Benefits to a snail in having a shell are apparent, but what about costs?  The obvious costs are those of manufacturing the shell, including organic matrix and mineralisation, and transporting the shell about.  But what about another cost, not so obvious, that a limit may be set on tissue growth by the maximum rate at which the shell is produced?  This would have important evolutionary implications, not just for whelks, but for any organism with a mineralised skeleton, such as corals and barnacles. A comparison of growth in 2 morphs of the whelk Nucella lamellosa at Friday Harbor Laboratories, Washington shows that both have similar soft-tissue mass and both produce shell material at the same rate but, because 50% more shell material is required for a thick-shelled individual, its rate of tissue growth is actually constrained.  In the schematic representation above Right,which best fits the data obtained in the study, note that neither morph has reached a maximum rate of soft-tissue growth, and neither has reached its maximum rate of food ingestion.  Thus, it appears to be limitation on shell growth that is holding back soft-tissue growth, possibly in both morphs, but certainly most evident in the thick-shelled one.  Palmer 1981 Nature 292: 150.

NOTE the 2 morphs are a thin-shelled, fluted form and a thick-shelled, smooth form

As in all molluscs, shell growth in whelks occurs at the free edge of the mantle.  Damage, such as that from non-lethal predator attack as, for example, crabs, can be repaired at the shell edge or anywhere on the inside of the shell that the mantle contacts. Such internal repairs are often indicated by blisters on the nacreous part.  Mollusc shells have 2 components: an organic protein matrix that is surrounded and perfused by crystalline calcium carbonate.  Studies at the Bamfield Marine Sciences Centre, British Columbia on shell repair in different species of gastropods after experimental damage¹ show that rates of regeneration are inversely related to organic content of the shell.  Thus, the greater the organic component, the slower is the regeneration (see graph on Right).  Since these experimental snails² are not fed and have to rely on what is likely to be a “standard" level of energy store for the regeneration, the inverse relationship suggests that the production of the organic matrix is more metabolically³ demanding than the crystallisation of calcium carbonate.  Palmer 1983 Mar Biol 75: 287.

NOTE¹  shells are damaged by clipping away a goodly proportion of the last body whorl with pliers.  Regeneration is measured as mass of shell added over roughly 40-d periods at 12-14^oC.  Proportion of organic matter is determined from mass loss of dried shell after ashing

NOTE²  the 15 species used in the study include 7 neogastropods

NOTE³  the author cites unpublished data suggesting that protein synthesis may actually be up to 30 times more costly than calcification 

Shell repair in a mollusc is energetically costly and would be predicted to lead to decreased growth and locomotion, and possibly enhanced vulnerability to predators. During reproductive season the recipient of shell damage may be faced with a potential tradeoff between subsequent reproduction and shell repair.  As noted in a study on Nucella ostrina at the Bodega Marine Laboratory, California, if repair detracts from reproduction, then it should occur only to the extent that unrepaired injuries decrease fitness.  It is surprising, then, to find that experimental damage to shells of N. ostrina collected at Bodega Head, California actually leads to increased growth, especially if the snails are fed ad libitum on barnacles Balanus glandula during the 34d of recovery (see histogram above). Damage, moreover, leads to increased production of egg capsules although, in this case, nutritional status does not significantly affect the results (see histogram lower Right).  

The laboratory part of the study thus shows that food availability can limit shell growth.  In the field part of the study, 367 snails are collected, some are damaged, then all are released.  Recapture of 139 of these after 31d shows that the shell-damaged specimens grow significantly more than undamaged ones (3.2 and 2.4mm, respectively).  Mortality of shell-damaged snails is also higher than undamaged ones.  Geller 1990 J Exp Mar Biol Ecol 136: 77. Photograph courtesy Nate Charbonneau and Dave Cowles, Walla Walla University, Washington wallawalla.edu.

NOTE  the experimental treatment consists of grinding the outer lip of the shell approximately 3mm to the level of the withdrawn operculum.  Field surveys in the Bodega Head region disclose up to 6% damaged N. ostrina shells in tidepools and 20% in surge channels

NOTE  interestingly, while females invest significantly more energy into production of egg capsules when they are damaged, actual embryo production is unaffected. The author provides results of statistical analyses for this, but not the actual data

How costly is it for a marine mollusc to produce a shell?  Estimates for this are confounded by the fact that costs for shell manufacture cannot normally be separated from other metabolic costs.  However, the fact that the whelk Nucella lamellosa in the Barkley Sound area of British Columbia produces extra-thick shells under certain environmental conditions allows the extra food costs required for this to be estimated. By feeding both thick- and thin-shelled morphs on barnacles Balanus glandula in the field and laboratory, and measuring amounts of somatic growth, shell growth, and food consumed over periods of up to 94d, the author is able to cost out shell production by the differences observed.  Results show that calcification costs are 1-2 J ^. mg CaCO₃^-1 deposited, or about 5% of that estimated for the cost of the organic proteinaceous matrix on an equivalent mass basis.  The calcification cost equates to only about 6% of the overall respiratory costs for Nucella. This is low, in part owing to the ready availability of CaCO₃ in shallow oceanic waters.  Palmer 1992 Proc Natl Acad Sci USA 89: 1379.

NOTE  shell costs include not just accumulating, transporting, and precipitating CaCO₃, but also producing an organic matrix around which the calcarous crystals form.  Cost of the latter is estimated from other data and, as noted by the author, some “educated guesswork”

NOTE  these morphs occur on quiet-water shores where predation by crabs is more common

Thick-shelled morph of whelk Nucella lamellosa
showing what appears to be new shell growth 1.5X