Brachiopods: Aren’t they just mussels?

So, today we pick back up with our journey through various species.

We are moving on from worms today (no more gross videos for now!) Today I am going to introduce you to a phylum called Brachiopoda, and I should first show you some pictures so you can see that I haven’t sneakily decided to post about worms again! There will be two posts on Brachiopods, today is about the general history of them, and next post is going to be about their feeding habits and behaviour.

Lingula anatina, a brachiopod found today primarily in the Asia-Pacific region across  Image from Natural History Museum

Another image of Lingula anatina, this photo is from a beach in Australia. Image from wikipedia

These are very ancient creatures, and are sometimes referred to as “living fossils” in the way Coelacanths are, as they do not appear to have changed much over the course of time.  Here are some fossils of  Lingula, from rocks in the UK

Lingula fossil, found in the Mendip hills in the UK. Image from  British Geological Survey

There is a second type of Brachiopod, and they look a little different, as the images below will hopefully show:

An Articulate Brachiopod: Terebratulina retusa. Image from EOL (Encyclopedia of Life)

Articulated Brachiopod fossil, the one shown here is Oleneothyris. Image from Fossils of New Jersey

But wait, I hear you say, these are just like mussels or clams. Well, yes, I reply, I can see why you would think that, especially if I showed you a fossil like this:

A brachiopod from the order Orthida: Image from UCMP Berkerley

So if these are not mussels or clams (and they are not), then what are they, and why am I showing you them today?

The reason I am showing these organisms to you today is that recent studies have shown their closest relative to be the Nemertea which we covered before HERE  (Bourlat et al 2008), and so they are the next step along in our journey, and in the next post I will cover their similarities and differences with Nermertea (I promise, they are not worms!)

They are superficially similar to clams or mussels, as they have two halves to their shell, however, the symmetry of these shells is different, and the creature inside the shell is very different too!

If you cut open a mussel or clam shell along the joining line of the two halves of the shell, each half would be be a mirror image of the other.  In contrast, the shell halves of a brachiopod are not the same size, and they can be different shapes as well.  The line of symmetry is at 90 degrees to the line between the shells (This means that the line of symmetry is perpendicular to the hinge line). What this means is that if you cut a Brachiopod shell from top to bottom instead of along the hinge, the two halves there will be symmetrical.  This refers only to the shell, not to the creature inside.

That sounds quite confusing, and it took me a while to get it, but, I think this image explains it better:

Comparison of shell symmetry between a brachiopod and bivalve Image from Kansas Geological Survey

Brachiopods appear to have been much more common in the past than they are today.  Today there are around 300 species of Brachiopods, whereas the fossil record shows 12000 species. Whilst it is not certain why these became so reduced in numbers, and the molluscs became more prevalent, it has been suggested that the extinction event around 250 million years ago (The Permian-Triassic extinction event) affected brachiopods more than molluscs, and in the changed circumstances in marine environments after this extinction, molluscs were better able to adapt to the new ecosystems which were present.  Other theories suggest that as Brachiopods have a lower metabolism than bivalves, and a simpler circulatory system than molluscs, this played a role in the reduction of species.  Molluscs have gills, whilst Brachiopods use an organ known as a lophophore in gas exchange (more on that next time).  These factors are known to increase vulnerability of species today to changes in environment (Knoll et al 2007).

It is also thought that as brachiopods are not as well able to buffer against changes in their environment (increases in acidity for example), and their shell is larger in relation to their soft tissue than mussels, they were more susceptible to fluctuations in their environment. (Clapman & Payne 2011, Knoll et al 2007)

What this means for the few brachiopod species we have today is that they are especially vulnerable to any changes which are occurring, such as the increases in acidity in sea water, and increasing sea temperatures.  In the next post I will be covering other, behavioural, factors which contribute to their vulnerability compared to mussels.

References and Further Reading:

Bourlat et al; Testing the new animal phylogeny, a phylum level molecular analysis of the animal kingdon 2008

Clapman & Payne; Acidification, anoxia and extinction: A multiple logistic regression analysis of extinction selectivity during the middle and late Permian 2011:

Knoll et al; Paleophysiology and end-Permian mass exctinction. 2007:


5 thoughts on “Brachiopods: Aren’t they just mussels?

  1. Pingback: Brachiopods 2: Feet, shells and lophophores « skepticalsquirrel

  2. Pingback: Bivalves…Sucking and Sieving | Skeptical Squirrel

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