Population differences in the timing of diapause: adaptation in a spatially heterogeneous environment

Populations of the planktonic copepod, Diaptomus sanguineus, live in permanent and temporary freshwater ponds in Rhode Island. All ponds in which they occur become uninhabitable at some time during the year, but the nature and timing of the harsh period varies both spatially and temporally. Females produce discrete clutches either of subitaneous eggs which hatch immediately or of diapausing eggs which hatch the following season. The two egg types show distinct chorion morphologies under transmission electron microscopy. In permanent ponds the copepods start making diapausing eggs in March, one month before rising water temperatures induce planktivorous sunfish to become active. In temporary ponds diapausing eggs are produced, in a complex pattern from May to July, before the water disappears in late summer or early fall.We investigated the spatial scale at which D. sanguineus is adapted to this complex environment. In a reciprocal transfer experiment between temporary and permanent bodies of water, female copepods placed in new ponds made subies of water, female copepods placed in new ponds made subitaneous and diapausing eggs in the same sequence as control females retained in their home ponds. The copepod populations enter diapause at times appropriate for the local habitat conditions they experience, but inappropriate for other, nearby ponds. Transplanted females were unable to sense a change in pond type or to adjust egg production accordingly. We conclude that D. sanguineus populations are adapted to the specific conditions of isolated ponds rather than to a broader geographical region containing several pond types.