Clumps of northern rock barnacles are commonly seen attached to rocks, pilings, and docks.
Clumps of northern rock barnacles are commonly seen attached to rocks, pilings, and docks.
E. Zabel, courtesy of URI

Northern Rock Barnacle (Balanus balanoides)

Alternate common names: Barnacle, Acorn Barnacle.
Color: Shell is white to grey; interior is darker.
Size: 1 1/2 inches in diameter.
Habitat: Permanently attached to rocks, pilings, or any hard substrate in intertidal and subtidal habitats.
Seasonal appearance: All year.


Upon first glance, there does not appear to be much more to a barnacle than the outer shell. They are actually quite complex organisms. Barnacles permanently attach their heads to a substrate by a cement that they produce. They have flat, irregular tops and resemble small volcanoes. The outer shell of a northern rock barnacle consists of white overlapping shell-like plates that grow with the animal. A soft cuticle covers the inside of the shell.

Life History and Behavior

Northern rock barnacles are crustaceans like green crabs and American lobsters. In fact, they are the only crustaceans that remain fixed in one spot for life. When the northern rock barnacle opens its mouth, featherlike feeding legs emerge to gather phytoplankton from the water. These feathery legs are also used as gills to extract oxygen from the water. These feathery appendages are also jointed. Northern rock barnacles have an exoskeleton, like all crustaceans. They grow by shedding the exoskeleton through molting. Molted exoskeletons can be seen floating on the water's surface.

Northern rock barnacles begin life as planktonic larvae in the water column, often aggregating in large clusters when they settle to the bottom. Reproduction is sexual, which requires the animals settle in close proximity to one another. They will often attach themselves to slow-moving organisms, such as Atlantic horseshoe crabs, American lobsters, and even whales. Northern rock barnacles are sensitive to temperature extremes and desiccation, and they can die if the tide leaves them high and dry for too long. The animal closes its shell when the tide goes out, and opens it again when the tide comes in.

Special Notes

Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.