Two studies published together in the same 1940 volume of the journal American Midland Naturalist present differing views of homing in Lottia scabra.  In an investigation conducted at Moss Beach, California one pair of authors (Villee & Groody) report not being able to see large L. scabra individuals moving, perhaps because the limpets only do so when splashed by waves on the incoming tide.  In a “whoops, did I say that?” kind of way, these authors conclude that “It is conceivable, but hardly probable, that those L. scabra could get out of their pits and back into them so exactly as not to be detected”.  The other publication by Hewatt is a report on L. scabra being found in scars on the shores of Monterey Bay.  Although uncertain that the limpets actually excavate the scars in which they are found, this author nevertheless correctly describes feeding excursions out and back on the same tracks, returning not just to the same spots, but also to original orientations.  Villee & Groody 1940 Amer Midl Nat 24: 190; Hewatt 1940 Amer Midl Nat 24: 205.

NOTE  the state of flux in west-coast limpet taxonomy from the 1940’s to present day, coupled with the possibility of uncertain identification, make it difficult to interpret some of the older literature such as this study

Lottia scabra is the only west-coast limpet species that truly homes.  It occupies a home scar to which it returns after feeding excursions. An unpublished report notes that 298 L. scabra over a 24d period make 691 movements during high tide, 99% of which results in a return to a home scar.  Report by Brant cited in Haven 1970 Veliger 13: 231.

How does Lottia scabra create its home scar?  Possibilities include: 1) dissolution by secretions, 2) radular scraping, and 3) abrasion by the shell.  The last is dismissed by the authors of this study after close observation showing that while the shell does fit neatly into its own almost perfect depression, there is an inner, deeper, depression into which the foot fits.  The centre of this depression is raised to accommodate the hollow in the foot.  If shell abrasion were responsible, how could the inner foot-depression be created?  Also, the shell has only a hardness of 3 on the Mohs scale, which is only slightly harder than gypsum and talc, and much less even than sandstone (where scars are most evident).  The second proposal, radular scraping, is theoretically possible because the principal component of the radular cusps, hematite, with a hardness of 5-6 on the Mohs scale, is hard enough to scrape sandstone, but the authors question how the limpet could determine a template for grinding, especially to 2 different depths.  Instead, they propose that chemical dissolution is responsible.  Glands secreting carbonic anhydrase and acid mucopolysaccharides (combined pH of 3.6) are scattered over the surface of the foot, but less in its centre, and in the mantle edge.  Although carbonic anhydrase is more usually implicated in drill-hole formation in whelks and other drilling snails, the location of the glands do correspond with the features of the scar, even to explaining the raised area on which the foot sits.  Lindberg & Dwyer 1982 Veliger 25: 229. Photo courtesy Dave Cowles, Walla Walla University. rosario.wallawalla.edu.

NOTE  diamond, the hardest substance, is classified as 10 on the Mohs scale, quartz is 7, and gypsum and talc are slightly less than 3

Lottia digitalis

What about the ribbed limpet Lottia digitalis?  Is it not also a homing species? Numerous investigators have addressed this question and provided these conclusions:

Frank 1964: there is a “home range”, but strict homing does not occur
Galbraith 1965: 54% of a population in Los Angeles Harbor exhibits homing over a 7-d period, but the proportion rises to 68% if only those individuals that actually make excursions are included
Millard 1968: 53% of individuals remain in the same clump for up to 32d
Miller 1968: 25% show a tendency to home
Breen 1971: there are 2 behavioural types: one that homes, and one that doesn’t

These appraisals are quite varied. What seems common in the species’ behaviour is clustering during low-tide periods on vertical rock surfaces.  When the tide comes in, individuals disperse to feed and later some return to the same clustering area, but others don’t.  What seems to be also evident is that individuals do not show fidelity to an exact spot on the rock, nor are scars created and occupied as in “true” homing species.  Is it possible that after dispersing to feed the ones that return to their “home” site are attracted by the same cues that led them there in the first place (presence of conspecifics, shade, protective hollows or rugosities in the rock surface)?  Based on the evidence at hand our conclusion has to be that “strict homing does not occur” in L. digitalis.  Frank 1964 Am Nat 98: 99; Galbraith 1965 Amer Midl Nat 74: 245; Frank 1965 Ecology 46: 831; Millard 1968 Veliger 11(Suppl.): 45; Miller 1968 Veliger 11(Suppl.): 30; Breen 1971 Veliger 14: 177. Photo courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.