Wet Beast Wednesday: box jellyfish
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.
(Gif: footage of the rare spotted box jellyfish. It has a large bell with stripes and rings on it and the tentacles are thick and reddish. It is swimming, showing off the movement of the bell in how it propels itself. End ID)
