A Peek at Ligia pallasii, a Supralittoral Crevice Dweller

Peer into an inhabited crevice. Ligia will give you a peek- dim and incomplete, but intimate. You could stare for hours. Instead, you disrupt the stillness, and the peek is over.

I’ve learned a few things from my impatience with crevice-dwelling Ligia. They’re not solitary. Many more than I expect scuttle away from an intruding finger tip. They scramble confusingly up the cliff face, so fast a good photo is out of the question. A few, if you reach for them, drop off, supposedly to avoid capture. My experience is that most are are climbers, not droppers.

Morning light sometimes finds Ligia searching nooks in plumes of Ulva intestinalis, abundant at seeps. What are these algal excursions? T. H. Carefoot (1973) studied the L. pallasii diet at Port Renfrew, on the southwest coast of Vancouver Island. He reported,

the principal foods were encrusting diatoms, insect larvae, occasional members of the same species, and a variety of red and green seaweeds growing in the upper intertidal tidepool habitat.

In his 1973 food preferences paper, Carefoot described his study site as an “ampitheatre” carved from a cliff face, a description that matches my experience. The habitat photos below illustrate the ampitheatre concept.

Ligia pallasii, like most Ligia, are nocturnal, spending the day in crevices above the splash zone. At night, especially at low tide, they venture lower, into the splash zone, where decaying wrack and other food is more abundant than on barren cliff faces. That’s the story. Ricketts and Calvin mention a Japanese species whose movements between high rocks and lower beach are orderly, with members following established routes. They go on to say,

The casual impression gained from watching our own Ligia is that of an aimless, disorganized rabble, dispersing in various directions.

Ligia pallasii sticks to rocky shores. Sandy backshores host their own supralittoral isopod, shy little Alloniscus perconvexus.

References

Carefoot, T. H. 1973. Feeding, food preference, and the uptake of food energy by the supralittoral isopod Ligia pallasii. Marine Biology 18(3): 228–236.

Ricketts, E. W., and J. Calvin. 1968. Between Pacific Tides. 4th ed., revised by J. W. Hedgpeth. Stanford University Press.