The worms of Portobello
I recently took a walk along Porty beach at low tide. If you've done the same, you might have noticed thousands of little protrusions from the wet sand in the intertidal zone that look like the frayed ends of thin rope. They only stick up by an inch or two, but they absolutely cover a certain range of the beach once the tide is out.
Not for the first time, I wondered what might have left these in the sand, and whether they were even man-made. So I finally looked them up. Ten minutes later I was reading about all the different worms we have in our sands.
Sand mason worms
As you've probably guessed from the intro, the little rope-like structures are created by a type of worm known as the sand mason worm, or Lanice conchilega.
They're called mason worms because they build! They're called sand mason worms because they build with sand (and bits of shells). They grow to around 30cm long and they burrow into the sand, where they build a straight tube from cemented sand to live in. This tube protrudes a couple of centimetres from the sand surface and has a characteristic "frayed" look to the end (which is why I originally thought they looked like old fishing rope).
Mason worms are suspension / surface deposit feeders, meaning that they eat bits of microorganisms and organic matter from the water and sea bed around their head, which has a crown of spindly white tentacles that come out when the worm is underwater.
They can very quickly shrink down inside their sand tube if they're in danger, and will happilly build it up higher if the top gets buried (question: do they do this when exposed by low tide? I might try lightly covering the top of one in wet sand and see if the worm gets to rebuilding it straight away).
So, now you get to feel extra bad for crushing hundreds of these carefully constructed little worm houses underfoot on your way out for a low-tide swim!
Blow lugworms
These are the worms you probably expected me to talk about if you read the title. They leave cute little piles of sand (called casts) on the surface of the beach that look just like a big coiled up worm.
There are a few types of lugworm, but we predominantly seem to get blow lugworms, or Arenicola marina on Porty beach (the other type that exists in the UK is the Black lugworm, or Arenicola defodiens).
Once I tell you how they live and eat, you'll see the hidden structure of their burrows everywhere. Blow lugworms make a U-shaped burrow and settle themselves in the bottom of the bend. As with many such animals, one end is the head where the food comes in, and the other end is where the poo comes out.
This U-shaped burrow emerges into the sand surface at two points: above the worm's head is a usually conical cavity / crater with a dark hole at the bottom. At the other end, there's the distinctive worm-shaped cast. As bits of sand and other particles fall into the cavity (and from there the worm's mouth), it processes them through its body, extracts whatever food it can manage, and poos it back out the other end. This extrudes the worm-shaped pile of sand onto the seabed or beach.
Take a look next time you come across a big expanse of lugworm casts: you'll notice that they pair up nicely with the slightly less distinctive conical holes in the sand. Usually the two ends of the burrow are around a foot away from each other: you can imagine the horseshoe-shaped tunnel and happy little worm under the surface! It's easy to work out which burrow goes where near the edge of the population, where they're a bit more sparse and it's clear which "mouth" pairs with which cast pile.
Anglers will already know all of this, because they use lugworms as a free source of bait when fishing by the sea: they're easy enough to dig up once you know how to locate their burrows and get over the squeamishness.