Here are three common intertidal ones that live commonly together on sandy intertidal flats. M. secta tends to occur deeper in the sediment, communicating to the sediment surface by means of long siphons. M. inquinata tends to occur in muddier sediments than the rest. This amphipod was found living on sea lettuce, Ulva sp. Its color probably derives from the chlorophyll in the sea lettuce.
Many seaweeds have chemical defenses.
This shrimp lives in deep y-shaped burrows in muddy sand tidal flats of the west coast of North America. It is a suspension feeder. Here is a close up, showing the feathery feeding appendages, presumably for trapping detrital particles. During the breeding season, the parapodia become better developed and males and females swim into the water to spawn and die. There are many species of fiddler crabs, but in all the male has an exceptionally large claw, used for displays to attract females and for intermale combat.