Marine Wildlife

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Detailed image of a tidepool

NPS Photo

The rocky shoreline of Acadia National Park is a stunning landscape where land and seawater meet. The tides rise and fall twice daily. Something is also moving at the seashore, crashing waves, crawling crabs, curious seals, and an array of marine wildlife living together in a diverse neighborhood.

Snails and Barnacles on Rock
Snails and barnacles attached to a rock

NPS Photo/Julie Flores

Marine Invertebrates

Marine invertebrates, animals without backbones, live on the part of the shore that is exposed during low tide and submerged during high tide and must survive both in and out of seawater. Different organisms inhabit certain intertidal zones. Their hard exteriors prove that shore wildlife must survive harsh conditions when exposed to sun, wind, and waves.

Some shell bearers include bivalves with soft bodies and live inside a two-piece shell joined by a hinge, like the blue mussel. Mussels have their own way of hanging on, fastening to rocks with strong threads they spin themselves. A filter feeder, mussels slightly open their shells and move water through its body cavity trapping microscopic plants and animals.

Snails are another shell bearer that live inside a single coiled shell. Snails use their muscular foot to travel, pushing that foot in front of them and pulling themselves up to where their foot is. The common periwinkle is an abundant snail that varies in color with a teardrop shaped opening. They feed by scraping algae of rocks with its numerous teeth.

Hermit Crab
Detailed image of a Hermit Crab

Photo by Ashley L. Conti, Friends of Acadia, NPS


Crustaceans have jointed legs and a hard exterior called an exoskeleton covering their soft bodies. Commonly seen crustaceans are barnacles and crabs. Nothing in the world can hang on like a barnacle who live inside volcano shaped shells. When they are young, they swim freely, then eventually cement themselves to a rock to settle, adding minerals to its shell plates overtime to grow. They feed by extending shrimp like legs out of its opening to feed on tiny animals and plants floating in water.

Hermit crabs have a soft abdomen; therefore, they seek shelter in empty periwinkle shells to call home. Other crabs include the green crab and Asian shore crab, two species that are not native to Maine. These invasive species have been transported by human activities, posing significant threats to the ecology of coastal Maine.


Sea stars are boneless echinoderms; spiny skinned animals who depend of their tough skin, made up of calcium carbonate, and plates for protection like a coat of armor. They use their rubbery tube feet to move around. Sea stars also use their feet to collect food, for example, pulling open mussels.

Tidepooling is a great way to discover the microcosm of ocean life. During low tide, explore Acadia’s tidepools as they reveal these creatures that are often overlooked.

Harbor Porpoise
Harbor Porpoise

NPS Photo

Marine Mammals

Seals are common along the shores of Maine, gray and harbor seals being the most common. Identifying the difference between them is displayed by their head shape. Harbor seals have a dog like shaped head with V shaped nostrils, while gray seals have more horse like heads with parallel nostrils. Seals are curious creatures and them often lift themselves up out of the water to observe. Often females will leave their pups alone on shore to feed herself. Although it may seem a seal pup needs rescuing, this is a natural occurrence and should be left alone and observed from afar as to not scare off the returning mother.

On the other hand, harbor porpoise are shy creatures, but can be spotted in the same areas as seals. They have a small, but solid body and blunt beak but the most easily seen characteristic is their triangular shaped dorsal fin that is slightly sloped and curved. Harbor porpoises travel in groups up to 10 and occasionally come close to boats since they swim around and underneath them. They mainly eat schooling fish, but occasionally they will eat squid or octopus.

Although the harbor porpoise is in the whale family, it is rare and unlikely to see a whale in the bay or from shore. These larger creatures are spotted further out in the ocean and are best viewed from whale watching tours and excursions.


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