Sunday, 29 April 2012

Sturgeon

Image: david.torcivia via Flickr
Sturgeon! With a T! Don't mix him up with a surgeon because this guy just isn't cut out for heart surgery; he has no proper backbone, no hands and I swear he needs glasses. Does he remind anyone else of Mole from The Wind in the Willows?

Image: Robertson, D Ross
Sturgeons are 20-something species in the family Acipenseridae. They are found in much of the northern hemisphere, right up to Siberia. Some of them are proper freshwater fish, living in lakes and rivers. Others are brackish, taking advantage of nutrient rich estuaries.

Most of them can also enter coastal marine waters. The Green Sturgeon for example, ranges from Mexico to Alaska to Russia to China.

Really, these adventurers have the kind of choice most fish could only dream of. And they use it, too. Make no mistake, these are active, roaming fish who can travel miles to find good eating. This is important when you consider the amount of food they'll need due to their sheer size.

Sturgeons range from pretty big to utterly massive. Smaller ones, like the Sterlet, are quite popular as pond fish because they slowly grow to only about 1 metre (3.3 ft) long. Others will commonly reach 2 or 3 times that.

Image: Kenneth Sturman
The biggest one is known as the Beluga. It comes from the Caspian Sea and can reach 5.5 metres (18 ft)! That's like three average Dutch men! Or three average women wearing whatever heels necessary.

Even more ridiculous is that a monster like that isn't even a record breaker. A seriously, amazingly huge Beluga is even bigger. An average Dutchman bigger. When it comes to buying fish for the aquarium or pond, never has it been more important to ensure you get the right species.

Sturgeons are ancient fish, living fossils even. They first emerged some 200 million years ago and have scarcely changed since. That's probably a good explanation for why they're so weird!


For example, they have no scales. Instead, they have five rows of bony plates called scutes running down their body.

Image: Yu Diving via Flickr
Despite being perhaps the world's oldest living ray-finned fish, they don't actually have bony skeletons. It's made of cartilage instead, like a shark.

In fact, they don't really have a proper backbone at all! It's one, long, continuous rod known as a notochord. All vertebrates have this at some point in their embryonic development and it gives us the phylum name "chordata".

Most of us get it broken into bits and ossified to form the bones of the vertebral column. Salps get rid of it entirely as they grow. Sturgeons leave it as is.

But there's another thing about the Sturgeon and its notochord. In other fish, the backbone stops at the base of the tail and then the fin rays extend symmetrically. Sturgeons are more like a shark outside and in.

Image: Yu Diving via Flickr
Their tail fin is asymmetrical, with a longer upper lobe than lower, as in sharks. Also, their notochord continues up to the tip of the upper lobe of the tail fin. Again, more like a shark. Also a bit like icthyosaurs, except they had backbone in the lower lobe instead.

There's one significant difference between your average shark and a Sturgeon, though...

Image: Frank Peters via Flickr
Sturgeons have no teeth! They really are quite old. At least it provides a reassuring limit on the size of food they can eat.

Image: Theron Trowbridge via Flickr
Sturgeons are carnivorous. They'll feed on pretty much any fish, mollusc, worm or crustacean they can swallow whole. Their rostrum, what I like to call the schnoz, is used to stir up the riverbed and reveal tasty nibbles.

Four barbels in front of the mouth detect the tell-tale signs of the edible. As with catfish, this allows them to survive in the murk and dirt at the bottom of lakes. Those tiny eyes are about as useful as their size suggests.

Not only are Sturgeons old as a whole, but individuals can also be really old. Some can live to be well over 100! This is quite a sensible and understandable age to human ears. Enough to be impressive, but it's not like 1,000 years or whatever. It just gets silly after that.

With that kind of lifespan it's probably not surprising that they take quite a while to reach sexual maturity, too. It differs from species to species and also due to temperature and food availability. When the going's good, some males may take just a few years, but some females could be nearing 30 before their first dalliance.

It's now that they'll migrate to their favoured river to spawn. And it can be quite a journey. One Yangtze Sturgeon was seen to travel 3,200 km (2,000 mi) from the Chinese coast to its favoured part of the Yangtze River.


It sounds a bit like those salmon and it is, but there's a significant difference - they don't all die when they're finished. Sturgeons can spend several years out at sea between each spawning, feeding and preparing. You can really take your time when you live for over a century.

Image: Magalie L'Abbé via Flickr
But who will they spawn with? This isn't as simple a question as it usually is. Sturgeons of one species can produce viable offspring with Sturgeons of other species, even ones in a different genus.

With all these perfectly fine hybrids around, you can imagine that it took quite a while for everyone to sort out how many species there actually were. But it gets worse...

Sturgeons are one of those animals that are deeply affected by their environment. The same species in different areas can grow to significantly different lengths, have a differently shaped rostrum and a different number of scutes on their skin. And with them venturing out and travelling for thousands of miles, they can find themselves all over the place. It all turns out to be a bit of a nightmare for the people who deal with that sort of thing.

Image: Joel Abroad via Flickr
Anyway, when all's said and done (or just done, Sturgeons don't say much) females lay their eggs. Depending on species, size and age there could be tens of thousands or several million of them. They're sticky such that they adhere to plants and the river bed. Or to your spoon. Yeh, these Grandma fish are a major source of ridiculously priced caviar. It's not the kind of thing you can smear on your toast in the morning.

Sturgeons probably aren't too triumphant about the monetary value of their unborn children. If all goes well, the eggs will hatch to reveal a ridiculous number of tiny Sturgeons. They grow quite quickly at first, maybe reaching about a metre (3.3 ft) in 5 or 10 years. By now they'll be big enough and strong enough to check out the estuaries and maybe even start investigating the sea.

But it'll be decades of slow growth before they become real Freshwater Leviathans.

Image: U.S. Fish and Wildlife Service - Midwest Region via Flickr
We all have to start somewhere.

4 comments:

TexWisGirl said...

i know folks fished for sturgeons in the Great Lakes when i was growing up in Wisconsin. such prehistoric beings...

Comment1 said...

Yup! It's amazing that things like this still exist!

Chloë Langley said...

Some people are farming these beasts here, I bought a fillet of a small one at the marketplace. The skin feels like a shark + has the bony plates on it for extra effect. The skin is very slimy but the meat tastes great! It is unfortunate that they are quite expensive...

Comment1 said...

Haha! That's pretty weird that it has the scutes in the place. I guess it adds interest! One of these I'll have to do a post about all the weird things you've eaten!

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