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Dive Dry with Dr. Bill

#682: A Very Boring Life!

I must be feeling very "clammy" lately as this will be another column about a species of bivalve mollusc. Molluscs include clams, snails, octopus and squid. Bivalves are those like clams that have two shells that enclose and protect all the soft tasty parts inside. Last week I wrote about a species of piddock clam that burrows into soft bottoms at depth off our island. This week I'll focus on a more shallow kind of piddock.

Back when I was burning the midnight oil to earn my PhD at UC Santa Barbara, I often walked the beach near campus. Actually, I did that during daylight rather than in the dark hours since my lab was right along the shore. Those of you who have visited the beaches there have probably come across rocks with many holes bored into them and wondered what the heck did that? Well, read on... I'm about to tell you.

The holes bored through rocks like shale and sandstone and even subtidal structures of concrete were made by another species of piddock clam, most likely in the genus Penitella. Several species in this genus may be found in our waters including the flat tipped piddock (Penitella penita), P. gabbii and P. conradi. They are known from the chilly waters of Alaska down into southern California or Baja and to depths of about 300 feet.

Of course I've never gone that deep on SCUBA, but fortunately I've observed this bivalve bored into our rocky reefs in areas like Sea Fan Grotto, Twin Rocks and Long Point. These piddocks are fairly small with shell lengths up to about 2 3/4 inches. Divers can see the circular holes in the rock often with the tiny siphons partially exposed to feed.

Like last week's piddock, this one avoids predators by boring into the substrate. However, it has chosen something much more difficult to penetrate than the muddy bottom. To bore into the hard rock is a much more grueling task. The muscular foot holds on to the bottom of the burrow and the rough edges of the two shells act like a rasp against the rock as the piddock rotates its body back and forth. Morris, Abbott and Haderlie report that boring rates may be on the order of 4-50 mm (up to about two inches) per year depending on the hardness of the rock.

Up until they are sexually mature, these bivalves expend most of their energy in growth and boring activity. Upon reaching "the age of consent," energy utilization shifts toward reproduction. Munching becomes dedicated to mating. The foot wastes away and that end of the shell is closed off, thus preventing further excavation.

Hopefully my readers don't think these piddocks climb out of their burrows and seek a mate at the Marlin Club. No, they are broadcast spawners just like many attached and free ranging invertebrates. When the gametes unite and fertilization occurs, the egg develops into a tiny planktonic larva that is free to explore the world... for about two weeks before settling down to a boring life themselves.

Penitella penita is an important contributor to the erosion of shale along the coast of western North America. While less common in California, up to 90% of the boring clams along the Oregon coast may be of this species. The rock eventually gets its revenge as the erosion gradually reduces the length of the piddock's burrow and mature adults can no longer dig deeper to save themselves before they become exposed to the elements... and predators!

© 2016 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Piddock clams with siphons extended above the bottom (top); siphons being withdrawn as I film and voila... clammed up!

This document maintained by Dr. Bill Bushing.
Material and images © 2015 Star Thrower Educational Multimedia