What is a mud scud and where does it live?

Nick Baker takes a look at the life of a mud scud?

By Nick Baker

Published: Friday, 03 February 2023 at 12:00 am

Mudflats and estuaries are some of the easiest habitats to overlook. What’s so special about cold, grey, stinky mud anyhow? Well, if you’re a wintering wading bird, they are essential.

A vast, flat, highly productive dinner table that resets every 12 hours. The main course for many is a surprisingly insignificant-looking creature: a crustacean that goes by the deliciously quirky name of mud scud, Corophium volutator.

What is a mud scud?

While sometimes referred to as a mud shrimp, it is actually an amphipod and similar to the sand hoppers or fleas that you might find on the strandline.

The most obvious difference is that the mud scud has a very large pair of limbs that curve out in front of its head like two massive claws. Bigger and chunkier in the males, these appendages are a second pair of antennae, and look more like legs than even the mud scud’s legs do: they’re at least as long as the rest of the invertebrate’s body (even longer in the males).

Why the antennae have evolved this way is anyone’s guess, but for a mud scud they are essential tools of the trade. Their most obvious function is for digging, used like bulldozer buckets to shift and shovel the mud into shape and create their home: a U-shaped tunnel up to 10cm deep, which opens onto the surface by way of two entrances. Once the burrow is dug, the amphipod secretes a proteinous goo to hold the shifting sediment particles in shape.

How does the mud scud feed?

To feed, the mud scud uses its burrow and stays out of sight, continuously fanning its rear appendages, which creates a current that draws water down into one entrance and out through the other. This pulls in organic matter, as well as fresh oxygenated water, which passes over the base of their walking legs, where thin, flap-like gills can be found.

Their big antennae are also useful for feeding. At low tide they will reach out of their burrow as far as they can over the surrounding surface to rake in chunks of what appears to be just more mud. However, the mud scud is actually harvesting a thin biofilm – an invisible living skin of bacteria and diatoms (algae with a hard silica case) – on the mud’s surface.

Can you see them?

If you want to see a mud scud, you must be dedicated. As well as being quite small, growing to only about 1cm long, excluding their fabulous antennae, the species spends the majority of its life hidden away. It’s a strategy that makes sense if you live on a featureless, flat landscape, swarming with things that want to eat you. Even if you can access mudflats, the most you might see is a twitch or swirl of wet silt as the mud scud ducks down into its burrow.

While it is undoubtedly an interesting creature, its real secret power is in its numbers. It is so numerous, and its influence on the life and habitat all around it so vast, that it has been described by some as a keystone species – the little mud scud finds itself in the same club as wolves and beavers, it is that important.

In some rich estuaries, if you scooped up a square metre of mud, you might find a staggering 100,000 slightly annoyed mud scuds staring back at you. Their ability to irrigate the mud at 100ml per hour means that the mud itself is being turned over, aerated and fertilised, making it a better habitat for many other small creatures.