There will be about 60 days this summer with “minus tides,” tides that are below the average height of the lower of the two tides.
During particularly low minus tides during daylight hours, clam diggers flock to the bays and estuaries of the Oregon Coast seeking empire clams, cockles, and other dinner-bound mollusks.
Estuarine mud can be a very challenging place to live: salinity and temperature may change drastically with the weather and the tide; the shoreline is regularly exposed by low tides; currents and wave action move sediment, burying, scouring, or dislodging residents.
Further, very fine sediments in quiet waters can exclude oxygen, suffocating aerobic (oxygen-requiring) animals and fostering anaerobic (oxygen-avoiding) bacteria. The anaerobic bacteria break down the rich organic material in the sediment and produce black, smelly -- and rather toxic -- hydrogen sulfide.
Though a good place to hide, and a place often bearing collected organic nutrients, in the mud is a very challenging place for aerobic animals to live.
Fossil evidence of multicellular animals large enough to see date back 575 million years and genetic evidence pegs multicellular animals as far back as 635 million years, but the first fossil evidence of animal burrows in mud date back about 550 million years. Which means animals were living on top of the mud for at least 25 million years -- and more likely up to 85 million years -- before evolving the means to burrow into the mud.
Most mud-dwelling animals need to reach the surface for the oxygen- and food-laden water. Clams use their siphons (necks) to stretch to the top of the mud; other animals, including those most ancient burrowers, stay near the surface or construct burrows that bring seawater down to their hiding places.
Filtering microscopic, drifting plankton from the water is one way tideflat animals (such as clams and most burrowing shrimp) gather food. Some tideflat residents (such as many of the worms) glean organic material from the sediment, others feed on larger plants, and still others feed on larger animals.
As clammers pursue their quarry, they encounter many other denizens of the estuarine tideflat.
By mid-summer, the surface of the mud is often brownish and slick from the large populations of microscopic plants -- diatoms. Great beds of slender green eelgrass extend up into the lower intertidal. Tufts of delicate, bright-green sea lettuce and tough, olive-brown rock weed grow attached to exposed rocks and partially buried wood.
Many of the larger holes in the upper intertidal may fool inexperienced clammers. Several types of burrowing shrimp excavate and maintain U-shaped burrows in the sediment. The pinkish ghost shrimp (Callianassa californiensis) makes burrows with volcano-shaped tops; darker mud shrimp (Upogebia pugettensis) digs burrows without volcano-shaped tops.
Huge numbers of many different kinds of worms burrow through the mud or work their way along the surface. Notable segmented worms include the lug worm (Abarenicola pacifica), that leaves squiggled coils of fecal castings around its burrow entry; and the fat innkeeper worm (Urechis caupo), so named because of the other creatures that live inside the pudgy worm and its burrow.
My favorite segmented worm of the estuarine tideflat is the hooded worm (Pista pacifica). Hooded worms sport a durable tube that arises from perhaps a foot beneath the surface to a couple of inches above, where the top of the tube flares and curves to form a heavily fringed hood over the opening. At high tide, the worm's tentacles gather plankton from the water.
Sometimes clammers find competition–massive-footed moon snails (Polinices lewisii) prowl the tideflats seeking clams to drill into and eat. The rubbery, coiled collars found on the tideflats in late spring are masses of their eggs, cemented with mucus and sand.
Unseen to most clam diggers, many animals live in the burrows of shrimps and lugworms, and scale worms and pea crabs make their homes inside other animals. Also unseen are the overwhelming numbers of microscopic life living between the sediment particles.
Pools and channels in the tideflats often trap small fish and crab. Nudibranchs, pile anemones, snails, and many other non-burrowing animals may be found on plants rooted in the mud or attached to exposed rocks and wood.
Even if you’re unsuccessful in gathering dinner, digging for clams is a great excuse to explore this fascinating environment and encounter some very remarkable inhabitants!