Fab four Phyla part three: Mollusca

An empty butter clam shell.
If you look closely, you can see a tiny snail laying eggs in the center of the shell.

  By many, this would be considered the most delicious of our fab four phyla. Mollusca is filled with edible organisms like clams, mussels, chitons, oysters, snails, octopus and squid.  People routinely go clam digging on the beaches here and there are several oyster farms around the bay as well.  It behooves people to harvest with care though; these animals, though numerous, are not unlimited in number and we can have a detrimental effect on their populations if we harvest unchecked.

A lined chiton, an organism that scrapes algae from rocks.

Mollusks are characterized by a having a strong muscular foot, a soft body, a mantle and usually, a shell.  Of the Mollusks we find on our beaches, it is the exceptions to the rule that people are most interested in.  The mollusks without shells are the flashiest and most interesting,  organisms such as nudibranchs and octopuses.  Nudibranch means "naked shell" and these organisms are commonly referred to as sea slugs.

The opalescent nudibranch.
The feathery cerrata on its back hold  scavenged nematocysts.

Nudibranchs have some truly amazing adaptations and seem to specialize in eating things that no other animals will.  One of our most beautiful nudibranchs, the opalescent nudibranch, eats sea anemones and passes their nematocysts (stinging cells) untriggered through its digestive system, places them on its own back adopts the nematocysts for its own defense.  Other species of nudibranch eat equally difficult prey; animals that are poisonous or seemingly undigestible are preferred by sea slugs.

Two (fully grown) opalescent nudibranchs.
There is a tiny snail just to the left of the nudibranchs.

Our other most popular mollusk, the octopus, is also without a shell.  Octopuses are the smartest invertebrates and are smarter than many mammals as well.  They can solve puzzles, learn from one another and even use tools.  On our beaches we have the giant pacific octopus, which nestles beneath rocks to wait for the tide to return. The ones we have are fairly small, no larger than 6-8 feet across.  As they grow larger, the octopuses will abandon their cramped dens and move out to deeper waters where their large size affords them some protection.  Eventually, over the course of just a few years, they may grow to have 20 foot arm spans and achieve weights of 160lbs.

Tentacles of a giant pacific octopus.
Notice the empty crab shells outside the den.

 The octopus dens on our beaches are fairly easy to spot because the entrances are often littered with the carcasses of their prey; crab, snail and clam shells lie broken and discarded at an octopus den's opening.  Usually we can only see a few tentacles but occasionally we will find a baby octopus under a rock or see one free swimming in the water.  They are extremely curious animals and will often shake 'hands' with you if you touch the water near the entrance of their dens.

Curiously poking around the entrance to its den.

Mollusks are a group of animals deserving of attention.  Not only are they a commercially and economically important group of animals but they are also important for the health of our bays and oceans.  Clams and mussels can filter upwards of 25 gallons of water a day keeping bays clean.  They are also an extremely important food source for both humans an animals.  For example, there is a tiny pelagic (mid water) snail called a pteropod that serves as the main food source for young salmon.  Mollusks are particularly at risk from ocean acidification which occurs as more and more released CO2 is absorbed into the oceans.  The excess of CO2 in the ocean forms carbonic acid which releases more H+ ions into the water and drops the ocean's ph level.  The acidification of the oceans hinders the formation of the calcium carbonate shells formed by nearly all mollusks and, if it continues, could see the collapse of many of the oceans food webs (including salmon that thrive on pelagic pteropods).  This is one of many environmental challenges facing the world today and it is one that is very deserving of our attention.