At the end of the last ice age large areas of this landscape became flooded as the ice melted. In the process , over 70 marine lakes were created.
Cut off from the outside world, these lakes produces some unique animals. One of these was an ocean predator with long tentacles, but here it evolved into a harmless, graceful wanderer.
Jellyfish normally feed on small fish, but in the lakes there was little prey, so their bells have become home to millions of tiny photosynthesising algae.
When exposed to sunlight, these algae produce sugars which, in turn, provide their hosts, the jellyfish, with food. Now, each day, the jellyfish migrate across the lake, following the arc of the sun....
So, with little danger and a never-ending supply of food, the jellyfish have multiplied ... and multiplied ... and multiplied...
Cue some hypnotic pictures of teeming hosts of jellyfish pulsating through the sun-drenched blue waters (as the BBC footage presumably won't be on iPlayer for very long, here's some more footage of the jellies that somebody's posted on YouTube). A great spectacle, but after watching I got a glimpse into the mind of a creationist. I just couldn't get my head round the evolution that was supposed to be going on here. This stuff happened comparatively recently, at the end of the last ice age - say, about 14,000 years ago, so it's quite recent in evolutionary terms. The lakes flooded, jellyfish somehow got into them and then became cut off. So far, so good. But then, I seemed to be hearing, the food ran out, so the jellyfish evolved from being carnivores into creatures fueled by photosynthetic algae. This boggled my mind a bit.
It almost sounds as if the food ran out and before they staved to death, these guys start using algal byproducts for food instead. Evolution between meals. Now that's not going to work. As usual, the real story's a little more subtle and complicated than that. There's an excellent article entitled Darwin's Jellyfish in National Wildlife Magazine which fills in the missing parts of the story making it less mind-boggling but more interesting and understandable:
Palau’s first marine lake formed just 12,000 to 15,000 years ago after the last ice age ended and sea levels rose. Palau’s rock islands were limestone peaks riddled with erosion-carved channels, fissures and depressions. Seawater seeping through the limestone transformed the largest depressions into marine lakes and swept in the larvae of spotted jellyfish and other sea creatures. In a mere moment of evolutionary time, the landlocked jellyfish radiated into five different subspecies, each attuned to its own isolated “island” of seawater. The jellies in the deepest lakes, which filled first and are therefore the oldest, diverged the most from their lagoon-living ancestor...
The myth of Palau’s “stingless” jellyfish has a certain beating-swords-into-plowshares charm: The supposedly predatory ocean jellies, confined to the peaceful lake, became gentle algae farmers. Not true, says Martin: “The lake jellyfish do have stingers, and they do use them to prey on zooplankton. Lake jellies actually get more of their energy from prey than lagoon jellyfish.”
Luckily for snorkeling tourists, the lake jellies’ sting-power is geared toward flea-sized crustaceans called copepods. Your lips might tingle if you smooched a jellyfish, but nothing more. As for the farming legend, the jellyfish do not “eat” algae. Like their lagoon ancestors, the jellyfish simply absorb their algae’s photosynthetic leftovers. The jellies get about three-fourths of their energy from algal excretions and the remainder from prey. In essence, the jellyfish are landlords that hunt a bit on the side.
The jellyfish-algae partnership did not originate in the lakes, either. Ancestral spotted jellyfish brought the arrangement with them. “Spotted jellyfish in the lagoon have basic behaviors that help ‘sun’ their algae,” Martin explains. “They move eastward in the morning. The lake jellies have adapted this migration to each individual lake. The most spectacular migration is in Jellyfish Lake.”
So, fast evolution, to be sure, but the jellies haven't actually stopped hunting and their ancestors were already making use of algae to sustain them, long before a few of them became trapped in the marine lakes. They are "harmless graceful wanderers", but only from a human point of view - their reduced stingers aren't harmless to the tiny zooplankton they still hunt. The BBC film is still stunning, but it makes a lot more sense if you read the full story of the jellyfish.
Because the jellyfish stings are harmless to humans, a lucky few get to swim in the middle of these massive migrations. Whilst looking for details of these creatures, I stumbled across this Flikr memoir of swimming with the jellies. I also discovered a new expression. Because the jellies' sting is too feeble to hurt humans, you can discard your protective wetsuit and swim among them wearing only your budgie smugglers.
budgie smugglers plural noun Austrailan (coll) a stretchy garment worn over the buttocks and genital area when swimming; speedoes (trademark); tight swimming trunks.
Isn't Aussie slang wonderful? I don't know what the lifespan of the phrase "budgie smugglers" will be, but I'm guessing it's going to easily outlast "staycation." Survival of the fittest - it happens to words as well as jellyfish.
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