Exploring the Intertidal

Check out the intertidal zone and its biodiversity!

Arthropoda

 

external structure of a crab

75% of all living species belong to this phylum! It includes diverse groups such as insects and crustaceans.

All arthropods have 3 body segments: head, thorax, and abdomen. These animals have a hard exoskeleton that is secreted from the epidermis (skin). Advantages of exoskeletons are that they are waterproof, and prevent dehydration and are good structures for protection from predators.

The major component of the exoskeleton is a polysaccharide called chiton. When an arthropod grows, the exoskeleton must be molted and a new one is formed.

Most marine arthropods belong to the Class Crustacea. Some examples of crustaceans are crab, lobster, shrimp and small planktonic creatures such as copepods and euphausids (krill). The head and thorax are fused in crustaceans, forming a single structure called a cephalothorax. The main body shell of the crustacean is called the carapace. They have 5 walking legs (periopods) and two pinchers (chelae). Crustaceans also have 2 sets of antennae and stalked eyes. One crustacean that was once believed to be a mollusc is the barnacle! Barnacles are only found in the marine environment and they are sedentary (do not move). Barnacle larvae settle on their heads and stick their feet in the air to feed! They were confused with molluscs because of their calcium carbonate shell.

Subclass Malacostraca

Hemigrapsus oregonensis (Green Shore Crab)
Description:Grey/green in colour. Juveniles are mottled.
Range: Alaska to Baja, California.
Habitat: Rocky crevices and eel grass beds. High to low intertidal.

Cool Fact: On some beaches, you may lift up a rock to find dozens of these tiny crabs hiding underneath!

Hemigrapsus nudus (Purple Shore Crab)
Description:Purple to red-brown. Spots on chelae.
Range: Alaska to Turtle Bay, Mexico
Habitat: Under rocks and in algae. Mid to low intertidal.
Cool Fact: The name "nudus" comes from the fact that this crab has a smooth shell, with no hairs - unlike the similar H. oregonensis.

Pagurus samuelis (Blueband Hermit Crab)
Description: Dark green with blue bands around walking legs in adults, and white bands in juveniles.
Range: Vancouver Island to Baja, California.
Habitat: Exposed rocky intertidal. High intertidal.
Cool Fact: Often uses old black turban snail shells to make its home in. Constantly on the look-out for potential new homes as they grow!

Subclass Cirripedia

Balanus glandula (Acorn Barnacle)
Description: White to grey. Cone-shaped.
Range: Aleutian Islands, Alaska to southern California.
Habitat: On rocks and shelled animals. High to mid intertidal.
Cool Fact: Since competition for space is fierce, these tough crustaceans often grow in an elongated fashion in crowded situations.

Semibalanus cariosus

Balanus nubilus (Giant Barnacle)
Description: Dirty white. Purple, red or yellow tissue near centre plates.
Range: Southern Alaska to Montery Bay, California.
Habitat: Low intertidal to 91m deep.
Cool Fact: Coastal First Nations used to eat roasted giant barnacles.

Pollicipes polymerus (Goose Neck Barnacle)
Description: Cream plates and flexible, brown stalk.
Range: Sitka, Alaska to Baja, California.
Habitat: Exposed mid intertidal and subtidal.
Cool Fact: There used to be a British Columbia fishery for goose necks. They were exported to Europe as a delicacy.

see also hermit crabs, heart crab, Puget Sound king crab,
and sea spiders pages!

---

 to Intertidal Field Guide directory