I enjoy watching MonsterQuest, but it tends to irritate me.  My feelings towards the show were handily summed up by a quote from the latest episode I watched.  "If ____ happens, the results would be frightening."  Well, sure!  And if crocodiles learned to drive cars, the results would be frightening, too.  What's your point?

The point of their episode "Killer Jellyfish" is that if jellyfish take over the ocean, the results would be frightening.  (Except for some of the clips from night diving in Australia, the point of which was "if these sharks were hungrier, the results would be frightening."  I mean honestly, sharks have it hard enough without the narrator throwing around terms like "feeding frenzy" to describe sharks swimming around peacefully, over a suite of horror movie music, don't you think?)

Aside from some of the annoying bits, the show made some excellent points about the way the oceans' biological landscape is shifting in response to climate change.  Although massively destructive overall, climate change is proving to be a boon for jellyfish.  Jellyfish are resistant to poorly oxygenated water, and their ranges are largely limited by ocean temperatures.  When ocean temperatures rise, jellyfish follow along for the ride.  

Jellyfish also tend to prosper in "dead zones," which are areas of the ocean with little to no oxygen, where plankton and algae tend to boom, and higher predators are either killed or avoid the area entirely.  Sadly, these dead zones are becoming increasingly more common in the world's oceans.

Nomura's Jellyfish (Nemopilema nomurai) have been terrorizing coastal Japan and China in recent years.  Well, "terrorizing" might be a little strong.  "Slowly clogging" is probably more accurate.  The Nomura's Jellyfish is gigantic, one of the largest jellies in the world, being up to six feet in diameter and weighing up to 660 pounds.

Although many species of jellyfish have experienced a dramatic increase over the last ten years, Nomura's Jellyfish tend to attract a lot of attention.  Because of their size, an increase in their numbers can cause huge logistical problems.  Nomura's Jellyfish blooms have been responsible for clogging intake valves and bays all over coastal Asia, and even caused a nuclear power plant to have to be shut down.

According to MonsterQuest, Nemora's Jellyfish have experienced unusual population spikes in Japan about every 20 years.  These population surges were recorded in 1920, 1958, and 1995 as you would expect.  However, between 2002 and 2009 no fewer than five population spikes were recorded - in other words, there were more invasions in the first decade of this century than there were the entire century before.

Unfortunately, some of the impact of the show was undercut by its own assertions.  The show repeatedly talks to a researcher who ascribes a level of consciousness and intent to the movements of box jellyfish which most people would find… unlikely.

(Incidentally, the show's assertion that box jellyfish sleep on the ocean bottom at night has largely been debunked.  It was shown to be more a result of the presence of a big heavy tracking sensor attached to the jellyfish, than a demonstration of the jellyfish's normal behavior.)