Thursday, 3 May 2012

More about mussels ~

Obtaining this dramatic image prompted me to set down more of what I've learnt about mussels in the months since I wrote my earlier article "Mussel gathering and the joys of the local beach".  

Each time Rewi gathers mussels for a meal I carefully pick off the wildlife that inevitably comes home with them: limpets, chitons, anemones, barnacles, snails and seaweeds,  as well as the little crabs that sometimes hitch a ride either outside or inside the mussels themselves.  I set up one or two bowls on the kitchen table, fill them with seawater, and use them to house these little creatures until I can return them to their rock pools.  Sometimes I put live mussels in as well if there is too much other growth on them for me to easily pick it off.  The one pictured above was one of these.  I was delighted to see the byssus threads it produced overnight which anchor itself to the side of the ceramic bowl.  These can be seen clearly after I drained the bowl in order to fill it with fresh sea water.  The tuft of broken threads sticking out from between them is what it would have been attached to its rock face before it was harvested.  More about that later.  

Note that if you are setting up your own temporary aquariums you will need to change the water often.  I change mine three to four times a day. 

There are three to four major groupings of mussels that live on our stretch of beach here in South Otago:
Of the larger ones the most common are:
I'm not confident I could tell these apart as the ones collected are often so weathered that the green coating which distinguishes the green-lipped ones may have been eroded away or been overgrown.  The pair below is clearly the green-lipped variety.  The cone on the upper one is a vacated barnacle shell.  The ones we get here are usually about 12 cm in length.

The middle-sized mussels can grow up to 8 cm but are often smaller.  These are:
The little crabs nestling with it are pea crabs and were found inside two of the large mussels when they were shelled.  They are relatively common.
The little mussels, which grow to about 3 cm in length, often grow in abundance on rocks in the mid-tidal zone.  These are called:

What do mussels eat?
Mussels are described as bi-valve molluscs, which feed on plankton and other microscopic sea creatures which float about in seawater. These are too tiny for us to be able to see.  The mussels feed by drawing the water in through a siphon and filtering it for nourishment.  The NZ underwater life site says that adult mussels can filter up to 15 gallons of water a day, which must make them an important part our shoreline ecosystems.  Below you can see the same mussel pictured above, but re-immersed in seawater and open to feed:  

If you look closely at the near edge you can see the green edge of the upper shell and the pink inner lip which sits inside the lower shell.  The opening is perhaps a quarter of an inch in width, which is as wide as I've seen them.  Any wider and they would be more vulnerable to predators.  

Byssus thread is the means by which mussels anchor themselves to their chosen spot:
This very special substance is made by the secretion through the part of the mussel's anatomy described as its foot.  Byssus does not rot or distort in seawater, and in the time of the ancient Greeks was used in weaving!  You can read more about that in the Wikipedia article linked to here.  

I am delighted to be able to share with you a series of images of a number of mussels all at work in their shallow bowl of seawater:  
The foot looks like a long fleshy tube, which indeed it is.  When the mussel withdraws its foot it commonly leaves the thread attached to the surface.  In these photos however, the mussels  seem to use these 'feet' almost entirely for the purpose of moving about!  These images were taken within an hour of each other.  You can see that the green anemone on the tip of the large mussel shell also moved...

The small mussel at the left is clearly a ribbed mussel.  The one that is largely white may also be one, but as its shell is very worn I'm not sure.  The little ones I don't know either.  Since they came attached to the larger mussels I assume they are most likely to be juveniles of either of the others pictured. 

The large mussels are less mobile, I imagine due to their weight!  The one pictured at the top of the page was fairly active though, and spent much of its time rocking itself back and forth as it busily siphoned the water in which it lay. 

Here is another image which shows the point of byssus attachment particularly:

When mussels are prepared for eating the remaining tuft of byssus is removed from its source inside the shell.  Here is a plate of them:

They don't look like a very likely weaving fibre to me, but the Wikipedia article states that the usual source of byssus used for weaving, was the large Mediterranean pen shell, Pinna nobilis, which produces rather longer strands of it!

Mussel's beard is another thread-like substance often found on mussel shells.
It is a term commonly used for byssus threads but also often refers to exterior growth on the shell itself of something rather different.  The NZ Te Ara Encyclopedia says that:
Mussel’s beard is the light-brown hairy growth on mussels or tidal rocks and seaweeds. It is a colonial organism known as a hydroid, made up of chains of tiny polyps.  Some people confuse mussel’s beard with the threads (byssus threads) that attach mussels to rocks. These [byssus] threads are darker, feel like plastic, and grow from the interior of the mussel.
The Otago University's Marine Studies website identifies it more particularly as Amphisbetia minima

From the references I've looked up above it appears that this term of reference is used interchangeably by many people.  Here are two of my favourite images of wonderful exterior growth of mussel's beard:

Pea crabs, which you can also see in the image higher up the page, are the other hitch hiker associated with the larger mussels.  
It seems incredible that they live inside the mussel shells.   They are roly-poly little things and have very soft shells.  If their shell is broken they die very quickly.  Here is one which I put back in a rock pool.  How tiny it is!

And here it is in its pool - already almost invisible:

The mussel shells themselves are infinitely varied and adapt themselves to the environments in which they are lodged.  Here are some of the more unusual ones I've come across:
This one has been damaged resulting in a hole on its side:

Inside it has coated the hole with nacre:

This one has created a whole series of beautiful bumps, in response to some kind of intrusion into its environment:

The exterior of the same shell gives no clue as to what this may have been, but it does have an interesting attachment on its tip: another shell:

This pair is awkwardly shaped, probably from growing wedged in amongst others. The little orange spot is a closed anemone:

I passed this beautiful fragment on the beach - and left it there for the sea to enjoy...

Finally, for the sake of those readers who have not read previous articles, I'm a vegetarian, so the mussels consumed in this household are not eaten by me.  I say this so that it's clear that I'm not being inconsistent in my regard for and treatment of the sea life that comes into the house.  However, the regular harvesting of these handsome creatures has provided me with ample opportunity to observe all the inhabitants of the rocky pools and the inter-tidal zone of the nearby beach generally.  This has brought me a much pleasure - and greater knowledge of this fascinating part of our living world.

I hope you too enjoy it!

More useful links:
You can find my other articles about exploring the beach and its rock pools via the link below: 

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