The sight of thousands of sand dollar sea urchins washed up on a beach in the US state of Oregon has left experts puzzled.
The live sand dollars washed ashore on the south end of Seaside Beach, and as a result, are drying up and dying.
In a Facebook post, Seaside Aquarium in Oregon said experts were not sure why this mass die-off was happening.
"At this time, we do not know what has caused this, and these types of incidents usually have several contributing factors."
"We are also unaware if this is an isolated incident or if this is happing on other beaches."
What are sand dollars?
Sand dollars are related to sea urchins.
"The outside of their shell is covered with millions of tiny spines which look like 'fuzz' or hair," the Seaside Aquarium's Facebook post said.
On one side of the sand dollar, you can see their mouth and five small teeth, which they use to eat plankton.
"Each tooth closely resembles the shape of a bird, and many people refer to them as 'doves'," Seaside Aquarium said.
"Sand dollars are found worldwide and there are many different species, each with their own unique characteristics."
How can you tell if a sand dollar is still alive?
The best indication is if the sand dollar is still 'fuzzy'.
But it is advised to leave the fuzzy ones on the beach if you ever come across them, as they can smell quite badly.
