Imagine: you’re swimming in one of our many Great Lakes, enjoying a bit of socially-distanced fun while staying cool during one of Chicago’s hottest summers ever. Something long and thin brushes your leg. You begin to panic. You’ve heard about sea monsters, especially krakens and giant squid, and other tentacled beasties that will drag their prey down to the murky depths below. Well, I hate to burst your bubble, but there’s no way a tentacled monster is going to make you a snack while you’re taking a jump off Navy Pier. 

Now that summer’s over, and you’re reflecting on your attempts to take a dip without going near anyone else, or perhaps your goal to pick up fishing as a hobby (a good,  socially distanced sport), you may wonder what’s actually lurking down in the depths of our freshwater lakes around the Midwest. Typically, the fish you’ll see will have fins, tails and scales. We’re talking bluegill, largemouth bass, perch, maybe even a muskie or a northern pike, if you’re lucky. If you’re not so lucky, you might get an alien-like invader, such as a lamprey eel or an Asian carp. But, have you ever wondered why we don’t see anything with multiple tentacles, like an octopus, within our freshwater lakes? 

First, let me explain what these tentacled beings, called cephalopods, are. The Cephalopoda class includes octopuses, squid, and cuttlefish – all your favorite tentacled, aquatic beasts, minus maybe Ursula the sea witch. There are so many different species of cephalopods found in all oceans on Planet Earth. Cephalopods are unique in the marine biology community: they are all easily identified by their fairly large heads, their higher-level body symmetry, and their many tentacles. They’re also known for their ability to produce ink as a defense mechanism, to the point that their nickname is ‘ink fish’ in many languages. Cephalopods are predators, eating aquatic animals, from tiny shrimp to giant fish, using their many limbs and fast speed to hunt and kill their prey.

Their structural engineering is even more complex, with cephalopods having at least two hearts to maintain the complex cardiac structure of their bodies. (Think about that for a second…Imagine having more than one heart within your body–but you’d also have to imagine having four legs and four arms at the same time!) And they have one more characteristic in common: they all avoid freshwater bodies. 

In their many different forms, from the mighty Pacific octopus to the petite Hawaiian bobtail squid, cephalopods display multiple features that explain why you’ll never see one of these creatures hanging off Chicago First Lady on a river cruise. One these features is their high protein, large prey diets. Some of the biggest cephalopods eat some of our biggest marine species on Planet Earth—such as sharks and small whales– this is why you’ll see them congregating strictly in ocean water. However, the vast majority of cephalopods aren’t gigantic, but they still prefer eating species that thrive in the sea, such as oysters, one of the favorite food of octopi.

Hawaiian bobtail squid.

Well, hold on! I’m hearing you say, “there’s plenty of bivalves, like mussels, in freshwater! Heck, sometimes sharks are found in freshwater, like that bull shark in the Mississippi! Can we find squid and octopus in places like that?”

Not quite. Like sharks, some tentacled sea creatures can be found in brackish waters: the water found in the bays, rivers, and estuaries that exist at many ocean coasts, where the water isn’t quite fresh, nor is it 100% salt water. The semi-saltiness allows some species of sea animal to live in them, usually as a safe spot to lay eggs, reproduce, or seek shelter during large storms, such as hurricanes (source). However, there is only one species of octopus found in this type of environment, and its within Chesapeake Bay, off the coast of Maryland. When you compare this to the hundreds of other cuttlefish, octopuses, and squid swimming in the ocean, the number and coincidence of this one little octopus is a rare one, indeed. 

The Atlantic brief squid is the only cephalopod that is known to tolerate brackish waters. This particular one was found Chesapeake Bay.

Believe it or not, another one of the reasons squid and octopuses can’t survive in lake or river water also has to do with their own engineering, according to some scientists. All marine animals need the right salt balance in their blood (like us fellow humans). Cephalopods’ freshwater, boned, comrades, have trouble keeping their salt levels up, so they have pumps in their gills that pump salt into their bodies and push water out; if it weren’t for these pumps, they’d get bloated and die. Cephalopods and other marine animals have the opposite problem: they have trouble keeping their salt levels down. So, when they drink the available water around them, their kidneys pump out excess salt. This way, they fill up with freshwater and don’t dehydrate. If you stuck a cephalopod in Lake Michigan, their bodies would keep pushing salt out, because that’s just what its kidneys do, and the sad little animal wouldn’t make it.

I know by this point, you’re excited about these incredible creatures. I was when I was at the Oregon Institute of Marine Biology, where we saw these fascinating invertebrates on a near constant basis. For many of them, climate change affects their habitats, as well as their ability to eat and reproduce. One of the best ways to seem them locally is in the Shedd Aquarium, as well as models of them at the Field Museum. Go visit both, or support them by going here to make a donation to these institutions:


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Total Solar Eclipse on April 8, 2024

Total Solar Eclipse on April 8, 2024

On April 8th, 2024, a total solar eclipse will sweep across North America, from Mexico to the Maine-Canadian border. For those who experienced the spectacular solar eclipse of 2017, this one will be similar, crossing the United States from west to east and passing through or near several major metropolitan areas. And while its path is quite different this time, Carbondale, Illinois, a reasonable destination for Chicago-area residents, will once again be on the line of totality.    

Just a little background on eclipses:  Lunar and solar eclipses are not uncommon – they each occur about twice a year when the moon is crossing the ecliptic, the path of the sun in the sky.

Two women representing the Illinois Science Council at an event.

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