Guard Duty Turns Dad into Cannibal

**SPEAKER_2** (0:08)
You're listening to Strange by Nature, your guide to the strange, weird, unbelievable and improbable wonders of the natural world.

**Kirk** (0:23)
Hello, everyone, and thanks for being here today. I'm Kirk Mona, and joining me are Rachel Gimza and Victoria Thompson. We're all naturalists who together have scoured the world for weird and wonderful wonders, just to please your mammalian brain's desire for novelty. Isn't that nice?
Let's do this.

**Victoria** (0:41)
Well, hello, everybody. Welcome to the show. Good to see you guys, as always.

**Kirk** (0:46)
Good to be seen. Good to see you, too.

**Victoria** (0:48)
I am starting things off this week.

**Rachel** (0:50)
Or, you know, heard, at least.

**Victoria** (0:51)
Well, all right. So I'm starting things off, and I'm going to talk about a beautiful region of the world. If you spend any time on sandy beaches in tropical or subtropical areas of the Indian and Pacific Oceans, you may notice a strange phenomenon.
Tiny spheres of sand that are scattered across the beaches.

**Rachel** (1:20)
Okay.

**Victoria** (1:21)
Little balls of sand, but not randomly. They form radiating lines and sunbursts, sometimes spirals or concentric circles. These works of art last for a few hours until they're washed away by the next tide.
And these patterns are the work of small crabs.

**Kirk** (1:45)
Huh.

**Victoria** (1:48)
Known as sand bubbler crabs.

**Rachel** (1:50)
That's so cute.

**Victoria** (1:52)
Yes. There are various species in the genus's Scopemera, Scopemera, I don't know, and Dotilla. The crabs are quite small. They're usually about one centimeter wide across their carapace, carapace.
And otherwise, they look, you know, very crab-like. They just look like typical crabs. They are sand colored and they camouflage really well against the beach. And they make small burrows where they hide during high tide.
When the tide goes down, they emerge from their burrow and look for food. Well, what do sand bubblers eat? They eat sand.

**Rachel** (2:37)
Of course. Why wouldn't they eat sand? They're only sand bubblers.

**Victoria** (2:42)
Like small children, they eat sand.

**Rachel** (2:44)
Oh, oh, oh, oh, oh, oh. Nightmare.

**Victoria** (2:48)
Of course, they are not actually eating the sand grains because that is hard rock. What they eat is microscopic, organic material from the sand, detritus, tiny animals, single-celled creatures.
That's what they dig.

**Kirk** (3:06)
Okay.

**Victoria** (3:07)
Their front claws point downward and are especially adapted to scoop up sand into their mouths. They also have specially adapted mouth parts that are lined with bristly citae, sort of like hair-like fringes.
They store water in their gills, and as they scoop sand into their mouth with their claws, they vibrate the water through the sand and use the citae to filter out the organic material in between the sand grains.

**Kirk** (3:38)
Okay.

**Victoria** (3:39)
So then they swallow the food, but as they release waste sand, clean sand, it forms a small ball, and when it reaches a certain size, they spit it out on the beach and put it behind them.

**Rachel** (3:56)
Yeah, sure. Why not?

**Kirk** (3:58)
Very cool.

**Victoria** (3:59)
The sand balls can be one to nine millimeters across, depending on the size of the crab.
So yeah, they just push it back, and then they move on to the next bit of each. Okay.

**Rachel** (4:12)
Makes sense, I guess.

**Victoria** (4:13)
Yeah. What about the patterns?

**Rachel** (4:16)
Well, why are they in fun patterns, Victoria?

**Kirk** (4:18)
Yeah.

**Victoria** (4:19)
The center of the pattern is always the crab's burrow, and the crab moves forward as they filter the sand, they place the balls behind them.
When they've gone as far as they like, they head back to the burrow and start again on a different trajectory. This creates the starburst pattern. And by making these balls, the crabs then have a clear visual representation of sand that has already been processed.

**Kirk** (4:47)
Nice.

**Victoria** (4:48)
So they're not going to waste time and energy on areas that have already had all the nutrients extracted.

**Rachel** (4:53)
Right.

**Kirk** (4:54)
Yeah.

**Rachel** (4:54)
Okay.

**Kirk** (4:55)
Brilliant.

**Victoria** (4:56)
The pattern is going to vary depending on the species of crab and also how the nutrients are distributed. They're not going to spend as much time in a location that's nutrient poor. They're going to spend more time in an area that has a lot of nutrients. And there are also numerous crabs on the beach usually. So their patterns run into each other and overlap. So it can create these kinds of intricate combinations of patterns.
Of course, the crabs have a limited amount of time to do their work. They can only feed at low tide. And as the tide comes back, it will wash everything completely smooth again, but also distribute more food in the sand for them.