title for learn-about section on whelks & relatives in A SNAIL'S ODYSSEY
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Explanations for answers to quiz on possible causes of inter-population differences in capsule-wall thickness in whelks Nucella ostrina:

Different exposure to the sun.  Possibly, but this would need to be tested.

Different levels of potential predation.  Yes.  For example, proximity to and density of predators is known to affect shell thickness and degree of shell-ornamentation in several species of whelks.  It happens that the thick-capsule population of Nucella ostrina in the foregoing study lives amidst abundant predators (not many crabs, but lots of isopods and chitons), while the thin-capsule population lives in an area with less variety of predators (lots of crabs, but no isopods or chitons).  These observations plus other experimental data showing that isopods Idotea wosnesenskii and Gnorimosphaeroma oregonense preferentially eat thin-walled capsules, suggests that predators such as isopods may have been a strong force leading to the selection of thick-walled capsules. Rawlings 1994 Evolution 48: 1301; Rawlings 1990 Biol Bull 179: 312. 

Different exposure to wave action.  Possibly, but this would need to be tested.

Different seawater salinities.  Possibly.  Studies on a related species, Nucella lapillus show that the egg capsules are readily permeable to salts and water, but whether permeability relates to capsule-wall thickness would need to be investigated.  Pechenik 1983 J Exp Mar Biol Ecol 71: 165.

Different sizes of females.  Possibly.  Snails depositing thicker-walled capsules tend to be about 25% larger than ones depositing thinner-walled capsules, but the author of the Research study in question suggest that further research is needed to demonstrate this.