A Pacific blood star (Henricia leviuscula) © Josh McInnes
OCEAN CHRONICLES
SPECIES IN THE SPOTLIGHT
SEA STARS
Wonders of the Seashore JOSH M C INNES
We lay motionless 18 m below the surface, only
Most sea stars possess a well-developed skeleton com-
our air bubbles hissed with each breathe. The group of
posed of calcium carbonate that gives them a rough
four divers, including myself, were on a dive off Og-
exterior. The main disk forms the body of the organ-
den Point, Victoria when we stumbled upon a giant
ism, which is split into an oral side (bearing a mouth)
sunflower sea star (Pycnopodia helianthoides), stalking
and aboral side (not bearing a mouth). Surrounding
a smooth pink scallop (Chlamys rubida). The sea star
the body are arms that aid the sea star in moving and
must have been close to 1 m in diameter! As the sea
finding food. Movement is highly specialized and relies
star creeped closer, the scallop responded by opening
on a water vascular system (WVS) which pumps water
and closing its shell repeatedly, forcing a jet of water
throughout the animal. On the aboral side, water en-
out propelling the scallop through the water column.
ters through the madreporite, a canal that pushes water
The sunflower sea star kept advancing and over time
through fluid filled canals that lead to hundreds of tu-
the scallop tired and succumbed to the sea star.
bular structures called tube feet. This system also allows sea stars to pick up chemical signatures or pheromones
Sea stars (often misleadingly termed “starfish”) belong
that are excreted by prey.
to the morphologically diverse phylum Echinodermata, which include the sea lilies, feather stars, brittle stars,
In the coastal waters of the Pacific Northwest, 83 species
sand dollars, sea urchins, sea biscuits, and sea cucum-
and sub-species of sea star are known to occur in the re-
bers. Approximately 6500 species of echinoderm have
gion. This includes familiar species like the purple star
been classified, with almost all species being predomi-
(Pisaster ochraceus), blood star (Henricia leviuscula),
nantly marine. Sea stars belong to the sub-class Aster-
leather star (Dermasterias imbricate), and mottled star
oidea, and approximately 1600 species have been de-
(Evasterias troschelii). Most like the voracious sunflower
scribed worldwide.
sea star mentioned above are major predators in marine
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