Aaron - The Seasonal Ebb of the Medusae

My stint at Friday Harbor Laboratories on San Juan Island is now over and I’m freshly arrived at the Oregon Institute of Marine Biology for the rest of the summer’s work. I've just begun to meet an exciting new group of students and researchers and  have just started to adjust to a whole new reach of coastline.

At the time of my transition south the medusae of Mitrocoma cellularia have also been moving on. This is the time of year that populations of these medusae start to fall off in their numbers. After they've spent the spring mating they start to die off around the beginning of July. Last week, these jellyfish were plentiful and it was easy to capture 20 of them off the dock in well under an hour. This last week, it took multiple searching sessions throughout the day to acquire the same amount. Soon they won’t be found at all until next spring.

Jellies in the tank
foreground: Mitrocoma cellularia
midground: Aurelia aurita
background: Aequoria victoria

Watching the jellies disappear helps give a concrete lesson about the jellyfish life cycle: the conspicuous forms that awe us with their undulating movements and drifting tentacles are in fact only a temporary emergence from a more persistent form, the polyp. Polyps are sessile forms that attach to substrates under the water’s surface (i.e. ocean floor, rocks, etc.) They can live for years and are totally self-sufficient, complete with nematocyst-equipped feeding structures (nematocysts are the stinging cells on tentacles that ensnare and paralyze prey). They give rise to medusae by asexually budding them off. The new-born medusae are called ephyra. These mature into the “jellyfish” we all know. A single polyp can produce dozens of these genetically identical forms. The male and female meduasae release sperm and eggs when they reach maturity, and the rest is history.

Aequoria victoria in its natural (turbulent) environment

It’s been fascinating to watch this cycle play out. The thing to remember is this: if you have your heart set on some jellyfish sightings next time you go to the beach, make sure that you’re in the midst of their breeding season!

I'll hunt for some polyps in the meantime and see if I can get some pictures to post.