Are direct developers more locally adapted than planktonic developers?

The hypothesis that populations of direct developers exhibit greater geographic differentiation in life history features than populations of planktonic developers, was tested with two species of grazing snails of the genus Littorina from 1986 to 1987. Littorina sitkana (direct developer) and L. scutulata (planktonic developer) coexist on sun- and wave-sheltered beaches from Alaska to Oregon, USA. Seasonal patterns in growth, survival and reproduction were monitored for samples from four geographically separated populations of each species grown in population cages at a common site, Friday Harbor, Washington, USA. The environmental and population effects on growth in the two species were determined in a four-way reciprocal transplant experiment with the same populations. Both the direct and planktonic developers exhibited geographic differentiation in life history features. Differentiation in the direct developer occurred over distances shorter than 30 km, while differentiation in the planktonic developer occurred over the 500 km distance examined (greater than their larvae would likely travel).