When going on a diving trip, we can enjoy a unique and wonderful experience. Naturally, we need to consider a series of recommendations to ensure the dive goes perfectly. Even so, we might occasionally encounter some issues, but simply paying attention will ensure everything turns out fine.
What is a jellyfish?
Cnidarian comes from the Greek word 'knidé', meaning nettle. Their main characteristic is possessing stinging cells called cnidocytes or nematocysts.
The word jellyfish originates from one of the three Gorgon sisters, Medusa, a sea divinity from Greek mythology who had snakes instead of hair, resembling the form of these curious creatures.
What are they like?
Their body, almost always transparent, consists of 95% water, giving them perfect camouflage. They have three main parts: the bell, oral arms (surrounding the mouth) and stinging tentacles. They are radially symmetrical animals. They have an internal cavity where digestion occurs, called the gastrovascular cavity, which has a single opening serving as both mouth and anus. Jellyfish are composed of two tissue layers, the ectoderm or outer layer and the endoderm or inner layer, separated by a connective tissue layer called the mesoglea.
They possess a primitive nervous system, with nerve cells grouped into two rings that control rhythmic pulsation. On the margin of the bell, there are sensory organs: ocelli (which allow them to detect different light intensities) and statocysts (very simple balance organs).
Classification
Cnidarians can exhibit two different morphologies:
- The polyp form, a sessile or fixed form on the substrate, with a tubular body and tentacles and mouth facing upwards.
- The jellyfish form, free-living with tentacles and mouth facing downwards.
There are over 9,000 known species of cnidarians, grouped into 4 classes: polyp and jellyfish phases, true jellyfish, class Cubozoa and class Anthozoa.
Where do jellyfish live?
With the exception of hydras and other freshwater hydrozoans, cnidarians are marine creatures. They inhabit everything from tropical seas to the cold waters of the Arctic.
They have been traversing the seas for over 650 million years. Jellyfish, also called sea nettles, are pelagic animals (of the open sea) and part of the zooplankton, as although they can propel themselves through rhythmic movements of their bell, they mainly drift with ocean currents.
Reproduction
Jellyfish have separate sexes, meaning there are male jellyfish and female jellyfish. To reproduce, males and females release sperm and eggs into the water (sexual reproduction). After fertilisation, larvae (planulae) develop, which will either become new jellyfish or settle and form polyps. From these benthic polyps, through asexual reproduction, new free-living jellyfish may develop.
The largest jellyfish...
The lion's mane jellyfish (Cyanea capillata) has tentacles that can grow longer than a blue whale. They're found in both temperate and Arctic waters.
Voracious predators
Jellyfish are carnivorous and can grow quickly, forming large numbers when food is abundant. But if food becomes scarce, they can shrink again.
They feed mainly on zooplankton, small crustaceans like krill or copepods, though small fish or other jellyfish also form part of their diet. It's quite curious to see their latest catch through their translucent body before digestion.
Their tentacles, equipped with stinging cells, serve as both defence and a powerful weapon for capturing prey. Upon contact with victims, the nematocysts in the tentacles fire the harpoon-like filament they contain coiled inside, releasing a toxic, stinging substance that paralyses prey. The oral arms assist in capturing and ingesting food.
Precautions
It's not advisable to swim at beaches when jellyfish presence is warned. Even walking along the shore can be dangerous, as tentacle remnants may remain in the sand. Jellyfish can still sting hours after death.
What to do if stung by a jellyfish?
Never use fresh water to wash the affected area, as the change in salinity can cause any remaining stinging cells on the skin to rupture and release more venom. It's best to apply cold to the sting using ice cubes, but always in a plastic bag, never directly on skin.
Any attached tentacles can be removed with tweezers, but never with bare fingers. If complications arise, such as breathing difficulties or heart rhythm abnormalities, it's necessary to seek medical attention for proper treatment.
