Spotlight on Howe Sound Marine Life: Radial Symmetry Animals

The Echinoderms are animals with radial symmetry, usually five-armed and with no left or right sides. They also have an internal hydraulic system of fluid canals that often includes many rows of conspicuous tube feet. Included are the sea stars, brittle stars, feather stars, sea urchins and sea cucumbers.

Sea stars were rather badly named starfish, but they are not fish in any way. Sea stars usually have five arms, but that number can go higher than twenty in the sunflower star, and other species can have from six or eight to 11 arms. The shape and colour of the arms can help in identification, as well as the texture of the skin which has an embedded skeleton, as well as small pincers.

Sea stars move along the bottom of the ocean floor with their tube feet, and they can pull open mussel or clam shells by hydraulic power, clinging with tube feet and pumping seawater to apply force in opening the shells so that their stomach can protrude in and digest their prey inside its own shells.

Brittle stars have long, delicate arms that can trap particles from the water or mop up bits of debris from the bottom. The name “brittle” refers to the ease with which the arms or rays break off, only to be regenerated or re-grown in the same way that sea stars can.




There is only one feather star in Howe Sound, but it is large and this species can be very abundant. Feeding in a manner somewhat like brittle stars, the feather star can release its grasp on a rock and swim by waving its arms. The feather star lives sort of like an upside-down brittle star, with its arms up in the water.

The sea urchins have their radial symmetry evident on their hard, globular shell, but that shell is surrounded by moveable spines that resemble a sort of pin cushion. Sand dollars are a type of urchin with a flattened shell and spines that lie flat over the shell; they burrow into sand rather than cling to rocks the way other sea urchins do. Sea urchins are often found eating seaweed, and some areas are called “urchin barrens” when all the seaweed has been mowed down and hordes of urchins remain.

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The sea cucumbers also have five-way symmetry with their tube feet, but they resemble large sausages, sometimes living on top of rocks and sometimes burrowing under rocks. They mop food into their mouths with sticky tentacles.

With the support of Sitka Foundation, the Vancouver Aquarium is embarking on a two-year project to train divers to identify marine life in Howe Sound, as part of our commitment to the research and conservation of this area. The information they glean on Howe Sound’s sea life will be presented in this series of blogs, and will be used to educate students taking part in the Aquarium’s school programs and AquaVan visits to inspire the next generation to keep learning more about marine biodiversity in British Columbia.