In moderate to large pleural effusions, it’s not uncommon to see lung atelectasis on the ultrasound. This collapsed lung within the effusion moves with respiration and appears like a jellyfish.

Important things to note:

Jellyfish sign favors atelectasis over pneumonia as the collapsed lung can change its shape with respiration whereas pneumonia is more firm.

It also favors transudative etiology of the effusion as the viscosity of exudative effusions tend to hamper mobility of the lung tissue.

Here are more examples