There is a common belief that certain creatures are insentient blobs, twitching unconsciously at the ends of the strings of their biological predispositions. We suspect insentience in many of the more primitive creatures, the liver fluke for instance, or slime mould, but the poster child for the “insentient blob” in the common understanding, are jellyfish.
Jellyfish have no brain, no heart, no eyes or ears, an extremely simple body plan and a relationship to their immediate surroundings that seems little more than simple instinctive reactivity.
They don’t hunt, chase or follow, they can’t swim against a current and they appear to have no social life despite occasionally appearing in large “smacks” of effectively independent individuals who share the current-line into which they have been concentrated.
With no vision, they do not appear to react to nearby prey, so when they eat, it is because some unlucky critter didn’t avoid their passive, but very effective, traps.
We now know that they do have a bit more agency than originally credited. They can detect chemical cues and prepare themselves for feeding or reproduction and they can detect light in order to orient their bodies in the water column, but still, not much about them suggests anything beyond simple programmed reflex.
This impression is shattered when you spend a bit of quality time with box jellyfish.
The first thing you will notice about these amazing animals, is that they are not beholden to the currents they are exposed to. They swim with what looks like direction and purpose. They defy currents by swimming against or across them.
And they are quick. A boxy on the move is quicker than a human swimmer.
The next thing you notice is that they actively avoid obstacles, like pier pilings and parked boats and you. They will stop, turn and avoid touching anything that they don’t consume and, if you’re lucky, you will see them appear to “notice” and then “target” the small fish and crustaceans that they do consume.
They even appear to collaborate with others of their kind in the hunting of schools of fry in places where their prey can be herded. The marina in Port Douglas is one such place where this behaviour is easily witnessed. There, you can see pairs of our very own local species of box jellyfish working the edges of the pontoons. One will take the lead, swimming parallel to the floating docks, with another tailing just behind and closer to the wall. The small fry that shelter around the pontoons actively avoid the lead box jellyfish by moving closer to the structure. However, with both hunters tentacles fully extended such as to form a moving box trap, the small fry become trapped between the pier and the pair of hunters until, inevitably, some end up hitting the curtain of tentacles that surround them.
I have had the great fortune to have witnessed this behaviour on many occasions, so I know it is not just a coincidence that I’ve misinterpreted.
I have also had the opportunity to sit in clear water that was literally full of our small varieties of box jellyfish, some species of which we call “Irukanji Stingers”. I have watched as they actively avoided me until I sat still enough for them to appear to “get used” to my presence, whereupon they “allowed themselves” to approach me. As soon as I moved my hands towards the animals though, they reacted by rapidly retracting their trailing tentacles and either halting or retreating to a safe distance with a clear intention not to make contact with a giant they could not eat.
So, how could an animal with no brain, no eyes and a blob for a body, engage in the behaviours I have just described?
Well, it turns our that almost everything we know about jellyfish is wrong when it comes to the box jellyfish group, the Cubozoans.
Firstly, these animals are neurologically, highly advanced. Not satisfied with a single brain, Cubozoan jellies have four “brains”! There are dense concentrations of neurological tissue at the top of each tentacle cluster. These “ganglia” are connected to each other by a thick cord of nervous tissue, and to the complex network of nerves that extend throughout the animal’s body. The whole creature can be thought of as a kind of mobile weaponised brain. Animals that learn and think must also sleep to consolidate memories and maintain their neurological networks. Box jellyfish sleep. At night, they pull in their tentacles and drop to the seafloor and just lie there until morning.
They also have eyes. Twenty four of them in fact. Up to sixteen of these eyes, depending on the species, are lens and retina based “camera eyes” like our own, while the rest are light detectors, pinhole cameras and strange slit-like optical devices unique to the Cubozoans.
It has been demonstrated convincingly that they can identify shapes and follow movement with enough resolution to separate prey signals from their complex background environment.
They are also “jet propelled”.
Yes, that’s what I said. Jet propelled! Ordinary jellyfish move by effecting weak, peripheral contractions of their outer bell. This method of locomotion “puffs” a diffuse stream of water from below the animal which enables it to adjust its position in the water column. Such a propulsion system is unable to resist anything but the weakest current, so a jellyfish generally ends its life wherever the currents it encounters carry it.
Box jellyfish, by contrast, possess a structure called a “velarium”, a thin, muscular flange of tissue that extends inward from the outer edge of their bell. Upon effecting their locomotory contractions, the velarium contracts forming a siphon through which the propulsion stream is ejected, thus forcing a powerful jet out of the animal. This jet is powerful enough to shoot the box jellyfish forward faster than a person can swim, and fast enough to actively chase down many kinds of highly mobile prey.
Now, on the face of it, a jet propelled, keen-eyed, predatory brain with deadly tentacles that chases down and kills it’s victims, might sound a bit ominous. Or actually terrifying. The good news though, is that all of these formidable features actually make these animals much less dangerous to you and me than you might think.
Firstly, they can’t eat you. Even if they wanted to.
However, due to the automatic nature of their stinging mechanism (see my previous article) they can’t “not sting” you if they come into contact with you. When they do sting, they stick like glue to the flesh they brush against. So if they touch something too large, they become physically stuck to it.
That is not ideal. The last thing that an animal with the consistency of firm custard wants, is to become accidentally adhered to a panicking, flailing giant who will proceed to thrash them to snot in the interaction.
And when you have four brains, you can want plenty.
With excellent vision and a jet siphon they have every opportunity to detect and avoid obstacles like you and me.
In deep, clear water, their preferred mode of egress is to simply retract their tentacles, flip over and head vertically down and out of your sphere of influence. This is why, despite their global ubiquity, with over forty species and counting, almost no-one ever gets to see one in open ocean.
There are circumstances in which their ability to detect and avoid you are challenged though. Shallow inshore waters are murky and energetic, and in the tropics, full of semi-naked people who occupy the entire water column. There is no “dipping down” option to avoid an obstacle with its feet on the sand, so they must try to go around a bather. In turgid waters, visual detection and subsequent avoiding action, happens at the last minute which can lead to people and jellyfish having unfortunate encounters.
With such a range of incredible features, this group of animals are really not like jellyfish at all, other than in their consistency. As such, they are now considered to occupy a group of their own, related, but distinct from the “true jellyfish”.
They are officially the most toxic organisms known, but luckily for us they have no interest in running into you and more than enough brains and agility to avoid you if you apply a bit of common sense considering what you now know about them.
With “stinger season’ now beginning, it’s a great time to head to the marina in the early morning to get a safe look at their real beauty and some of their amazing behaviours. Look for the rhythmic pockmarks on the water’s surface tracing the lines of the marina fingers and then squint hard to see the glass-like cup shaped body, trailing a beautiful set of white streamers as they hunt between the boats. Polarising sunglasses will much improve your view once you’ve found one, but begin the search without so the reflected sky highlights their punctuated wake.