Stephen A. Chun TTROD TION A major effect of grazing on the marine community was previous¬ ly found to be the selective elimination of foliose macrophytes (Foreman, 1977). Grazing also appears to be an important factor in controlling the overall abundance of algae in the high intertidal community (Cubit, 1975) and in keeping rock surfaces free of lit¬ toral diatom populations (Castenholz, 1961). The purpose of this study is to provide some information of the relationships between herbivorous gastropod grazers and the epiphytic microalgae growing on the fronds of the erect, red macroalga, Rhodoglossum affine (Harv.) Kylin, in the rocky intertidal community. MATERIALS AND METHOD This study was conducted in the rocky intertidal zone near Mussel Point, Pacific Grove, California, within the research reserva¬ tion around the Hopkins Marine Station of Stanford University in May. 1979 (illustration 1). Four 39 by 39 cm study areas were marked off on two vertical rock faces in the mid zone of Rhodoglossum affine at the same tidal height of 1.5 to 3.0 feet above mean low low water on the shoreward faces of the rocks. Ninety-five percent of the area in this zone was covered by R. affine, while the remaining 5 % consisted of granite substrate and other erect red algae: Gigartina papillata (C. Agardh) J. Agardh, Gelidium sp., and Iridea sp. Inclusion and exclusion fences 3 inches high were made of resin -3- Stephen A. Chun coated, 1/2 inch marine plywood boxes. They were fitted and bolted to the granite substrate using lead anchors. The bottoms were seal¬ ed to the substrate with quick-drying cement and epoxy putty, and a fine wire mesh 5 cm wide was nailed to the tops. One fence was placed on each vertical face while the un-fenced study area which was 30 cm to the right of each fenced area served as a field control. The high snail density study area was created by placing the following numbers of prosobranch gastropods in the algal holdfasts in one fenced area twice each week: 200 Tricolia pulloides (Carpenter, 1865); 100 Barleeia haliotiphila (Carpenter, 1864); and 25 Tegula funebralis (A. Adams, 1855). These snails were col¬ lected from different areas and kept in aquariums a few days before their addition. The low snail density study area was created by re¬ moving all gastropods and other grazers which were found while exam¬ ining the algal holdfasts, the algal fronds, and the granite sub¬ strate within the other fenced area once each week. Four to five randomly selected fronds were collected from all four areas each week. The top portion of the frond above the second branching was removed and the remaining area was scored under a dissecting micro¬ scope at 30X magnification for the percentage of area which was cov¬ ered by grazing tracks and by epiphytes. The vertical distribution of these tracks and epiphytes was also recorded. Student t-tests were computed to compare the means of the total number of fronds collected from each area on weeks 1, 2, and 3 1/2. Stephen A. Chun -4- On the day of the last collection, snail densities of each study area were determined by removing the algae in 10 randomly selected 16 cm" areas in each study area. Densities were determined by the number of snails per gm of dry weight of algae dried at 60* cen¬ tigrade for 12 hours. The epiphytes were identified by Dr. I. A. Abbott. Grazing tracks were identified at 30X magnification under a dissecting mi- croscope by comparisions with known grazing tracks made on pieces of Rhodoglossum affine by herbivorous gastropods kept in the lab. RESULT As figure 1 indicates, the largest percentage of frond area examined was covered by the green alga, Endophyton, which grows into the cuticle and cortex of the fronds, and by dense growths of pennate diatoms which seemed to be imbedded in the cuticle. Pennate diatoms were also found in other areas under the compound micro¬ scope, but only the dense growths (illustration 2) were clearly visible as shiny specks under the dissecting microscope. Covering a much smaller percentage of frond area examined were: the green algae, Pseudodictyon sp. and Enteromorpha sp.; blue-green algae growing in cocoid clusters; a filamentous red alga, Acrochaetium sp.; a small coralline red alga; and centric diatoms. As figure 2 illus¬ trates, Endophyton was found mainly low on the fronds while the dense pennate diatom growths were found mainly on the upper portions of the frond area examined. -5- Stephen A. Chun The types of grazing tracks which were identified on the fronds were the clusters of dots made by either Tricolia pulloides or Bar¬ leeia haliotiphila, and the larger, thin, and somewhat sickle-shaped tracks made by Tegula funebralis or Tegula brunnea (Phillippi, 1848) (illustration 3). Figure 2 shows that grazing tracks made by T. pul- loides and B. haliotiphila were found mainly high on the frond area examined while grazing tracks made by Tegula sp. were found all over. In the area where the grazers were removed, the percentage of frond area examined bearing Tricolia pulloides and Barleeia halio- tiphila grazing tracks was significantly lower than its field con- trol (fig. 3), while the percentage of frond area examined which was densely covered by diatoms was significantly higher than its field control (fig. 4). Figure 5 shows that when compared to its field control, this same area had no significant change in the per¬ centage of frond area which was covered by other epiphytes during this time period. The relationship of Barleeia haliotiphila and Tricolia pulloides grazing tracks with the dense pennate diatom growths is shown in figure 6 where the study area with a high percentage of T. pulloides and B. haliotiphila grazing tracks had a low percentage of dense pen- nate diatom cover, while the area with a low percentage of B. halio- tiphila and T. pulloides grazing tracks had a correspondingly high percentage of dense diatom cover. Figure 7 shows that the study area where the snails were re¬ moved did have a significantly lower density of Tricolia pulloides -6- Stephen A. Chun and Barleeia haliotiphila than its field control, and that the study area where the snails were added did have a significantly higher T. pulloides and B. haliotiphila density than its field control. Fig- ure 7 also shows that the percentage of frond area bearing B. haliotiphila and T. pulloides grazing tracks increases with snail density while the percentage of frond area covered by a dense growth of pennate diatoms decreases. This relationship between snail den¬ sity and percentage of frond area densely covered by pennate diatoms is plotted in figure 8. These points best fits a power function showing an inverse relationship between B. haliotiphila and T. pul- loides density and the percentage of frond area covered by a dense pennate diatom growth. DIS( CUSSION The positive relationship between snail density and percentage of frond area examined bearing Barleeia haliotiphila and Tricolia pulloides grazing tracks indicates that grazing pressure was de- creased in the study area where the snails were taken out. The in¬ verse relationship between grazer density and the percentage of frond area densely covered by pennate diatoms further suggests that this decreased grazing pressure of B. haliotiphila and T. pulloides had a positive effect on the percentage of frond area densely covered by pennate diatoms. The hypothesis that high T. pulloides and B. halio- tiphila densities reduces diatom populations is further supported by the finding that both T. pulloides and B. haliotiphila grazing tracks Stephen A. Chun -7- and these dense pennate diatom populations were found mainly on the upper portions of the frond area which was examined (fig. 2). At the same time of this study, Mooers (1979) found that pennate diatoms make up 90 - 95 % of the diet of T. pulloides, and Foster (1979) found tha T. pulloides will move to the upper portions of algal fronds under optimum conditions. All of these findings seem to support the hypothesis that the grazing of T. pulloides and B. halio- tiphila reduces the percentage of frond area covered by a dense growth of pennate diatoms and suggests that T. pulloides moves out to the upper portions of the fronds because that is where its major food source is very abundant. SUMMAR) 1. Grazing pressure on the epiphytes on the fronds of the intertidal red alga, Rhodoglossum affine, was manipulated by adding and re- moving herbivorous gastropods. 2. The green alga, Endophyton, and dense pennate diatom populations were found to be the most abundant épiphytes in percentage of frond area covered. 3. The primary grazers were found to be the prosobranch gastropods, Tricolia pulloides, Barleeia haliotiphila, Tegula funebralis, and Tegula brunnea. 4. Grazing by Tricolia pulloides and Barleeia haliotiphila was found to reduce the percentage of frond area covered by dense growths of pennate diatoms. Stephen A. Chun ABSTRACT Grazing pressure on the epiphytes on the fronds of the inter¬ tidal red alga, Rhodoglossum affine, was manipulated by adding and removing herbivorous gastropods. The green alga, Endophyton, and dense pennate diatom populations were found to be the most abundant epiphytes in percentage of frond area covered. The primary grazers were found to be the prosobranch gastropods, Tricolia pulloides, Barleeia haliotiphila, Tegula funebralis, and Tegula brunnea. Grazing by T. pulloides and B. haliotiphila was found to reduce the percentage of frond area covered by dense growths of pennate diatoms. -8- Stephen A. Chun -9- ILLUSTRATION 1. Map showing study site at Mussel Point, Pacific Grove, California. 2. Scanning Electron Microscope photograph of dense pennate diatom growth on the frond of Rhodoglossum affine. Drawing of grazing tracks as viewed under 30X magnification under a dissecting microscope. C Stephen A. Chun o5 . : . ... ... .*.. 5 8 -10- Stephen A. Chun 7. - :: * . 4 tata + R —-- Illustration 2 -11- Stephen A. Chun TRICOLIA PULLOIDES BARLEEIA HALIOTIPHILA 1 mm mm Illustration 3 TEGULA FUNEBRALIS TEGULA BRUNNEA -12- Stephen A. Chun -13- FIGURES 1. Histogram of epiphytic abundance (measured in % of frond area) on fronds which were collected in the control study areas. 2. Histogram showing the vertical distribution of the epiphytes and grazing tracks found on R. affine fronds. The solid bar denotes that it was found only on the bottom portion of the frond; the striped bar denotes that it was found only on the top portion of the frond area examined; and the white bar indicates that it was found all over the frond area examined. Graph plotting the % of frond area bearing T. pulloides and B. haliotiphila grazing tracks over time in the low snail density study area and its control. X is the mean for all the fronds collected on weeks 1, 2, and 3 1/2. The bars denote the standard of error. (p .02). 4. Graph plotting the % of frond area covered by dense pennate diatom growths over time in the low snail density study area and its control. X is the mean for all the fronds collected on weeks 1, 2, and 3 1/2. The bars denote the standard of error (p.001). Graph plotting the % of frond area covered by epiphytes other 5. than dense pennate diatom growths over time in the low snail density study area and its control. X is the mean for all the fronds collected on weeks 1, 2, and 3 1/2. The bars denote the -14- Stephen A. Chun the standard of error (p£0.9). Histogram showing the % of frond area covered by dense pennate diatom growth (striped bar) and the % of frond area bearing T. pulloides and B. haliotiphila grazing tracks (white bar) for the low snail density study area and its control for weeks 1, 2, and 3 1/2. Histogram showing T. pulloides and B. haliotiphila density and the % of frond area which was covered by dense pennate diatom growths (striped bar) and by T. pulloides and B. haliotiphila grazing tracks (white bar) at the end of the 3 1/2 week per- iod. The bars denote the standard of error. Graph plotting B. haliotiphila and T. pulloides density vs. the % of frond area covered by dense pennate diatom growths at the end of the 3 1/2 week period. The points best fit the power 1.4 12 - 0.84 function: diatom coveragel - 5.3snail density 9 Stephen A. Chun -15- % OF FROND AREA EXAMINED 6 Stephen A. Chun % OF TOTAL FRONDS EXAMINED 8 H D Z -16- Stephen A. Chun — O % OF FROND AREA BEARING T PULLOIDE S AND B. HALLOTIPHILA GRAZING TRACKS +e1 — o— -17- Stephen A. Chun o C o o -18 % OF FROND AREA COVERED BY DENSE GROWTHS OF PENNATE DIATOMS e —— Stephen A. Chun + — — O % OF FROND AREA COVERED BY EPIPHYTES OTHER THAN PENNATE DIATOMS 5 p Fo+ — o -19- Stephen A. Chun 20 -20- LOW SNAIL DENSITY LOW CONTROL STUDY AREAS Figure 6 Stephen A. Chun ". pulloides and B. haliotiphila DENSITY (4 snails / gm dry wt. algae) 20 10 — — — Figure 7 -21- % OF FROND AREA COVERED 10 20 LOW SNAIL DENSITY LOW CONTROL HIGH CONTROL HIGH SNAIL DENSIT Stephen A. Chun -22- 30 20 10 - 10 Tricolia pulloides and Barleeia haliotiphila DENSITY (4 snails / gm dry wt. algae) Figure 8 Stephen A. Chun -23- ACKNOWLEDGEMENT I wish to thank the entire faculty of Hopkins Marine Station and especially Dr. Robin D. Burnett, Dr. Isabella A. Abbott, and Dr. Donald P. Abbott for their wonderful support and insight. A special thanks to my advisor Mr. William H. Magruder whose guidance was invaluable throughout the project. -24- Stephen A. Chun TERATURE CITED Castenholz, Richard W. 1961. The effect of grazing on marine littoral diatom pop¬ ulations. Ecology 2(4) : 783 - 794 (Autumn 1961) Cubit, John David. 1974. Interactions of seasonally changing physical factors and grazing affecting high intertidal communities on a rocky shore. i-xi + 1 - 122; Dept. of Biol., Univ. of Oregon (March 1975). Foreman, R. E. Benthic community modification and recovery following 1977. intensive grazing by Strongylocentrotus,droebachiensis. Helgol. Wiss Meeresunters 30(1-4) : 468 - 184 (August 1977) Foster, Kelly. Movement of Tricolia pulloides on Gigartina papillata. 1979. (Unpublished manuscript on file at the Hopkins Marine Station Library) (Spring 1979) Mooers, Mary G. 1979. Diet and reproductive biology of the rocky intertidal prosobranch gastropod, Tricolia pulloides. (Unpublished manuscript on file at the Hopkins Marine Station Library (Spring 1979.