Freshwater Cephalopods?

For anyone who puts enough thought into animals and environments (particularly if they think about cephalopods), they will eventually wonder why there are no freshwater cephalopods. If you think about   other families of mollusks you know that they dwell in water both salty and fresh water, as well as on land. So why not cephalopods? They are most of them top predators in their ecosystem.

The simplest factor that I could think of and find was how much salt there is in the water. The reason for this is simple. Freshwater species of fish actually conserve salt, while salt water species expel it. A more physical explanation for this is that cephalopods don't have a sodium pump.

Now for a crazy theory of my own: My theory is that even if cephalopods did develop the proper organs for freshwater life they could never move in the ways that made them top predators in the first place. The octopods, squid, and cuttlefish could never keep up with the rest of the creatures in the rivers because saltwater makes you lighter. Also the nautilus would be robbed of its controlled buoyancy ability (once again because you are more buoyant in saltwater. Please keep in mind I am neither a marine biologist nor have any proof behind what I say, as it is just a theory.

Sodium pump:

The Nautilus in Literature and Art

In Literature:

A poet by the name of Oliver Wendell Holmes wrote a peom with the chambered nautilus (one of the most commonly known nautiluses) as it's subject and title. And of course a submarine named The Nautilus appeared  in two of Jules Vernes' novels.

In Music:

Interestingly enough a russian band was named Nautilus Pompilius, nautilus pompilius being the chambered nautilus's binomial name.  Also the amerisan composer Deems Taylor wrote a cantata called The Chambered Nautilus.

In Art:

The artist Andrew Wyeth made a painting called "Chambered Nautilus". Interestingly, nautilus shells were used to make cups that were really more for decoration rather than use.

Nautilus cup:

Nautilus Related to Geometry?

Ok nautiluses are not related to geometry, but their shells are a great example of the golden spiral and the Fibonacci spiral. To start off with, what is a Fibonacci spiral? A Fibonacci spiral is made when you take the square root of the area of a square and add that to the square next to it's square root. Expressed mathematically the dimensions would look like 1x1, 1x1 (because there is no square next to it so it is 0+1), 2x2, 3x3, 5x5, 8x8, 13x13, and so on and so fourth. The nautilus's shell does not mirror this exactly, but it is almost the same.

Fibonacci spiral:

The golden spiral is a logarithmic spiral (a self-similar spiral curve) who's growth ratio is the golden ratio (that being a+b is to a as a is to b). Golden spirals (approximate) are also found in galaxies some times. Nautilus shells are not perfect Fibonacci or golden spirals, but they are just about the closest thing in nature to them.

Nautilus Overview

The nautilus is the oldest cephalopod at 5 million years old and is the only cepalopod with an exterior shell. Their shells average eight inches in diameter and are stripped and smooth. Each male nautilus has 90 slim tentacles with no suckers and a large eye, while females only have 50 tentacles. Although their arms are less powerful than other cephalopods they are so numerous they can easily overpower prey. Most live to be 15 years old. Nautiluses reproduce annually. It takes 9 months to a year for them to hatch. The nautilus lives in tropical waters, coral reefs by day and deeper waters by night. They eat hermit crabs, crustacean's exoskeletons, and small fish. Octopods, sharks, triggerfish, and sea turtles prey on them. But the real predator they need to watch out for are humans. The nautilus is endangered because of its beautiful shell.