How Speedy are Spineless Swimmers?

Last week a producer from the BBC's show More or Less contacted me with a question from a listener: Can jellyfish swim as fast as an Olympic swimmer? (I'm guessing this question was in part spurred on by Michael Phelps', umm, race with a shark during Shark Week in 2017.)

I LOVE thinking about jellyfish swimming. (Can you think of a better distraction from the bad news right now?) In Spineless, I wrote one of my favorite chapters on the amazing hydrodynamics of jellyfish. I had always considered jellyfish rather pokey, but I hadn't ever looked carefully at how fast they can go. So, I basically dropped everything to find out.

First step: How fast does an Olympian swim?  The answer is pretty easily found with a quick Google: about 2.1 meters per second.

Next step: Dig into the scientific literature. There are two categories of jellyfish swimming: in the ocean and in the lab. The first includes all the intricacies of navigating currents and tides, vertical movement as well as horizontal movement, not to mention avoiding predators and potentially stopping for a snack. The other is like being the only one racing in a pool.

In recent years, scientists have begun adorning jellyfish with satellite tags, like putting a little necklace just under their bell, which can measure swimming speeds in the real world. One nice study was done in the sea near France with the jellyfish Rhizostoma octopus. The range of speeds was 0.03 to 0.08 meters per second with an average of 0.05 meters per second.*

And luckily for anyone who wants to delve deeper into jellyfish swimming, a big review paper on jellyfish hydrodynamics was just published online and will be out in print in January.** In the lab, a moon jellyfish clocked in at swimming 17 millimeters in 2.5 seconds, which calculates out to a dawdling 0.007 meters per second.

Now, I haven't been in the pool for a while, but I'm pretty certain even I could outswim either of those jellies.

I think, though, that the BBC listener was thinking about the zippiest group of jellies, box jellies. Although I couldn't get to the information first-hand because the book is out of print, the first person to discover just how dangerous the sting of the box jellyfish could be, Jack Barnes, reported a box jellyfish out in the sea swimming fast as 2.1 meters per second.*** So Olympic level.

(As a diversion from this diversion, you might want to dig into Jack Barnes' life. He was a military-commando-turned-medical-doctor-slash-marine -toxicologist who laid box jellyfish on his arm--and that of his son--to prove its toxicity.)

Later authors pointed out that no one was accounting for currents when Barnes made his measurements in 1960, so it's not clear how much drafting was going on. When scientists took a more careful look at box jelly speeds in the lab, averages were around 0.05 to 0.06 meters per second.**** The fastest was 0.115 meters per second, about double that of other jelly measurements, but nowhere close to that of an Olympic swimmer.

But what’s super interesting is what jellyfish lack in actual speed, they make up for in efficiency. On a per body-weight basis jellyfish are the most efficient swimmers every studied. They use so little energy, it would be like us running 100 strides on the energy of just 1. (See Figure 1 in this paper. Because of copyright laws, I'm a bit worried about reproducing it here.)

There’s one other way to look at it too. If you bring things down to the relative size of a jellyfish, rather than the absolute speed, jellies can swim anywhere from 1 to 4 bell-lengths per second.***** If an Olympic swimmer is 2 meters long, she or he swims at just over 1 body-length per second.

So, depending on how you look at it, a jellyfish could be pretty dang competitive.

*Fossette et al., Current-Oriented Swimming by Jellyfish and Its Role in Bloom Maintenance, Current Biology (2015),

**Costello et al., The Hydrodynamics of Jellyfish Swimming, Annual Review of Marine Science (2021) 13:1,

***Kinsey, B., Barnes on box jellyfish. (1986) Cairns: James Cook University of North Queensland.

****Shorten, M. est al., Kinematic analysis of swimming in Australian box jellyfish, Chiropsalmus sp. and Chironex fleckeri (Cubozoa, Cnidaria: Chirodropidae) J. Zool., Lond. (2005) 267, 371–380

*****Dabiri, J. et al., A wake-based correlate of swimming performance and foraging behavior in seven co-occurring jellyfish species (2009), The Journal of Experimental Biology 213, 1217-1225

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