We've been experiencing a marine heatwave recently in Cornwall, affecting our marine life.
Reports and footage have shown the world’s largest turtle, the leatherback, spotted in St Austell Bay. Hordes of jellyfish have appeared across the coast, with hundreds of barrel jellies washing up on Cornish beaches. And we’re witnessing an incredible bloom in octopus numbers, the largest influx seen in Cornish waters in decades. The last such event on England’s south coast occurred in the summer of 1948, and before that, a ‘plague’ of octopuses was recorded in 1899.
These so-called octopus blooms are natural but rare events, triggered when conditions favour the survival of octopus larvae.
The common octopus (Octopus vulgaris) has a short but dramatic life cycle, typically living around two years. In their second year, they devote all their energy to reproduction. After mating, females retreat to lay thousands of tiny eggs, guarding them devotedly without feeding until they hatch.
Once hatched, the larvae, at just a few millimetres long, drift freely in ocean currents, vulnerable to predators and the elements. Most don’t survive, but when sea conditions align, with warmer temperatures, stable currents, and abundant food, more larvae survive, leading to a population bloom.
As they grow, common octopuses become formidable and efficient predators. They feed on crustaceans such as crabs, prawns, and shrimps, as well as bivalve molluscs like mussels and scallops. Their soft, muscular bodies allow them to squeeze into tight spaces and grow quickly, eventually reaching an arm span of over a metre (up to around 1.2 m) and weighing up to 10 kg.
Octopuses are remarkable creatures, alien looking but perfectly adapted to their environment. Each of their eight arms operates independently; scientists have shown each arm processes sensory information before sending it to the central brain, effectively allowing each leg to “think” for itself.
They have excellent eyesight, with eyes structurally similar to ours but even more refined, and can change colour and skin texture instantly thanks to a complex system of pigment cells (chromatophores) and muscles that create smooth, bumpy, or spiky camouflage.
Inside, their adaptations are equally fascinating. Octopuses have three hearts and blue, copper-based blood containing hemocyanin, which helps transport oxygen efficiently in cold or low-oxygen waters, an important adaptation in a changing environment.
This year's exceptional numbers may be linked to the marine heatwave. Warmer sea temperatures, especially during winter, can increase larval survival, while changing prey populations and ecosystems also play a role. We’ve also seen increasing numbers of spider crabs around our coast, another species benefiting from warmer waters.
While blooms like this are spectacular to witness, the changes in our marine ecosystems that they signal require careful monitoring so that we can understand the impacts. As the climate warms, such events may become more frequent.
If you’d like to learn more about our marine conservation work and how you can help, find more information here.
Snorkellers and divers across Cornwall have been heading to the water to capture some of this incredible marine life in action. Check out our social media for more footage and check out Seasearch to see how you can get more involved.