Southwick Research

An Autobiographical Collection of Observations and Investigations

by J. Wanless Southwick, Ph.D.




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Seashore Isopod



Behavior of a Seashore Isopod

Insect behavior always fascinated me. While visiting Puerto Penasco on the Gulf of California in Mexico, I noticed thousands of large, cockroach-like isopods crawling about a rocky seashore beach at low tide. My curiosity was aroused when they weren't there at the next low tide, but then they showed up en-mass at other times. Why? The observations grew into a research project. I returned to that same beach several times at different seasons of the year to see what factors controlled the timing of these en-mass beach migrations of this isopod, Ligia occidentalis.

The tides at Puerto Penasco reached their extreme highs and lows approximately at full moon and new moon each month. These spring tides exposed large expanses of boulders on the study beach. Mature and juvenile Ligia crawled over the boulders and down the beach where they fed on algal growths that encrusted the boulders. Predatory birds, particularly willets and curlews, feasted on the exposed isopods. Ligia seemed to be as much at home hiding in cracks and crevices in dry boulders above the water line as they did underwater when the tide came in. Underwater their abdominal gill plates would beat rhythmically to flush water over their gills. On land, the gill plates covered the gills to prevent excessive evaporation of gill moisture. When traveling from dry hiding places, the isopods would stop at little pockets of water, insert their posterior uropods into the water, like a straw, to siphon water onto their drying gills. After tanking up on water, they would continue on their terrestrial treks.

To study their daily migratory patterns, I placed cameras on the beach from high tide level down to low tide level. Each camera focused on a particular boulder and took a photo every 15 minutes. I kept myself busy trying to keep cameras in place as the tides rolled in and out. After I developed the rolls of black and white film, I counted the number of Ligia on each boulder for each 15-minute time period, thereby developing a time and space sequence of their presence or absence at each location. Movement times were superimposed on a tide calendar and a consistent pattern appeared.

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