Plankton sign on chest ultrasound?

Q: What is the plankton sign on chest and abdominal ultrasound?

Answer: The plankton sign, while performing ultrasound, refers to swirling, punctiform internal echoes within an otherwise anechoic pleural effusion. It appears slow and whirling, which may be buffered by cardiac and respiratory movements. It is highly suggestive of debris within effusion or even hemothorax, hemoperitoneum, or hemorrhagic ascites. 

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Reference:

Lichtenstein DA, Mezière GA. Relevance of lung ultrasound in the diagnosis of acute respiratory failure: the BLUE protocol. (2008) Chest. 134 (1): 117-25. doi:10.1378/chest.07-2800