Coral Reef – The Maldives – Chris Packam
Across the planet, coastal ecosystems, like the Sunderbans, are essential for both the land and the sea. They act as barriers, protecting the land from storms. And they provide vital nurseries for ocean-going fish. But, more importantly, they trap much of the silt and sediment, so that clean water flows out to sea. And in the tropics, this has a profound effect on the world’s richest marine habitats… the Coral Reef.
And one of the finest on earth is here – the Maldives. There’s something very odd about a coral reef. Look at the water – it’s clear. It’s absolutely crystal clear.
I can see a vast and colourful coral city. And across the world’s oceans, these are home to a quarter of all marine species… from tiny clown fish… To the black-tipped reef shark. This really is the ocean equivalent of a rainforest. But it’s also a puzzle. The waters around this reef aren’t just low in sediment, they’re consequently low in nutrients. So, how on Earth can a coral reef support so much life?
It was a puzzle that stumped the world’s most famous biologist, Charles Darwin. And thus it became known as Darwin’s paradox. And it took science more than 100 years to figure it out. And guess what the key was?
Connections. Wonderful connections between the species that live here. Take the coral itself. It’s not made from one, but from two organisms. First, tiny creatures, just a few millimetres in length, called polyps.
Polyps spend their lives filtering microscopic particles in the clear waters. But up to 90% of their food comes from their coral partners. Sheltering within the safety of the polyps, are colourful specks – algae. Like plants, these tiny algae get most of their food through photosynthesis, to make sugars, powered by the sunlight. So, whilst the polyps provide the algae with protection, in return, the algae supplied the polyps with food.
But this ecosystem just doesn’t add up. As Darwin knew, this fabulous diversity of life here can’t be sustained by just sunlight alone. It also needs vital nutrients, things like nitrogen, phosphorus and potassium. And if that doesn’t get these from silt, then where does it get them from?
Well, all of the animals that live in these waters excrete valuable nutrients. But the constant tidal currents quickly wash them away. So, what the reef needs is something that can hold onto those nutrients. Well, there is such a thing and there’s also a very special creature that’s going to lead me straight to it.
Coral Reef Inhabitants
The Hawksbill Turtle. They’re strong swimmers, and I’m going to see where it goes. And I’m really hoping that this one is hungry. It’s a rather odd-looking meal – a sponge. It’s tough, but the sharp beak of the hawksbill can cut through its sinuous flesh. What is clearly much harder is actually keeping hold of it in the swirling currents. So turtles love to eat them, but why does the reef need the sponge?
Well, sponges are creatures that live embedded amongst the coral. And there are thousands of species. They are amongst the most bizarre animals on the planet. They don’t have eyes, a heart, or a nervous system. But the weirdest thing about a sponge is the way that it feeds.
A sponge sucks the coloured water out of this syringe. The plunger isn’t even being touched. As sponges siphon water through their bodies, they extract the nutrients. And although those nutrients are in tiny concentrations, if the sponge pumps quickly, it can get enough. Their secret is the scale with which they can do this.
A sponge like this one can pump five times its own volume of water through its feeding canal in just one minute! And a sponge, just 60 cm in length, can filter the equivalent of an Olympic-sized swimming pool in just five days!
So, as the animals on the coral reef excrete nutrients, it’s the sponges that capture and concentrate these as viable food. It’s these nutrients that help feed the reef, benefiting everything that lives here, from the coral, right up to the top predator.
This large-scale recycling of nutrients helps keep these nutrients around the reef for longer, delaying the inevitable leaking away into the open ocean. For that reason, it’s the sponges that are my coral reef heroes.
What I’ve learned here, is nothing short of a revelation, really. Everything is connected – the fish, the turtles, the corals. But it’s not just these animals – it’s the sponges, too. And further upstream, the tiger and the snails.
And when all of these things come together, the connections make this place work. And it really does work, because this is one of the richest ecosystems on our planet.
Around the world, ecosystems in shallow seas like these, convert diverse nutrients in the water, to provide a haven for a huge variety of sea life. But the most miraculous place of all is further out to sea… In the deep ocean.
External Links
Coral Reef – Wikipedia Page
