radio tracking marine invertebrates

After spending more than a decade conducting field work in the intertidal zone, I really began to wonder what the organisms were doing when field biologists weren’t present. This question seemed even more important to answer when I considered that intertidal biologists virtually ALWAYS do their work at low tide, when these organisms are exposed to air and sun.

In order to find out what marine snails do when no researcher is out watching, I use radio frequency identification tags–also known as PIT tags. While these are well known in wildlife and freshwater fisheries studies, the electrical conductivity of seawater interferes with their use in seawater. New tag technology developed over the past few years enabled their use in the ocean.

I tagged intertidal snails naturally present in my areas of interest and returned them to the rock. Shown below is the whelk snail Nucella ostrina with a radio frequency identification tag (12 mm length) epoxied to its shell. The tag has no power on its own, but when “pinged” by an antenna, responds with a unique ID code which gets recorded along with time and location. Whether the tide was high or low, I knew when a tagged snail was present in my intertidal areas of interest.



The intertidal areas of interest that I chose were regions high on shore that offered little protection from sunlight and desiccation, but were still home to one of the toughest marine creatures: the barnacle. In the San Juan Islands of Washington State, Nucella ostrina preferentially eats barnacles. When all of the tasty barnacles low on shore or in sun-protected crevices have been devoured, this snail may venture up to the high shore regions to eat more. I wanted to know if they did this when it was hot and risky for their little wet bodies, when it was cool and dark as I suspected, or even during high tides when intertidal biologists weren’t looking. below, you can just barely see the white body of the snail shoved into the opening at the top of the barnacle. After rasping their way into the barnacle shell (usually through softer ligament), the snails consumes and partially digests the barnacle in place.


I placed radio tag detection antenna in these high shore barnacle regions. Shown below are two green antenna loops–the spotted patches are epoxy holding them to the rock–and the circuit boards that power the antennas, hidden in protective white tubes. Most of the other materials seen are serving to keep the equipment attached to the rock or to help waterproof it. The large pipes in the background are part of the intake for the recirculating seawater at the University of Washington’s Friday Harbor Labs.



I confirmed that snails are no dummies. I found that they visited high shore regions at times that reduced their chances of getting fried and dried by the sun. This was primarily for a few days of each two-week tidal cycle when they would only be exposed to air during the cool, dark night. It would have been harder to detect the nighttime presence of these small animals by flashlight search alone, but the radio tags didn’t require light or alert researchers to find snails. The radio tags also helped me learn that with few exceptions, the snails were not making quick “runs” to and from high shore areas with high tides. Their movements were as slow as generally imagined.

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