Cool critter of the month: Sea Mat (Membranipora membranacea)

Phylum: Bryzoa Family: Membraniporidae

Where do they live? This epiphytic (meaning it grows on, but is not a parasite) encrusting critter can be found on living on macroalgae – particularly kelps – in the intertidal and sublittoral (permanently covered by the sea) zones.  The sea mat originates from the North Pacific, but has since reached both sides of the Atlantic, probably transported in ships ballast water.

Why are they awesome? It doesn’t look like an animal does it.  Actually the sea mat is a colonial animal, made up of single critters known as zooids.  The oldest zooids are found roughly in the centre of the structure, with younger ones growing around them.  Being a sessile sort, the sea mat can’t make a speedy get-away from any predators that fancy a nibble on them.  This leaves the colony very vulnerable to being completely wiped out.  One of it’s secret to survival lies in the colony’s rapid growth - several millimetres per day!  Small to us, certainly but for the sea mat, that’s pretty huge.  Well that might be enough to think that this is one awesome critter but there are many more!

Gerroff my frond

So here’s the deal.  Space to expand a colony is limited on kelps.  And limited space means competition, and competition often means aggression.  Yes, even for the humble and seemingly non-threatening sea mat.  Don’t let appearances deceive you.  The sea mat, when faced with an oncoming colony, will produce stolons – branched stem-like structures - to slow down and in some circumstances completely stop the growth of an enemy colony.  In such battles, size does not necessarily matter either.  Without stolons, even a large colony may not be able to halt an oncoming smaller colony taking over vital space. (1)

Weakening its habitat Originating in the North Pacific, this sea mat is not a native to Nova Scotia in Canada, but it has managed to do quite well there.  This is quite unfortunate for the kelp of Little Duck Island in Nova Scotia, on which the sea mat is quite happily living.  Duck Island’s kelp beds have suffered extensive defoliation, apparently caused in large part by the sea mat.   Although the sea mat is not parasitic, it is thought that the sea mat is making the native kelp brittle, and thus more vulnerable to storm damage.  Recovery of the kelp is not always possible either, because regrowth can be slowed down and even completely stopped because another, more rapidly growing invasive algae - Codium fragile takes its place. (2)

Come and have a go if you think you're hard enough Just because they can’t run away doesn’t mean sea mats are helpless in the face of predation.  Just like it can try and defend valuable space, the colony can slow down critters that want to eat it - like nudibranchs.  Instead of the stolons that are used for ‘claiming territory’, the zooids use flexible chitinous spines for this task.  Chitin is pretty tough – it’s the primary component of crab and lobster shells, so ideal for warding off hungry mouths.  Each individual zooid can produce two spines, but they are costly to produce both in terms of energy and in terms of claiming territory.  Spine production slows colony growth, which is not ideal when, as many a sea mat does, live in a space-limited environment.   This may explain why not all colonies have the spines at all times, and why not every zooid in a colony produces spines.  The sea mat only produces spines where and when it needs to – when a predator starts to have a nibble.  (3)

References (all open access!) 1:



ImageMembranipora membranacea on Pleurophycus gardneri  inTaku Harbor State Marine Park.  Credit Mike LaBarbera/Flickr (CC BY-NC-ND 2.0)