An unusual number of sea slugs have been washing up this spring on beaches along the eastern shores of San Francisco Bay in California, reports say. Many people visiting the beaches have been calling police, mistaking the purple creatures for human body parts or other strange discoveries.

The sea slugs -- also known as sea hares -- produce noodle-like long eggs and live for about a year. According to the Metro, slugs can grow up to a length of 15 feet, 30 inches. Even though the creatures looks strange and unpleasant, they are considered to be harmless to humans.

The invasion of the big purple blobs has raised curiosity among naturalists. According to Morgan Dill, a naturalist with the East Bay Regional Park District, sea slugs are often the size of a human organ and can look strikingly similar to one. “There was a population boom about a year ago and what we’re seeing is, after a year, they lay their eggs and they die and we’re seeing them wash up on shore,” Dill said in a statement.

Dill added that by the time sea hares reach a beach, they are dead. However, she confirmed seeing a live slug at a beach recently, as she picked it up to closely look at it. 

According to the naturalists at the East Bay Regional Park District, this is the second biggest unusual die-off of the sea slugs reported in the last 15 years. The researchers are unsure what is causing the problem. However, they think that warmer temperatures in the bay must be a factor.