Water-line Isopods: Scourge of the Surf-line

When jellyfish wash up on the shore at Long Beach in Pacific Rim National Park Reserve they don’t last long. At first glance, this stranded Moon Jelly (Aurelia labiata) appears forgotten. However, a closer look reveals that water-line isopods (Cirolana kincaidi) are hard at work dealing with the jelly’s remains. These marine isopods look similar to terrestrial isopods like wood bugs.

Moon Jelly
Moon Jellies often wash up on sandy beaches on the West Coast of Vancouver Island.

Water-line isopods are one of the beach’s most aggressive scavengers. They commonly follow the line of the tide as it moves up and down the beach, covering anything that is dead and devouring it very efficiently. In fact, they are so aggressive that if you stand motionless as the water washes over your feet you may soon be covered with nasty red bites!

Water-line Isopods (Cirolana kincaidi)
Water-line Isopods (Cirolana kincaidi) are active scavengers along the surf-line.

Because water-line isopods are small they are difficult to see. The speckled, semi-transparent bodies blend in well with grains of sand. They move quickly with the waves and bury down into the wet sand when the water retreats. Searching pools of water for swimming isopods or looking closely at the remains of jellies and other detritus are good techniques to find them. Water-line isopods also leave networks of thin lines of tracks in wet sand.

Water-line Isopods (Cirolana kincaidi)
Water-line Isopods (Cirolana kincaidi) quickly devour anything dead that has washed up on the beach.

Keep an eye out for these superb scavengers the next time you’re visiting the surf pounded sandy beaches of Pacific Rim National Park Reserve. They’re fascinating to watch, at least if you’re safely wearing rubber boots!