Image of the Day: On the Rocks

Otters leave distinct patterns of wear on rocks they use for pounding mussels.

Mar 15, 2019
Carolyn Wilke
Natalie Uomini. Haslam et al. 2019., Wild sea otter mussel pounding leaves archaeological traces,.Scientific Reports

Sea otters leave their mark on shoreline rocks they use to crack open shells, researchers reported yesterday (March 14) in Scientific Reports. They are the only marine mammals known to use stones as tools, which help them break the shells of mussels and other mollusks. Sea otters use rocks to get to their food while floating along on their backs, but also bash mollusks on fixed “anvil” stones along the shore.

In 10 years of observing wild sea otters (Enhydra lutris) at a California site, the authors noticed a distinct pattern to the damage the animals left on anvil stones. The authors also observed that mussel shells at these sites were generally cracked in the same way, with a diagonal break on the same side of the shell. These two clues may help scientists find other sites otters once frequented, according to the authors. 

M. Haslam et al., “Wild sea otter mussel pounding leaves archaeological traces,” Scientific Reports, doi:10.1038/s41598-019-39902-y, 2019.

Mussel shell breakage patterns: (A) Outer and (B) inner faces of each valve; (C) drawing of the exterior of a mussel shell showing the typical sea otter breakage pattern (illustration by Neil Smith); (D) broken mussel shells in situ
Image (C): Neil Smith; Images (A), (B), & (D): Michael Haslam. Haslam et al. 2019. Wild sea otter mussel pounding leaves archaeological traces. Scientific Reports