Jellyfish in decline this year, compared with previous summers
Many animals living on the Vineyard and in its coastal waters experienced abnormalities in their annual life cycles this summer, according to local scientists and field experts.
One creature in shorter supply this season is the jellyfish, in particular the lion's mane jellyfish (Cyanea capillata), which are recognizable by their red center and long spindly tentacles, and the comb jellyfish (Ctenophora), which look like iridescent, gelatinous blobs. Mane jellyfish sting; combs do not.
Sean Colin, an associate professor of biology at Roger Williams University who is conducting jellyfish research this summer at Woods Hole Oceanographic Institute (WHOI), confirmed the decrease in jellyfish in a telephone conversation. He explained the jellyfish life cycle, and gave various reasons that might explain their recent decline.
"There is definitely a decline in jellyfish, especially compared to last summer," Mr. Colin said. "There are far less lion's mane jellies. I've only seen a couple and that's accounted for all summer long."
An expert in the jellyfish field, Mr. Colin studies jellyfish morphology and their form, and how that behavior relates to how they feed and what they feed on. "Basically, we video them and look at how they interact with the water and with particles around them," he said.
Jellyfish of all sorts have a complex life cycle, according to Mr. Colin. "When we're not seeing them in the water near the surface, there's polyp stage, when they are on the ocean floor," he said. "These polyps, when the conditions are right, release medusae. But, there are a combination of conditions that affect how many jellyfish they spawn."
Mr. Colin likened the jellyfish's lifecycle to a plant's, and explained that factors such as temperature, food availability, and rainfall affect seasonal jellyfish numbers: "Much like a tomato plant, jellyfish can have a good or bad harvest."
Not a huge amount of research has been done on lion's mane jellies in the Northeast, Mr. Colin said, but scientists have done general studies about jellyfish that have given them more insight on specific types. For instance, as with many types of jellyfish in New England, the lion's mane's primary purpose is predatory, feeding on tiny particles in the water, like zooplankton. However, Mr. Colin said that comb jellies also eat zooplankton, and are actually much more affective predators within the Vineyard's ecosystem. "The thing is, we don't pay as much attention to comb jellies because we don't get stung by them," he explained. "They are so good at eating things, and have a much more profound effect on ecosystems, surprisingly."
Though not as abundant as last year, comb jellyfish do not usually peak until late summer or fall, Mr. Colin said. In spite of the definite decline in jellyfish this season, however, it has been difficult to predict whether the species' numbers will be affected in the future. "I think it is kind of important for people to realize that with jellyfish, just like anything that grows seasonally, it is difficult to make any conclusions about one or two years," he said. "This has been a funny year."