hanging down like washing on a line.  The laterals have leaf-like structures (laminae) and air bladders (vesicles) and between these emerge the reproductive structures or receptacles.  The receptacles are cigar shaped and covered in pits (male and female conceptacles) within which develop the eggs and sperm.

During spring tides the eggs and sperm are released from the male and female reproductive parts (conceptacles).  The eggs are held close to the receptacle on jelly like strings and the sperms swim actively to fertilize them.  Over the next couple of weeks the fertilized egg develops into a multicellular germling.  It is still attached to the receptacle at this point and has developed a short thallus (1-2mm) and tiny rhizoids (root-like structures).  It looks a bit like a diminutive hot-air balloon at this point.  The receptacles begin to look hairy because they are covered with tiny germlings.

At the next set of spring tides these partially developed germlings drop off and because they are quite hydrodynamic “glide” away a short distance from the parent (probably no more than a metre).  The rhizoid initials are extremely sticky and as soon as they land they stick to the substrate.  Now they have an advantage over other macro algae.  They are already half developed so can grow like the clappers and avoid getting grazed up by herbivores.  (If they fail to drop off they stick to the receptacle, this eventually rots and they float off but can’t reattach).  Releasing at spring tides will give them a longer period of calm conditions to attach and start growing.   Thousands of germlings are produced so it’s very likely that some of them will strike it lucky and land in a suitable spot.

Sargassum muticum fronds are capable of reproducing even when detached from the parent plant because of the many small air bladders which keep them afloat and therefore alive.  During the late Summer and Autumn the primary laterals are cast off from the perennating base.  So long as they remain afloat they can continue to grow and reproduce.

A key element in its success is the fact that it is the only monoecious, self-fertilizing species of its genus. Male and female gametangia are present on the same receptacles.  This means that fronds can continue to grow and reproduce as long as they remain afloat.  Germlings can be released over anywhere the loose frond floats over.  Just a single receptacle can produce many offspring.  Should they settle in a suitable spot (wet and not too exposed) they will attach and develop into adult plants next spring.

This explains the rapid spread and extraordinary success that Sargassum experienced on the south coast of England since its introduction on the Isle of Wight.

  Sargassum is capable of surviving attached to stones while buried in soft mud and can tolerate variable salinity (down to 17 parts per thousand in Lake Vlissingen in the Netherlands). It might find conditions in Milford Haven eminently suitable for its growth.  Having said that, the population at Black Rock near Dale has virtually disappeared.  I have been unable to find anyone claiming to have removed it.  Maybe some factor or factors unknown caused its demise?  If anyone has any information I’d be interested to hear it.  Most of the stuff you read about Sargassum seems to take the popular press viewpoint, i.e. It’s entirely a bad thing.  This is not necessarily the case.  On the South coast of England large areas that where once mudflats now have a major primary producer.  Not only that, its highly complex morphology provides an array of attachment surfaces for epibionts of all kinds.  An early study by biologists from Portsmouth Polytechnic found 80 animal, 52 plant and 9 fungal species.  I well remember from my own work in the early 1980s the stunning variety of sea squirts, bryozoans and other creatures that lived on the plants from late Summer.  It is well known that the structural complexity of ecosystems plays a major part in determining their diversity.  Sargassum muticum can certainly enhance the physical complexity of a system.  In my own studies on the ecological effects of clearance programmes I found that epibionts of Sargassum survived the clearance better than non-target species.

In summary, I’d say that Sargassum has been given a harsh press.  It’s an interesting attractive plant that supports a vast diversity of other species.  It’s certainly not going to go away, so we’d better get used to it.

Here is my short video of how it all works:

Watch out for blog number 6 for more interesting stuff you didn’t know you wanted to know.




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