Help! Did this fish steal grandpa’s dentures?

Produced by: Science Popularization China

Produced by: Su Chengyu

Producer: Computer Network Information Center, Chinese Academy of Sciences

This is a toothy fish called sheepshead snapper (Archosargus probatocephalus).

Image source: Wikipedia

The people who first discovered it thought that the teeth of this fish looked like sheep's teeth, so they named it sheephead. In addition to sheepshead snapper, it has another name - prisoner fish, because its black and white "coat" has the texture of a prison uniform.

Image source: britannica.com

Sheepshead snapper with "good teeth"

The sheepshead snapper has bright eyes and a ferocious appearance. If it had a mouth full of sharp teeth like a shark, it wouldn't be strange, but it has a row of large incisors and molars for chewing. Rather than looking like a sheep, it looks more like a mouth full of human "big white teeth". Its mouth looks like a human trying to speak, which is quite "ghostly".

Looking at its grinning appearance like a human, it is really a bit scary, as if it is Frankenstein's monster.

In late September 2021, an angler caught a sheepshead snapper weighing about 4 kilograms. When he posted a "headshot" of the sheepshead snapper with its mouth wide open and teeth showing, netizens commented:

"I think grandpa lost his dentures and this fish picked them up."

"It seems that the dentist who treated its teeth was not very skilled."

Image credit: Jennette's Pier/Facebook

As shown in the picture, an adult sheepshead snapper not only has "big teeth" similar to those of humans, but also has three rows of molars in the upper jaw and two rows of molars in the lower jaw.

Image source: Document 1

Is it possible to use sheepshead snapper as a portable shell opener?

With a good set of teeth, of course everything you eat tastes delicious.

From shrimps with soft shells to oysters, crabs, barnacles and clams with extremely hard shells, sheepshead snappers prey on everything from shells to meat. If you try to bite open a hard clam, you'll probably break your teeth. The sheepshead snappers have good teeth.

So the question is, with sheepshead snapper’s teeth being so powerful, can they open walnut shells?

First, we need to know how strong the bite force of sheepshead snappers is. Scientists caught 190 sheepshead snappers from the mouth of Tampa Bay in Florida, ranging from juvenile fish to adult large fish, and then tested their bite force.

In order to get closer to the hunting scene in natural environment, scientists also bought some mussels and oysters to conduct bite experiments on sheepshead snappers.

Initially, scientists calculated the bite force based on the anatomical structure of different sheepshead snappers, measuring their bite lever distance, muscle mass and muscle density.

The result is obvious - the larger the sheepshead snapper, the greater the bite force. As can be seen from the curve in the table below, the larger the fish, the longer the lever distance of the bite, and the greater the bite force. This is a simple lever principle.

As the fish gets bigger, the lever arm of the bite gets longer, and the bite force of the incisors and molars gradually increases.

(Image source: Document 1)

As you can see, the maximum bite force of an adult molar can reach more than 400 N, while the maximum bite force of an incisor can reach about 250 N. If a sheepshead snapper bites you with this force, how painful would it be?

Assuming a little girl weighs 40kg, 400N is roughly equivalent to the feeling of a little girl stepping on you with her toes, while 250N is equivalent to the feeling of an old-fashioned bicycle with its front wheel not touching the ground and standing upright, with one contact point of the rear wheel pressing on you. To be honest, it still hurts.

If the growth of sheepshead snappers is divided into five stages, then in the first stage, when the sheepshead snappers are just born, their bite force is not that strong yet, so they can only eat worms and shrimps;

In the second stage, sheepshead snappers can eat small crabs;

In the third stage, you can eat stone crabs with a shell diameter of about one centimeter;

In the fourth stage, stone crabs with a carapace diameter of two centimeters can be caught;

In the fifth stage, hard-shell oysters and clams, which are delicious and crunchy, were added to its menu.

Later, scientists directly used sheepshead seabream for bite tests. The maximum actual bite force of sheepshead seabream with a bite lever of 170 mm long was about 65N, the maximum actual bite force of sheepshead seabream with a bite lever of 172 mm long was 74N, and the maximum actual bite force of sheepshead seabream with a bite lever of 315 mm long was about 196N.

Image source: Document 1

Knowing the bite force of sheepshead snapper, we also need to know how much force is required to crack a walnut.

In 2008, some scientists conducted a study on fracturing walnut shells. The purpose of this study was to better develop a walnut shell fracturing machine.

There are several factors that affect whether the walnut shell can be cracked successfully, such as the thickness of the shell, the moisture content of the walnut, the direction of the force applied to the walnut, etc.

The direction of the force applied to the walnut, z is the edge (Image source: Reference 2)

He found that for walnuts with a geometric mean diameter of about 3 cm, normal thickness, and a water content of 20%, the best place to break the shell is at the edge of the walnut, which is the "edge" we can observe when eating walnuts. Here, a force of about 400N can be used to break the shell.

Image source: Document 2

Another study on walnut shelling was conducted in 2004. The result was that if a walnut is cracked from the edge, a force of about 377N is required.

Based on these two studies, it can be roughly estimated that for a normal walnut, it takes several hundred newtons of force to crush it from the edge.

Of course, these studies are only for shell-breaking machines. The force applied by the machine is applied to a very small area. If the area where the force is applied is too large and the pressure is too small, the force applied in the study will not be enough.

Take squeezing an egg with your hands as an example. If you hold an egg completely in your hand, even if it is a crispy egg, it will be difficult to crack it no matter how hard you use.

From this perspective, the maximum actual bite force of a 30 cm adult sheepshead sea bream is not enough to crush a walnut, even if the bite is applied from the most vulnerable edge.

An adult sheepshead snapper needs at least twice as much force to barely crack a walnut. Our dream of using it as a walnut-cracking tool is likely to be shattered.

But if you find a large enough sheepshead snapper, theoretically it can have a bite force of over 400N, and it might be able to act as a "walking shell opener".

Large sheepshead snapper (Image source: YouTube)

These are all theoretical discussions. In reality, if you really find a sheepshead snapper that is nearly half a meter long, keep it in a fish tank, and use it to crack walnuts for you, you still have to consider many situations.

For example, can you keep enough sheepshead snappers alive to "help" you?

In the previous study on sheepshead sea bream, scientists only used three fish for testing. It was not because they were "reluctant to spend the experimental funds" and only bought three fish, but because many of the sheepshead sea bream they raised died, and only three could be used for testing.

Even if you keep the sheepshead snapper alive, whether it is willing to open the walnuts for you is a problem... After opening the shell, you still have to find a way to take the walnut kernel out of its mouth to prevent it from swallowing the walnut kernel. Assuming that you have done all the above and you have been lucky enough to take the walnut kernel out of the fish's mouth, then you still have to get used to eating walnut kernels with a fishy smell...

Anyway, I can’t imagine how delicious this thing is, yue.

References:

1. Fernandez LPH, Motta P J. Trophic consequences of differential performance: ontogeny of oral jaw‐crushing performance in the sheepshead, Archosargus probatocephalus (Teleostei, Sparidae)[J]. Journal of Zoology, 1997, 243(4): 737-756.

2. Sharifian F, Derafshi M H. Mechanical behavior of walnut under cracking conditions[J]. Journal of Applied Sciences, 2008, 8(5): 886-890.

3. MA Koyuncu, K. Ekinci, and E. Savran. “Cracking Characteristics of Walnut.” Biosystems Engineering 87.3 (March 2004): 305–311.