Drifting with the ocean currents, these creatures are often misunderstood. We talk to scientists at The Deep aquarium in Hull and uncover why we shouldn’t fear them
We talk to jellyfish scientists at The Deep aquarium in Hull and uncover why we should marvel at – and not fear – these strange aquatic animals
With no brain, blood, heart or bones, a jellyfish’s body is made up of 95 per cent water. So how do they function? This jelly-like, slimy body is actually an excellent conductor for nervous energy. With many lacking even primitive eyes, they have a complex net of nerves throughout their bodies and can sense changes within their environment in order to navigate the seas.
From polyp to medusa, jellyfish undergo a complex lifecycle. Starting life as an anemonelike stalked polyp, they attach themselves to rocks, extending their tentacles to capture plankton passing in the current. When the conditions are right these polyps reproduce asexually, releasing tiny jellyfish called ephyrae, which bud off singularly or in long chains into the open ocean. At only two millimetres (0.07 inches) wide, they form part of the planktonic soup, growing rapidly and taking advantage of having prey within a tentacle’s reach.
Once they’ve developed into an adult medusa, jellyfish reproduce sexually – so a jellyfish is born either a male or female. The male releases sperm into the water to fertilise the female’s eggs. Once fertilised, they undergo the embryonic development typical of all animals and hatch as free-swimming planula larvae. The oval-shaped larvae drift through the ocean for a couple of days before sinking to the seabed, where they settle and root themselves to a rock before transforming into a polyp ready to begin the cycle again.
With a continuous breeding cycle and having to rely mostly on the ocean currents to move through the seas, it’s no wonder jellyfish are able to form such infamous and spectacular blooms. However, as humans continue to populate the world’s coastlines, our presence has resulted in a rise in the occurrence of these blooms, often with negative results. Human contributions to marine pollution and rising sea temperatures are generating an increasing influx of plankton blooms, and where there is more food, there are more jellyfish. However, jellyfish have been forming blooms in the ocean for over 500 million years, so we must not lose sight of their necessity within the ocean ecosystem.
Jellyfish, hydrozoans, siphonophore and ctenophores form an important part of the ocean food chain, maintaining an everfluctuating balance by consuming algae and zooplankton. They are also prey, not only on the menu for larger jellyfish species but a crucial meal for the survival of many sea turtles, sunfish and spadefish. They also prove to be a tasty treat for some crabs and other large crustaceans, and in some parts of the world, humans too.
Many young fish and small crabs use the stinging tentacles of the jellyfish to their advantage. Unaffected by their potent venom, they are able to hide among the flowing appendages, protecting themselves from predators until they are big enough to
“Marine pollution and rising sea temperatures are generating blooms”
defend themselves. Rather selfishly, they also take a share of the jellyfish’s food, forming a commensal relationship rather than a symbiotic one, meaning that while one animal derives a benefit from the arrangement (in this case the creatures hiding in the jellyfish’s tentacles), the other is neither helped nor hindered by it.
Scientists are working to further understand both the benefits and impacts of jellyfish blooms on our ocean ecosystems, but with so much ocean and miles of coastline to study, more eyes are needed. Beachgoers, divers and fisherman – to name a few – can all help provide the vital data needed to successfully map sightings and track the environmental changes that may be triggering them.
Jellyfish are also the staple diet of the leatherback turtle, which are threatened throughout their range. Seasonal visitors to the UK, research has revealed that these giant reptiles feed on several species of UK jellyfish. By comparing the distribution of jellyfish with factors such as sea temperature, plankton production and water currents, scientists hope to gain a greater understanding of what influences the seasonal distribution of jellyfish in UK waters and the important relationships they form within the ocean ecosystem.
Report your jellyfish sightings
Spotted a jellyfish? We want to know! Your reports help scientists monitor the state of jellyfish and their blooming events throughout the UK.
Jellyfish hitchhiker: A rock crab (Metacarcinus gracilis) hitching a ride on the oral arms of a Pacific sea nettle(Crysaora fuscesens)
Feeding time: Using powerfulstinging nematocysts, the jellyfish paralyses its prey beforeconsuming it