Dive Dry with Dr. Bill

#183: Soft Bottoms and Purple Olives

No, I'm not talking about baby (or "baby") buttocks and something to put in your next designer martini this week. Soft bottoms refers to sandy habitats as opposed to rocky reefs. Purple olive refers to the snail of that common name which is found on soft bottoms. As a kelp forest ecologist, I usually dive rocky habitats where kelp is capable of attaching and growing. During a recent trip on the King Neptune, we had an extended stay at Hen Rock to allow divemaster Chris to take several "newbies" on Discover Scuba excursions. Since Capt. Tony had anchored the boat closer to shore than usual, I decided to spend my time floating over soft bottom habitat to see what I could find.

Many divers avoid soft bottoms. Superficially they look much like their terrestrial desert counterparts. Both habitats are made of shifting sand and are somewhat unstable for life to gain a foothold. However, there is plenty of food for sand dwellers in the sea since plankton don't reside strictly over rocky habitats, and much organic debris collects on the ocean floor. Because of the shifting nature of the sand, marine life there is often buried beneath it and therefore not always readily apparent from the surface. Let's dig a little deeper into this, so to speak.

Often the first evidence of life on the sandy ocean floor is through indirect signs. One may see the trails created when critters tunnel just beneath the surface of the substrate, or the breathing siphons of clams extending up out of the sand, holes or burrows, or even piles of poop expelled from the burrow. By looking carefully for such clues, we soon discover that soft bottoms are often very much alive with life. We call living things that burrow under the sand "infauna" since they are critters that live in the substrate rather than on top of it (those are "epifauna").

My three dives at Hen Rock that day provided a wealth of discoveries I really hadn't expected. I was thrilled to find sea pens easy to film thanks to the high light levels in the shallows, a nursery for the young of the white sea urchin, and several types of flatfish. However, one subject that really caught my eye was the purple olive snail. I'd seen evidence of their burrows at this site before, but on these dives there were a number of them cruising along the surface of the sand in plain view.

Purple olives are small snails, usually less than 1" long. Their smooth shells are most commonly white but may be dark gray, lavender or even yellow or orange. They are often purple at the base. Purple olives may be very abundant on the sandy bottoms of protected waters including bays and lagoons. They are usually found at depths shallower than 160 feet. This species may be found from Vancouver Island in British Columbia down to Magdalena Bay on the Pacific coast of Baja.

The purple olive's fleshy foot is adapted to crawling on sand. It has wide extensions forward and to each side that give it a broader base for traction... kind of like using wide tires for off-roading. Mucous glands secrete a slippery coating that aids in locomotion. Their smooth shells make it easier to plow through the sediments. A tubular siphon at the animal's forward end can be extended up above the sand to obtain water to pass over the gills for respiration. This siphon reminded me of the old siphon on my waterproof 1953 M-38A1 military jeep. I could literally drive it into several feet of water with the tube sticking out to allow air into the carburetor... if I didn't get stuck in the soft sand that is.

As mentioned, most of these snails are burrowed under the sand during the day. At about dusk, they come up to the surface where they are active at night. They are omnivores and have a fairly broad diet including algae and kelp detritus, as well as fresh or decaying animal matter. They may also feed on fine particles of organic matter on or in the sand. In turn the purple olive is eaten by octopuses (not octopi), other snails like the moon snail and California cone, sea stars, shore birds and most likely fish. Purple olives can sense an approaching sea star and will react by fleeing, burrowing under the sand, or even somersaulting away.

Mating and spawning occurs throughout the year in our waters. The males detect their mate chemically, following the mucus or slime trails laid down by ripe females. Hmmm... I doubt Essence of Slime would make a good perfume for humans. I was able to watch as one male tracked the smaller female for several minutes, finally catching up with her. When he made contact, he "stomped" his soft foot down on hers. It reminded me of how I used to dance with my partners back in 8th grade dance class... a skill I've retained to this day! Once trapped, the female may struggle for a while to escape (don't they all?), but the male quickly secretes a mucus coating that envelopes both of them. Actual mating may last for three days (without benefit of Cialis I might add).

The female lays her fertilized eggs one at a time on stones, empty shells and other hard surfaces. She may keep quite busy for a month or more laying in excess of 4,000 eggs. Development of the eggs is highly variable, taking 10 to 30 days, and does not appear to correlate with water temperature. Upon hatching the young swim to the bottom and therefore lack a planktonic stage with which to disperse and colonize new areas. The lifespan of a purple olive may be as long as 15 years.

Other than the occasional shell collector, modern humans have little use for these snails. Native Americans, including those living on the Channel Islands, saw them much differently. The shells were often used for food, as ornamentation to decorate soapstone items and in burial objects, and were also used as money. Now if this were still true, I could make a living harvesting the purple olives at Hen Rock. Unfortunately Wells Fargo, my mortgage banker, has indicated they will not accept purple olives in payment. I guess I'll just have to wait until the exchange rate improves before I can view myself as a rich man. Of course just watching these interesting snails has made me a richer man in terms of experiences! Tell that to my potential dates.

© 2006 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, "or "Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Purple olive snail on sand showing extended siphon, group of purple olives at Hen Rock;
male "putting his foot down" on female before mating, male chasing female to mate. .

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia