Fab four Phyla part four: Arthropoda

Arhtropoda is the largest and most diverse phlyum on Earth.  The pre-fix Athro refers to joints (as in arthritis) and pod means foot, so the name arthropod refers to animals with jointed feet or legs. The phylum arthropoda encompasses organisms such as insects, spiders and crustaceans.  Besides having jointed legs, athropods also have segmented bodies and a tough chitinous exoskeleton that they occasionally molt.  On our beaches we see mostly crustaceans; i.e. crab, shrimp and barnacles.

Black Clawed Cancer Crab
Its large claws are used to crush barnacles

Most people don't realize that barnacles are so closely related to crabs and shrimp.  Barnacles begin their life as a plankton and as they grow larger they change through various stages until they become a cyprid.  The cyprid's task is to find a good spot to settle on the sea floor and become a barnacle.  They land head first and begin to create a hard, 6 plated calcium shell around themselves.  To feed, they kick out their feet, which are shaped like tiny fans, and collect their food from the water.  We just recently had a barnacle fall out in our area and now everything is coated in the brittle white shells of young barnacles.  You feel bad crushing so many animals underfoot but our beaches are just so packed with life that it is impossible to do otherwise while tidepooling.

Empty molt of a helmet crab

There are several species of crabs on our beaches, though none of them are popular commercially.  Decorator crabs are my personal favorite.  Decorator crabs do not have large claws to protect themselves with, instead they rely on camouflage.  Using their narrow claws, they hang algae and other things from the tiny hooks on the carapace until the blend seamlessly into their environment.  It is nearly impossible to see a decorator crab unless it is moving.  Taking one out of water, they become spider-like, with long narrow legs and a proportionately small body.

A friend holding a decorator crab

Hermit crabs are another popular arthropod found on our beaches.  We have a few different varieties ranging in color from a drab brown to a brilliant orange.  Hermit crabs appropriate the empty shells of other organisms (usually snails) to give themselves additional protection.  Their body curls to fit into the shells and they have legs specifically adapted to hanging onto the shell.  One of their claws is generally over sized and acts as a shield to cover the entrance to their home.  We keep several large hermit crabs in our touch tanks at the field station and you can often watch them squabbling over new shells.

Helmet crabs mating

  At one point there were several species of crabs in Kachemak bay that were commercially important to Homer.  Tanner crabs, Dungeness crabs and King crabs could all be caught in the bay during the late 70's.  No one is exactly sure why the fisheries collapsed; whether it was due to mismanagement, a fuel spill, or just part of a natural cycle, these species no longer exist in Kachemak bay in harvestable numbers.  The Tanner crab and Dungeness crab populations are slowly recovering and hopefully they will continue to do so.

An Orange Hermit crab from our touch tanks