Decorator crabs: The amazing spider crustaceans found in Hampton Beach tidepools

Summer has arrived on the Seacoast and although it may still seem cool to us it is warming for the Gulf of Maine’s inhabitants.  

The temperature in the water along Hampton Beach this past weekend was in the low 50s.  The animals that live in the intertidal zone (the area along the beach that is covered with water at high tide and then exposed as the tide recedes) need to adapt to the changing water.  During the spring and summer, the sun’s rays heat up the water left in the tidepools allowing temperatures from 50 to 75 degrees in one day. 

The rain, if it occurs during low tide, changes the water from seawater to brackish water (rainwater dilutes the seawater diminishing the salinity or saltiness).

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Normally animals and plants that live in the ocean do not respond well to any changes in salinity or temperature. Most of the animals living in the Gulf of Maine are tied to areas of constant temperature and salinity and cannot survive in the changing environment of the intertidal zone.

Life in the tidepools can be precarious and dangerous.  Predators are everywhere, only the adaptable survive. One of these amazing creatures is the decorator crab. 

This small crab or spider crab can be found along the east coast of North America from Florida to Atlantic Canada. There are several species of decorator crabs here all belonging to the superfamily Majoidea. These are specialized spider crabs that display a unique behavior. 

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First, let’s describe a spider crab.  Spider crabs are a true crab, an Arthropod and crustacean which has 10 appendages, eight legs and two claws. They have an exoskeleton and must molt or shed to grow. During this process, if they have lost a body part it will regenerate or grow back. This is a very handy adaptation for aggressive animals as they appear to lose legs at an alarming rate due to confrontations with other creatures. 

These crabs have internal fertilization, and the females carry eggs internally, some for up to a year. Then the eggs are deposited under the female's tail where they develop and hatch. This process can take anywhere from less than a month to over a year depending on the species. Spider crabs are very distinctive with spindly, long legs and small claws. The largest spider crab on record is a Japanese spider crab that measured over 6 feet across all its spider-like legs. No worries, most of the spider crabs found along our coast are less than 4 inches. 

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Most of the ones found in our tidepools are small enough to hold in the palm of your hand.

Decorator crabs are specialized spider crabs that appear to actively engage in behavior to hide from predators. They cruise the seafloor for plants and small animals that they place on their back (carapace) where it attaches to fine hairs and begins to grow.  Most decorator crabs appear to prefer specific seaweed or anemones and will reject a multitude of things before placing the “correct” specimen in just the right place on themselves.  Some scientists have observed specific species of decorator crabs prefer certain plants and will reject anything else. Unfortunately for the crab, each time they molt they must begin again to find just the right decorations to adorn their body.  

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The Oceanarium is open for the season, and we have two adorable decorator crabs in our tanks. They have adorned themselves with seaweed that gives them the appearance of a tarantula when held in your hand.  Underwater they look like a walking rock.  What a perfect camouflage from their enemies.  When approached by a predator they hunker down and disappear into the seafloor. This is an amazing adaptation for these unassuming crabs allowing for a long and prosperous life in the dangerous tidepools.

Ellen Goethel is a marine biologist and the owner of Explore the Ocean World at 367 Ocean Blvd. at Hampton Beach.