Mud-flat Crab      Hemigrapsus oregonensis



An aquatic pugilist of the first order, the common Mud-flat crab can be found throughout Puget Sound on, where else, mud-flats.  When approached this tiny, quarter inch to one inch crustacean rears up on its rear legs with it's boxing gloves, oops pincers, at the ready.


At low tide the crab can be found scurrying about the exposed mud flats looking for bits of plants or animals to eat.  When threatened it will often run to the nearest hole or under a convenient rock, but if caught it can put up quite a fight with its strong pincers.  It retains some water under its shell in the gill chamber to allow it to breath out of the water.


Breeding in the spring and summer the female crab carries it's eggs attached to the underside of its tail using specially designed legs. The babies hatch into the water at high tide and drift off as a member of the plankton.  After spending weeks floating about they settle to the bottom where they begin their adult life.


The thousands of crabs actively convert detrital material from the mud flats to crab tissue and then, if unlucky, into bird, fish or Raccoon food.  Prowling the shore when the tide is out, Great Blue Herons and crows capture many an unwary crab.  Even the occasional Raccoon will hunt for these delicious creatures as the tide recedes.  As the tidal waters rush in to reclaim the land, the crabs are still not safe as fish follow the water looking for stragglers.


In rocky areas of Puget Sound you will find a close cousin of the Mud-flat crab called the Purple Shore crab (Hemigrapsus nudis).  Unlike the Mud-flat crab which has noticeable hairs on the outside of its legs, the Purple Shore crab lacks such hairs hence the nudis part of the scientific name. It has a purple coloration but otherwise it's life history is similar.