Monday, 29 September 2008

More dances with bivalves: flexible mussels

It was a simple question:

Those big blue edible mussels farmed in Port Phillip Bay. What are they?

And it should have had a simple answer:

It's Mytilus [insert specific epithet here]. I'm surprised you didn't know that. It's astonishingly common. [Insert gratuitous reference to questioner's intellectual ability here.]

But there's nothing simple about these Mytilus. They've been the subject of continuing disagreement about their identity. Over the years, they've been identified as M. edulis and M. galloprovincialis, which are both smooth-shelled blue mussels with antitropical distributions. (That is, they occur in cool and temperate waters of both N and S hemispheres but are absent from the tropical zone between.) They've also been called M. planulatus, which is an Australian endemic. And M. edulis planulatus, by those whose who like to hedge their taxonomic bets.

So which one is it?

I did say it wasn't simple. Where M. edulis and M. galloprovincialis meet, they often hybridise, which results in genes from one ending up in the other. This was thought to have happened with Australian populations. Tasmanian mussels, which are the best known, were revealed as mostly M. galloprovincialis with a little bit of M. edulis mixed in. The mixing wasn't a recent event — not the result of an itinerant mussel hitching a ride into town on a ship's hull — but was the product of some ancient inter-species hanky panky.

But the story turns out to be more complicated than that. (As if you hadn't suspected ...) A different set of genetic evidence suggests that the southern mussels separated from the northern ones before they diverged into what we currently know as M. galloprovincialis and M. edulis. The southern mussels are further divided into three distinct groups — Australia (exemplified once again by Tasmanian specimens), South America and New Zealand — all genetically distinct from one another, as well as from the Northern Hemisphere populations.

Which is all very well, but what should we call our fine Australian blue mussels? Well, the SE Tasmanian population, anyway. As Gérard et al. (2008) wrote in their paper
Under [the second] scenario the distinction, based on nuclear markers, of M. edulis-like and M. galloprovincialis-like mussels in the Southern Hemisphere would be merely virtual, constrained by human's wish to always assign new samples to reference populations assumed to represent the genetic composition of a given taxon …

And as I wrote in my note book after reading the papers:

Maybe it's M. planulatus after all?


Borse, P, Daguin, C & Bierne, N. (2007) Genomic reticulation indicates mixed ancestry in Southern-Hemisphere Mytilus spp. Mussels. Biological Journal of the Linnean Society 92: 747 – 754

Gérard, K, Bierne, N, Borsa, P, Chenuil, A & Féral, J-P. (2008) Pleistocene separation of mitochondrial lineages of Mytilus spp. Mussels from Northern and Southern Hemispheres and strong genetic differentiation among southern populations. Molecular Phylogenetics and Evolution 49: 84 – 91.

Hilbish, TJ, Mullinax, A, Dolven, SI, Meyer, A, Koehn, RK & Rawson, PD (2000) Origin of the antitropical distribution pattern in marine mussels (Mytilus spp.): routes and timing of transequatorial migration. Marine Biology 136: 69 – 77


Mosura said...

Woe! I always thought mussels were confusing but now I see why. So for the dummies (me), is saying that "the distinction is merely virtual" the same as saying that "it's virtually impossible to make a distinction". Are the above mentioned mussels basically all mongrels to one degree or another?

Denis Wilson said...

Personally, I have always failed to see the charm of mussels in a bowl. Tough and full of grit, I have found.
But I have fond memories of them on the piers around Port Phillip Bay, in Melbourne, when I was a kid, with all those wonderful shiny blue shells, all jammed together. But they made swimming around those piers quite challenging, to avoid getting cut and scratched.
Thanks for the memories.

mick said...

This sounds a bit like the argument I have heard - that you can't really know something without knowing its name. But, I don't think that's necessary for simple enjoyment!


I wonder if they taste different. Taxonomy with a little bit of help from gastronomy?

Anonymous said...

For aydin örstan,

There were a lot of complicating factors when I had some delicious beach suppers of mussels steamed open on fires, around Port Phillip Bay decades ago. Sewerage outfalls not the least :-)
However, the ones we dived for on Mordialloc pier were a lot plumper hence tastier than those from Brighton's - which is quite close along the bay to Mordialloc.

Pippis - clams - are the other mongrels.
Good luck with working out specifics with them ;-)
Also extremely delicious.

With shellfish, perhaps why they've thrived in spite of being so thoroughly predated by man, fat is tasty and skinny is inedible, so only healthy populations have been plundered etc etc.
So that stressed populations of bivalves, unlike say, teleosts which are taken all sizes and condition, have been left alone to get healthy again.
Just saying, of course, I am not a scientist.


Snail said...

Been speed-limited ... grumble

Mosura, I think what it means is that the southern mussels contain genetic markers for galloprovincialis and edulis because the N and S hemiphere mussels were all one happy mixed group at a stage before those two mussels diverged into separate species. So if we try to shoehorn in our mussels using those markers we're going to come up with some confusing tale of mongreldom. We need a different approach for the S ones. A clean slate, maybe?

What I want to know is: to what species do the S mussels belong? It looks as though there's between 1 and 3 based on the genetic data. The Tas specimens are distinct but the small sample from WA appear to be recent immigrants from the N. I'd love to see data from Vic, NSW and SA specimens.

Snail said...

Denis, pier piles are great spots for watching the 'small life'. Portsea Pier is a winner, with the occasional sea dragon dropping in. At least, it used to be. Dunno about now.

Mick, it is handy to be able to put a name on something, but in this case it looks like we've all been trying to force them into the wrong pigeon holes!

Snail said...

Aydin, if we can work out how many species there are, maybe we could try a taxonomic taste off! It should be easy on your side of the equator with th taxonomy well-known. On this side, we'll have to compare Tas with NZ with Chile.

I'm up for that.

Snail said...

D***y, fresh mussels ... Does Homer Simpson drool. Not sure I'd be plucking them straight out of the Bay any more.

About twenty years ago, I was chipping oysters off the rocks at Etty Bay, near Innisfail in Far North Qld. Bloody marvellous. But I don't think I'd do that now. Pesticides, fertilizer run off ... (Not to mention the rezoning of the Great Barrier Reef Marine Park.)