Why are there no freshwater squid?
March 1, 2007 4:13 PM   Subscribe

Why are there no freshwater squid?
posted by jonesor to Science & Nature (9 answers total) 1 user marked this as a favorite

I suspect it's because of how they reproduce. Tens or hundreds of thousands of them collect together in a single location and everyone lets go at once. The resulting murk in the water contains billions to trillions of fertilized eggs, the vast majority of which won't make it to adulthood. The survival rate is very low, but the number of eggs released is so huge that enough make it for the next generation.

They're relying on overwhelming the predators who prey on their young through sheer quantity.

In fresh water settings (e.g. rivers), not enough squid could collect together, and the fertilized eggs wouldn't be produced so that a sufficient quantity could survive predation. I suppose it could be done in a really large body of fresh water (e.g. Lake Michigan) but there aren't enough of those kinds of bodies around, and they're not easily accessible for squid to migrate to.

It's difficult, evolutionarily speaking, to make the transition from salt water to fresh water. If you take a salt water fish and drop it into fresh water, it'll explode over the course of an hour or two. (Putting a fresh-water fish in salt water causes it to die of dehydration.) Migrating from the Atlantic Ocean to Lake Michigan would be a trip taking thousands of years, and the squid would have to become adapted to reproducing in a river first. (Then they'd have to figure out how to get up Niagara Falls.)

Some fish make the journey each year, e.g. salmon. But the transition from fresh-to-salt and salt-to-fresh takes weeks. The fish hang out in the area near the mouth of the river and as their internal biology gradually adjusts for different levels of salt in the ambient environment, the fish stay in areas where the local salt concentration is comfortable. (A big river creates a large zone of fresh and brackish water near its mouth. The Amazon was discovered by Europeans when they realized that the water they were sailing through wasn't salty. They turned west and finally got close enough to see it.)

But in those kinds of fish, the change is genetically programmed. If they're forced to make the change before they're ready, they'll die. And most fish are not genetically programmed to be able to make the change at all, including squid.
posted by Steven C. Den Beste at 6:00 PM on March 1, 2007

Another way to look at this is, is to take a step back and go ... that's just the way they evolved. Mollusks like snails, slugs and freshwater clams are a better evolutionary fit in some areas, in the current geological timeframe, than octopii and squid, and vice versa, for an innumerable amount of reasons.
posted by frogan at 6:40 PM on March 1, 2007

All cephalopods (octopi, squid, etc) are strictly carnivorous. Most lakes and rivers are too nutrient poor to support a complex carnivorous food-chain, so most freshwater animals are either herbivorous or filter-feeders.
posted by randomstriker at 9:13 PM on March 1, 2007

And most fish are not genetically programmed to be able to make the change at all, including squid.

Squid are mollusks, not fish.

Frogan has it right. I hope you aren't disappointed, but the question cannot be answered correctly in another way. The alternatives are unverifiable just-so stories.
posted by grouse at 10:55 PM on March 1, 2007

I was using "fish" generically to refer to "things that live by swimming"; I'm aware that "fish" technically are all chordates.
posted by Steven C. Den Beste at 11:41 PM on March 1, 2007

An aside: you've probably seen this already, as it's the first Google result for 'freshwater squid', but Jeff Vandermeer has written a whole bunch of short fiction pieces about the mythical beast. Here's one piece on the Festival of the Florida Freshwater Squid.
posted by painquale at 12:37 AM on March 2, 2007

Lolliguncula brevis, the Atlantic brief squid, is euryhaline in nature and can tolerate salinity as low as (warning, .pdf) 8.5 ppt. Compare this with a salinity of 32 to 35 ppt for seawater. This isn't quite freshwater, but certainly counts as brackish or estuarial water. These squidlets are also osmoconformists. They match their internal salinity to their immediate environment, unlike other cephs.

Me, I just like saying "lolliguncula".

At the risk of seeming cephalopedantic about the plural form of octopus, ahem.
posted by Sallyfur at 2:27 AM on March 2, 2007 [2 favorites]

I was using "fish" generically to refer to "things that live by swimming"; I'm aware that "fish" technically are all chordates.

Personally I don't find it helpful to extrapolate biological truths about all swimming things (including mollusks) from examples that are only about bony fish. Especially when there is no indication that any of the existing freshwater mollusks managed the transition by switching from hyperosmotic to hypoosmotic regulation within a lifetime. It would be a lot less complicated to gradually change the type of osmoregulation through evolution over many generations.
posted by grouse at 6:29 AM on March 2, 2007 [1 favorite]

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