Box jellyfish (class Cubozoa) are cnidarian animals 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. Some species like the Carybdea murrayana produce a painful sting but are not fatal to humans.
Taxonomy and systematics Historically, cubozoans were classified as an order of Scyphozoa until 1973, when they were put in their own class due to their unique biological cycle (lack of strobilation) and morphology.
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.
Class Cubozoa Order Carybdeida Family Alatinidae Family Carukiidae Family Carybdeidae Family Tamoyidae Family Tripedaliidae Order Chirodropida Family Chirodropidae Family Chiropsalmidae Family Chiropsellidae
Description
The medusa form of a box jellyfish has a squarish, box-like bell, from which its name is derived. From each of the four lower corners of this hangs a short pedalium or stalk which bears one or more long, slender, hollow tentacles. The rim of the bell is folded inwards to form a shelf known as a velarium which restricts the bell's aperture and creates a powerful jet when the bell pulsates. As a result, box jellyfish can move more rapidly than other jellyfish; speeds of up to 1.5 metres per second have been recorded.
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 gonads are located in pairs on either side of the four septa. The margins of the septa bear bundles of small gastric filaments which house nematocysts and digestive glands and help to subdue prey. 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 retinas, corneas and lenses. Their eyes are set in clusters at the ends of sensory structures called rhopalia 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 cnidocytes, containing nematocysts, a harpoon-shaped microscopic mechanism that injects venom into the victim. Many different kinds of nematocysts are found in cubozoans.
Distribution
Although the notoriously dangerous species of box jellyfish, such as Chironex species, are largely restricted to the tropical Indo-Pacific region, various species of box jellyfish can be found widely in tropical and subtropical oceans (between 42° N and 42 °S), including the Atlantic Ocean and the east Pacific Ocean, with species as far north as California (Carybdea confusa), the Mediterranean Sea (Carybdea marsupialis) and Japan (such as Chironex yamaguchii), and as far south as South Africa (such as Carybdea branchi) and New Zealand (such as Copula sivickisi). It was only in 2014, that the first ever sightings of Tripedalia cystophora were officially published in Australia, Thailand and the Indian Ocean. There are three known species in Hawaiian waters, all from the genus Carybdea: C. alata, C. rastoni, and C. sivickisi. Within these tropical and subtropical environments, box jellyfish tend to reside closer to shore. They have been spotted in near-shore habitats such as mangroves, coral reefs, kelp forests, and sandy beaches.
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.
Detection
Cubozoans are widely distributed throughout tropical and subtropical regions, yet the detection of these organisms can be quite difficult and costly due to a high amount of variation in their occurrence and abundance, their translucent body, two different life stages (medusa and polyp), and vast amounts of size variability within the different species in the class Cubozoa.
Understanding the ecological distribution of cubozoans can be difficult work, and some of the costly methods like visual observations, a variety of 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.
Ecology Age and growth It has been found that the statoliths, which are composed of calcium sulfate hemihydrate, exhibit clear sequential incremental layers, thought to be laid down on a daily basis. This has enabled researchers to estimate growth rates, ages, and age to maturity. Chironex fleckeri, for example, increases its inter-pedalia distance (IPD) by per day, reaching an IPD of when 45 to 50 days old. The maximum age of any individual examined was 88 days by which time it had grown to an IPD of . In the wild, the box jellyfish will live up to 3 months, but can survive up to seven or eight months in a science lab tank.
Behavior The box jellyfish actively hunts its prey (small fish), rather than drifting as do true jellyfish. They are strong swimmers, capable of achieving speeds of up to 1.5 to 2 metres per second or about . and rapidly turning up to 180° in a few bell contractions. Some species are capable of avoiding obstacles.
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 prawns and bait fish) and for defence from predators, which include the butterfish, batfish, rabbitfish, crabs (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.
Reproduction
Cubozoans usually have an annual life cycle. Box jellyfish reach sexual maturity when their bell diameter reaches 5 millimeters. Chirodropida reproduces by external fertilization. Carybdeida instead reproduces by internal fertilization and is ovoviviparous; sperm is transferred by spermatozeugmata, a type of spermatophore. Hours after the fertilization, the female releases an embryo strand that contains its own nematocytes; both euryteles and isorhizas. Cubozoas are the only class of cnidarian that contains species that perform the "wedding dance" to transfer the spermatophores from the male into the females, including the Carybdea sivickisi species.
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.
Genetics