Box jellyfish

Box jellyfish belong to the class Cubozoa. They are invertebrates distinguished by their unique, cube-shaped bodies. Unlike many other jellyfish that simply drift with currents, box jellyfish are active hunters. They can swim at speeds of 1.5 to 2 meters per second. This rapid movement allows them to engage in fish-like behaviors. Some species possess potent venom that can be fatal to humans. For example, species like Chironex fleckeri and Carukia barnesi cause extremely painful stings. Other species, such as Carybdea murrayana, produce painful stings but are not lethal.

The anatomy of a box jellyfish is highly specialized for movement and hunting. The medusa form features a squarish bell. At the rim of this bell is a folded shelf called a velarium. This structure restricts the bell's opening and creates a powerful jet when the jellyfish pulsates. This mechanism enables their impressive swimming speeds. From the four lower corners of the bell, short stalks called pedalia hang down. These stalks hold long, slender, hollow tentacles. In the center of the underside, a mobile appendage called a manubrium hangs near the mouth. The interior of the bell is the gastrovascular cavity. This cavity is divided into a central stomach and four gastric pockets by four equidistant septa.

Box jellyfish possess a sophisticated sensory system. They have a ring nerve at the base of the bell to coordinate pulsing. They are unique among jellyfish for having true eyes with retinas, corneas, and lenses. These eyes are located on sensory structures called rhopalia. Each of the four rhopalia contains six eyes, totaling 24 eyes. Each rhopalium has two image-forming lens eyes. One lens eye looks upward to help with navigation, such as observing tree canopies in mangrove lagoons. The other lens eye helps the animal avoid obstacles. They also have pit eyes for light detection and slit eyes to sense vertical movement. A heavy, crystal-like structure called a statolith keeps the eyes oriented vertically.

Taxonomy and the history of discovery have evolved as scientists learned more. Historically, cubozoans were classified as an order of Scyphozoa. However, in 1973, they were moved into their own class. This change was due to their unique morphology and a biological cycle that lacks strobilation. As of 2018, at least 51 species were known. These are organized into two orders and eight families. In 2023, a new species named Gershwinia thailandensis was discovered in the Gulf of Thailand. It was named after scientist Lisa-ann Gershwin. This species is unique because it has specialized horns and lacks a common digestive system.

These animals are widely distributed across tropical and subtropical oceans. They live between 42° N and 42° S latitude. While many dangerous species stay in the Indo-Pacific, others inhabit the Atlantic and east Pacific. Species have been found as far north as California and Japan. They also live as far south as South Africa and New Zealand. They often reside near shores in habitats like coral reefs, mangroves, and kelp forests. Because they are translucent, they are difficult to detect. Scientists now use environmental DNA, or eDNA, to find them. This method involves filtering water through a cellulose nitrate membrane to find genetic material.

The life cycle and hunting methods of the box jellyfish are quite complex. To hunt, they extend their tentacles and swim upward. Then, they turn upside-down and stop pulsating to sink slowly. Prey, such as small fish or prawns, becomes entangled in the tentacles. Each tentacle can have about 15 tentacles per corner and contains 500,000 cnidocytes. These cells hold nematocysts, which are microscopic, harpoon-shaped mechanisms that inject venom. Researchers use statoliths, made of calcium sulfate hemihydrate, to study their age. These structures grow in daily layers. For example, Chironex fleckeri can reach an inter-pedalia distance of 100 mm by 45 to 50 days old.

Reproduction in Cubozoa also shows fascinating variety. Most species follow an annual life cycle and reach maturity when their bell is 5 mm wide. The order Chirodropida uses external fertilization. In contrast, the order Carybdeida uses internal fertilization. Some species, like Carybdea sivickisi, even perform a "wedding dance" to transfer sperm. While it was once believed they were semelparous—meaning they reproduce only once before dying—new research suggests otherwise. The species Chiropsalmus quadrumanus may be iteroparous, meaning it can reproduce multiple times. This discovery changes how we understand the life history of these incredible marine invertebrates.