Article: Fossette, S., Gleiss, A.C., Karpytchev, M., Hays. G.C. (2015). Current-oriented swimming by jellyfish and its role in bloom maintenance. Current Biology 25, 342-347. doi:10.1016/j.cub.2014.11.050
Jellyfish are some of the most visible and well-known marine organisms to the public. A simple trip to the local aquarium could say as much – the pulsing bells of scyphozoans (a fancy term for jellyfish) have mesmerized aquarium goers for decades now. Despite this movement though, jellyfish are commonly thought of as drifters, passengers subject to the will of the currents. To their credit though, jellyfish do have some control over the direction they swim in, if only minor. But as far as how jellyfish employ their limited swimming skills is still relatively unknown to us.
The vast ocean, as it turns out, is patchy in its resources and jellyfish, like all other animals in the world, cannot survive in any patch that does not contain the necessary resources (food, other jellyfish to mate with, etc.). This type of drifting lifestyle can be very risky as whether or not they end up in a favorable patch of water (or stay in it) is up to chance. Thus it’s purely up to the direction of the wind and the traveling water that allows jellyfish to find suitable areas and hope that they don’t end up somewhere inhospitable (or even beached on land). Such a life history strategy makes animals like jellyfish vulnerable to random occurrences like storms, tidal waves, and strong winds (scientists call these type of events stochastic, or random). To offset any dangers stochastic events may have on jellyfish populations, they grow their populations as fast as possible when conditions are good, on the off chance that an approaching stochastic event will not wipe out the entire population. This results in (sometimes) very large and dense occurrences of jellyfish, called blooms (such as the recent Velella velella bloom that occurred in California last year).
Again, as with much else involving the detailed workings of jellyfish, scientists don’t have a clear grasp of how blooms of jellyfish persist and seem to stay localized for so long, despite being passive drifters in the ocean. Well, a team of researchers led by Drs. Sabrina Fossette and Adrian Gleiss think that what limited swimming abilities jellyfish have might be used in a strategic manner to help keep their position within the ocean and, at least for a short amount of time, allow them to control their distribution.
How’d they do it?
Drs. Fossette and Gleiss attached data loggers onto 18 Rhizostoma octopus jellyfish in the Pertuis Breton in the Bay of Biscay, France (Fig. 1). These wearable apparatuses were equipped with acceleration loggers, which record diving behavior, activity, and body orientation for up to 6 hours at a time. In addition to this, field crews ran 19 half hour transects through the bay during flood tide, ebb tide, and slack water to observe the direction the jellyfish were swimming in. They used current profilers on buoys to measure current speed and direction.
Based on the data collected by the attached data loggers and from the transects, the scientists modeled jellyfish bloom development over time in the Bay of Biscay and analyzed the model runs to see if it aligned with the many blooms that have been observed in the bay over the years. To test whether or not directional swimming in jellyfish has any impact on bloom development over time and space, they ran two scenarios: one with virtual jellyfish that are passive drifters, and one with jellyfish that orient swimming direction based on current movement.
What’d they find?
From the data gathered from the attached loggers, the team of researchers found that the average activity level of R. octopus was significantly impacted by the tide. The data loggers showed that the jellyfish were most active during slack tide (no tide) and were less active during ebb and flood tides.
Despite this burst of activity during slack water, transect data shows that there was no correlation of this increased activity with any particular direction. However, the activity levels observed by the data loggers during food and ebb tide showed a statistically significant correlation with direction of movement. During ebb tide (tidal current heading from east to west), the jellyfish traveled predominantly against the tide. During flood tide (when water was moving from west to east) the jellyfish were observed to move predominantly against the current as well (though there were particular groups of jellyfish moving along with the flood tide – though these jellyfish were found to be in deeper water farther away from shore). The directionality of jellyfish movement during slack water was much more variable (Fig. 2). This shows that tidal currents (in particular, the direction of the current) has some sort of modulating effect on jellyfish orientation. They seem to be able to sense (somehow) what direction the current is moving and will orient themselves against the current to maintain their position.
This is the first time that jellyfish have been observed to orient themselves in response to current direction. Drs. Fossette and Gleiss wanted to see whether or not this directionality would have an effect on bloom maintenance. They ran different simulations of jellyfish blooms in the Bay of Biscay and found that models that had current-oriented jellyfish had different bloom distributions than simulations with passive jellyfish and that, in general, passive jellyfish tend to have blooms further south in the bay than active jellyfish (Fig. 3). Furthermore, they found that simulations with current-oriented jellyfish had bloom distributions that were more closely matched with what was observed in Biscay Bay based on aerial photographs or boat surveys.
What does this mean?
This is an important publication for a couple of reasons. First of all, this is the first time jellyfish of any species have been tagged with accelerometers. Additionally, they’ve shown that jellyfish, while brainless and weak swimmers, are able to detect stimuli in their environment corresponding to water movement and make adjustments in their own motions to compensate and remain relatively stable in their position. The exact mechanism of how they perceive or pick up these stimuli still needs to be investigated. This research isn’t specific to jellyfish either – the researchers think that many types of animals, from zooplankton, to fish larvae, to flying insects, and even birds, may all use similar mechanisms and can be studied in similar ways.
While important for our understanding of jellyfish ecology, this has real world implications too. Accurate predictions of jellyfish bloom distributions has great potential as a tool for coastal area management. Understanding how possibly harmful jellyfish (or even phytoplankton) blooms disperse or remain in place would improve our abilities to forecast beach closures, protect aquaculture facilities, and even protect the public from such events. The authors argue that a deeper understanding of the controls on jellyfish blooms is important in reducing negative socioeconomic impacts they might have.