Field Guide to Marine Invertebrates - Arthropods - Crustaceans

Jonah Crab
Cancer borealis
Cancer borealis ("Jonah crab") is found in the mid- to lower intertidal zone under algae and among rocks. This predator eats blue mussels and other invertebrates. The remains of this crab are often found in the upper intertidal zone, evidence of bird predation.
Green Crab
Carcinus maenas
Carcinus maenas ("green crab") is found under algae and small rocks, and it eats other invertebrates in the mid- to lower intertidal zones. This crab is native to Europe and is an invasive species. The green crab is smaller than the Jonah crab.
Lobster on rocks
Homarus americanus
Homarus americanus ("American lobster") is a bottom-dwelling predator and scavenger. The American lobster is found throughout the Gulf of Maine, where it is commercially important. The lobster is found in rocky, subtidal habitats but, rarely, can be observed in lower intertidal pools.
Isopod on aglae
Idotea balthica
This herbivorous crustacean, an isopod, feeds on a variety of algae including large rockweeds (Ascophyllum, Fucus). It is common in the mid- and lower intertidal zones but is hard to see because its color usually matches the algae it is feeding on. This behavior helps it resist predation by fish. Individuals vary in color from yellow to brown to deep burgundy.
Close-up view of barnacles

Semibalanus balanoides
emibalanus balanoides ("barnacles") are sessile filter feeders. Larvae settle in the spring and attach head down to the rock. They make heavy calcified plates, which protect them from predation and drying stress, and begin to sweep phytoplankton into their digestive system by catching them with modified, feathery limbs that sweep out into the seawater from the plates. Semibalanus balanoides can be found from the low to high intertidal zones. Individuals lower in the intertidal zone and in high intertidal pools are larger than individuals that settled in the high intertidal zone. The larger individuals are in habitats where they are covered by water for a longer period of time each day, and this protects them from stress and provides them with more opportunities to feed.

All photos by Sarah Hall, except for American lobster (NPS/Todd Edgar photo).

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