Sharks Unknown with Jasmin Graham

I've been studying sharks for nine years now, and even still, there are many shark secrets to discover.

Today, we'll be looking closer at my favorite shark species, the bonnethead shark.

These adorable little sharks were discovered to be the first known omnivorous shark.

And they exhibit differences between males and females not seen in other sharks.

And these could just be clues to a larger question, which is could these sharks be two species hiding in plain sight?

Bonnetheads were part of my undergraduate research and even today, I assume working with them in the field in Tampa Bay.

Today well be meeting with Kathy Liu, my former mentee who was comparing bonnetheads in Tampa Bay to those in Biscayne Bay to unlock some of their secrets.

It all started when Jasmine came down here.

She saw that our bonnetheads looked different.

They were slightly smaller and she was like, Why are your bonnetheads weird? And we were like, What do you mean? And me and Jasmine have been working on that this whole time that I've been here in Miami.

Kathy's curiosity about the sharks was piqued when she was doing fieldwork with me.

While Kathy was gathering data in Biscayne Bay in the summer of 2023.

We had a chance to go out with her.

A few months later we met to get an update on her progress.

So I'm really excited to talk to you about bonnetheads.

Yeah.

Me too.

I love them and I'm happy you love them.

Yes, because bonnetheads are the best shark, obviously.

And anyone that says differently is wrong.

They're really cool because they're omnivorous, so they eat seagrass in addition to other things which most other sharks don't do.

They also are the only known shark species to show sexual dimorphism in their head shape.

So when we think of sexual dimorphism in birds, for example, it's usually pretty dramatic where the males have this really pretty coloring.

They got a lot of flare and the females are sort of drab and brown and a little bit boring.

That has a lot to do with attracting mates.

So the girl birds go, Ooh, look at him.

I'm going to go mate with him. I wonder if that's true for the bonnetheads.

Right.

So with sharks, females are generally larger than males, usually to kind of accommodate for all the babies that they're going to have.

But with the bonnetheads, they're kind of the only known shark so far that shows difference in their head shape.

And we don't really know the reasoning why, but my project looks into that head shape and seeing if it's prevalent here and if it changes across different subpopulations.

We we're looking at bonnetheads and Biscayne Bay more in depth, which hadn't really been done before, as well as looking at their diet in comparison to those in Tampa Bay.

The sharks that I use were caught with long lines and gill net.

We bring them on board and place a small PVC pipe in their mouth to flush ambient water over their gills.

And that provides very oxygenated water to help facilitate respiration.

We would take various body measurements and we took a picture of their head or their cephalofoil on a grid board.

And then I analyzed those photos and looked at the curvature of the head through that analysis.

And then with diet, one of the samples we collect in the field is a small muscle biopsy, and we facilitate a very fast and efficient workup so that these bonnetheads are spending as little time outside the water as they need to.

Kathy wound up going home with a lot of data, along with plenty of hands-on experience as a scientist working in the field.

I have gotten over 460 sharks over 90 days during my time here getting my masters and 120 something, I think, have been bonnetheads.

So a quarter, which is really exciting in a lot of diversity and learned a lot of new skills.

After all that time on the water and in the lab, I was excited to see what secrets Kathy had uncovered from our favorite shark species, and there were a few surprises.

So what all did you find through your research?

So we found that for both the Biscayne Bay and Tampa Bay bonnetheads, the males do have that significantly pointier cephalofoil than the females.

It's kind of the opposite of what the previous study hypothesized, though they thought that the males developed that point as they matured.

But what we were finding was that point was the pointiest in the immature males and everybody kind of rounded out in their head shape as they matured, which leads us to asking some more questions about that.

And then we also are seeing with stable isotope analysis that the bonnetheads are eating different things.

So what do you think might be driving this sexual dimorphism in the bonnetheads?

We don't know, but we do need some more research to kind of finalize that finding.

Kathy had uncovered a new secret about the differences between male and female bonnetheads.

Kathy's discovery with the male bonnetheads is a huge breakthrough that has the potential to show completely new findings that the scientific community didn't know about.

If males and females are showing sexual dimorphism most when they're babies, then obviously this sexual dimorphism in their heads has nothing to do with finding a mate.

So what's really going on here?

Kathy may have uncovered a new mystery surrounding bonnethead sharks.

So this made me curious about whether our sharks in Biscayne Bay and the ones in Tampa Bay were experiencing another rare phenomenon, one known as cryptic speciation.

Cryptic speciation is when you have two or more species that look physically similar or even identical.

But when you look at their genetics, they're actually genetically distinct.

So they are different enough in their genomes that scientists declare them different species.

The cryptic part of cryptic speciation is hidden a species hidden within another species that looks the same but is actually different.

So clearly, male and female bonnetheads heads look different.

But the Tampa Bay bonnetheads and the Biscayne Bay bonnetheads also look quite different to me.

So I was curious whether Kathy had considered whether there might be cryptic speciation within these boneheads and if other folks had found cryptic speciation within boneheads before.

Have you ever considered the cryptic speciation side of things?

Yes.

So with the literature searches that I've been doing, there is a subspecies of bonnethead that they've found to be genetically distinct in the Caribbean.

And we don't know if other bonnethead subspecies might exist, but they have a pretty broad range along both coasts of the Americas.

And if they're not migrating and intermixing with each other, that kind of would start that process of separating them as different species.

So you did all of this great work on bonnetheads, but were often in science communication asking this, So what?

So what?

Why do we care if these bonnet heads are a distinct species?

Why do we care if they have differences in their head shape between males and females?

Why do we care if they're eating different things?

Why does all of this matter? Yeah.

So with bonnetheads, they are fairly widespread and they are currently mainly looked at as a species and sometimes having management plans that are broad don't always help all of the individuals if they're living in different places and experiencing different impacts, having better understandings of multiple subpopulations and even multiple species is important to have those more localized management measures to help conserve and protect a species, an organism, or the habitat that species rely upon.

After all the data is gathered, Kathy was a huge hit at our Diversifying Ocean Sciences workshop, and some of the members of her fan club even came to her master's thesis defense.

I had the best time.

Really happy to have been a part of Minorities in Shark Sciences throughout this whole thing and really enjoyed talking about experiences that I didn't really see reflected in media or anywhere really growing up and so happy to be a part of being a mentor and a person of color in shark science and helping create space for a welcoming environment.

This is my last official field school intern day of my masters and that this was the best course to end it on.

And you all are awesome.

And then glad I know you now.

Aw Kathy!

Hammerheads are a really unique family, not only because they have this really strange morphology with their heads being super wide.

They also have a lot of cryptic speciation, which isn't super common in the animal kingdom.

We don't know what the sexual dimorphism situation is with the bonnetheads.

But you never know when you go in researching a hammerhead species what you're going to find.

They're hiding a lot of secrets which is why they're the best This program was made possible in part by the National Science