Are jellies taking over?

Some news reports: Tampa Bay has been hit by an invasion of jellyfish or “jellies” and stingrays so thick in some areas “you could almost walk across them,” a county biologist says. “They’re thicker than I’ve ever seen them before.”

Public and private beaches on the Great South Bay are beset with the heaviest jellyfish plague in years.

“Portuguese Men-of-War” Found on Beaches for First Time in 20 Years, Lifeguard Says.

Jellyfish of many species have been plentiful in the waters about the island for some time, but the phenomenon of clouds of minute ones, scarcely larger than a large pea, has not been noticed here before.

You are probably not surprised by stories like these, but you might be surprised that these articles appeared in 1973, 1959, 1948, 1937, and 1906.

Headlines from 2009 tout the threat of the huge Nomura’s jellyfish in Japan, in an article picked up by Discover, National Geographic, and countless others. The problem with these reports is that according to the Japan Times, the jellies have actually undergone dramatic decreases this year:

“Up to last year, 3,000 to 5,000 of the jellyfish would get tangled up in a single fixed net in some cases. But this year, only one or two were reported to have been caught.”

I’ve wanted to write this post for a while, but even though I’ve been stalling, it hasn’t seemed to become any less relevant. The news and blogs are full of doom-and-gloom stories of jellyfish taking over the oceans, mostly attributed to global warming. While there certainly are cases of jelly blooms, what is less certain is whether these are higher than usual, and what data there are to support such claims. If blooms have reached anomalous levels, it is also not at all clear what the causes might be. News reports and even lifeguards are unreliable sources of historical context of population dynamics. Of course they’ve never seen so many jellyfish before! It is almost like the perceived rash of shark attacks that get reported every few summers, even though shark numbers are in serious decline.

I just returned from a cruise where we conducted blue-water dives in the Bahamas. This happened to be the 20th anniversary of my first collection dives in that area, and we revisited a few of the same sites and cays where we dove in years past. (One slight difference, Gorda Cay, having been bought and developed into a resort, is now called Castaway Cay <ack>.) I expected to find the same wondrous assortment of ctenophores, siphonophores, and plankton that we’d collected on previous trips. What we found instead was an absolute desert. The incongruity between the beauty of the warm azure water and the absolute lack of planktonic life – even in plankton tows – was shocking and wore on my spirits. It seems that if the marine ecosystem is crashing, it is taking the jellies down with it.

The most abundant jellies that we did find were an interesting pair of genera, both of which have algal symbionts inside them, just like corals. The photo here shows a fluorescence photo of one of these species, Dipleurosoma ochracea, taken under blue-light. The green spots are the GFP that I’ve written about here before. The red in the canals comes from fluorescence of chlorophyll that the jellies harbor. Is it coincidence that jellies benefiting from photosynthesis are the ones that are doing well in these oligotrophic waters? One thing that bothers me about the “sea of slime” proponents is that they suggest that jellies like warm, acidic, polluted waters. In fact, most jellies enjoy eating plankton too, just like fish do. If the plankton go away, the jellies won’t be far behind.

I was amused to find an article in an Indian newspaper entitled “Govt bid to save jellyfish.” It seems that even jellies are being overfished in Indian waters, and as a result the sea turtles “are being deprived of their favourite food.” All of the species play a role in preserving the biodiversity of these ecosystems, and any manipulations or imbalances are likely to have cascading effects.

Jellyfish come in all shapes and sizes, so it is hard to generalize about what conditions they might like. But it’s clear that they are important members of marine communities, and should be studied, not demonized. Don’t believe every blog you read – including this one – without finding the facts behind the frenzy.

Update: April 2010. Visit the site jellywatch.org to report jellyfish sightings. Your reports will become a part of a publicly accessible database.

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