A giant nomad jellyfish (Rhopilema nomadica), photographed while snorkeling off the coast of Mafia Island. Although, quite captivating to see in the water, be sure to steer clear of these guys. They can cause very painful injuries to humans, since they have vermicular filaments which are covered in venomous stinging cells in the mouth arms. These jellies can grow up to 10 kg (22 lbs) of weight, and its bell is commonly 40-60 cm (16″-24″) in diameter, but can be up to 90 cm (35.4″). The European Union lists them as one of the worst invasive marine species as they have moved up the Suez Canal into the Mediterranean and are now causing problems as far west as Malta. Make sure to steer clear as they can cause very painful injuries to humans.
Usually Nomad jellyfish or jellies as they are more correctly called come inshore in large numbers on the Tanzanian coast every November and December. In October 2016 large numbers moved into the coastal waters. They were seen in heavy densities as far out as Latham Island which is 34 nautical miles from the mainland.
The image below was taken from around 500 feet in height and gives one an idea of the numbers that arrived. Each white dot is a Nomad jellyfish. Coupling was observed between some of the jellyfish and this can be seen in the image below. You can learn more about Jellyfish here.
Below is some footage taken of the Nomad Jellyfish