Jellyfish

Jellyfish are members of the phylum Cnidaria and are among the simplest multicellular animals. Their bodies are approximately 95% water, with no brain, heart, bones or blood. They have a basic nervous system — a nerve net — that allows them to respond to light, gravity and chemical signals. The bell-shaped body contracts rhythmically to provide gentle propulsion, though jellyfish are largely at the mercy of ocean currents. Their tentacles are armed with specialised stinging cells called nematocysts that fire harpoon-like structures to immobilise prey. Jellyfish feed on plankton, small fish and other jellyfish.

The jellyfish life cycle is one of the most complex in the animal kingdom, alternating between sexual and asexual reproduction. Adult medusae (the familiar bell-shaped form) release eggs and sperm into the water. Fertilised eggs develop into free-swimming larvae called planulae, which settle on the seafloor and transform into polyps. Polyps reproduce asexually, budding off tiny juvenile jellyfish called ephyrae that grow into adult medusae. One species — Turritopsis dohrnii, the 'immortal jellyfish' — can revert from its adult medusa stage back to a polyp when stressed, theoretically allowing it to cycle through its life indefinitely.

Many jellyfish species produce their own light through bioluminescence — a chemical reaction that produces a cold, blue-green glow. This light serves various functions: attracting prey, deterring predators, and communication. The crystal jellyfish (Aequorea victoria) produces a green fluorescent protein (GFP) that has become one of the most important tools in molecular biology and medicine, allowing scientists to tag and visualise proteins within living cells. The discovery and development of GFP from jellyfish earned the 2008 Nobel Prize in Chemistry.

Jellyfish populations are increasing in many parts of the world, with blooms becoming more frequent, larger and longer-lasting. This is linked to a combination of factors: overfishing of jellyfish predators and competitors, ocean warming, eutrophication (nutrient pollution) and the creation of low-oxygen dead zones where jellyfish thrive but fish cannot survive. Jellyfish blooms can have significant economic impacts, clogging fishing nets, blocking cooling water intakes at power stations and desalination plants, and affecting tourism. Understanding and managing jellyfish blooms is an increasingly important area of marine science.