how to ruin your crustacean karma

A few weeks ago, a local Alaskan was kind enough to drive my coworkers and me up a mountain to a particularly scenic spot that is hard to get to. I rode in the backseat, and within the quarter mile up the mountain, was overcome with my first-ever bout of (severe) motion sickness. The only relief was to close my eyes and try to think happy thoughts of kittens and Anthropologie. I never saw that scenic spot, unfortunately. But I did leave my lunch, which I had eaten 2 hours prior, on the roadside. It happens.

This experience caused me to recall an experiment where an ingenious but somewhat evil scientist discovered the mechanism by which most invertebrates sense up from down. Let me explain.

In the late 1800’s, people observed newly-molted crawfish picking up small rocks and particulate from their surroundings and placing them in little cavities in their heads (called statocysts).  In 1893, a zoologist by the last name of Kreidl decided to solve the mystery behind this strange behavior. His hypothesis was that it functioned in balance or orientation, so he came up with his malevolent plan.

Kreidl placed newly-molted crawfish in terrariums with iron filings at the bottom instead of sand. As predicted, the crawfish placed the iron filings into their statocysts. And then started the fun.

He took an electromagnet and brought it close to the crawfish. When the magnet was brought close to the animal’s right side, the crawfish tried to turn over so that it’s belly was facing the magnet. Vice versa when the magnet was exposed to the left side. When held right above the crawfish, it tried to flip over onto its back. Clearly, the crawfish associated the position of the iron filings within their statocysts with the direction of gravitational pull. Ah hah!

When at rest, the ventral setae are excited.

But there’s more to this story than simply acclimating towards gravity. The inertia of the statolith (grain of sand, rock, iron filing, whatever) can also tell the crawdaddy which way it is moving. If the posterior setae are excited, then the animal experiences the sensation of moving forward.

When scuttling forward, the posterior setae are excited.

In Kreidl’s experiment, the animals showed general disorientation when the magnet was brought to the posterior. This is likely because they weren’t locomoting forward, but still felt like they were moving forward. They were all “what the hell?!” and started flipping around, searching for the sensation of gravity and rest.

Come to find out, every time a crustacean molts, it loses the lining of the statocysts and therefore the statoliths within. This explains why the statolith-gathering behavior is observable after molt. How strange this is to me: physiology dependent upon the addition of external objects to a sensory system, i.e. having to stick rocks in your head every so often. Strange, but pretty neat.

But seriously, those poor crawfish- if only they had more sophisticated cognitive abilities and inner ear complexity, I’m sure they would have felt about like I did on that mountain. If Kreidl had a mustache, I bet he twirled it and laughed the whole time.

In reality, Kreidl was not evil but super smart. His experiment was a seamless and inspired piece of scientific work. Well done, Kreidl.

Resume petting your evil cat.