PHYSIOLOGICAL ECOLOGY AND INTERTIDAL ZONATION IN LIMPETS (ACMAEA): A CRITICAL LOOK AT "LIMITING FACTORS"

Abstract

1. This study tests the hypothesis that physical factors limit the ranges of five specíes of limpets (Acmaea) inhabiting the splash zone (Zone I) and upper midtidal (Zone II) of the Central California rocky shore. Two criteria are considered necessary to establish the hypothesis. First, interspecific differences in adaptation to physical factors must permit exploitation of different portions of the intertidal gradient. Secondly, physical conditions, by occasionally causing mortality, must set the limits of ranges at different levels depending upon the interspecific differences in physiological tolerances. 2. Dilution by winter rainwater runoff probably presents no osmotic threat to Acmaea. No pronounced differences are evident in either resistance to or tolerance of dilution. All five species tolerate immersion in fresh water for periods in excess of the maximum exposure expected in nature. Under exceptional circumstances some Zone I limpets may experience entrapment and lethal dilution in fresh water seeps. 3. Interspecific differences in tolerance to high temperatures are clearly correlated with solar heating occurring in the species' natural microhabitats. Thermal tolerances show no pronounced seasonal acclimation or intraspecific variation in limpets from different zones. 4. During three years, maximum field temperatures never exceeded the thermal tolerance of any of the limpets. Furthermore, all of the species tested, with the exception of A. scabra, seem to avoid maximally exposed microhabitats. It is concluded that high temperatures do not of themselves limit distributions of these limpets, but contribute to desiccation. 5. The Zone I limpets show higher desiccation tolerances and will tolerate drying conditions much longer than Zone II limpets. The limpets have no mechanism for avoiding the osmotic effects of desiccation; as expected, the Zone I limpets also have higher tolerances to hyperosmotic solutions and will tolerate elevated concentrations longer than will Zone II limpets. 6. Tolerances of desiccation and of hyperosmotic solutions in all 5 species are extremely high, ranging from about 70% to about 82% total water lost, and from 400% to 600% sea water. 7. Mortality during desiccation can be attributed entirely to the concentration of internal fluids resulting from evaporative water loss. Contrary to earlier reports, there are no increased water reserves in Zone I limpets. Thus high desiccation tolerances depend on adaptation to high electrolyte concentrations at the cellular level. 8. Desiccation rates of Zone I limpets are as low as 1/10 those of Zone II limpets. The reduction is due almost entirely to a mucous diffusion barrier produced between the shell margin of Zone I species and the substratum. Non-"homing" A. digitalis, by virtue of the mucus sheet, have the same desiccation rate as "homing" A. scabra suggesting that the principal advantage of homing may not be desiccation resistance alone. 9. Environmental conditions in Zone I exceed tolerances of the Zone II limpet species, and therefore potentially limit their ranges. However, behavioral adaptations apparently prevent upward migration and occupation of potentially lethal microhabitats. Hence, although differences in adaptation to drying conditions exist, the second criterion is not satisfied, and desiccation is not considered limiting for Zone II Acmaea. 10. The greater adaptation of Zone I limpets to drying conditions permits their exploitation of the high intertidal. Furthermore, desiccation does kill A. digitalis and A. scabra high in Zone I, especially small individuals, during periods of unusually warm weather or reduced splash. Thus, desiccation mortality does limit upper ends of intertidal ranges and apparently influences age structure in populations of these species. 11. The hypothesis is advanced that in general, range limits of animal populations should not be expected to be limited by physical factors, but by behavior, competition, or some other biotic factor. Only where the range borders on an unexploited resource would selection pressure favor range expansion to the limits of physiological toleramices. The data presented for Acmaea support this hypothesis: the Zone II species, with ranges overlapping those of the Zone I limpets above, are limited by behavior; the Zone I species, bordered above by a visible algal film, are limited by desiccation.