The greatest migration on the planet

Did you think your commute was tough? Kate Feller from Cambridge University tells Chris Smith about the largest migration on the planet...

Kate - It’s called a diel or daily vertical migration and it is the largest migration that happens on the planet. It happens every day and it’s maybe only countered by human commuters as far as biomass.

Chris - Well go on then, put some numbers on it. What's happening, who’s moving, and how far are they travelling?

Kate - So, a lot of stuff is moving. So you’ve got living at the surface at the top 200 metres are your phytoplankton that’s kind of like the grass that all of your herbivores are going to be eating. And so, the tiny zooplankton like copepods like plankton from SpongeBob SquarePants, they want to eat this phytoplankton. However, there's a lot of bigger things that want to eat them and they use vision in order to locate these copepods. And so, as a way to avoid being eaten by a visual predator, they go down to where it’s darker at greater depth in order to avoid these predators, and they do that during the day, and then at night, they come up to feed. And so, it helps them eat while not being eaten.

Chris - How far do they travel? When you say they go down, are we talking hundreds of metres that these little animals might move up and down?

Kate - Yes. So that’s interesting. The really little ones I believe are more restricted to the top 200 metres but diel vertical migrations have been documented down to 200, 800, and even 1400 meters which counts as the deep sea, and it’s all based off of the sun cycle.

Chris - It’s quite impressive for a small creature that you'd really need a magnifying glass to see clearly.

Kate - So, it’s more like a chain reaction. So the little guys will be more towards the surface and then the bigger ones that eat them will migrate with them, and follow them, and then the bigger guys who eat the bigger guys – it’s like a chain reaction.

Chris - And everyone is going up and down every day.

Kate - Everyone is going up and down and everyone is dying, and everyone is pooping. And so, all that death and poop just rains down on the deeper parts of the deeper layers of the ocean, and that’s what everyone else eats. So they're going to move up and down with all the poop and death, and stuff that’s coming down because that’s how they get the things they need to survive.

Chris - Ultimately, is there rainfall that lands on the ocean floor because if you’ve got lots of stuff at the surface capturing energy because you’ve got plants up there, you’ve got animals up there, they're capturing energy from the sun, they're capturing carbon from the air in the form of carbon dioxide, some of them are not going to get eaten. Are some of them going to land on the seafloor and effectively carry carbon down?

Kate - Yeah. So that’s how this biological pump of carbon works, is that a carbon-based organism near the surface dies and then just starts to slowly rain down on the animals beneath it, and then somebody swims by and goes, “That's delicious! Thank you.”

Chris - So it’s actually very important sort of drawdown mechanism for planting carbon seafloor-wards. If we disturb that, we could have trouble.

Kate - Well, at least the animals in the deep sea will have trouble.

Chris - Now you mentioned that some of these animals can use their visual system to spot these things going up and down and follow them. How developed are their visual systems then and what are they looking for? What can they see?

Kate - Oh, they're fantastic! We’re talking baby crustaceans, other types of crustaceans that are fully adults and just swim around in the open ocean. We’re talking larval fish, full grown fish, sharks, dolphins, whales. I mean, everything is swimming around looking for something to eat all the time. So you’ve got the full spectrum of eye types. So you’ve got eyes that are just simple little eye spots that can detect a shadow versus not a shadow, all the way up to a great resolution eye that’s more like ours, designed like a camera that can really see something in great detail.

Chris - I often wonder when I look at something like a shrimp or a crab and it’s got these little eyes on storks or whatever, how good is its vision?

Kate - Well generally, their resolution is a bit poor. They have what's called a compound eye. So it’s an eye that’s made up of a whole bunch of little units. Imagine a bouquet of straws and each straw is almost like a little camera itself where it’s got a lens on the end and then a piece of film at the base. And so, it’s sampling each point in space like a pixel. So, each one of those pixels gets assembled into a picture and it’s just going to be a low resolution image, so there's not going to be a lot of pixels or as many as you would have in a camera-type eye like ours that has much, much smaller cells in it so you can pack a lot more pixels into it.

Chris - But obviously, good enough that it can find something to eat and avoid being eaten.

Kate - Yeah, exactly. So you don’t need a lot of resolution in order to see something coming at you that you don’t want coming at you, or see something that you want to pursue.