Shrimpfish swim strangely

Head down, tail up: How vertically-oriented Shrimpfish swim

Picture a fish swimming. What image came to your mind? Maybe it was a speckled trout, hastily swimming upstream or your pet betta circling its fishbowl. Regardless of your imagined fish, I can almost guarantee you did NOT picture a fish hovering in a vertical position, with its head pointed down and tail straight up. This is not a common orientation displayed in our fishy friends. Shrimpfishes (Aeoliscus punctulatus) however, can be found swimming together in synchronized schools in this strange vertical position.

Pictured by: Bernard Dupont

Why would shrimpfish orient themselves “head down, tail up”?

It is likely that these small fish are using this not-so-fish-like orientation to camouflage themselves, hiding from predators in other vertically oriented flora and fauna, like seagrass, coral, or even the long spines of sea urchins.

While this is a clever disguise, this orientation is not something just any fish can do and requires a specific design crafted by evolutionary processes. The challenge of a vertical orientation is that the shrimpfish has to remain steady in this position without rotating or rolling one way or the other. Conversely, if the shrimpfish needs to make a quick get-away, they also need to be capable of making rapid turns.

New research by R. Holzman and F. Fish investigates the mechanisms of stability and maneuverability that keep this fish “down-right.” To better understand the shape and design of this fish, the researchers studied and measured museum specimens. To study their swimming behavior the researchers used high speed cameras to quantify the movements of the fish’s body and fins.

What did they find?

1) Shrimpfish have streamlined bodies

These fishes are laterally compressed (squished sideways), similar in shape to a peapod, which helps them reduce drag. Cross sections of the fish’s body also revealed shapes that were strikingly similar to engineered airfoils.

Holzman and Fish, 2019
Holzman and Fish, 2019

2) They have active and passive ways to stabilize their head-down position

To stay stable in this downward position with the changing currents and waves, Shrimpfishes must continuously beat their pectoral fins. This requires a lot of energy, but Shrimpfishes also have passive ways to maintain their vertical position, one is called hydrostatic stability. Less complicated than it sounds, this simply relates to the different densities of tissues distributed along the fish’s body, specifically, the position of the center of mass and center of buoyancy. If the center of mass and the center of buoyancy are in close proximity, the forces pulling the fish down (mass/gravity) and pushing it up (buoyancy) are more balanced, helping the fish stay in place or reorient if disrupted. If the center of mass and buoyancy are father apart or misaligned, this results in destabilizing forces that can cause the fish to roll.

3) They are maneuverable  

Despite their stabilizing forces, Shrimpfishes are fairly maneuverable and able to pitch their heads upwards into a horizontal orientation. Additionally, they can use their tail fin as a passive rudder and turn quickly about their long axis (head to tail).

4) They can swim fast for short durations

Shrimpfishes were observed swimming very quickly for short spurts, called “burst swimming.” Shrimpfishes were found to swim at speeds up to 14 body depths per second (body depth used instead of body length due to their downward orientation).

To summarize: Shrimpfishes employ a swimming orientation not often seen in fishes and to do so they rely on their unique evolutionary design, including a hydrodynamic shape, hydrostatic and active stabilization forces, and the placement and beating of their fins.

Kelsi Rutledge is 2nd year Ph.D. student at UCLA studying the functional morphology and fluid dynamics of olfaction in batoid fishes. Check out her website here or follow her on Twitter here.

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