SPACE, TIME AND SHELLS: USING STABLE ISOTOPES TO DISENTANGLE GEOGRAPHIC VARIATION IN THE SEASONAL TIMING OF GROWTH IN THE KNOBBED WHELK, BUSYCON CARICA

Many marine ectotherm species inhabit large geographic ranges, and as a result different populations experience both widely varying temperature ranges and different annual timing of temperature changes. These variations in the abiotic environment are expected to exert a first-order control over the biotic environment and individual organism’s ecology, placing limits on when throughout the year organisms may be most active in terms of feeding, growth, reproduction, and so on. Busycon carica, the knobbed whelk, is a carnivorous marine gastropod species that ranges from Cape Cod, Massachusetts to Cape Canaveral, Florida along the east coast of the United States. B. carica’s range spans ~10 degrees of latitude and includes two different biogeographic provinces with strong annual differences in both water temperature and in the seasonal timing of temperature change. We used stable isotope schlerochronological records to investigate the timing of growth for six individuals of B. carica from two different populations (one high latitude and one low latitude population).

High and low latitude populations showed substantial variation in the annual timing of growth. Low latitude individuals appear to grow primarily in the early spring, whereas high latitude individuals add shell material later in the season. The shift in timing of growth within different populations likely reflects adaptation to different temperature regimes as well as different lengths of growing seasons at low and high latitudes. The addition of shell material appears to primarily occur over a short period of time (on the scale of two to three months) each year. Restricting the addition of shell material to a small temporal window may be an adaptation to minimize whelks breaking their shells during feeding, which whelks accomplish by chipping open prey with their shell margins.