What Made These Holes?

Pitted shell found at Chesapeake Bay, Virginia Beach, 28 Nov 2019 (photo by Kate St. John)

Last month I found this pitted shell at Chesapeake Bay in Virginia Beach and assumed the holes were made by sand and waves. But that can’t be true. If it was, most shells would look like this. So what happened here?

A Google search of shells with similar holes revealed the likely cause: a boring sponge.

Boring sponges make their homes by boring holes into the calcium carbonate shells and skeletons of animals like scallops, oysters and corals. Using chemicals, they etch into the shell and then mechanically wash away the tiny shell chips, slowly spreading holes within the skeleton or shell and sometimes across its surface. Eventually, these holes and tunnels can kill their host, but the sponge will continue to live there until the entire shell has eroded away.

from Smithsonian Magazine article about boring sponges: Drill Baby Drill

Cliona celata is a common boring sponge that lives on oyster reefs in Chesapeake Bay and around the world. It’s considered a major pest by Bay oyster harvesters. Here’s what it looks like underwater and in two closeups.

Underwater view of Cliona celata in France (photo from Wikimedia Commons)
Boring sponge, Choptank River watershed, Eastern Shore of Maryland (photo from Wikimedia Commons)
Closeup of boring sponge, Choptank River watershed, Maryland (photo from Wikimedia Commons)

Interestingly, a 2013 study of scallops and boring sponges found that the sponges thrive in the warmer more acidic seawater that results from climate change. This spells bad news for oysters, corals and other shells.

In the future we’ll find more shells like this.

Shells on beach at Cayo Costa Island, FL, including one pitted by boring sponge (photo from Wikimedia Commons)

(first photo by Kate St. John, remaining photos from Wikimedia Commons; click on the captions to see the originals)

2 thoughts on “What Made These Holes?

  1. Thank you! I’ve been going down to the bay in VA my whole life. I have always seen shells like that. Now I know!

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