Jellyfish are curious things, but some are curiouser than others. Most jellyfish are members of the class Scyphozoa, which is a very diverse clade with plenty of jellies to choose from. But then there are the Cubozoa, a separate class of jellyfish that have a lot of interesting and unique features, such as being some of the most venomous animals in the world. Try not to get stung.

(Image: a box jellyfish. It is a mostly translucent jellyfish with a box-shaped bell. From the corners of the bell hand four tentacles that are a striped white and orange. End ID)

Like the Scyphozoa, the approximately 50 known species of box jellyfish have a body plan consisting of a bell and trailing tentacles. While the Scyphozoa have round bells, box jellyfish have cuboid bells, which give them their name. At each lower corner of the bell is a stalk that leads into one or more tentacles. The interior of the bell is divided into four regions by structures called septa. In each region is a gastric pocket that helps direct food into the central stomach and has structures that help water flow into and out of the animal. Each septa is lined with digestive filaments and two of the jellyfish's eight gonads. In the center of the bell is a flexible, trunk-like appendage called the manubrium which contains the mouth and stomach. When thebox jellyfish eats, it draws prey into the one of the septa where the digestive filaments begin digestion. The manubrium then maneuvers the mouth over to the prey and eats it. Prey is captured with the tentacles, which are lined with stinging cells called cnidocytes which contain structures called nematocysts that inject venom into prey and threats. The tentacles then help move the food into the bell. While Scyphozoa come in a variety of colors, box jellyfish are almost always translucent.

(Image: a box jellyfish, this one having a squatter bell and multiple tentacles at each corner. The manubrium is visible through the bell as a curving, trunk-like structure. End ID)

Box jellyfish have a developed nervous system consisting of two portions: the nerve ring and the rhopalia. The ring nerve stretches around the base of the bell and controls how it moves. The rhopalia is a system of nerves that controls sensory structures. In particular, it controls a sense of gravity and the eyes. While some Scyphozoa have simple light-sensing eyes, the box jellyfish have true eyes with retinas, corneas, and lenses. Like the bell, the rhopalium is divided int four segments. Each segment has two true eyes as well as two simple pit eyes for detecting light levels and two simple slit eyes that likely detect movement. The true eyes can be oriented in different directions and they always appear to keep one pair pointing up. Thanks to their good eyesight and the shape of the bell allowing for rapid bursts of speed, the box jellyfish can actively hunt its prey instead of just drifting and waiting for food to come to them. What's more impressive is their ability to actively navigate their environment and learn to avoid certain threats. It was long thought that animals without brains would not be able to truly learn, but multiple types of cnidarians have shown learning behavior, with the box jellyfish seeming to be the most advanced.

(Image: a scientific diagram showing pictures of a box jellyfish eye and a drawing of its anatomy. End ID. Source)

Box jellyfish reproduce every year and are semelparous, meaning they only mate once before dying. However, there is some inconclusive evidence that the species Chiropsalmus quadrumanus may be able to mate multiple times. Reproduction can be both internal and external depending on species. In external reproduction, the male and female release gametes into the water. In internal reproduction, the pair will maneuver themselves so the openings of their bells face each other and the male will pass a packet of sperm to the female. In these species, the female retains the eggs internally until they hatch. The juveniles hatch as a type of swimming larva called a planua. The planua will eventually settle and become fixed to a hard surface, metamorphosing into a polyp. The polyp is a non-swimming stage found in all members of the clade Medusuzoa, of which the box jellyfish are a member. Polyps will grow for months to years and during this period, they can reproduce asexually by budding off clones of themselves. Eventually, the polyps will metamorphose into the adult medusa stage

(Image: photos of a box jellyfish at different stages of its life cycle. It starts as a bean-shaped planula, progresses to a worm-like creeping polyp, then to an anemone-like sedentary polyp, then to a newly released medusa, which looks like a tentacle-less box jellyfish. The medusa them matures to a small juvenile and finally a mature adult. end ID)

Box jellyfish are found worldwide in tropical waters, though most known species are from the Indo-Pacific. It is likely that many species ave been overlooked as their transparency and frequently small size makes it easy to miss them. New studies using e-DNA, DNA that is shed and collected through the environment, may help identify nee species and populations. Box jellyfish are known to most people as being some of the most venomous animals in the world, but only a few species are dangerous to humans. The species Chironex fleckeri is the most dangerous to humans. Its venom causes extreme pain and can lead to death within 2-5 minutes if enough venom enters the system. C. fleckeri has caused over 60 recorded deaths in Australia (because where else would it live?) and while there is now an antidote to the venom, getting it to the victim fast enough can be challenging. A singe C. fleckeri is often said to carry enough venom to kill 60 people. Most reported cases of stings have not ended in death as not enough venom was introduced to the system. Also from Australia (of course) are the Irukandji, a group of about 16 similar box jellyfish that can also be lethal, but while C. fleckeri at least has the decency to be big, the Irukandji jellyfish are all tiny. Their venom causes a reaction called Irukandji syndrome that is delayed for up to 2 hours after the sting and can lead to cardiac arrest if not treated. Jellyfish nets are common sights on Australian beaches. They enclose parts of the water with nets that have mesh small enough to keep most jellyfish out. Application of vinegar to the sting is the current recommended treatment and many beaches in Australia are required to stock it, though some research has indicated that vinegar may actually make the sting worse.

At first glance, some may mistake this as the infamous box jellyfish. But actually- this is its cousin, the ✨️box jellyfish✨️! Confused?

Jellies are a prime example of why common names are often misleading or cause confusion. In actuality, "box jellyfish" has been given to dozens of species of jellies that belong to the class Cubozoa. The most infamous are the irukandji and Chironex fleckeri, which often pop up in the news as simply "box jellyfish" or "sea wasps."

The specimen in this video, while possessing a sting you would certainly want to avoid, is not of the lethal sort. While difficult to identify conclusively, it is likely Alatina alata or a close relative. Also known as the sea wasp or box jelly - like so many other jellies are.

Jellyfish fall under three classes;

Scyphozoa (true jellyfish)

Phylum Cnidaria (this class also includes other species such as coral and sea anemones) and

Cubozoa (box jellyfish).

Jellyfish are the medusa phase of a few members of the phylum Medusozoa clade.

The medusa phase is the sexual reproduction phase in which jellyfish produce planula larvae.

Most species are fast growing with a short life cycle of only a few months. They breed then die shortly after.

Jellyfish are found all over the world, from shallow waters to the deep seas.

All jellyfish have cnidocytes also called stinging cells. Depending on the species the deadliness and intensity of the sting can vary. These stinging cells are located on its tentacles. The top of the jellyfish, called a 'bell', remains safe from the stingers.

-Edited by our amazing partner system.

Cubozoa. This class is known as the box jellyfish, which have a cube-shaped bell which allows it to move faster than other jellyfish

Hydrozoa. This class is of predatory cnidarians, some of which are colonial, include hydras and the Portuguese Man o' War

Sooo... jellyfish.

You may have seen this video circulating recently. The original was posted on Facebook by Scuba Ventures Kavieng; this is a colour-corrected edit [link to the original in the post source, because Tumblr is stupid about external links]. It caused a small stir on the internet after being tentatively identified as Chirodectes maculatus, a monotypic genus known only from a single small specimen captured and filmed in 1997, off the coast of Queensland. (That footage is sadly not available on the Internet.)

If you know a little about jellies, you’ll recognise this as a box jellyfish or Cubozoan. Box jellies are wonderfully interesting animals, and differ from the more familiar “true jellyfish” or Scyphozoa in a number of ways:

a more developed nervous system allowing more complex behaviour, including the capacity to learn from experience

true eyes (with retinas, corneas, lenses, the whole shebang)

the ability, unlike true jellies which mostly drift, to actively propel themselves around obstacles and towards prey

...a concerning trait in an animal most notorious for being extremely fatal to humans. (c.f. Wasp Jellyfish, Viper Jellyfish, the delightfully named Common Kingslayer, Irukandji*) * sting symptoms include excruciating pain and “a feeling of impending doom”, and let’s raise a glass to the researcher who first documented Irukandji syndrome by deliberately testing it on himself, a lifeguard, and his nine-year old son: “Eschewing animal models and laboratory studies (not to mention all common sense), Barnes took the two specimens, and proceeded directly to human experimentation.” please read the article linked in my reblog, it’s hilarious. (A lack of self-preservation is a trait apparently common to all jellyfish scientists - the 2005 article on Chirodectes maculatus notes that it failed to sting, or adhere to, the hand and forearm of an incautious volunteer.)

The point is that box jellies are deeply, deeply cool in every respect except basic body plan, which goeth thusly:

box

tentacle on each of the four lower corners

....and while many cubozoans find that four is not enough and multiply to get 4n tentacles, that’s about it. Ultimately, what you end up with is still just a translucent jelly box with string hanging from each corner, like a very sad, very dangerous piñata.

Compare scyphozoans / "true jellies": [top to bottom: Pacific sea nettles Chrysaora fuscescens aka windows screensaver, barrel jellyfish Rhizostoma pulmo, sea cauliflower Cephea cephea, moon jelly Aurelia aurita, lion's mane jellyfish Cyanea capillata]

They may not be all that bright, but by god they’re fancy.

Now let's see the box jellies: [Bonaire banded box jellyfish, which rejoices in the name of Tamoya ohboya; sea wasp Chironex fleckeri; Copula sivickisi; Tripedalia cystophora]

And by cubozoan standards, it is big: according to the diver, "a bit bigger than a soccer ball" (which have a ~21cm diameter), which would make it larger than the 15cm in the original species description.

Where am I going with all this? My point, dear jelly lovers, is that Chirodectes maculatus took a body plan like an inverted plastic bag and made it into the fanciest, most ostentatious chunk of jelly that ever wobbled the seven seas.

Look at it. It’s like a 1970s lampshade set off to colonise the ocean. And isn't it just fabulous.

Cambrian Explosion Month #06: Phylum Cnidaria – Medusozoa

 The medusozoans are a group of cnidarians that includes modern true jellyfish, box jellyfish, stalked jellyfish, hydrozoans, and the weird fish egg parasite Polypodium.

Due to their soft gelatinous bodies their fossil record is very sparse. While vague fossilized blobs tend get interpreted as jellyfish fairly often, many of them turn out to be trace fossils or inorganic structures, and definite preserved medusae are only found in a few sites of exceptional preservation.

Among those rare examples of fossil jellies there are some amazingly well-preserved specimens known from the mid-Cambrian, discovered in the Marjum Formation in Utah, USA (~505 million years ago).

None of these species have been given their own names, and they're all tiny, only around 1cm in diameter (0.4"). But their anatomy is still preserved in enough detail to tentatively classify them into known lineages, including the box jelly, narcomedusan, and semaeostomean shown here.

Much larger Cambrian jellyfish have been also found in Death Valley, California, and in Wisconscin, representing preserved mass stranding events on ancient shorelines. Some of these jellies were up to about 50cm in diameter (20"), indicating that large soft-bodied animals were much more common in Cambrian seas than previously thought.

———