Physiological Ecology and Intertidal Zonation in Limpets (Acmaea)
Introduction
Intertidal gradients from marine to terrestrial conditions correlate with spatial partitioning of habitats by animals.
This leads to the assumption that terrestrial physical factors during tidal emersion determine the upper range limits of intertidal populations.
This study tests this assumption, framing it as a hypothesis.
The hypothesis posits that if physical factors limit intertidal partitioning, then:
Interspecific differences in physiological tolerances exist.
Micro-environmental conditions exceed these tolerances at range fringes, causing mortality and preventing range extension.
High temperatures and desiccation are often cited as limiting factors.
The assumption relies on correlations between:
Physiological tolerances and zonation.
Fair weather and animal death or disappearance in the field.
Many studies demonstrate a relationship between laboratory physiological tolerances and zonation.
High intertidal animals often exhibit:
Higher lethal temperatures.
Higher desiccation tolerances and lower desiccation rates.
Greater tolerance of osmotic extremes.
However, there's a lack of data on microclimate extremes experienced by animals in the field.
Without demonstrating that field conditions exceed physiological tolerances, it's hard to definitively say physical factors are limiting.
Field measurements of body temperatures and desiccation levels are often below laboratory-determined lethal limits.
While animal deaths have been noted during warm, dry weather with minimal wave splash, a causal relationship between physical conditions and death is hard to establish without field microclimate measurements.
The finding of dead, dry animals doesn't confirm desiccation as the cause of death.
Despite adaptation studies, the importance of physical factors in limiting population distribution remains unclear.
This study focuses on the physiological ecology and zonation of five limpet species (Acmaea) from central California to examine the effects of physical factors.
The hypothesis tested states that upper distribution limits are determined by physical environmental factors.
Laboratory tolerances to environmental conditions are measured to see if interspecific differences allow exploitation of the intertidal gradient.
Environmental conditions in microhabitats of each species are monitored for three years to see if they exceed limpet tolerances and limit ranges.
Materials and Methods
Five limpet species commonly found on rocky central California shores were studied.
Three species inhabit the splash zone (Zone I):
Acmaea digitalis: small, greenish-gray to brown limpet on vertical or overhanging surfaces. Occupies the zone up to 30 feet above mean lower low water (MLLW).
Acmaea scabra: small, heavily ribbed gray to white limpet on horizontal surfaces exposed to the sun. Exhibits homing behavior, returning to the same spot on the rock.
Acmaea persona: large, smooth olive-shelled limpet in dark crannies and under boulders, sheltered from the sun. Negatively phototactic, active at night.
Two species are found in the upper mid-tidal zone (Zone II):
Acmaea pelta: high-peaked brown limpet found among macroscopic algae on surfaces experiencing wave action.
Acmaea testudinalis scutum: moderate-sized limpet with a flat shell profile found on shaded walls or boulders at the bottom of surge channels or in tidepools.
The study was conducted at the University of California's Bodega Marine Laboratory.
The rocky shore is composed of rugged diorite granite.
Biotic zonation aligns with descriptions for exposed rocky coasts.
Water surface temperatures range from 9^
circ C in May to 16^
circ C in September.Mean daily maximum air temperatures range from 10^
circ C in January to 20^
circ C in late August.Weather patterns include:
Fog and little wind, especially in summer.
Sunny weather with high (30-100 km/hr) northwest winds.
Rare clear and calm days, with temperatures exceeding 20^
circ C (less than 10 days per year).
Tides at Bodega Head are mixed semi-diurnal, with two unequal high and two unequal low tides each day.
During spring and early summer, lower low tide occurs in the morning, followed by the lower high tide. Zone I and II may remain exposed all day, submerged only a few hours at night.
Zone I limpets may experience a maximum of 4 hours immersion once per day or no immersion for consecutive days.
Zone II limpets may expect a minimum of 4 hours immersion at least once per day.
Limpets were collected from rocky shores near the Laboratory.
Capture was done using a quick prying motion with a stainless steel table knife.
Limpets were placed under running sea water immediately after collection and used within 48 hours, except for some A. scabra maintained for weeks on glass plates with artificial tides and microscopic algae.
Field measurements were designed to evaluate microclimate extremes experienced by the limpets.
High-temperature data was collected in Zone I on the south-facing side of the cove adjacent to the Laboratory between 12:00 and 15:00 on clear, calm, exceptionally warm days.
Temperatures of air (shaded), rock surface, and limpet body were measured using:
Yellow Springs Instruments Teletheiinometcr with a hypodermic needle thermistor probe.
12-channel Leeds-Northrup Speedomax thermocouple recorder powered by a portable 115 V generator.
Limpet temperatures were obtained by prying the animals off the rock, inserting the needle probe into the center of the visceral mass, and pressing the limpet back onto its original location.
Since limpet body temperatures were virtually the same as adjacent rock surfaces (mean deviation of 12 specimens of A. scabra from rocks was -0.33^
circ C \pm 0.19 (S.E.) at 15:00, 8 August 1971 ), rock surface temperatures were often used.Recordings of temperature fluctuations over an entire day were obtained using thermocouples fastened to rock surfaces.
Interspecific differences in maximum temperatures attained in the field were examined by recording temperatures of individuals of each species in a study area where all five species occurred.
Windspeeds were measured with a Hastings-Raydist portable heated-thermocouple anemometer with an omnidirectional probe.
Relative humidities were measured with a Hygrodynamics, Inc. electric hygrometer indicator.
Thermal tolerances of the five species were compared in the laboratory using a technique similar to that of Fraenkcl (1968).
Six to ten limpets were allowed to adhere to the walls of a 500 ml beaker, the cold sea water in the beaker was then replaced with aerated sea water at the desired test temperature and the beaker placed in a water bath at the test temperature for 15 minutes.
The thermal lethal limit was defined as the lowest temperature killing more than half of the sample tested.
Survival was assessed after a 24-hour recovery period in running sea water, based on the resumption of normal locomotion.
Most thermal tolerances were determined with limpets collected during the summer.
Variation of thermal tolerances throughout the year was checked by collecting A. digitalis and A. scabra from Zones I and II in winter.
Lethal limits were also determined in air with a slow temperature rise to simulate natural conditions.
The water was emptied out of the beakers and a few hours allowed for the surface to dry. The beakers were then placed in a recirculating wind tunnel with slow (0.5 m/sec) air movement, and the temperature of the limpets (monitored via thermocouples) was raised over a period of 5-6 hours to the lowest desired test temperature, as would naturally occur with limpets exposed by a morning tide.
Effect of size and desiccation state on thermal tolerance was assessed by including samples of very small and of moderately desiccated A. digitalis.
Determination of desiccation tolerances and rates required weighing or similar manipulations of limpets without disturbing them.
These operations were carried out with the animals attached to discs of transparent 0.004 inch Mylar plastic film.
Desiccation rates were always determined with moving air.
Progress of desiccation was followed by periodic weighings.
Desiccation rate studies in the laboratory were carried out in a closed, re-circulating wind tunnel within a constant-temperature enclosure, with humidity controlled by pans of appropriate salt solutions.
Drying of limpets prior to determination of desiccation tolerances took place at 20^
circ C and at 1.0-1.4 m/sec airflow and the humidity was that of the outside air drawn into the ventilating system (usually 50-80% R.H.).The lowest desiccation levels causing 50% or higher mortality were designated the desiccation tolerance limits.
Tolerance of A. t. scutum was also determined under less severe drying conditions to check the effects of rate of desiccation on desiccation tolerance.
Since initial hydrated weights of limpets in the field were not available, a graph was prepared from laboratory data comparing water loss, measured gravimetrically, with chloride concentration in mantle-cavity water/urine of 78 specimens of A. digitalis.
The chloride determinations were carried out on 1 microliter samples of fluid with a Buchier-Cotiove Chioridometer. The resulting curve (Fig. 1) was used to translate chloride concentrations of extra-corporeal water obtained from animals in the field into per cent body water lost at the time the sample was collected.
Equilibration rates and tolerances to fresh water were determined by allowing groups of limpets to attach to plastic film, immersing them in fresh water at 15^
circ C, and sampling the population at intervals.Still water was used to provide identical conditions for all limpets.
Animals removed from the fresh water were allowed about two hours in still air to equilibrate body fluids with the mantle cavity water and urine ( extra-corporeal water or E.C.W. ).
Chloride concentration was determined with a Buchler-Cotlove chiloridometer.
The animals were subsequently assessed for survival.
Comparison of concentrations of blood and E.C.W. in freshwater stressed animals was made both with the Chloridometer and with a nanoliter freezing-point osmometer.
Blood samples were obtained after all extra-corporeal water had been expressed and blotted away.
For ease of comparison, all concentrations are expressed as per cent sea water, where 100% sea water has a salinity of 35%.
Tolerance to acute immersion in solutions hyperosniotic to sea water was tested by immersing limpets for 5 hours in aerated solutions, followed by return to running sea water for subsequent assessment of survival.
The effects of a gradual rise of salinity were examined by gradually adding concentrated sea water to a small quantity of normal sea water containing the experimental animals.
Rate of equilibration of linipets to 500% sea water was determined by suspending four specimens of A. digitalis by a fine wire from the pat-i hook of at-i analytic balance, and a beaker of the hypcrosmotic solution was raised to cover the limpets, and their weight while immersed was recorded at intervals for 24 hours.
The degree of volume regulation, or of osniotic dehydratioti, was checked by immersing Acnu.tea digitalis on plastic sheets overnight in 400% sea water and comparing loss in wet blotted weight with initial total water (initial wet blotted weight minus dry weight with shell).
Fresh water dilution or evaporative concentration of limpets in the field was measured by drawing 1 microliter samples of E.C.W. into capillary micropipettes from limpets freshly removed from the rock surface.
Results
Air, rock surface, and limpet temperatures fluctuated similarly in both Zones I and II.
Nighttime temperatures reached a minimum between 03:00 and 06:00.
On a few occasions, mild frosts (-2^
circ C) occurred during the winter, but limpet temperatures never dropped below freezing.In the morning, temperatures rose until the sun was past its zenith, then fell gradually until the returning tide caused an abrupt drop to sea surface temperature.
The highest temperatures of rock surfaces and individual limpets were recorded on clear, calm, sunny days (Table I) when the lower low tide occurred during the late morning and left much of the intertidal exposed during the hottest part of the day.
The temperatures of limpets and the rock stirfaces adjacent to their roosting sites depended primarily on their orientation to the sun, amid to wimid and spray. Intertidal height was of secondary importance, serving principally to determine the length of exposure to high temperatures.
Since Zone II is exposed for shorter periods, the probability of tidal exposure coinciding with the hottest part of the day is lower than in Zone I, and maximum temperatures usually were lower in Zone II.
On occasions when both Zone I amid Zone II were exposed during the heat of the day, maximum temperatures of sites similarly exposed to the sun could be virtually identical iii both zones (Table I).
In a given area, maximum temperatures of those individual limpets most exposed to solar radiation showed pronounced interspecific differences.
Maximum temperatures of A. scabra were higher than those of the most exposed A. digitalis, which in turn were higher than those of A. persona, A. pelta and A. t. scutitnt in the same area (Table II).
Thermal tolerances of the five species, determined during itiiiniersion itt sea water, are lower for the Zone II species, intermediate for A. persona, and higher for ti-ic remaining Zone I species (Table III ).
Also thermal tolerances of A. scabra arid A. digitalis, which experience the widest range of tiiicroclimates, were essentially constant regardless of seasot or collecting site : those of winter-collected samples were at most 1^
circ C below those of summer samples ; those of A. scabra and A. digitalis collected in Zone I I were at most 10^
circ C below those of Zone I samples.Thermal tolerances determined under simulated natural conditiotis were about 5 ^
circ C higher than tolerances determined during immersion (Table III ) for A. digitalis, A. pelta, and A. t. scutum, and presumably for the remaitiing species as well.Thermal tolerance seems to be it-idependent of size ; no differences were evident between small (9.0 \pm 0.27 mm) and large specimens of (16.4 \pm 0.28 n-in-i) A. digitalis.
Removal of 20% to 60% of the total body water by desiccation did reduce the thermal tolerance of A. digitalis by about 3^
circ C.Both the usual and extreme values of the factors pertinent to desiccation—temperature, humidity (as vapor pressure deficit), and windspeed—are summarized in Table IV.
Desiccation rates of all five species of limpets, living or dead, or limpet shells filled with 15% gelatiti, or evet small glass vials filled with 15% gelatin, showed a l)attert sit-i-iularto Figure 2 tinder constatt conditions.
A high initial rate of water loss, which after a time declites to a lower, fairly constant rate, is characteristic of water-cottainitg permeable bodies exposed to constant dryitig conditions.
All of ti-ic species shiowed a lifting of the shell at high temperatures, cited by Segal at-id Dehnei ( 1962) as a mechianism for evaporative cooling.
Under desiccating conditions ti-ic Iimi)ets tend to conserve water rather that-i using it to regulate body temperature : ti-ic shell-lifting response is probably evidence of impending heat con-ia.
Average desiccation rates utdcr conditions approximating a cool windy clay at Bodega Head, using 25-30 individuals of each species, were markedly lower iii the Zone I limpets than in the Zone IT species (Table V ).
Also Ac;naea scabra was omitted since the serrated shell margins of this species would not fit closely to the plastic discs, as they would to the rock.
Under more rigorous conditions, simulating those occurring during unusually warm weather, all four species tested lost water at higher rates. The increase of rate in the Zone II species was roughly proportional to the increase in vapor pressure deficit, as expected from the general evaporation formula:
Evaporation = K ( vapor pressure deficit ) C( \frac{windspeed}{ length of evaporating surface } )^nThe increase in desiccation rate of A. persona is nearly three times that predicted by the evaporation formula, possibly implying the breakdown of a regulatory mechanism at an elevated temperature not usually experienced by this shade-dwelling limpet.
Doubling the windspeed increased the desiccation rate of A. digitalis by a factor of 1.5, suggesting that the exponent “n” in Leighly's formula is about 0.5
Desiccation continues even when conditions during exposure do not appear at all stressful ; 20 specimens of Acmaea digitalis, pre-dried until 20 to 75 % total water had been lost, amid then exposed outside overnight in fog, continued to lose water (a maximum of 5% total water lost overnight).
During the course of the study found an ability of the Zone I limpets to form a mucus sheet between the shell margin and the rock surface.
To assess the importance of this mucus sheet in siowitig desiccatioti. Desiccation rates of A. digitalis with the normal smooth shell margins, A. digitalis with chipped shell margins, A. scabra with smooth shell margins from the artificial tide system, and A. scabra with ti-ic normal rough shell margins were cotiipareci.
These results indicate that ti-ic normal desiccation rate of A. scabra on its homesite is about the same as that of A. diqitalis which does not home.
The effect of removing the mucus sheet was investigated with four groups of A. digitalis.
Desiccation tolerances of the five species of limpets under conditions approximating a cool, breezy day (20° C, 1-1.4rn/sec wind) , expressed as per cent of total body water lost and as corresponding internal osmotic concentrations (Table VII), are correlated with the intertidal height of each species' normal habitat.
To examine the implications of rainwater runoff, all five species of lirnpets were exposed to standing fresh water for 4, 14, 28, and 42 hours.
Extra-corporeal water concentrations at which limpets died indicate that ti-ic limit for all five species is around 30% sea water.
Dilution of E.C.W. caused a decline in l)lOOd chloride concentrations to values below those of ti-ic E.C.W.
A single tidal exposure in a seep of 1.7% sea water depressed E.C.W. chloride in 5 individuals of A. digitalis to 38-40%sea water-near the lethal limit.
The line represents data calculated for evaporation from at-i ideal salt solution ; the fit of experimental limpet data to this line indicates that salts are being neither voided into the mantle cavity water and urine compartments, nor sequestered in the body during the progress of desiccation.
Total Water, Hydrated/Weight of Soft Parts, Hydrated) was similar for all five species at-id for both large and small A. scabra (Table VIII).
All five species tolerated 5-hour iiiimiiersion in comicentrations up to 400% sea water ; at 500 and 600% sea water the majority of A. pelta and A. scutu@n perished while the majority of the Zone I limiipets survived.
Equilibration of A. digitalis to 500% sea water, as shown by changes in weight while immersed due to water loss amid salt tmptake, was 90% complete in 4 hotirs (Fig. 8).
The osmotic stress produced by immersion in 400% sea water is equivalent to that produced by evaporative loss of 75% of the total water.
Gradually raising salimiity over 20 hours to 250% sea water simulated the osmotic effect of evaporatively removing 60% of the total water, a level well below the acute tolerances of even A. pelta and A. t. scutuin. Nevertheless, both A. pelta and A. t. sczttunt began to (die early in the second day of exposure and all were dead within 33 hours.
Discussion
The acmaeid limpets present a convenient system it which to approach this problem, because there are several abundant species which collectively occupy ti-ic entire intertidal range.
The response of all of these himpets to tidal exposure is immobility-the clamp down response of McAlister and Fisher ( 1968).
The aquatic microenvironment of such roosting limpets is the small quantity of water retained under the shell.
The physical environmental factors representing potential stresses to limpets differ according to season.
Limpets appear to have no defense, other than ti-ic clamp-downresponse, against dilution by fresh water.
Despite their apparent vulnerability to dilution by fresh water trickling down the rock surface, the various species of Acrnaea show no pronounced differences in either resistance to dilution or tolerance of depressed internal electrolyte concerti trations (Fig. 6) , suggesting that no strong differential selection for these traits is taking place in the field.
Thus lethal, or eveti stressful, dilution by winter rainfall appears to be at-i extremely unlikely situation for even the highest of the limpets.
Exceptionahlv wart-i-i weather was associated with all of ti-ic limpet kills previously reported (Orton, 1933 ; Hodgkin. 1959 : Frank, 1965a : Stitherland, 1970), and with tiost of those observed (luring the course of this study.
A hypothesis consistent with all of these data is that cumulative, eventually lethal desiccation is ti-ic limitingfactor determining the partitioning of ti-ic intertidal by these five species of Acnzaea.
However, Davies (1969) ; high-shore Patella vulgata show lower desiccation rates than low-shore P. vulgata or P. aspera.
The ability to form a mucus sheet between shell margin at-id substrate ( Fig. 3), which occurs in the species with low desiccation rates, is by far ti-ic most important adaptation.
The homing habit restricts A. scabra to the area within which it can forage at-id return to its homesite during a single tidal immersion.
It seems clear that the mechanism by which desiccation causes death in Acniaea is through concentration of the body fluids to lethal levels.
Upper range limits of two otherwise very similar species, A. pc/ta and A. t. scutusn, are determined by their behavior, not desiccation mortality.
The data presented for Acinaea stipport 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 alga! film, are limited by desiccation.
Summary
This study tests ti-ic hypothesis that physical factors limit ti-ic ranges of five spedcs of himpets (Actnaea) inhabiting the splash zone (Zone I) at-id upper mid tidal (Zone II) of the Central California rocky shore. Three years of data suggest both mortality relating from exposure and behavioral adaptations influence range.
Dilution by winter rainwater rumioff probably presents mio osmotic threat to Ac,naea.
Interspecific differemices in toleramice to high temperatures are clearly correlated with solar heating occurring in the species' natural niicrohabitats.
During three years, niaximtmni field temperatures never exceeded the thermal tolerance of any of the !impets.
The Zomie I himpets show higher desiccation tolerances amid will tolerate drying conditions mtmch longer than Zomie I I himpets. As expected, the Zomie I linipets also have higher tolerances to hyperosmotic soltmtions and will tolerate elevated concentrations longer thami will Zone II himpets.
Tolerances of desiccation amid of hyperosmotic solutions in all 5 species are extremely high, ramiging from about 70% to about 82% total water lost. and from 400% to 600% sea water.
Mortality during desiccation cami be attributed entirely to the concentration of internal fitmids resulting front evaporative water loss. Contrary to earlier reports, there are no increased water reserves in Zone I linipets.