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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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