Sedimentary, my dear Watson
At low tide there’s little sign of the metropolis of filter feeders hiding in the sediment (sand and mud).
Look a little harder however, and evidence of the creatures of the sediment can be found. Worm casts litter the shore, caused by lugworms excreting the waste from sieving through the sediment for food (tiny plankton). These filter feeders are what the birds are after when scouting the retreating tide line, and are also dug up by fishermen to use as bait- because the fish of the estuary eat the poor lugworm too.
The lugworm’s neighbour’s in the mud include ragworms, flatworms and millions of microscopic invertebrates. Dig up a spadeful of mud and you could be carrying up to a million nematode worms in it!