Why don’t fish swim upside-down? Come on, don’t tell me you’ve never wondered about it. True –out of sight, out of mind and everything- but don’t you find it a puzzle? I mean, if you think about it, their vertebral columns make them top-heavy, so apart from the occasional fin they would probably flip belly up anyway. Everything would still work.
I think Retirement does this to me. I have more time to think inside the pond, as it were, to consider those things I postponed when I was younger, to boldly go where no one has gone before… Uhmm, that may be hyperbolizing it a little, but you take my meaning. Unfortunately, in terms of the fish, people have been there. In fact, an article in the Smithsonian magazine brought it to mind… again: https://www.smithsonianmag.com/science-nature/why-dont-fish-swim-upside-down-180967192/
It’s truly sobering to realize that there are very few unthought things anymore -but sometimes not with the solid answers I might have wished. For example: ‘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.’ To say that ‘The leading explanation is that fish began life right side up, evolutionarily speaking, and so most never had a reason to change’ does little to inspire my allegiance.
I am more comfortable with another explanation, albeit one I would likely not have used in grade 6: ‘In most invertebrates, the nerve cord runs down the belly. The nerves have little difficulty connecting to the eyes and brain, which are more dorsal, since invertebrates don’t often have hard parts inside their bodies to get in the way, Sallan says [he studies early vertebrate evolution at the University of Pennsylvania].
‘In contrast, even the most primitive vertebrates have hard tissues, such as bones, that form internal barriers within the body. As a result, their body plan is much less adaptable. The nerve cord runs along the animal’s back and its brain develops on its dorsal side to link to the nerves. Since the eyes are in the skull near the brain, they end up on top as well. With the top of the body unavailable, the stomach and mouth end up on the bottom.’
Nice, but unfortunately, it still doesn’t answer the question about the orientation of the fish in the water. Of course, there are the basic tenets of evolution: ‘[…] if a new trait doesn’t provide a distinct advantage, things tend to stay the same.’ In other words, laziness –if I may be permitted to generalize from my own experience- is seductive.
But there is a brave attempt at recoup -a Parthian maneuver, I suspect: ‘Scientists have teased out some of the perks to swimming top side up. For example, gravity will help move blood from the brain back to the heart, says Flammang. Fish also benefit from having their eyes above their mouths, says Love [a semi-retired marine zoologist at the University of California, Santa Barbara], as it keeps those sensitive organs away from food and debris, and on the lookout for predators. Even their top-heaviness confers fish some benefits: it increases their maneuverability, Flammang says.’
And, of course, there’s the usual escape clause: ‘And, some fish have learned to swim upside down, while others flirt with the idea. Many fish that live near structures such as reefs or piers will orient their bellies against vertical walls or even ceilings, says Wainwright [who studies fish morphology and behavior at the University of California, Davis]. The fish seem comfortable swimming this way near the structures, but they will swiftly turn right side up when they swim away.’ One guesses that this might be a temporary effort for protection from predators, or local conditions such as storms or turbulence.
I was still ruminating on the article on a crowded bus the next day. I was just sitting there quietly minding my own business when I happened to eavesdrop on two old ladies in the seat in front of me.
“Flossie’s being such a clown, you know…I mean, for a fish.” This from a woman in a purple coat and tight white curls stuck to her scalp like they were drawn there. Who would have thought that other people actually talked about fish? Who would have guessed that old people saw them differently? Knew different things about them?
“You call your goldfish ‘Flossie’?” the woman beside her said, with obvious surprise. She, too, was dressed in a dark coat, but was sporting a blue toque-like hat that enveloped her head like a decorative sac, so I couldn’t see her hair.
“Yes,” the first one replied. “I named it after my sister –the one who passed last year,” she added, for clarification.
Hat lady nodded at this. “It’s always nice to have a reminder, isn’t it Ellie?” They were both silent for a moment, and then the hat spoke up again. “And was your sister a real card?” I loved the metaphor.
“Oh yes, Jane.” I could see her nod with a memory that had just surfaced. “And she so loved swimming…” Ellie sighed and turned to Jane. “But only on her back… She said she liked to float like that to look up at the sky.”
Jane glanced at her with a puzzled look on her face, but Ellie seemed unfazed.
“I thought of her again today, you know.”
“Of your sister, you mean?”
Ellie nodded. “Flossie –the fish, I mean this time- has been swimming on its side a lot lately. My sister used to do that sometimes, too -when she got bored, I guess.”
Jane’s face suddenly softened and the one eye I could see from my seat, looked concerned. “And do you think the fish is okay?”
Ellie nodded confidently. “Well, it’s been swimming like that on and off for a few days now… I guess they get bored, too.”
Jane looked down at her lap for a moment. “I suppose all fish are different, eh?” she said, but I could tell she didn’t really believe it.
Ellie chuckled. “Boy, I’ll say. As I was going out the door this morning, it was even swimming on its back.” She shook her head at the wonder of it. “I certainly named her well, don’t you think?”
Jane didn’t look up from her lap as she nodded her assent. “You certainly did, my dear.”