Freaky Animals

The Frog Pond #27: Blame ancient fish for your sensitive teeth

Did you know sharks aren't attracted to human blood? Plus ancient jawless fish, exoskeletons becoming teeth, and more in this month's Frog Pond!

Sarah L. Hawthorn

31 Oct 2025 — 6 min read

Photo by Leigh Miles / Unsplash

Let's talk sharks, blood, senses, and where teeth come from!

Sharks just aren't that into you

Did you know sharks aren't attracted to human blood? While fish blood will draw in a predatory shark, they can tell the difference with human blood and aren't particularly interested¹.

Sharks, even the great white sharks that come by Nova Scotia's waters in the summer months, are a miniscule risk to humans compared to nearly anything else you'd find on a day at the beach².

Still, if you're up close and personal with a great white shark, they may test out a bite to figure out what you are. I'd always thought they used their noses, but it's their teeth that hold the sensitive nerve endings needed to take in information from the ocean around them. Most sharks will release a human after the first bite so long as it doesn't mistake you for a grey seal³. Getting too close to seal territory or where there are active fishing activities can confuse a shark into hunting you instead. If you're really worried, skip ankle jewelry and swim at dusk and early in the morning when there's less chance of the lighting on the water making you appear scaley².

I grew up with Shark Week (somehow every other week on the Discovery Channel) throwing conservation caution to the wind to promise me the immediate danger of sharks. It's nice to see how much the conversation on sharks has shifted over the years to understand and respect them.

Super-Every-Sense

Sharks can sense through more than our own five senses, and they do a lot with them!

Smell

Up to 66% of the total weight in a shark's brain is dedicated to smell³. While it's true that they can detect fish blood in tiny amounts compared to their surroundings, the "one drop a mile away" is a stretch. One teaspoon in an average swimming pool is more accurate, from about a football field's distance away³.

Sight

The iconic glowing/shiny eyes of a shark come from a reflective layer of cells, like a cat's, behind the retina to improve their vision in the low-light conditions underwater³.

Taste-Touch

A shark's mouth is an important tool for their sense of touch, though their sense of taste is not as highly adapted. "Test bites" are common and may injury creatures or objects not normally deemed prey just for the shark to understand what's in front of them³.

Electricity

Sharks have another sense called electroreception. Tiny pores on their head and snout are extremely sensitive to electrical fields to feel everything from Earth's geomagnetic field helping them migrate across vast oceans to the muscle contractions in prey³.

Flow

Another row of pores make up a shark's "lateral line" that runs from snout to tail. Water flows through these pores and sensory cells inside can sense pressure changes³. This line helps give sharks a better sense of their surroundings when the waves from its own movement bounces off obstacles - kind of like an underwater form of echolocation!

Even more?

Sharks also have a "pit organ", two oversized denticles (flat, V-shaped scales) that cover a small pocked in their skin. These can be on their back, sides, or lower jaw. While the exact function hasn't been determined, researchers believe these may be used to register water currents⁴.

From jawless to Jaws to jaws full of teeth

Those denticle sensory organs that sharks have had for hundreds of millions of years have a connection to our modern day chompers! The first tooth-like structures on skin on early fish would eventually evolve into the sensitive to touch teeth we know and bite with today⁵. While fish and mammals have teeth as backboned vertebrates, these tooth-like structures also appear on invertebrates, though the structure underneath differs⁵.

In fossils from the Cambrian Period (539 - 487 million years ago), researchers had always interpreted the tubules as being made of dentine. In humans now, dentine is the yellow layer beneath enamel that can sense pressure, temperature, and pain⁵. But recent research has found that these early fossils were of invertebrate arthropods, not vertebrate fish. They were more like small hairs on the skin and had no dentine, so they were not evolutionarily connected⁵.

This means the oldest known teeth are actually found in the Ordovician Period (487 - 443 million years), with fossils that had odontodes similar to the Cambrian Period tubules with dentine on the skin. Specifically the researchers studied Eriptychius, an ancient jawless fish⁵.

Fun fact: Only the front of Eriptychius' skull was mineralized, the rest was made of cartilage⁶!

The "outside-in" hypothesis

When some fish became more active predators, they needed to hold their prey⁵. Hard odontodes migrated towards the mouth, which predatory fish used to bite. Like a lot of things in evolution, teeth weren't "made" with the purpose they ultimately got, but they happened to be the approach that worked well for the creatures that had them. The researchers called it an "oral invasion"⁵ of the structure, which delights me.

This leads to the "outside-in" hypothesis, which counters the long-standing "inside-out" hypothesis. Basically: Sensitive structures in evolution began as points of sensory glands on the outside of exoskeletons, then "moved inward" to eventually become teeth⁷.

A word from a passing frog

It's been a minute, huh? I've had this Frog Pond half-done for the past couple of months while deadlines and day-job have taken over my life. Mainly good things! Just a lot of things!

Writing

Last we spoke (in MAY, oh my god) I was in my first round of developmental edits for A FATE WORSE THAN DROWNING. I've since finished a second round of dev edits and finished my copy edits! I've also gotten to see what the inside formatting is going to be (I cannot WAIT to gush about it!!) and, of course, the cover. I'll send out a special edition of The Frog Pond just for that, don't worry, and it should be soon!!

The next step for A FATE WORSE THAN DROWNING is the bound galleys/Advanced Reading Copies that are sent out to authors I'm hoping will want to blurb the book (those quotes on the front/back of the book) then we'll move on to the final proofreading. From there, I take a backseat as editorial work becomes production of the physical books themselves!

It's an exciting and nerve wracking thing to think of all the authors I admire so much being asked to read my work and, in essence, say nice things about it. Luckily the years of beta reading, querying, and editing have prepared me plenty for rejections. I'm not sure I'll be as ready for any yeses we get!!

I've got a few other projects up my sleeves, excited to share as we get closer to them :)

Reading

I had a real deep-dive into classic r/nosleep and creepy-pastas the last few months. Tales from the Gas Station, MotherHorseEyes, Ted the Caver, The Left Right Game, etc.

The Annual Migration of Clouds by Premee Mohamed: This was such a great post-apocalyptic eco-horror novella, set in Canada! I loved how deep it dived into a fictional fungal infection passed down genetically. Premee is a phenomenal writer and I'm looking forward to reading the sequels!

I Who Have Never Known Men by Jacqueline Harpman: Read this dystopian 90s classic a few months back when it came up on Libby. There's something so haunting about this narrative and the lack of answers gives such an eerie feeling in the second half of the book. Also a short one!