OCEAN CHRONICLES

A Pacific blood star (Henricia leviuscula) © Josh McInnes

SPECIES IN THE SPOTLIGHT

SEA STARS Wonders of the Seashore

JOSH MCINNES

We lay motionless 18 m below the surface, only

our air bubbles hissed with each breathe. The group of four divers, including myself, were on a dive off Ogden Point, Victoria when we stumbled upon a giant sunflower sea star (Pycnopodia helianthoides), stalking a smooth pink scallop (Chlamys rubida). The sea star must have been close to 1 m in diameter! As the sea star creeped closer, the scallop responded by opening and closing its shell repeatedly, forcing a jet of water out propelling the scallop through the water column. The sunflower sea star kept advancing and over time the scallop tired and succumbed to the sea star.

Sea stars (often misleadingly termed “starfish”) belong to the morphologically diverse phylum Echinodermata, which include the sea lilies, feather stars, brittle stars, sand dollars, sea urchins, sea biscuits, and sea cucumbers. Approximately 6500 species of echinoderm have been classified, with almost all species being predominantly marine. Sea stars belong to the sub-class Asteroidea, and approximately 1600 species have been described worldwide. Most sea stars possess a well-developed skeleton composed of calcium carbonate that gives them a rough exterior. The main disk forms the body of the organism, which is split into an oral side (bearing a mouth) and aboral side (not bearing a mouth). Surrounding the body are arms that aid the sea star in moving and finding food. Movement is highly specialized and relies on a water vascular system (WVS) which pumps water throughout the animal. On the aboral side, water enters through the madreporite, a canal that pushes water through fluid filled canals that lead to hundreds of tubular structures called tube feet. This system also allows sea stars to pick up chemical signatures or pheromones that are excreted by prey.

In the coastal waters of the Pacific Northwest, 83 species and sub-species of sea star are known to occur in the region. This includes familiar species like the purple star (Pisaster ochraceus), blood star (Henricia leviuscula), leather star (Dermasterias imbricate), and mottled star (Evasterias troschelii). Most like the voracious sunflower sea star mentioned above are major predators in marine

Brooding Sea Star (Henricia Sp) © Josh McInnes

ecosystems like the intertidal zone. They are known to feed on a wide variety of prey, including mussels, clams, scallops, marine snails, sea anemones, seaweeds, small fish, and other sea stars (in the case of the sunflower sea star). Once prey are found, sea stars extrude their stomachs through their mouths onto their prey, where they digest them externally. For species, like the hard to open clams, which have shell valves that clamp shut, sea stars can extrude their stomachs through an opening of 1 to 2 mm. At this point the sea star excretes digestive enzymes that weaken the muscles of the clam, enabling

Josh McInnes is a cetacean ecologist and a Research Scientist with Juan de Fuca Marine Research. He taught ocean sciences at the Bamfield Marine Science Centre from 2014-2016. He lives in Victoria, BC. the sea star to feed. Ecologically sea stars are important predators in the intertidal ecosystem. They control the populations of highly competitive species like the Pacific blue mussel (Mytilus trossulus) and thatched barnacles (Semibalanus cariosus), which outcompete other marine organisms.

In the mid 2000s sea stars along the west coast of North America were hit by an unknown pathogen that decimated multiple species. Populations crashed to an extent not previously documented in marine ecosystems. The infected sea stars deteriorated quickly over time, with limbs and body part falling off. The disease was labeled “sea star wasting disease.” Multiple research organizations began intertidal surveys to try and understand the transmission between communities and laboratory studies were conducted to try and identify the pathogen. It was soon discovered that the pathogen was a highly infectious virus in the family of densovirus. The virus has been recorded affecting 40 different sea star species, and on the British Columbia coast the Sunflower sea star in particular was severely affected.

The factors resulting in such a population decline are poorly understood, but increasing warm water temperatures and over population have been hypothesized. Currently it is believed that multiple pandemics have occurred over the decades, and that populations do bounce back after such episodes. So while the population was affected, it is hopeful that sea starts will rebound from such events.

ashcroftassociates.ca

Courtenay Office:

206-501 Fourth St. Courtenay, BC V9N 1H3 Phone: 250 334-6068 Underwater view of a Dall's porpoise © Josh McInnesFax: 250 338-6068