The Live Science blog reports that whale watchers in Japan were recently treated to a rare wildlife encounter when they saw, not one, but two white orcas swimming side by side.

The pair and their pod were spotted by a group on a Gojiraiwa Kanko Whale Watching boat off the coast of Rausu on the northernmost of Japan’s main islands called Hokkaido on July 24, according to a Facebook post.

The white coloration of orcas, also known as killer whales, can be the result of either albinism or leucism.

“Albinism is caused by defects of melanin production and equally affects the whole skin, fur and eyes,” Erich Hoyt, a research fellow at Whale and Dolphin Conservation (WDC) in the U.K., told Live Science. “Leucism results from defects in pigment cells and can be patchy.”

Albinism in orcas is the result of inbreeding, but there is no clear evidence to suggest that this negatively impacts the killer whales in any other way, Hoyt said. Leucism is the result of a random genetic mutation and also doesn’t seem to impact the orcas’ health, according to WDC.

“Roughly 1 in 1,000 orcas in the western North Pacific [are white],” Hoyt said. “That is probably the highest ratio anywhere in the world.” In 2016, Hoyt and others published a study in the journal Aquatic Mammals highlighting the unusual abundance of white orcas in the region.