Language has added to the challenge. In common use, the term jellyfish lumps together organisms that can be quite different from one another.
For the purposes of this study, the researchers included records for true jellies, the type most familiar to beachgoers; their relatives the hydrozoans; comb jellies, which use tiny hairs, called cilia, to swim;
and another group of free-swimming invertebrates called salps.
From around 1940 to present, the records show the 20-year rising and falling cycles. Prior to that, researchers saw signs of oscillations in regions where data were available; however, this isn't enough information to draw conclusions about global patterns, said lead researcher Rob Condon, a marine scientist at Dauphin Island Sea Laboratory in Alabama.
Multidecadal cycles are not uncommon in nature, whether in organisms' growth patterns and populations, or physical phenomena, such as the oxygen concentration of the oceans.
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