Effects of Rock Temperature on Limpet Densities

E. T. Kosman

 

 

 

 

Abstract:

            Temperature plays an important role in the zonation of the intertidal community.  To date, very few studies have examined the role of substratum temperature in zonation.  Rock temperature and species density was recorded to determine if there was such an effect.  This study found no daily effect of substratum temperature on zonation, but found a possible seasonal effect on zonation.

 

Introduction:

 

            Zonation in the intertidal has been attributed by two causes, physical stresses and predation.   Tolerance to physical stress is believed to set the upper limit of the distribution of a species.  These physical stresses are desiccation, osmotic stress, oxygen deprivation and temperature stress (Somero, 2002). 

In particular, resistance to temperature stress has been shown to be correlated with an increasing tidal height range (Somero, 2002).  Temperature stress has often been linked with the growth rate of mollusks, showing an important sub-lethal impact on species (Kenny, 1983).  While studies have been conducted on lethal temperatures of intertidal species (Miller, 2005) and the roles that temperature plays in zonation (Bertness et al, 1999), these studies have used ambient temperatures in their calculations.  Few studies have focused on the role that microhabitat temperatures may play in the distribution of intertidal invertebrates.

Helmuth (2002) stated that the color of the shell and how much contact it had with the substratum help dictate the animal’s internal temperature.  Thus, the temperature of the microhabitat may play an important but relatively undocumented role in species distribution.  In a study done on barnacles, it was noted that by artificially shading the rocks the temperature of the substratum significantly increased survivorship and decreased the temperature of the rock (Bertness et al, 1999).

Another set of intertidal species that have a high surface area in contact with the substratum are limpets.  Limpets are found in all intertidal zones and have known lethal temperatures which determine their placement in the intertidal (Wollcott, 1973).  Previous work on limpets has shown that the internal body temperature of the animal in equal to that of its substratum (Wollcott, 1973).  Four species of limpets common off the coast of California will be the focus of this study: Lottia pelta, Maclintockia scabra, L. limatula, and L.digitalis.  L. pelta and L. limatula inhabit the low intertidal, M. scabra inhabits the mid intertidal, and L. digitalis inhabits the high intertidal, although there is some habitat overlap among the four species.

The purpose of this study is to determine if there is a difference in the densities of limpet species based on the temperature of the substratum.  By examining the effects of the rock temperature on limpet density, I hope to determine the importance of the microhabitat in zonation studies. 

 

 

Methods:

Two sites were selected, one at Cabrillo Beach and the second at Wylder Annex.  At each site at least 20 rocks were selected along a transect line.  A 0.16 x 0.155 m² quadrat was used to estimate limpet densities on the rock.  The quadrat was placed on the side of the rock that was closest to the water line to account for wave action, and placed 0.16 m above the ground. 

Adults were identified as one of four limpet species (M. scabra, L. digitalis, L. limatula, or L. pelta) and counted. Sub-adults and other species were counted, but not identified.  Limpets were counted on two separate days; September 18 and November 13 in 2005.  Limpets were counted when mean low tide was less than +0.4, and no earlier in the day than 12:30 p.m.(Pacific Standard Time). Rocks temperature was measured within the quadrat using a Raytek ST Pro Non-Contact Thermometer.

              One-way ANOVAs were performed to determine if total limpet densities varied by date or by site.  One-way ANOVAs were also used to determine if there was a significant interaction between temperature and date or site.  A two-way ANOVA was used to determine if there was any significant interaction between site and the total densities of each limpet species.  As number of rock sampled for each date was different, a GLM was performed to determine if there was an interaction between the total density of each limpet species and sampling date.  A regression analysis was performed for each species to determine if temperature of the rock affected the density of the species.

 

 

 

Results:

            No L. pelta were sampled at either site, so this species was excluded from the analysis.  There was no significant difference in the total number of limpets sampled by date (Fig. 1).  However, there was a significant difference in total numbers of limpets found between sites (Fig. 2), with more limpets found at Wylder Annex than at Cabrillo Beach.     

Fig. 1:  There was no difference in the number of limpets found, regardless of species, between the two sampling dates (P=0.37, F=0.83, DF= 63). 10.15 is September 15th 11.13 is November 13th

Fig. 2:  There was a significant difference in the number of limpets found, regardless of species, between the two sampling sites (P=0.016, F=2.61, DF=63). 1 is Wylder Annex 2 is Cabrillo Beach

There was a significant effect on temperature due to site and date, with Wylder Annex having a higher temperature than Cabrillo (Fig. 3).  This could be due to the fact that Cabrillo was sampled second, when the temperatures had begun to decline.

Fig. 3: Temperature significantly different by site (P<0.001, F=16.33, DF=63). 1 is Wylder Annex 2 is Cabrillo Beach

 

When the effect of site on temperature was removed there was a significant effect date on temperature for Wylder Annex (Fig. 4) with the temperature lower during November than in the September sampling period, but no effect of temperature by date for Cabrillo Beach.  Cabrillo Beach rock temperatures were widely variable during the November 13 sampling period, possibly due to the windy and calm periods in weather experienced at that time. 

There was no effect of site on the densities of each individual limpet species (Fig. 5), even though total limpet densities were found to be lower at Cabrillo Beach (Fig. 2).

 

^{Fig. 4:  Wylder Annex showed a significant difference in rock temperature due to date (P=0.001, F=15, DF=31), whereas Cabrillo beach did not (P=0.105, F=2.8, DF=31).}

 

There was a significant effect of sampling date on individual species density.  Less L. digitalis and more L. limatula were counted on November 13 as opposed to September 15 (Fig. 6). 

Fig. 5: Box plots showing the densities for each species by site.  There was no significant difference between the sites (P=0.076, F=2.61, DF=2). * are outliers 1 is Wylder Annex 2 is Cabrillo Beach

  

Since there was a significant effect of temperature on sampling day and no effect of species density by site, samples were combined by site and not date for regression analysis.  Regression analysis showed that there was no significant correlation between temperature of the rock and limpet species for any of the three limpet species for both days (Fig.7).

Fig. 6: Shows the mean number of L. digitalis found per rock decreased, while the mean number of L. limatula increased on November 13 sampling date (P=0.007, F=5.10, DF=2).

 

 

 

Fig. 7A-E:  Scatter plot of rock temperature and individual species density, with a line of best fits.  Regression analysis showed no significant effect of rock temperature on species density after taking sampling date into account (P>0.05).  R-squared values= 0.004 (A), 0.039 (B), 0.004 (C), 0.036 (D), 0.064 (E), 0.020 (F).

 

 

 

 

Discussion:

            Although there was no discernable effect of rock temperature on the distribution of limpet species, this study shows a possible effect of seasonal temperatures on limpet distribution.  Daily rock temperature seems to fluctuate considerably based on the weather conditions of the site, as evidenced by Cabrillo Beach’s wide temperature range during November 13.  During that day, there was a shift between windy and calm periods which resulted in rock temperatures ranging from 19º-27ºC, a much wider range than the other sampling periods.  Daily temperature fluctuations may be too short a time span for limpet reaction to be recorded.  On the other hand, Wylder Annex showed a consistent rock temperature decrease between the two sampling dates, with November being much cooler than October. 

This cooling trend may allow for migration upwards intertidally of the limpet species.  Seasonal migrations in the intertidal have been recorded for chitons (Harper and Williams, 2001) and limpets (Liu, 1994) in Hong Kong.  L. digitalis has also been noted to migrate upwards during October to gain more food, and downwards during spring summer season to avoid desiccation (Breen, 1972).  In this study the same area in the intertidal was sampled, and by finding an increase of L. limatula, a lower intertidal species, and a decrease in L. digitalis, a mid intertidal species, there seems to be some support for the idea of seasonal migration being the cause of this difference.

Breen (1972) noted that L. digitalis was able to move five meters or more away from its initial starting point in a month while migrating.  Takada (1999) observed that mobile gastropods were able to evaluate their environment and move into shady areas within two weeks.  Thus, it is very probable that L. limatula and L. digitalis were able to move from the study area within the time frame of the two sampling periods.  Harper and Williams (2001) noted that chitons who were able to conform better to their crevice did not exhibit a migration pattern, as they were able to use their crevice as a refuge from heat.  This may explain why there was no difference found in the densities of M. scabra, which resides in a home scar (Sept, 2002).  While daily rock temperatures may not affect limpet densities, monthly and seasonal temperatures may.

Further studies are needed to validate this hypothesis.  A semi-permanent transect across the different zones of the intertidal and monthly measurements of limpet densities by species may accomplish this.  In addition, marking limpets and observing individual limpet movements would help us understand rates and direction of migration seasonally. 

           

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Literature Citied:

 

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Breen, P.A. 1972. Seasonal migration and population regulation in the limpet Acmaea (Callisella) digitalis. Veliger. 15:133-141

 

Harper, K.D., G.A. Williams. 2001. Variation in abundance and distribution of the chiton

Acanthopleura japonica and associated molluscs on a seasonal, tropical, rocky shore. Zoological Society of London. 253:293-300

 

Helmuth, B. 2002. How do we measure the environment?  Linking intertidal thermal physiology and ecology through biophysics. Integrative and Comparative Biology. 42:837-845

 

Kenny, R. 1983. Growth characteristics of intertidal limpets in relation to temperature trends. Pacific Science. 37:37-44

 

Liu, J.H. 1994. The ecology of the Hong Kong limpets Cellana grata (Gould, 1859) and Patelloida pygmaea (Dunker, 1860): Distribution and population dynamics. Journal of Molluscan Studies. 60:55-67

 

Miller, L.P. Effects of high temperature and desiccation stress on the survival of the giant owl limpet, Lottia gigantea. Conference Society for Integrative and Comparative Biology. San Diego, CA. 4-8 Jan 2005.

 

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Takada, Y. 1999. Influence of shade and number of boulder layers on mobile organisms on a warm temperate boulder shore. Marine Ecology Progress Series. 189:171-179

 

Wolcott T.G. 1973. Physiological ecology and intertidal zonation in limpets (Acmaea): a critical look at "limiting factors". Biological Bulletin. 145:389-422