What is that rock in a fish's skull cavity?

So like many asians, I eat fish whole. I get fresh tilapia and I usually steam it with some garlic and green onions on it. Then we eat the whole thing. My favorite part of a tilapia is the head, because you can eat the cheek meat, the eyeballs, and kind of chew up the whole head.

If you take the head of a fish off, there’s this little open cavity on top of the head that connects to the spine. I’m pretty sure it’s where the brain is. When I steam it, I always make sure to drink that (the brain liquifies). However, there is always a rock in there. Not a round smooth rock, but a little jagged, irregularly shaped piece of mineral or bone. What is that?

I’ve noticed this in other fishes too, but I mostly eat tilapia and I’m sure it’s always there. It’s shaped like an arrowhead, jagged, pointy, and not attached to anything at all. It’s just sitting there floating in the brain juice. When I bite it, it’s much harder than the rest of the fish. I know fish doesn’t really have bones, they have softer cartilage, so the appearance of this really really hard rock is puzzling. More puzzling is why it’s in the skull cavity where the brain is. Does anyone know what that thing is?

When I eat the eye balls of fish, sometimes I would come up agains this hard, white, partially translucent ball. Is that the lens of the fish?

Fish (well, most of them) have bones. It’s just the sharks, rays, lampreys, and a few others that have cartilage instead.

Do you mean the Otolith? They’re structures that help the fish determine right-side up, and are usually near the brain. They’re also helpful to humans attempting to age the fish and/or identify the species from just the bones.

I’m also curious about what the hard eye balls are. The only structure I can see in the human eye that would be that big in relation to the rest of the eye is the vitreous humor, but fish eyes (fish eyes, roly-poly fish eyes) might not be that similar to the eyes of us east African plains apes.

The eyeballs shrink and harden when the fish is cooked. My kid likes to eat them on a dare.

The eyeballs are the luckiest part, according to a Chinese ex.

No clue what that rock is though. That Otolith thingy sounds promising.

We have a fish in Australia called a jewfish with the original name being jewelfish, the jewels being the otolith or ear bones, some people make them into a necklace or scientist can age the fish by counting the layers a bit like the growth rings on a tree.

It was my job for a few years in the early 90’s to cut open fish, remove the otoliths and count the rings! Also to measure and weigh the fish before checking it’s stomach contents. Fun times.

Where you’re finding them, behind the brain, close to the spinal column sounds right. They are in pairs but if you cut on even a slight angle, it’s easy to miss the second one.

The ones I collected tended to be a rough oblong and quite flat, though how much variation there is between species, I don’t know.

Thanks for reminding me, it’s going to be a nostalgic evening. Not so much the slicing and dicing, but the stuff all around it.

As others have said, its an otolith. Every vertebrate has them.

My undergraduate thesis was on reptilian inner ear sensors. The otoliths were a pain in the rear - I had to figure out a way to get rid of them without destroying the delicate sensory epithelium underneath.

Good times.

Thanks, I’ve never heard of an otolith before. It’s probably what it is, but maybe this weekend I’ll get a fish and take some pictures

Wow. On the one hand this is amazingly fascinating and has me consulting all kinds of anatomical references to see what parts of me anybody would be eating. On the other hand, blaaaaargh and I can’t believe you’re such a writer to give me enough of a tactile feel of eating something’s head to make me want to vomit just drinking a glass of wine.

“Blaargh”?? You mean “Yum!”

So I did go out and buy the fish last weekend. I call her Tilly, short for Tilly the Tilapia. I didn’t even wait to finish up her flesh meat before I went for the head.

Here’s the thing inside the eyeball I was talking about. I really like those giant eyeballed rock cods cause the entire eye is like almost a mouthful. Delicious!

Here’s most of the head gone. After you eat the cheek meat and peel back the layers of scales that cover the head, you get this. You can see the empty eye sockets and the piece of jaw I haven’t eat yet hanging down. The cavity where the brain and otolith is located is just behind the eye sockets. Fun fact: a lot of the most delicious fish head goo is trapped in the bottom half of head, so when you eat the bottom half of it, you’re not eating as much as sucking out the juices and slimy fish goo. It’s really good!

When you finish up the edible outer portions and take away most of the goo and flesh, you get this. It’s all the cartilage immediately attached to the brain cavity. It’s upside down so if you can make out the orientation, you see the cartilage bone in the middle separating the eye sockets. Crack that lumpy mass of cartilage on the right side and you get access to the sweet brain nectar within

And here are the otolith. They look exactly like the wiki but different than what I’m usually used to. I guess I got a textbook fish and not one of its mutant cousins.

That’s a great job you’ve done there. Now, if you could just remember to check the stomach contents next time.

And the sexual maturity! It was a big joke for us at the time that the local emergency number is 111 - which was also the code for a male, immature, no visible gonads.

Oh the fun we had at parties!

Wow, when you say you eat the whole thing, you ain’t kidding!

Maybe I’ll have to try that someday. Got a recipe?

I usually prepare tilapia very simply. Get the whole fish, gut it, sprinkle on some garlic salt (or regular salt), chopped green onions, and chopped garlic over it and underneath it on the plate. Steam that thing until it’s cooked and eat, that’s it. Takes about 10 mins to prepare and 20 to steam