Further discussion is merited on the adaptive significance of an asymmetrically
ID: 206374 • Letter: F
Question
Further discussion is merited on the adaptive significance of an asymmetrically coiled shell. Most gastropod shells are asymmetrical spirals. This enables a snail to use it as a portable retreat rather than having to depend on clamping tightly to the substratum. The change in shell design involved an increase in shell height and a decrease in shell aperture, thus changing the shape from a shield to a cone. Cones can be problematic too. They may become unwieldy to carry or make it difficult for the animal to exploit crevices and holes for food and shelter as the animals grow larger. Thus, both shield and cone shells may have constrained the evolution of gastropod body size. The problem of shield and cone shells can be overcome partially by spiraling (or coiling) the shell. Shells can be coiled several ways. One such pattern of shell coiling is called planospiral shells in which the shell is bilaterally symmetrical with each spiral, or whorl, located completely outside the one preceding it. It also lies in the same plane, like a hose coiled flat on the ground. One group of gastropods, the Bellerophontaceae that exhibited shells with planospiral coiling are now extinct.
Why do you suppose? In other words, can you deduce an explanation of the costs of this form of shell coiling (compared to asymmetric shell coiling)? (in short)
Explanation / Answer
Planiispiral shell coiling has each spiral or whorl located completely outside the one preceding it. Therefore, the shell is not compact and this arrangement leads to much greater diameter of the shell. As each whorl is outside, these whorls can not overlap and becomes larger as the organism grows. This would result in less mobility of the organism and hence, is disadvantageous.
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