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The maintenance of shore-level size gradients in an intertidal snail (Littorina sitkana)

University of British Columbia

The population size structure of many intertidal animals varies with tidal height so that the mean animal size is increasing or decreasing with tidal height. These size gradients could be produced by growth or survival varying with tidal height, or by animals moving to a preferred tidal level. The body size of the snail, Littorina sitkana, increases steadily with tidal height in rocky high intertidal habitats of British Columbia. To determine how size gradients were maintained in L.sitkana, I quantified how growth, survival, and snail movement varied with tidal height. I studied populations of L.sitkana found on sheltered pebble beach and exposed basaltic shelf habitats. Mark-recapture studies and experimental transplants showed that growth could not have produced the size gradients because snail growth rates in both habitats were as fast or faster at low tidal levels (where the snails were the smallest) than at high tidal levels. However, survival rates were lowest at low tidal levels. On pebble beaches, this was due to size selective predation on large snails by the pile perch, Rhacochilus vacca. On basaltic shelves, heavy wave action at low tidal levels may have caused the poor survival rates. Transplanted snails moved homeward on pebble beaches, but not on basaltic shelves. Reduced survival rates at low tidal levels cause size gradients on basaltic shelves. Size selective predation by fish together with snail movement maintains size gradients on pebble beaches.

Text

2010-03-26

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http://hdl.handle.net/2429/22512

Zoology

University of British Columbia

Graduate