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