Saturday, 2 May 2009

The Best Shark Dive in the World!: Husky Dusky? Maybe!

Remember the mystery Shark?
It's still a mystery - but at least, we've managed to narrow down the choices to two likely suspects. And having gone digging, I found some brilliant pics on Andy Murch's great Elasmodiver website.
The two suspects look like this.

That would be a Dusky (Carcharhinus obscurus) on top and a Silky (C. falciformis) below. See the alignment and the size of the fins? And that caudal keel that for so long has left everybody baffled? Except that, apparently, it aint really a proper "caudal keel"? Read below and you'll understand - it's complicated!

And now, compare them with the mystery pic on top: any preferences?

Like El Tiburon and after a knee-jerk reaction in favor of it being a Silky, I now root for the Dusky. I've never seen a Dusky, but I've seen plenty of Silkies - and although I can't quite put my finger on it, the mystery Shark just doesn't "feel" like one of them. I'm specially unconvinced by the first dorsal, but then again, who am I to say!

Talking of which, Juerg cautiously tends towards the Silky - but being the good scientist he is, he has passed on the question to a very prominent (and probably, the best) Shark taxonomist who has come back with the following.

"C. falciformis it is, nice pics."

but then, after reflection:

"I was a bit deprived of sleep when I made the call on the identification, so that your friend may be on to something.
I did a composite illustration of C. falciformis vs C. obscurus, and append it to this note for comparison with the best of your two images.

The only carcharhinids with prominent keels when alive and dead are Galeocerdo cuvier and Prionace glauca, so that your Carcharhinus when alive and swimming shows a keel but not when dead. Rather like observing a live bird in a tree through binoculars vs a live bird of the same species in hand from a mist net, and again the same species of bird as a study skin.
Fin size and shape change with growth in Carcharhinus. I was wondering about your C. falciformis in terms of second dorsal shape, position of the first dorsal, and pectoral fin shape and relative size.

Tooth shape of upper anterolateral teeth and vertebral counts are diagnostic for these species, but until we can pack underwater mini CAT-scan machines, we have to rely on dead animals to voucher live ones. Shark watching is not quite as advanced as bird-watching, but it's getting there"

There you have it! And yes, it's complicated! But fascinating, too - at least to me!
But whatever Shark that really was, the myth about Bronzies (C. brachyurus) prowling the waters of Fiji remains just that, a myth! As expected! For now!

Stuart: there you have it!
Well, sort of.

The Best Shark Dive in the World!: Husky Dusky? Maybe!

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