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Box jellyfish (class Cubozoa) are distinguished by their box-like (i.e., cube-shaped) body. Some species of box jellyfish produce potent venom delivered by contact with their tentacles. Stings from some species, including Chironex fleckeri, Carukia barnesi, Malo kingi, and a few others, are extremely painful and often fatal to humans.
At least 51 species of box jellyfish were known as of 2018. These are grouped into two orders and eight families. A few new species have since been described, and it is likely that additional undescribed species remain.
Cubozoa represents the smallest class with approximately 50 species.
Class Cubzoa
In the center of the underside of the bell is a mobile appendage called the manubrium which somewhat resembles an elephant's trunk. At its tip is the mouth. The interior of the bell is known as the gastrovascular cavity. It is divided by four equidistant septa into a central stomach and four gastric pockets. The eight are located in pairs on either side of the four septa. The margins of the septa bear bundles of small gastric filaments which house and digestive glands and help to subdue Predation. Each septum is extended into a septal funnel that opens onto the oral surface and facilitates the flow of fluid into and out of the animal.
The box jellyfish's nervous system is more developed than that of many other jellyfish. They possess a ring nerve at the base of the bell that coordinates their pulsing movements, a feature found elsewhere only in the crown jellyfish. Whereas some other jellyfish have simple pigment-cup ocelli, box jellyfish are unique in the possession of true eyes, complete with , and lenses. Their eyes are set in clusters at the ends of sensory structures called Rhopalium which are connected to their ring nerve. Each rhopalium contains two image-forming lens eyes. The upper lens eye looks straight up out of the water with a field of view that matches Snell's window. In species such as Tripedalia cystophora, the upper lens eye is used to navigate to their preferred habitats at the edges of mangrove lagoons by observing the direction of the tree canopy. The lower lens eye is primarily used for object avoidance. Research has shown that the minimum visual angle for obstacles avoided by their lower lens eyes matches the half-widths of their receptive fields. Each rhopalium also has two pit eyes on either side of the upper lens eye which likely act as mere light meters, and two slit eyes on either side of the lower lens eye which are likely used to detect vertical movement. In total, the box jellyfish have six eyes on each of their four rhopalia, creating a total of 24 eyes. The rhopalia also feature a heavy crystal-like structure called a statolith, which, due to the flexibility of the rhopalia, keep the eyes oriented vertically regardless of the orientation of the bell.
Box jellyfish also display complex, probably visually-guided behaviors such as obstacle avoidance and fast directional swimming. Research indicates that, owing to the number of rhopalial nerve cells and their overall arrangement, visual processing and integration at least partly happen within the rhopalia of box jellyfish. The complex nervous system supports a relatively advanced sensory system compared to other jellyfish, and box jellyfish have been described as having an active, fish-like behavior.
Depending on species, a fully grown box jellyfish can measure up to along each box side ( in diameter), and the tentacles can grow up to in length. Its weight can reach . However, the thumbnail-sized Irukandji is a box jellyfish, and lethal despite its small size. There are about 15 tentacles on each corner. Each tentacle has about 500,000 , containing Cnidocyte, a harpoon-shaped microscopic mechanism that injects venom into the victim.
Recently, in 2023, a new genus and species of box jellyfish was discovered in the Indo-Pacific region, specifically the Gulf of Thailand. Discovered and named after scientist Lisa-ann Gershwin, this new species of box jellyfish, Gershwinia thailandensis, is a member of the Carukiidae family. Gershwinia thailandensis is described as its own new species as it has sensory structures with specialized horns and lacks a common digestive system among box jelly, the stomach gastric phaecellae. Due to this and other observations, structural and biological, Gershwinia thailandensis was accepted as a new species of box jellyfish.
Understanding the ecological distribution of cubozoans can be difficult work, and some of the costly methods like visual observations, a variety of different nets, light attraction techniques, and most recently the use of drones have had some levels of success in locating and tracking different species of cubozoa, but are limited by both anthropogenic and environmental factors.
A new form of detection, environmental DNA (eDNA), has been developed and employed to help aid in the analysis of the populations of box jellyfish which can be implemented to mitigate the effects that box jellyfish have on coastal anthropogenic activities. This relatively easy and cost-effective method utilizes extra-organismal genetic material that can be found in the water column via shedding throughout the lifespan of an organism.
This process for identifying box jellyfish using the eDNA technique involves collecting a water sample and filtering the sample through a cellulose nitrate membrane filter to extract any genetic material from the water sample. Once the DNA is extracted, it is analyzed for species-specific matches to see if the eDNA sequences sampled correlate with existing DNA sequences for box jellyfish. Given the results, the presence or absence of the box jellyfish can be indicated through the matching of genetic material. If a match is found, then the box jellyfish was present in the area. The utilization of eDNA can provide a cost-effective and efficient way to monitor populations of box jellyfish in both medusa and polyp life stages, to then use the data to help understand more about their ecology and limit the effects on coastal anthropogenic activities.
The majority of box jellyfishes feed by extending their tentacles and accelerating for a short time upwards, then turn upside-down and stop pulsating. Then the jellyfish slowly sinks, until prey finds itself entangled by tentacles. At this point the pedalia fold and bring the prey to the oral opening.
The venom of cubozoans is distinct from that of scyphozoans, and is used to catch prey (small fish and invertebrates, including and bait fish) and for defence from predators, which include the Stromateidae, Ephippidae, rabbitfish, (blue swimmer crab) and various species of turtle including the hawksbill sea turtle and flatback sea turtle. It seems that sea turtles are unaffected by the stings because they seem to relish box jellyfish.
It is previously believed that medusa species only reproduce once in their life before dying a few weeks later, a semelparity lifestyle. Alternatively, in July 2023, the box jelly species Chiropsalmus quadrumanus, were found to potentially have iteroparous reproduction, meaning they reproduce multiple times in their life. Oogenesis appears to happen numerous times as oocytes are discovered in four stages; pre-vitellogenic, early vitellogenic, mid vitellogenic, and late vitellogenic. Continuous research needs to be conducted to determine if box jellyfish are semelparity or iteroparous, or if it is species dependent.
In Australia, fatalities are most often caused by the largest species of this class of jellyfish, Chironex fleckeri, one of the world's most venomous creatures. After severe Chironex fleckeri stings, cardiac arrest can occur quickly, within just two minutes. C. fleckeri has caused at least 79 deaths since the first report in 1883, but even in this species most encounters appear to result only in mild envenoming. While most recent deaths in Australia have been in children, including a 14-year old who died in February 2022, which is linked to their smaller body mass, in February 2021, a 17-year-old boy died about 10 days after being stung while swimming at a beach on Queensland's western Cape York. The previous fatality was in 2007.
At least two deaths in Australia have been attributed to the thumbnail-sized Irukandji box jellyfish. People stung by these may suffer severe physical and psychological symptoms, known as Irukandji syndrome. Nevertheless, most victims do survive, and out of 62 people treated for Irukandji envenomation in Australia in 1996, almost half could be discharged home with few or no symptoms after 6 hours, and only two remained hospitalized approximately a day after they were stung.
Preventative measures in Australia include nets deployed on beaches to keep jellyfish out, and jugs of vinegar placed along swimming beaches to be used for rapid first aid.
In Hawaii, box jellyfish numbers peak approximately seven to ten days after a full moon, when they come near the shore to spawn. Sometimes, the influx is so severe that lifeguards have closed infested beaches, such as Hanauma Bay, until the numbers subside.
The recently discovered and very similar Chironex yamaguchii may be equally dangerous, as it has been implicated in several deaths in Japan. It is unclear which of these species is the one usually involved in fatalities in the Malay Archipelago.
Warning signs and first aid stations have been erected in Thailand following the death of a 5-year-old French boy in August 2014. A woman died in July 2015 after being stung off Ko Pha Ngan, and another at Lamai Beach at Ko Samui on 6 October 2015.
In 1990, a 4-year-old child died after being stung by Chiropsalmus quadrumanus at Galveston Island, Texas, on the Gulf of Mexico. Either this species or Chiropsoides buitendijki is considered the likely perpetrator of two deaths in West Malaysia.
Vinegar is made available on Australian beaches and in other places with venomous jellyfish.
Removal of additional tentacles is usually done with a towel or gloved hand, to prevent secondary stinging. Tentacles can still sting if separated from the bell, or after the creature is dead. Removal of tentacles may cause unfired nematocysts to come into contact with the skin and fire, resulting in a greater degree of envenomation.
Although commonly recommended in folklore and even some papers on sting treatment, there is no scientific evidence that urine, ammonia, Papain, sodium bicarbonate, boric acid, lemon juice, fresh water, steroid hormone, ethanol, ice pack, papaya, or hydrogen peroxide will disable further stinging, and these substances may even hasten the release of venom. Heat packs have been proven for moderate pain relief. The use of pressure immobilization bandages, methylated spirits, or vodka is generally not recommended for use on jelly stings.
In April 2019, a team of researchers at the University of Sydney announced that they had found a possible antidote to Chironex fleckeri venom that would stop pain and skin necrosis if administered within 15 minutes of being stung. The research was the result of work done with CRISPR whole genome editing in which the researchers selectively deactivated skin-cell genes until they were able to identify ATP2B1, a calcium transporting ATPase, as a host factor supporting cytotoxicity. The research showed the therapeutic use of existing drugs targeting cholesterol in mice, although the efficacy of the approach had not been demonstrated in humans.
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