The ocean is full of weird and wonderful creatures, from the enigmatic handfish to the large and ancient Maugean skate. But what about all the organisms we can’t see?

As we zoom in on the oceans’ smallest life forms, just before we hit the realm of the microscopic, we’ve got the macrofaunal invertebrates - tiny creatures that lack a backbone, but display a mind-bending array of colours, shapes, and sizes.

Whether we’re talking amphipods, sea slaters, worms, micro-snails, or the many other creatures in between, macrofauna can go largely undetected in our oceans… unless you know where – and what – to look for.

"Basically, they're the small invertebrates that are just marginally too small to see with the human eye. You can see a little spot wriggling, but that’s about it,” said Graham Edgar, Professor of marine ecology and conservation science at the University of Tasmania in Australia.

“But they have an incredible range of body shapes – including crustaceans with green stripes, red eyes, and purple spots. People would be gobsmacked if they could see these animals.”

How big are marine macrofauna?

One of the trickiest things when it comes to classifying the tiniest organisms in the ocean is trying to sort them into well-defined categories.

We classify all invertebrates as belonging to one of four main groups, from smallest to largest: microfauna, meiofauna, macrofauna, and megafauna. But where the lines are drawn between them is something that’s been argued about for decades.

In a recent paper by Emil Ólafsson from the Spanish Institute of Oceanography, he lists the ballparks that he likes to work with when it comes to size:

Microfauna are the smallest, and tend to be less than 0.4 mm in size

Meiofauna are around 0.4 mm to 1 mm

Macrofauna are typically a few millimetres to several centimetres. But the system used in the 2009 reference text, Ecology of Marine Sediments, uses entirely different scales:

Microfauna < 0.063 mm (1 micron = one-thousandth of a millimetre, and a human hair is about 75 microns across)

Meiofauna 0.063 – 0.5 mm

Macrofauna 0.5 - 50 mm

Megafauna > 50 mm. Ultimately, what ends up being discovered during marine surveys depends on the size of the mesh screens scientists use to ‘sieve’ tiny creatures out of the water – for example, macrofauna are often collected using 0.5 or 1 mm mesh screens http://web.vims.edu/bio/shallowwater/benthic_community/macrofauna.html .

Where do marine macrofauna live?

Whether they’re found on the land or underwater, most macrofaunal invertebrates make their homes in sediment, or on rock, seaweeds or other animals.

If we’re talking about marine macrofauna specifically, they’re found in what’s known as the benthic zone – the region that begins right up at the shoreline, and plunges deeper than 4,000 metres depth in the ocean’s abyssal plain.

On the seabed, macrofauna prefer to hide in nooks and crannies or seaweeds, or they make their own tubes or burrow in the sediment.

What do marine macrofauna look like?

The term macrofauna describes an incredible range of life forms, including isopods (sea slaters), amphipods (crustaceans), small gastropods (snails), worms, and bivalves (clams, oysters, cockles, mussels, and scallops).

They often display brightly coloured bodies, boasting pinks, reds, yellows, and even purple hues, and strange, unique patterns across their shells and exoskeletons.

“They are quite spectacular in terms of their appearance,” said Professor Edgar, who specialises in Australian macrofaunal species.

“The diversity is astounding. In one hour in the shallows of southeastern Tasmania, I can find 200-300 species of amphipod, which is more than has been recorded in the entire UK.”

Here’s a ladybug amphipod from the family Cyproideidae:

Ladybug amphipod. Credit: nstermo/Shutterstock.com.

And here’s the scaly-foot gastropod (Chrysomallon squamiferum), which is only a few centimetres long. Its home environment in deep-sea hydrothermal vents can result in very different colours across separate populations:

Scaly-foot gastropods from two different vent fields. Credit: Kentaro Nakamura et. al. PLoS ONE 2012.

Why are marine macrofauna so important?

Being tiny means very few people will be thinking about marine macrofauna at any given moment, but they’re quietly underpinning the entire food chain of the world’s oceans. They are what young fish eat before they grow mature enough to feed on other fishes, crabs or seaweeds.

“They’re important because they’re at a pivotal stage of the food chain,” Professor Edgar said, likening them to plankton in terms of their importance as a universal food source in our oceans.

“Most of the fish that we catch have changed diets from when they’re small - they rely on macrofauna for food in their juvenile stage. Basically, without them, there’d be no fish growing to the size that we catch.”

Edgar says despite their crucial status, marine macrofauna have been largely overlooked in conservation research. He believes it’s important that future studies investigate their roles in food webs and specific ecosystems, and how populations will be affected by pollution and climate change.

“They’re really fascinating animals,” he said, “but because they’re out of sight, they don’t exist in the public consciousness.”

By Bec Crew.

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