Holey Rocks

PEI’s shores are full of interesting finds, including holey rocks like this one. At first glance, you may think this stone was shaped by waves and tides, but the creators of these holes are biological, not physical.

Rocks like this are sometimes called witches’ stones, hag stones, or adder stones, although those terms are more commonly applied only if one or more of the holes passes all the way through. According to legend, witches’ stones bring good luck by letting only good fortune pass through the hole to reach the finder, and allowing the owner to see through disguises, recognize bad intentions, and avoid evil spirits and spells.

It's not uncommon to find holey rocks around the Island’s coast, and we have several marine animals that make them. For example, Piddock Clams (aka Angelwings, Pholadidae Family) are bivalves that spend most of their lives within the protection of soft rocks. Free-swimming Piddock Clam larvae find a suitable rock (such as PEI sandstone) and begin the process of burrowing into it. Piddock larvae can detect the presence of other Piddock Clams, and so it’s common to find rocks riddled with holes.

As it grows, a Piddock Clam makes its hole wider and deeper, though the entrance size doesn’t change, meaning it can never leave. From within the safety of this hole, the Piddock sends its siphon into the surrounding water and filters out bits of organic matter.  Piddock Clams can live up to 20 years, and their holes tend to be larger than those seen here.

I suspect these holes are the work of Rock Borers (Hiatella arctica), rock-dwelling bivalves that are smaller than Piddock Clams. (Mature Piddocks can be as much as 15 cm [6 inches] long whereas Rock Borers are usually in the range of 4 cm [1.5 inches]). In addition to being smaller, Rock Borer holes are often more irregularly-shaped. If you look closely, you’ll see holes in this rock vary from round to oval, rectangular, and almost triangular.

Bivalves aren’t the only marine animals that create holes in rocks; marine sponges such as the Sulphur Boring Sponge (Cliona celata) can too! Unlike Piddock Clams and Rock Borers that physically scrape away at the rock, Boring Sponges secrete acid to dissolve holes and make a home. The tell-tales holes of Sulfur Boring Sponges are much smaller than those of Piddocks or Rock Borers – just 5 mm (about 1/5 of an inch) in diameter – and can be found in shells as well as rocks.

Rock and shell borers make several important contributions to their marine environments, including releasing nutrients locked up in those substrates and making them available to other organisms. Borers also increase the surface area and complexity of rock surfaces, creating habitats and attachment points for marine plants, animals, and algae.

Holey rocks may or may not bring us luck, but they do benefit the creatures that create and use them. They also contribute to the marine diversity that is part of PEI Untamed!