November 10, 2017 — There is no up or down in space, yet in shows like Star Trek, ships are always oriented the same way: right side up. It’s a scientifically unnecessary trope that has become a running joke among science fiction fans.

Yet here on Earth, fish find themselves in a strikingly similar situation. As a fish glides through its weightless, three-dimensional, watery world, it almost always stays right side up. The question—for both starships and fish—is why?

It’s an easier question to answer for fictional spaceships than real-life fish.

In movies or on television, directors show ships the way they do because it makes the scene more understandable to viewers, creatures accustomed to a gravity-bound world. “We have a fixed idea that everything should be right side up,” says Frank Fish, a functional morphologist at West Chester University in Pennsylvania. But what about fish? It’s a puzzle.

“I can’t get into the mind of a fish—despite my name—and determine why it would particularly do that,” says Fish.

Deepening the mystery is that scientists know few reasons why a fish would swim in any particular orientation—yet they clearly have a preference. Unlike land animals, fish don’t push against the ground to move. And, while moving, fish are no more streamlined in one orientation than another. What’s more, most fish are top heavy, says Brooke Flammang, who studies fish biomechanics at the New Jersey Institute of Technology. Like a child balancing on a beach ball in the pool, gravity wants them to flip. So why don’t they?

The leading explanation is that fish began life right side up, evolutionarily speaking, and so most never had a reason to change. “Just between us, yeah, they never bothered,” says Milton Love, a semi-retired marine zoologist at the University of California, Santa Barbara.

A fish’s preferred orientation “goes back to those very early steps of building a left side and a right side, a head end [and] a tail end,” says Peter Wainwright, who studies fish morphology and behavior at the University of California, Davis.

There are two groups of animals that have distinct left and right sides, Wainwright says. The first group, protostomia, includes most invertebrates, like insects and mollusks. Early on in development, as embryos, these animals develop a cavity that goes on to become the mouth.

Read the full story at Hakai Magazine