Jellyfish are sea-dwelling invertebrates of the cnidarian, or stinging tentacle, phylum. There are several thousand different species, distributed in most of the world’s oceans. They provide interesting examples of ways in which passive and active mechanisms of movement interact. Ocean currents and the wind cause jellyfish to move passively; this movement explains why they sometimes get trapped in large numbers in restricted coastal locations and harbours.
Jellyfish living free in the oceans periodically gather in very large numbers called blooms. Blooms appear to be a form of purposeful collective behaviour driven by reproduction, food abundance, or preferences for particular environmental conditions. When blooms occur in coastal regions, they sometimes lead to mass strandings on beaches, as you can see in Figure 1.
In some circumstances, such accumulations of jellyfish may attract turtles and other animals that feed on them.
Figure 2a shows how populations of jellyfish can be tracked using aerial surveys, acoustic trackers, and underwater cameras to give us an idea of their movement patterns. It is also possible to monitor the directional movements of individuals by fitting them with data loggers, as in Figure 2b.
Observations show that jellyfish may move away from the prevailing current and can even move in the opposite direction if the current is not too strong. Such movement is outlined by the different arrows in Figure 3. The distribution of jellyfish can also be strongly influenced by sea temperature and salinity, magnetic fields, gravity, and the low frequency sounds produced by predators.
Some jellyfish make complex behavioural decisions about their movements. Moon jellyfish (Aurelia spp.) off the coast of Washington State and in the Gulf of Mexico gather in circular or ‘hollow’ aggregations. Individuals move up and down in an orbital manner, producing different densities of jellyfish at different depths. This pattern of distribution is not caused by water currents, although the turbulence produced by their collective movement may help to concentrate the plankton on which they feed.
The helmet jellyfish, Periphylla periphylla (Figure 4), feeds on zooplankton in the deep waters of Norwegian fjords, as illustrated in Figure 5. Light penetration is especially poor in this environment and the jellyfish can forage at a range of depths without facing competition from vision-dependent fish. They change their depth over the day according to the amount of light and as the location of the food material changes — an example of an active positional adjustment. Individuals aggregate in these regions by entangling their tentacles, as demonstrated in Figure 6.