Prominent among ecosystem rearrangements of recent decades, one attributable in
part to fish stock reduction, is the increasing abundance of jellyfish, particularly of
scyphozoan medusae. Recall (Chapter 6) that this group alternates between a small
(<1 cm), attached polyp stage and very large, dome-shaped medusae trailing masses
of tentacles. The polyps “strobilate” producing larval medusae (ephyra) that grow to
substantial sizes – from serving platter-size (Aurelia) to dinner-table size
(Nemopilema). Reducing planktivorous fish stocks may reduce predation on ephyrae.
Trawling may remove predators on polyps. Seasonal pulses of abundant medusae, like
the summer outbursts of Chrysaora fuscescens off Oregon, USA, and in the Bering
Sea then compete for zooplankton meals and eat fish larvae, stabilizing the
community shift. The most dramatic outbursts have been those of the 2 m, 500 kg
Nomura’s jellyfish (Nemopilema nomuri) in waters surrounding Japan (Kawahara et
al. 2006). Like many scyphozoan polyps, those of this monster are unusually tolerant
of suboxia, which recently is prevalent in their benthic habitat beneath the outflow of
the Changjiang River draining northeast China. Thus, it is likely they suffer reduced
predation in this area where they are abundant on shallow bottoms, increasing
production of larvae. The ephyrae grow as they are carried across the East China Sea,
and turn north with the springtime Tsushima warm current into the East Korean/Japan
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