Boring Clam

Boring Clams (Penitella penita one of a half dozen different species at Cabrillo)

Contributed by TPERP Don Vaughn

Where to find them: When alive these molluscs occur from the mid-intertidal to 22 m subtidal depths. We commonly find their valves/shells (their remains) in cobbles and boulders on the beach in our rocky intertidal. They range from Prince William Sound, Alaska to Baja California.

What do they eat: They are filter feeders, also called suspension feeders. They take in seawater through their inhalant siphon and filter the organic materials for food, primarily phytoplankton.

Who eats them: Crabs, the Leafy Hornmouth (a gastropod predator), human beings, anything that can pluck them from their boring when exposed, e.g. some birds.

Adaptations: A specially modified shell and lifestyle that allows them to grind out a hole in the rock to live out their lives. The muscles move the shells independently to create a drilling action. The boring activity is entirely mechanical. They sense the boring activities of their neighbors and will change their boring direction or shape to avoid intersecting them.

Reproduction: “Broadcast spawning.” Separate sexes release vast quantities of sperm and eggs into the seawater. Like other molluscs they go through several free-floating larval stages. The last stage, the veliger, settles on an appropriate rock and starts the boring.

What’s their life like: After settling onto a rock they bore out a hole with their specialized shell in which they may live from 20 to 50 years. They alternate periods of boring and growth. When they stop boring entirely for physical or physiological reasons they become reproductively mature. They resorb their foot at that time and the shell becomes occupied with gonads. They can grow to sizes of 9.5 cm by 5 cm.

Interesting facts: Their rock grinding is the principal cause of erosion in some places in the marine environment! This “bio-erosion” can destroy rock outcrops over time. The study of clams is called “malacology.”

Phylum: Mollusca
Class: Bivalvia
Subclass: Heterodonta
Order: Myoida
Superfamily: Pholadoidea
Family: Pholadidae
Genus: Penitella
Species: Penitella penita

Boring Clams

A Not So Boring Clam—The Common Piddock

Penitella penita

 Don Vaughn VIP TPERP Summer 2013


How many times have you been asked on a busy tide day, “Why are there holes in this rock?” If you haven’t you will be! Some of these rocks are so perforated that they look like some volcanic rocks, full of holes due to the escape of hot gases from when the rock cooled as lava. But our holed rocks are not volcanic. A class of clams commonly called boring clams (Pholadidae) makes many of the holes in these common cobble size rocks found within our intertidal. Let’s explore this interesting creature.

First let’s talk about boring as a way of life. Many marine creatures bore into something to live part of their lives. (We will make a distinction here used by some researchers, between “borers” and “burrowers.” We will consider burrowers as creatures that bury themselves or feed in soft sediment. Borers physically create their borings in harder materials by mechanical or chemical means, sometimes both.)

If you think about it, it makes perfect sense to bore into something and live there relatively safe from your enemies, something that wants to eat you. Most boring organisms are invertebrates. These include molluscs (bivalves including clams and mussels), polychaete worms, siphunculid worms, isopod crustaceans (the gribble), and sponges, et al. They bore the holes as their home. Borers can penetrate different types of materials, too: wood, clay, coral, soft and hard rock, concrete, other organism’s shells, etc. Boring clams can be found in all of these materials.

In some parts of the ocean the activities of boring organisms are the main source of erosion! Warme, Scanland and Marshall published a paper in Science in 1971 wherein they described the effects of this “bioerosion” from rock samples removed from Scripps Submarine Canyon in La Jolla. They concluded that intense boring by invertebrate organisms is a greater cause of erosion in parts of the canyon than physical or chemical weathering. Chemical weathering of some hard rocks in water only extends to a depth of 1 cm into the rock, whereas borings from various organisms can penetrate several decimeters into the same rock. These organisms’ boring “…activities culminates in a network of passageways and eventual disintegration of the rocks.” They estimated an average attrition rate of rocks attacked by borers as 2 to 10 mm per year.

Many organisms make subsequent use of borings and tunnels created by borers. We refer to them as “nestlers.” These include other molluscs, sponges, bryozoans (moss animals), amphineurans (chitons), gastropods, crustaceans, ophiuroids (brittle stars), jellyfish, barnacles, algae, small fish and other creatures. Some worms even fill the borings with sediment then live in the sediment. Without the borings this community of organisms would not exist. The borers therefore are an important part of the marine ecosystem, providing housing and shelter not just for themselves but for other creatures as well.

Now let’s talk about boring clams. There are many kinds of boring clams and even boring mussels (e.g. the date mussel Lithophaga plumula). Some bore exclusively in wood, like Teredo or Bankia (referred to colloquially and erroneously as “shipworms”) and some bore into stiff clay. Others bore into rock or concrete and some bore into the shells of other marine invertebrates. The economic impact of their boring activities, especially the wood boring kind, is considerable. These clams destroy wood pilings and wharves costing millions every year. Even concrete is not immune to some species. Studies performed in Los Angeles Harbor found densities of 7 to 8 borers per square foot on concrete pier pilings!

Rock boring clams are generally referred to as the “Pholadidae” or “pholadids.” , or sometimes simply Pholads. There are at least 18 genera. One rock-boring clam whose former homes we find in cobbles scattered on our tidepool beaches at Cabrillo is the “Common Piddock,” also referred to as the “Flat Tip Piddock.” It is primarily a rock-boring clam though it can bore into wood, shell and concrete as well. Its scientific name is Penitella penita: Here is its taxonomy:

KINGDOM: Animalia

PHYLUM: Mollusca

CLASS: Bivalvia

SUBCLASS: Heterodonta

ORDER: Myoida

SUPERFAMILY: Pholadoidea

FAMILY: Pholadidae

GENUS: Penitella

SPECIES:Penitella penita

Penitella penita is the commonest and best known of all Pacific coast pholadids. It is widely distributed geographically and can be found on the west coast from Prince William Sound in Alaska to Baja California. Due mostly to the variety of substrates (materials) it can and will inhabit, it is extremely variable in form. It will bore into clay, shale, mudstone, sandstone (all in various stages of induration), though only rarely into other shells (e.g. Haliotis rufescens, the red abalone). It can be found in the fossil record as far back as the Pliocene (5.3 million years ago) and was particularly abundant in the Pleistocene.

Boring Clams
A robust boring clam shell, left and right valves.


They are found as living organisms within the intertidal at depths of approximately 2 feet, to the subtidal at depths of 60 feet. Where they are found at shallow depths however, it is primarily in quiet water. They will be difficult to see as a living organism because they have bored inside the rock. The part of the organism that is visible at low tides is their soft-tissue siphon, which protrudes from the rock and filters water in and out for their respiration and food. Penitella penita has a compound siphon, meaning the exhalant and inhalant siphons are joined as one structure.

Boring Clams
The exposed siphon of a boring clam. The rest of the creature is in the rock.

Penitella penita can reach a length of 9.5 cm and a width of 5 cm. It varies in form due to the variability of what it bores into but is a robust clam that grinds away at the rock to create its burrow. The burrow takes the shape of an inverted ice cream cone with a bulbous anterior end, which tapers to the siphonal opening. The clam bores the hole mechanically by using its foot, which has been modified to a sucking disc, to hold hard onto the substrate. Then the grinding end of the shell, which has been modified with ridges, is moved up and down and slightly rotated. The leading edge ridge is responsible for the abrasion of the rock at the bottom of the burrow. Another genus of rock boring clam (Zirfaea) has been observed to reverse its direction after each complete rotation. The debris from the grinding is taken into the clam and removed via its exhalant siphon.

Crowding can also affect the shape of the burrow. More pholadids can settle on a rock then can possibly live in it. Burrows in uncrowded conditions are relatively straight; those in crowded conditions are often twisted. Apparently pholadids can detect boring activity in neighboring burrows and can avoid other individuals by changing direction, elongating the burrow or stopping boring altogether.

Boring Clams
Boring clam growth diagram. Yellow clams are reproductive adults.

Boring and growth occur in a re-occurring two-stage cycle lasting about 15 days. Most growth takes place during a 7 to 10 day period of quiescence followed by a 5 to 8 day period of active boring with little or no growth. The Common Piddock can live 20 to 50 years.

Boring clams are filter feeders, also referred to as suspension feeders. They filter organic food material out of the seawater that is taken in by their inhalant siphon, mostly phytoplankton.

The Common Piddock has separate sexes and reproduces by releasing vast quantities of sperm and eggs into the water where they meet to create new free-floating individuals. A female can release ½ to 1 million eggs. This is referred to as “broadcast spawning” and is apparently controlled by water temperature so that sperm and eggs are released at about the same time. Other species of boring molluscs can keep juveniles in a brood pouch within the mother.

The larval clam goes through several free-floating stages. The final free-floating phase, referred to as the “veliger,” can last until the individual finds a suitable rock surface to settle onto. Initial attachment to the rock is by byssal threads. Boring begins after metamorphosis from the veliger stage and is concomitant with growth. Initial penetration of the rock is probably more of a plucking action than the grinding which occurs later.

The Common Piddock has several predators including various crabs or anything that can pluck them from their holes when they’re exposed at very low tide (e.g. birds), including human beings who harvest them with rock tools. Other gastropods, the Leafy Hornmouth, Ceratostoma foliatum actually in the north and Nuttall’s Hornmouth, Ceratostoma nuttalli from Santa Barbara south, drills into the clam’s shell, injects digestive enzymes into the clam then sucks out the dissolved juices.

Boring Clams
A Leafy Hornmouth attacking a nestler, a Littleneck Clam, Leukoma staminea living in sediment in a boring clam’s boring.

After the clam settles on the rock there are two stages in the clam’s life: the active or boring juvenile and the inactive and reproductive adult. In the first stage the hole is excavated until physical or physiological reasons spontaneously inhibit further boring. In the second stage the clam changes. It adds accessory shell plates, and its muscular foot is resorbed and replaced with gonads, which take up most of the shell. No further growth occurs.

Boring Clams
Common occurrence of boring clams on our tidepool beach.

What we see on the beach and within the intertidal are cobble to boulder size rocks, which display the holes the clams have bored. Sometimes the shells (the valves) remain in the hole. With a little searching these can often be found to make the point to a visitor that these are the remains of the organism that made the boring or are the remains of nestler molluscs living in the hole made by the boring clam.

Boring Clams

As noted earlier, in some places along the coast these clams are a significant source of coastal erosion, weakening coastal rocks to the point that they collapse. In other areas, where they are collected as food by diligent clammers who use sledges and picks to break open the rock the clammers themselves have become a potent geological force.

Afterward: I hope this little story has “fleshed out” the story you can tell when a visitor finds a holed rock on the beach and asks you what’s going on. The boring clam’s life is, indeed, not so boring! For more information you can refer to these Sources of Information I’ve listed here. Please note, any errors herein are not meant to be but if there are some they are solely attributable to the author.

Sources of Information

A Field Guide To Pacific Coast Shells by Percy A. Morris; A Peterson Field Guide, Houghton Miflin Company; 1966.

Between Pacific Tides Fourth Edition; by Edward F. Ricketts and Jack Calvin, revised by Joel W. Hedgepeth; Stanford University Press; 1968.

Submarine Canyon Erosion: Contribution of Marine Rock Burrowers; by John E. Warme, Thomas B. Scanland and Neil F. Marshall; in SCIENCE Volume 173 pp. 1127-1129; a publication of the American Association for the Advancement of Science; September 17, 1971.

West American Cenozoic Pholadidae (Mollusca: Bivalvia); by George L. Kennedy; Memoir 8 of the San Diego Society of Natural History; 1974.

George L Kennedy and N. Scott Rugh, Brian F. Smith and Associates; personal communication, 2013.

Last revised 29-Jul-13