Tenacity of 5 chiton species collected from different habitats around Pacific Grove, California shows good correlation with degree of exposure to moving water.  Thus, species inhabiting protected under-rock habitats, such as Stenoplax heathiana and Mopalia lignosa, cling to the substratum 2-3 times less tightly than species inhabiting wave-exposed habitats, such as Katharina tunicata and Nuttallina californica.  Linsenmeyer 1975 Veliger 18(Suppl): 83.

NOTE data are for lateral detachment force for individuals attached to Plexiglas in air, corrected for surface area of foot

A later study on Katharina tunicata at 3 sites at Trinidad, northern California shows, first, that as wave exposure¹ increases, mean body size decreases, but densities increase.  Measurements of tenacity² in the laboratory show that small Katharina cling relatively more tightly to the substratum than large ones, which the author interprets as meaning that small chitons are able to withstand shear forces of waves significantly better in their natural habitats than large ones, thus explaining their greater abundance in the more wave-exposed location (see graph). However, the data presented in the graph³ are corrected for foot size, and thus are relative estimates, not absolute ones.  In absolute terms, of course, large chitons would have greater tenacity than small ones.  Possibly what is needed to complete the picture is the relationship between body size and frictional resistance to shear forces of the waves, which would be predicted to be relatively greater in smaller individuals.  Stebbins 1988 Veliger 30: 351.

NOTE¹  wave exposure is expressed as the proportion of transect markers (5cm2 bolted to the substratum) lost over the 1-yr study period

NOTE² defined as adhesion strength or, in practical terms, resistance to shear force.  The author uses a mechanical, weight-loaded device that scrapes a test chiton horizontally from a 10cm diameter Plexiglas disc.  It is not clear whether the tests are done in air or water, but the first is the more likely

NOTE³ scaling principles predict that rather than being linear as shown in the graph, the data should be curvilinear.  Only a log-log plot of the parameters would yield a straight line.  The true relationship of foot-area corrected tenacity against body length would be predicted to have a slope of -2 , not a slope of -1 as shown by the author

Between a rock and a hard place, a black leather chiton
Katharina tunicata shelters during low tide against a
green anemone Anthopleura xanthogrammica 0.8X

Shell valves of chitons tend not to fossilise well.  The reasons for this are investigated by researchers from Indiana University working at Friday Harbor Laboratories, Washington.  Two species Katharina tunicata and Mopalia muscosa are studied at 2 sites on San Juan Island, Washington, one site wave-exposed; the other, wave-sheltered.  The authors collect shell valves from both locations and categorise them with respect to taphonomic features such as fragmentation, edge and surface modification, and general bioerosion.  The paper contains much information comparative preservation potential of  different valve-types in the 2 species, but more interesting are between-species comparisons in the 2 habitats. In general, valves of Katharina preserve better than valves of Mopalia, and both species, as expected, preserve better in the wave-sheltered site.  In both species medial valves preserve better than head or tail valves, and these biases are reflected in the fossil record.  Wear is greatest on the exposed (darker) parts of the valves than on the lighter coloured mantle-protected parts.  The former parts are also softer than the latter, which are have a cross-lamellar type of structure. The authors conclude that the rarity of chitons in the fossil record likely owes to poor survival of valves as they are knocked around on the shore, rather than to an absence of chitons in ancient environments.  Puchalski & Johnson 2009 Lethaia 42: 167.

NOTE  lit. “burial + law” G., referring to the study of decaying organisms over time and the process of fossilisation

NOTE  the 8 valves are categorised as head (1), tail (1), and medials (6) as shown in the photograph