San Diego ? Scientists say a rare species of dark purple jellyfish is showing up in San Diego Bay and washing ashore on beaches.

Dr. Nigella Hillgarth of the Scripps Institution of Oceanography said Tuesday the Birch Aquarium has four of the jellies for display.

Hillgarth says the black sea nettle has turned up in coastal waters more frequently in recent years. Oceanographers don’t know why, but think it may be due to warmer oceans or changes in the plankton populations the jellyfish eat.