Marine Creature of the Month November

Box Jellyfish

Photo from Nat Geo

Kingdom: Animalia

Class: Cuboza

Conservation Status: Not Evaluated (NE)

Found: Indo-Pacific ocean (dangerous species); Atlantic ocean, East Pacific ocean

Diet: Small fish

Weight: Up to2kg (4.4lbs)

Size: Up to: Side of bell: 20cm (7.9in); Bell diameter: 30cm (12in); Tentacles: 3m (9.8ft)

Habitat: Tropical and subtropical waters.

Box jellyfish are the most venomous creature in the sea. In the world, even. Well, three of the fifty-one species are the most venomous creatures. So they have a bit of a bad rap. Their venom can be deadly to humans but they are also fascinating and beautiful creatures who developed that venom for their own, non-human purposes. The three venomous species are: Chironex flickeri, the largest of the box jellyfish; Carukia barnesi and, Malo kingi. And they are venomous not poisonous because their toxins are transferred through sting not consumption. 

So let's start with the name. They are so named box jellyfish for the box-like shape of their jelly dome or bell, called a medusa. They have four lower corners along the rim of their dome from which a short pendalium hangs. From here an average of fifteen tentacles hang from each corner. The rim of their medusa fold inwards which allows the box jellyfish to propel itself along by creating a powerful jet of water as the bell pulsates. They can travel 6m (20ft) per minute which, for a jellyfish, is fast. This does mean they are not classed as "true jellyfish" as they do not travel through drifting. There are a few more biological differences which mean box jellyfish are not true jellyfish.

This includes having "true eyes". Most jellyfish, box included, have simple eyes called ocelli. These eyes do not form images but are able to distinguish light and dark. Box jellyfish have twenty of these ocelli eyes. But their true eyes, the rhopalia, are unique to box jellyfish. They are set in two clusters halfway up the outer surface of the bell. One of these eyes point upwards while the other points down and inwards. They allow the box jellyfish to see specific points of light. This means they demonstrate behaviour to suggest they can actually see - such as avoiding objects.

As mentioned before, box jellyfish propel themselves through the water. They actively hunt their small fish prey this way and use their tentacles to sting and trap them. Their mouth is at the tip of an appendage in the centre of their underside. The appendage somewhat resembles an elephant trunk! The interior of their bell is called a gastrovascular cavity. It is basically their stomach, which is divided into four gastro pocket and a central stomach. The segments are called septa, and each one has a pair of gonads located on them. The margins also contain gastric filaments which themselves are made of nematocysts and digestive glands. Nematocysts are cells with a secretory organelle. All of these help to subdue and presumably digest their prey. Part of this system are the septa funnels, located on the septa (funnily enough), which allow for fluids to flow in and out of the jellyfish.

When am I going to get to the good bit? The tentacles! Well, now. Of their maximum 3.1m (10ft) length, nine tenths of that is tentacle. The venom in their stings is used to catch prey, such as small fish and prawns, and to deter their predators, like butterfish, rabbitfish and sea turtles. Sea turtles do not seem affected by them, as box jellyfish are one of their favourite foods. The stings are activated by chemicals on the skin, so if you ever were to swim with box jellyfish cover your skin. Neoprene wetsuits work just fine. If their tentacles are removed from the bell, they can still sting so always be careful! Ultimately, I am not encouraging anyone to swim with box jellyfish. The ocean is theirs, respect that. 

They do not have long life spans, usually around a year. Their age and maturity can be measure and estimated by the layers in a calcareous particle called a statolith. These create layers of calcium sulphate hemihydrate and lay them daily. Scientists can use these layers to measure growth rate and postulate how old the creatures are and how long it takes them to reach maturity.

Diagram of a box jellyfish. From Alert Diver

Fun Facts!

• As of 2018, there are at least 51 species of box jellyfish.
• They are separated into two orders and eight families.
• Box jellyfish have the most developed nervous system of any jellyfish.
• They have twenty-four eyes in total, true and simple.
• They orient themselves using the statoliths which detect gravitational pull.
• Not all species are venomous.