Cubozoa
A cubozoan of the genus Chironex. Source: Guido Gautsch
Cubozoans are astounding animals. They resemble basic jellyfish, but they can swim quite rapidly, maneuver with great agility, and have good vision despite not having a brain...Believe it or not!
A caybdeid from Darwin, Australia. (Source: UCMP) In general, box jellies are similar in form to the "true" jellyfish, known as scyphozoans. However, it is relatively easy to tell the two groups apart. Cubozoans have a square shape when viewed from above. (Gee, maybe that's how they got their name.) They also have four evenly spaced out tentacles or bunches of tentacles and well-developed eyes. Not surprisingly, given their squishy nature, there are not many fossil cubozoans known. Today, there are about 20 known species found in tropical and semitropical waters. The Australian stinger Chironex fleckeri is among the deadliest creatures in the world, having caused human fatalities.
Contents
Fossil Record
Athracomedusa, 300 million years old. (Source: UCMP) Cubozoans are rare as fossils. A few probable cubozoans are known as fossils from the famous Mazon Creek locality (Pennsylvanian age) near Chicago, Illinois. One of these fossils, Anthracomedusa turnbulli, is shown to the right in a somewhat squashed condition; the bell is towards the top, and three bunches of tentacles can be seen at the bottom. Other fossils from the same locality more clearly reveal the square shape and four bunches of tentacles, features that indicate Anthracomedusa was very likely a cubozoan. A few other fossils that may be cubozoans have been found in the Jurassic Solnhofen Limestone of Bavaria, Germany. One Vendian fossil, known as Kimberella, was thought to be a cubozoan, but re-study of numerous specimens from the White Sea region of Russia has shown that Kimberella is unlikely to have been a box jellyfish.
Life History and Ecology
A group of box jellies. (Source: UCMP) Cubozoans are voracious predators. Many jellyfish and other gelatinous animals go unnoticed by those who visit the ocean. Like this "herd" of cubozoans in the photo, gelatinous animals are often difficult to see. However, they are important members of marine ecosystems. For instance, box jellies are known to eat fish, worms, and crustacean arthropods. When the tentacles, which can stretch to ten or more times the height of the bell, come into contact with prey, nematocysts fire into the prey's skin. Tiny barbs of the nematocysts hold onto and transfer venom to the prey. As the prey is immobilized by the venom, the tentacle contracts and pulls the prey near to the bell. Then, an amazing thing occurs. The muscular pedalium bends and pushes the tentacle and prey into the bell of the medusa. The manubrium then reaches out for the prey and the mouth expands and engulfs it. The entire feeding process is relatively fast, taking place in as little as one minute. Cubozoans are not quite at the top of the food web, as they are sometimes eaten by large fish and sea turtles.
Cubozoans are active and agile swimmers. Jellyfish are typically referred to as planktonic, meaning that their swimming is not strong enough to prevent them from drifting with currents. As a consequence, many jellyfish wash up on beaches. However, box jellies are very rarely found on beaches. This is because they are unusually strong swimmers. It has been reported that large specimens of Chironex fleckeri can swim as fast as two meters in one second. However, that speed has not been confirmed. Cubozoans (of various sizes) have commonly been observed to swim a meter in five to ten seconds. Presumably, cubozoans use strong muscles, as well as the Velarium, to achieve these speeds. Not only are cubozoans fast, but they are remarkably agile. They have been reported to maneuver around the pilings of peers and to flee would be human collectors. These behaviours can only be achieved because cubozoans have vision. Few experiments have been tried in order to determine how well cubozoans can see, but it is clear that they can see well enough to avoid swimming into things and being captured. It is also likely that they can see eachother, which might help explain some interesting observations about their mating.
Cubozoan Life Cycle
Jeuvenile Carybdeid Cubozoan. (Source: UCMP) Some species of cubozoans appear to pair up, male with female, in order to mate. The male puts his tentacles into the bell of the female and appears to pass packets of sperm. At least one other species has been observed in large mating aggregations. Mating appears to occur once every year. Fertilization takes place inside the females. In some species the fertilized eggs are released into the water column where they develop into planulae, while in others development into planulae occurs inside the female. Planulae swim in the water column for a few days and then settle on to the substrate. After settlement, the planulae grow into polyps. The polyps can move around, and they frequently bud off additional polyps. After a few months of feeding, the polyps are mature. Then, each polyp metamorphoses into a single juvenile medusa. During metamorphosis, the polyp tentacles are resorbed and four new tentacles and four rhopalia are formed. With a couple of contractions, the entire individual becomes detached and swims away as a juvenile medusa.
Systematics
There are two main groups of cubozoans, Chirodropidae and Carybdeidae. The two groups are relatively easy to tell apart. In carybdeids, each tentacle is connected to a single pedalium. Usually there are four pedalia each with a tentacle, however, in Tripedalia species, each corner of the bell has two or three tentacles each connected to a single pedalium. Chirodropids always have four pedalia, one at each corner, with multiple tentacles. Are you ready for the test?
The phylogenetic position of Cubozoa within Cnidaria has been the subject of a great deal of debate. Classically, cubozoans were viewed as a subgroup of the "true jellyfish" or Scyphozoa. However, when it was observed that the cubozoan polyp and life cycle were rather different than those of scyphozoans, Cubozoa was placed on its own. Some scientists have proposed that cubozoans are the closest living relatives of scyphozoans. Still others have argued that Cubozoa is more closely related to Hydrozoa.
More on Morphology
General Form of the Medusa
General Morphology of a Cubozoan. (Source: UCMP) The cubozoan bell is square in horizontal cross section. Inside the bell are the manubrium and mouth. A flap of tissue called the velarium is located along the underside of the bell. Muscular fleshy pads called pedalia are located at the corners of the bell. One or more tentacles are connected to each pedalium. Actually, that's the way to tell apart the two main groups of cubozoans (Chirodropidae and Carybdeidae). Carybdeids always have one tentacle per pedalium, while chirodropids have multiple tentacles attached to each pedalium. Take the test? Like other cnidarians, cubozoans are composed of two layers of cells, ectoderm and endoderm. The gut, or stomach cavity, is partitioned by septa and extends to the tentacles through pedalial canals. On the bell, located midway between the pedalia, are four sensory structures called rhopalia.
Cubozoan Sensory Structures
Inside the Rophalian Niche of a Cubozoan. (Source: UCMP) If you looks closely inside the rhopaliar niche of a cubozoan (left), one sees a remarkable thing . . . . looking back at you.
That's right, cubozoans have eyes, and surprisingly complex ones at that. In a close-up of a rhopalium, you can see six reddish spots, all of which are sensitive to light. The four smallest spots are relatively simple. However, the two larger regions actually contain lenses, corneas, and retinas, not so unlike those in your eyes.
It is still unclear how the images created by these lenses are interpreted by cubozoans since they do not have brains. A cubozoan can look both inward towards its mouth and manubrium and outward since each rhopalium dangles by a muscular stalk. Inside each rhopalium, located below the eyes, is an organ called a statocyst. Inside each statocyst is a hard nodule composed of calcium sulfate, the statolith. Statoliths appear to have daily growth rings. The statocysts are sensitive to orientation, and thus allow cubozoans to sense whether they are upside-down, sideways, or rightside-up.
Cubozoan Stinging Cells
Nematocyst of a Cubozoan. (Source: UCMP) Like all cnidarians, cubozoans are endowed with nematocysts, cells that fire a barb and transfer venom. As you can see to the right, the barb is coiled up inside a capsule. When a nematocyst touches something that might be prey or predator, the barb uncoils and fires from the capsule along with venom.
Nematocyst Rings on a Cubozoan Tentacle. UCMP Nematocysts are concentrated in rings on the tentacles of cubozoans. This makes sense. When the tentacles capture a prey item, they contract, and the rings allow for maximum contact and transfer of venom between nematocyst and prey. In some cubozoans, such as Chironex fleckeri, nematocysts are absent from the bell.
The Cubozoan Larva and Polyp
Unfortunately, we do not have (yet) any good pictures of cubozoan larvae or polyps. These stages are not very well known in cubozoans. In fact, it was not until about 1970 that the complete life cycle of a cubozoan was observed. The cnidarian larva is called a planula. Cubozoan planulae are pear-shaped (that's "pyriform" in scientist lingo), have pigment spots which may be sensitive to light, and swim for a few days using cilia. After a planula settles, it grows into a polyp. The cubozoan polyp can crawl around like an inchworm and bud off more polyps. Its form is relatively simple. It has a mouth surrounded by up to 24 tentacles. The polyps do not closely resemble the polyps of scyphozoans. These differences may indicate that cubozoans have a separate evolutionary origin from scyphozoans.
Finally, it is worth mentioning in case you go on to read more about cubozoans (or any other cnidarians for that matter) that the plains of symmetry in a cubozoan have special names. The interradii are marked by the pedalia; rhopalia occur along the perradii.
