Latitudinal compensation for temperature is seen commonly in physiological and other studies of invertebrates. Whelks, Nucella spp., inhabit rocky shores of Alaska and southern California, and seasonal seawater temperatures are greater by 6-10^oC in the southern habitats. Most scientists would predict that growth of embryos of Nucella emarginata would thus be faster in the warmer waters of the south, although not so fast as the temperature differences would suggest because of of such latitudinal compensatory adaptation. It is surprising, then, that growth measurements actually show the reverse, with rates about 9 times faster in Alaska than in southern California (see graph). The reason for the difference is not known.  Dehnel 1955 Physiol Zool 28: 115. Photograph courtesy Louis Gosselin, Thompson Rivers University, British Columbia.

NOTE  we now know that the experiments were likely done on different species of Nucella: ostrina in Alaska and emarginata in southern California.  Still, this is unlikely to have accounted for the large differences shown

Hatchlings of Nucella ostrina from Barkley Sound, British Columbia 50X

In the Lummi Island area of Washington, Nucella ostrina lives about 0.5m higher on the shore than N. lamellosa.  Both species are often openly exposed to solar irradiation and, as a result, tissue temeratures may be much higher than air temperatures. The authors note that lower humidities at higher tidal heights may aid in cooling through greater evaporative water loss.  Both species take up oxygen when emersed but, as shown here for N. lamellosa, at rates about 3 to 4-fold less than when immersed (see graph). Bertness & Schneider 1976 Veliger 19: 47. Photograph courtesy Dave Cowles, Walla Walla University, Washington www.wallawalla.edu.

NOTE  data are presented here only for large-sized N. lamellosa, but data for large-sized N. ostrina show similar differences

Nucella ostrina forages amongst prey barnacles Balanus glandula, Semibalanus cariosus, and Chthamalus dalli 1X

An unusual behaviour in whelks Nucella lamellosa is described by a researcher at Friday Harbor Laboratories, Washington.  The behaviour involves loss of the shell a few days to weeks after collection and transport to the laboratory.  The shell-less individuals crawl about and appear to exhibit normal behaviours, including, in 2 of 7 instances of shell loss out of a collection of several hundred, the drilling and eating of prey mussels and barnacles.  Although the author discusses the possibility that the autotomy is defensive, along the lines of limb loss in crustaceans or arm loss in sea stars, she acknowledges that it is more likely to be a response to stressful conditions, such as temperature or oxygen deprivation. Koy 2007 J Shellf Res 26: 267. Photograph courtesy the author.

Two shell-less whelks
Nucella lamellosa.

What is the geographic variation in thermal limits of a marine snail?  This is investigated in whelks Nucella canaliculata from 7 similar mid-intertidal sites ranging from Oregon to central California (see map).  By rearing eggs in capsules to maturity from each of the 7 populations under standard conditions at the Bodega Marine Laboratory, California, then mating up to 28 non-sibling pairs within each population, the researchers are able to produce test hatchlings minimally influenced by potential past field-acclimatisation and other non-genetic effects. 

Results of thermal-or lethal-tolerance (LT50) assays on the F2 progeny reveal that hatchling snails from central California are, surprisingly, less heat tolerant than conspecifics from Oregon (see histogram; capital letters indicate statistical homogeneity). The authors conclude that the differences in upper thermal limits existing among the populations of N. canaliculata are likely genetically based.  As to explanation for the differences, we have to conclude that Oregon whelks are exposed to higher potential thermal stresses than central California whelks.  Kuo & Sanford 2009 Mar Ecol Progr Ser 388: 137. Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE the temperature at which 50% of the test subjects are killed

Studies of temperature effects on feeding in intertidal animals are common, but it is rare for an investigation simultaneously to include rates for an organism both emersed and submersed.  Researchers at Friday Harbor Laboratories, Washington measure feeding rates of whelks Nucella ostrina on barnacles Balanus glandula at different and varying temperatures.  Results show, interestingly, that while feeding and growth over a 20d experimental period are greatest at higher submersion temperatures of 13^oC as compared with 11^oC, they are actually lowest at higher emersion temperatures of 28^oC as compared with 20^oC or 12^oC.  Thus, warm weather may lead to greater predation on the barnacle population, but is significantly released if air temperatures become too warm.  The authors suggest that at the highest experimental air temperatures the whelks may be so physiologically stressed that feeding becomes secondary to seeking shelter within their shells or in cooler microhabitats, or in devoting needed energy to cellular maintenance and repair.  Yamane & Gilman 2009 Mar Ecol Prog Ser 393: 27. Photograph coutesy Emily Carrington, Friday Harbor Laboratories, San Juan Island, Washington CARRINGTON LAB.

NOTE  temperature regimes imposed are in the range that the whelks normally experience during summer in San Juan Island, Washington

A whelk Nucella ostrina feeds on
a barnacle Balanus glandula 1X