ABSTRACT The subpopulation of Collisella scabra found on the shells of Lottia gigantea show significant differences from the general population of C scabra: population density is ten times less, shell/hody weight ratios are higher indicating slower orputh patterns possibly due to limited food availability, and denmity on the shell increases in areas of high wave astion possibly due to the faster rate of algal growth in these areas. Ihe limited foraging area on the Le gigantea shell may provide an upper limit to the number of riders it can support, as riders from crowded shells do not lower their metabolic rates. HE p tdtheea coat otherwise found on their shells. This loss of algae, the creation of the Cy scabra's home scar, the added weight of the riders, or the drag differences created by the riders cause no amparent advantage or disadvantage to the Li gigantea. While these remults are in ho way comprehensive, they provide a backorpund knowledge for further studies of the species interactions. Introdution In this study I examine the interactions between Collisella seabra and Lottia gigantea in particular the association between the subpopulation of Co scabra which live on the surface of the Le digantea shell (henceforth referred to as riders). What are the differences between this subpepulation and the genemal E scabra population (henceforth referred to as nonmiders) and, setund, what are the differences between Li gigantea with and without these C. scabma riders on their shells? Ihe existente of riders on limpets shells is not an entirely pew phenomenon. A surprising number of limpets live on the shells of other animals. Species of acmaeids and patellids live on the shell of conspecifics until they become too large, at which time they migrate onte the surrounding rock (Branch, 1971). Other spei suha Eatellidainiis and C are found to home on a series of other shells, changing host shells rapidly (Eikenberry & Wickizer, 1944, Creese, 1778). However, Om scabra are unique in that they ride on the back of a species othem than their own and in greater density and numbers than ather miders. In this study of the interactions between riders and the L disiamtes on which they are found, I maks preliminary field ubsemvations and measurements of shell and body welghte and sizes, C. scabra densities and metabolic mates, and tood availability on the Ey digantea shell surface. While the results t m tha theya th att later. Methods d studie All field observations and samples of kottia gigantea and Collisella scabra were conducted at Hopkins Marine Station, Facific Groye, CO. Sites 41 and 2 were on two sheer rack taces at Cabrillo Foint, directly facing the oncoming surf and site e nas a wall near Bird Hock at an angle of approximately 45 degrees to the ancoming sumf. I chose these sites because of their abundance of Lottia gigantea at the widest range of intertidal Heights to e fund at the Station. I observed the Li gigantea and Ce scabra during 30 daytime low tides and three high tides: 1.)a high high tide at 2100 a.m. 2y) a low high tide at 6100 a.m. (1/T heur after dawn) and d.)a hich high tide at 4:00 p.m. The high tide watches were done at sites i a T fom six hours, beginning three hours before high tidm. aand measured the lengthe and widths of all the Li gidam all of their pe scabra riders found at eath of the three sites. The pedal area of the limpets was then estimated to be mi (lenathith. t mesaumed in the field. However, within sach species the shapes af the shells, including the height of the apes, remains constant as a funntien of length and width. Therefore, surface area of the shells depende primarily on the length and width of the shell and the equation for pedal area was thus used as an estimation of ralative sumface amea of the shelle within each species. If it is again assumed that the shape of the limpet does not change as it grows (a reasonable assumption for these species), an estimate of relative limpet volumes is obtained by (pedal 1. area) The Da scabma density on the Li gigantea shells were calculated as (the of scabra riders on the bi gigantee shell)/(shell surfate area). (Note: Li gigantea pedal area was used as am estimation af shell suuface amea as noted above.) Spasial density of the Sa scabma in the area surrounding a lottia was malculated in two ways. First, as a measure of the sheem numbers af Dy sgabma in the area, I counted the number of scabra within a SOcm. X SO em. quadrat centered on each Ee didantea. Ca srabra demsity equalled (the of C. scabma in the transect)/(2500 em"). Second, as a measure of the numbers of C. scabna per area available for foraging, Cr scabma density equalled (the of C. scabra in the transect)/ (2500 emt - the zum af all areas within the transect bulldozed by by ginantea). The bulldezed grazing areas were measured using a grid divided into areas of 4 emt and counting the number of these divisions covered by the graing armas. kaiataauda messured at mite 2. This site was chosen betause it offered the greatest number of Li gidantea at intertidal heights unintergupted by large musmel beds. I meamured the length and Midthataltatta t taa a. divided the area inte &am horizontal transeats each 0 am wide beginning at the point of the lowest lottia. I computed and averaged the pedal areas of all Ly gigantea within each transect (4-7 luttia weme in each transect). I used the same method for measuring a scabma sise as a function of intertidal height except that I did not measure all the C. smabma at site H2. Instead, a 2Scm.-wide vertical strip was haphazardly chmsen at the site. Within the strip, I measured the lengths and widths of all the C. scabra at the same o herizontal transects marked off from the lottia measurements (4-9 scabra were in each transet). Laboratory Studies I measured the amount of standing algal growth on eight digantea shells and also noted the number of riders on each of these shells. On one shell was a fresh scar that appeared to be the homesite of a C. sgabra which was no longer on the shell. The sear appeared to be i-E dayz old as it lacked any algae om evidence of amosion and was still the lucid grey color of curgent C. scabma homesites. For the purpose of this measurement, I counted this scar as 1/2 of a rider because the missing ridem had recently beem present to eat some of the algae even if it hadm't mpased on the shell in the preyios days. As an estimate of shell suface area of Le gigantea in the laboratory studies, I measured the area of tissus paper needed to toyer the sumface of the shell. To measume the amount of algal arguth on theee same shells, I scraped the surface of the shell and dissolyed my scrapings in 70 acetone. A spectrophutometen was used to measure extinction coefficients at o4 nm. and 647 nm, and used in the spectrophotometric equations given by deffrey and Humphrey, 1975, tw establish the amounts of chlorophyll a and b present in the acetone solutions; Chlorophyll a - 11.93E4 1.E and Chlorphyll b- 20. E - 5.SOE. These chlormphyll ampunts indicated the amounts of greem algae present on the entire shell (Jeffrey and Humphrey 1975). The shell mass per murface area was used as the measurement of shell thickness. Mass of the shell was the weight of the L. gigantes shell after the body had heen completely cut away, the zhall pat-dried, and all algae had been scraped and bleached gff it. Shell stuface wam again taken by measuring the area of tizzue paper needed to cever the outside surface of the lottia zhell. Metabolic rates of C. scabra were estimated via the onygen consumption rates recorded in a differential respirometer. C. srabma found on Li gigantea with two or more riders were compared with C. scabra from the surrounding area. Gerial respiration rates of all the C. zeabma were measured within an hou after cullection from the field and conducted in the the dark to aveid any contribution the photosynthesis of algae on the amiht a meuapm the first hour showed erratic consumptions as the sa adjusted to their new envirenment in the respiremeter, only th spcund hour'a result were used. Ouygem consumption waz nurmalied t dry hedy mass (oven dried fom lo houms at ec dag The masses of various-sised C. scabra riders' shells were compared to the shell masses of the C. scabma in the surrounding areas. The shells were weighed after being pat-dried while the C. scabra bodies were oven dried at 60 degress celcius for 16 hours before being weighed. C RESULTS Field Ohservations Lattia gigantea were found only within a i meter Fange of intertidal heights, approwimately 1.Em to 2.Em above zero tide datum, and only in areas of high wave action on vertical rock faces. Collisella scabma inhabited these same areas but were also found in areas of lower wave action, on horizontal surtaces, and on sites up to .Em. above zero tide datum. Within the cne meter range of cohabitation hy these species (which included the entire La gigantea range), intertidal height made no difference in whether or not a lottia had riders or in the amount of riders ner lottia (r - .OS, n - 21). Instead, the number of riders per La digantea appears dependent on the Ly gidantes's microhabitats conditions. Casual field observations indicate that it the didantea is positioned in a crevice, behind a rocky on a rock face not directly facing the oncoming surf, or in any other microhabitat reducing its exposure to waves, it has none or fewer riders than a similar Ey gigantea in areas of higher wave action. C. scabra riders werd not observed to leave the LE didant shell. They remain immobile and positioned in their immediate homesites at all times except the daily high tides. When they attt mhellz. The piders Torage on any and all pante o the e didantea shall's enternal surface and return to their homesite kollsmingkitaeda amnemally only mebile and foraging for the twe hours surreumeing the hich tides. Casual observations note that time spent kaiatt oreater in times of rougher surf and more extreme high tides. Lottia gigantea foraging patterns were similar to those of the Cy scabra: Ee gigantea were mobile and foraging during the two hours surrounding high tides. Casual observations again huted that rougher sumf during the high tides increased the time spent fomaging, distance travelled, and the percentage st L aidantsa foraging in an area. In times of peak wave action the La diganteg were not only mobile, but ewhibited the unusual behavior previously noted by Morris, et al, 1980, of pulling its shell away from the rock surface rather than hunkering against the rock like other limpets. While this behavior has been noted to he advantageous for the Er gigantea because it allows water to flow around the La gigantea foot and decreases drag on the limpet as a whole (Morris, et al, 1980), it also effectively isplates the C. scabma on its back; the riders find themselves on a shell lifted uff the rock surface often as high as the P. acabma's own hody length. While this positioning of the Le gigantea away from the romk pan he a formidable barrier to a rider attempting to get off the Ly gigantea shell, it does not appear to be the cause of the Co acabma failing to leave the shall; the Le gidantea are only positioned away from the rock in this manner for a trastion aa ttth a not leave the shell despite the lack of the barrisr. The Ey soabma deneity on Le didantea is independent of both ktatakakaaita aataa in the area surrounding a Ex gigantea is not a factor in determining the density of rider C. scabra on the Ly gigantea. The average number of Cr scabra in the surrounding area is 1.O 10 Cscabra/m while the average number of C. scabra per E didantea shall area is l1 wy 10 Ce scabra/em". On averags there are ten times fewer C. scabra per area of Le gidantea shell than surrunding area. sittatatla function of intertidal height. Large Li gigantea were tound at the same low intertidal heights as the large C. scabra and small Ly digantea ware found at the same high intertidal heights as the small C. scabra (using basal area as an indicator of relative site in both speties) (see fig. la & b). Despite this equivalent size distribution of the two species, the larger Le gidantea did not necessarily have the larger riders (see fig. 2). While there is trend for some of the largest riders to be on the largest L. didantam and the smallest riders to be on the smallest Ladigantes, the trand is weak with a correlation of pi of .2es. Ihus, the size of the Co scabra is not a major determining factom in whether or nut its home is om particular sized y gidantea. There is a much stronger correlation (r - .45, n - 46) batusen the total arn of the areas of all the Fidere and twe an of the Le didantes (sæm fig. 3). If these dats are manipulated so that C. scabma measurements indicate siss (om') rather than amsa (omf) and by gigantea ares indicates available foraging ares ke diganteg area - area covered by the Cu sgsbra) the kaataita Field oheervations show that the amount of standing algal growth on the surface of the Li gigantea shell is inversely preportional to the number of riders present on the shell. Fig. S shows this observation to be quantifiably true; the presente of more riders implies the presence of less algae. In tact, with tue or more riders the ampunt of standing algae per square inch of La digantea shell is unmeasurably low. While both the Le gigantea with and without algae (i.e. without and with riders) increase the thickness of their shell as they increase its overall sise (see Fig. 4), the presente of algse (i.e, mome riders) did not cause any signiticant change in rate of increase in shell thickness for pe ginante. Ihe decreased presente of greem algaes on Ei gigantea with riders dome not make an apparent difference in the thickness of the a digantea shell. The average rate of oxygen consumption per dry body weight of the C. scabra on Le digantes with two or more riders was 77y ul Doghr (n - 7) while the average rate for O. scabra in the surrounding area was 767 ul Oo/g/hr (n - 6), with the difference peing insignificant. A t-test shows better than a 75 confidence in assuming the twe consumption rates to be the same. Despite thlaatat w aaa differente in the rate of oxygen consumption from e acabra feund en tha ack ubtrate. Eiderz ym, nonridems, however, did show a differente in the masz of their shells relative to mody size. While both L idass and nonrider orm heavem shells as them pody sie O enlarges (with r's of .75 and .40 respectively), rider Dy scabra have significantly heavier shells than the nonriders at all body weights (s fig. 7). Higher body weights of riders and nepriders show a greater difference in shell weights than at lower body weights. C DISCUSSION The subpopulation of E. scabra that lies within the one meter range of Li gigantea distribution is larger at lower intertidal sites than at higher sites (se Fig ib). This trend is opposite to that previously notes by Morris, et al, (1780) in which the general population of L are larger at higher intertidal sites. This reversal in Ca srabma size distribution within this one meter range can probably be explained by the presence and site distribution of Le gigantea in the same mange (see Fig. 1a). The Le gigantea bulldoze much of the area, which creatly affetts the distribution and population of C EA in the area. If a particulam C. scabra is able to resist the bulldozing force of a Le gigantea, it is more likely to be found in this range than if it were not able to resist dislodgement. W. G. Wright (1777) showed that homing species, including scabma, can sometimes resist forces in cese of the Le didantea's bulldozing force but only if the force is applied when the Cy scabma is on its homesite. Limpets without homesites could not resist the bulldozing at all. Assuming, then, that I.) The better fit the Co scabra is to its homesite, the better it atat respective fontes they tan enent on aer l themta distribution within this range can be explained by the fact that laeaaait were distributed as Morris, et al (1980), suggests, then the smallest M. zeamma would engounter bulldezing by the langest e O digantea and would be unlikely to be able to resist dislodgement because, first, its small sise would be unable to exert a large enough force, and, second, if its small size were due to a young age, then it would not have had much time tw establish a significant home scar. he tact that Ce scabra can resist dislodgement by L. gigantea may also be a factor in detemmining why it is the only species of limpets found on the backs of bi gigantea. Due to its homing behavior, it is one of the few species that cam resist bulldoring and exist within an area otherwise completely cleamed of limpets. Because this bulldozed area normally surrounds the E digantea, the Ca scabra may be the only species of limpete clase enough to scramble up on the Li gigantea's shell. In fact, Wright (1977) has shown that not only are Collissella digitalis unable to avercome the bulldozing force, but they alse tend to recognire and actively avoid Li gigantea grazing areas, which might explain why this species, which is similam in size, distribution, and grazing behaviors to C. scabra, is not found om L gidantea shells. The rigidity of Ca smabra's homing behavior (Jessee, 1968, Villee a Groody, 1740, Hewatt, 1740) may also explain its seemingly unique ability to live on the comfined amea aa the sams areas surrounding its hmmesite, the species may be able toamI andittl 1lat may selectively allow Eo szahma to live on the limited foraging aattaaaaiaa Froximity to Li gigantea may explain why the density of C. scabma per area of Li gigantea shell is independent of density in the measured surrounding area. Os only some can resist bulldozing (Wright, 1977), the density of C. scabra immediately surrounding the Li gigantea is a reflection of the number of D. spahma that could resist bulldozing. This number is not necessarily a function of the total number of C. acabma in the larger surrmunding area and is also conziderably less the G scabra density in this larger area. If further experiments show that approkimately one in ten Ci scabra could resist bulldoring and thus that the density of Ca scabra was on average 10 times less in the bulldored area than in the surrounding area, it could EMplain why density per area of Li gigantea shell is on average ten times less than in the larger surrounding area. Hider density would then simply be a function of of C. scabra density in the immediate surrounding graring area rather than in the langem ungrated surrounding area. The fact that there are ten times fewer . scabma per area of L. digantea shell than per surrounding area could be explained sy numeroum uther factors. Ferhaps the low density per area of shell is because: 1.) It is ten times more likely for a Q. scabma to forage on a rock than on a mobile shell (This is assuming Ca scabma are still able to forage onto a moving shell a . 2.) Unly one in ten Ee didante encountered by meving L scabme are stationemy (assuming C. smabne selectively clims only en Immobile shells), .) Survivability is on average ten time C lower for riders than nonriders, or 4.) The density of C. scabma is determined by the food availability and the surface of the smooth En gigantea shell provide only a limited area for algal growth while the surrounding rock'e surface is relatively etremely rough, providing a greatem surface area fom algal grouth, and cam therefore support an increased amount of foraging Laaa- Neither possibility l or seems likely to be the explanation for low rider density because they rely on foraging and homesite selectiveness of the the Ly scabra, a sturdy speties nut known te show much selemtiveness as they exist in a wide varisty of lecations under a wide variety of conditions. But these possibilities are difficult to disprove because it wmuld entail watching the Ea scabra as they mount L. gigantea shells, an umpredictable event at best and ome that would be difficult to see in significant numbers. Fossibility would entail a fecundity study af C. scabra which in the past have proved difficult and inconclusive in determining fastors for survivability (Farry, 1977, Wells, 1980). However, casual obzaryations pyer a seven week perind did not note a greatem disaaid which would probably he noticeable if it were large enough to euplain the ten-fold difference in demsities. Tt ntaa pamulte. Fig, 4 whows that there is a significant correlation Lattag sliattaa a deviations from this correlation are caused by the amount of riders being too small for a given sired Le gigantea and never in too large an amount of riders ewisting on the shell (Assuming the lipe in Fig. 4 to be specifying the limit of riders a Le gidantea shell can support). It appears, then, that the available foraging area of a Ly gigantea shell sets an upper limit to the total eum of riders' sizes whith can home and forage on the shell, but that it does not determine the amount of ridere Below that limit. If this is true, then only about half of the Le didantea are near the limit of the amount of riders its available foraging areas can support (see tig. 4). Howeyer, the shell's available foraging area is not uecessarily an indication of the available algse. The amount of avsilable algae is dependent on several factors, largely the rate of arputh of the algae. While the spectrophotometer data showed that the amount of standing growth of algae decreased with an increase in number of riders and that the amount of standing areuth was approaching zero on Le gidantea with two or more riderm (sse fig. E), it does not indicate the rate of growth of the algae on the shells. If the rate of algae growth on the Le siaantea with two or more riders is faster than the rate on the shells with fewer riders, then the C. scabra from the more trowded shells would have just as much algae available to them. aat that by gigantes with a bigh density of riders were in more saaakt enperiencing a greater amount of water flowing aver them. It nas been shown in several cases that increased ambient water flow¬ increases the rate of algal growth. Such an increase in alde growth rate could explain how Ca scabra can exist in numbess greater than twe on a Ei gigantea even if their is no standing algae on the shell. This hypothesis is further supported by the fact that there is no decrease in oxygen consumption in riders that are found on shells with two or more Co seabta. The rate of Mygen consumption is strongly correlated with metabplic rate (Parry, 1977) which has been shown to be inversely related to fGEd availability (Farry, 1978, Eranch 2 Newell, 1978, Davies, 1767). The lack of a decrease in respiration mates of riders on crowded Ei gigantea shells could indicate that there is sutticient food available om bi gigantee shells, even on shells with more than two riders and no apparent standing aldae. Ihis tood availability hypothesis is further supported by the tact that mider C. scabra have heavier shells per body weight than nonriders, which may indimate that they feed slower than the thinner-shelled nonriders and thus need less algae on which to feed. Falmer (1980) shewed that thicker shelled Thaid lamellosa fmed slower than thinner shelled ones, and this may also be true of L srabma. Palmer's study of In lamellosa also showed a slower rate of increase in bedy weight propertional to shell weight in thicker-shelled animals (1780), which is similar to the P Lahe propertional to body size than the thinner-shelled nonmiders. Palmer explains this by showing that the amount of shell material laid dowm over time is the same tor both the thick- and the thim- shelled animals, but that body size does not increase as much in thicker-shelled murphalogies becamse the animal cannut expand its internal volume faster than the shell is expanding. If this theory holds true for C. scabra, then the thicker-shelled rider C scabra do not need as much algae as the lighter-shelled nonrider Co scahma because, while they are expending the same amount of energy for shell material annually, the thicker-shelled ridera have lower metabolic costs because they grow and maintain a proportionally smallem bady weight. One could epand on this hypothesis in two ways. Fimst, it it were shown that rider C. scabra growth rates are food limited, ne could furthem hypethesite that the decramed fmad availability is only limiting body sise and shell production ntiuaa distribution of shell in relatively thicker layers would simply be due to the body failing to expand beneath it. On the other hand riders may inherently lay down thicken shells, perhaps REEESSItated y an increased expmure to waye astimn and predation because they are removed from their camouflage and the protection of rock protrusions. If this is true then the mody site of the riders is limited by the intemnal volume ar the shell, and an increase in food availability on the L ainantea shells would nut increase the riders' body mite. e that mthae dus to a decrease in medy weight relative to shell weight. Im aitamd to lomomotion. Wecause C. scabma never leave the shell of the E digantea, they are likely to never forage over long distances. C. scabma in the surrounding area, however, can forage over long distances and becaume of inter- and intra-species competition tor the local algae, they are more likely to need to forage over lang distances to get a sufficient food supply. Ereen (1971) shuwed that Collisella digitalis moved greater distances from its original site when food availability was low and the same may be true for Ca scabma. Calow, 1974, and Denny, 1980, have both indicated that locomotion in gastropods can he a signiticant metabolic cost. Calow (1974) reports locomotion costs to he as high as 26 of total food intake and if rider E. scabma dom't incur this metabolic cost them they can exist on less algae and still maintain the same respiration mate, which I have shown them to do. While rider C. scabma show differences in shell growth, foraging area, density of population, and pemhaps in di travelled compared to nonriders of the same species, appmar unaffected hy the existence om nonexistente ot rider on their backs. The major change incurred by the Li gigantea with the addition of a Cy scabra is the loss of the algal coat that i wau itaaaaalltaaaat- presence of this algae ys, the presence of midess hypothetically tould affect the i didentee in several ways: 1.) It could incrmaze the levels of biceresimn by increasing the ameunt at aldal borings am Okpan and Farrew l' E discuss, 2.) It cmuld deurmasz the layelz of biserssimn by increasing the avallability alt Farrow, 1985), J.) It could effect the drag coefficient of the limmet as a whole, 4.) The weight of the riders can increase the metabolic needs of the Le gigantea because of the increased load it muet carry in locomotion, or S.) The creation of a homescar by the rider Co scabma could significantly weaken the Li gigantea shell. In actuality, none of these hypathetical effects cause a significant difference between Ei didantea with and without riders. First, the Ly gigantea show no difference in the ratio of shell weight to sufame area (see fig. 6) between shells with and without riders. If borings, either caused by algae om a spunge, were significantly affecting the shells, a ditterence umuld have occurred in these ratios (Young amp; Nelsen, 1985). While the existence of riders did appear to be associated with an increase in the occurrence of the boring sponge gliona gelata on the My didantea shell, the microscopic bored holes were too small te make any substantial differente tu the Li gidantea. Likewise, the increass in drag coefficient due ta the algal coat on . digantea with no riders has beem shownn to be insigniticantly small and the loss of algae is likely to be countered by an aaatt drag amefficient independent of the nummem of .tes nit shell. In addition, the weight of the midens is unlikely to ingrmaze the metabolic needm of lomemotion as the proportionally liattaa metabolit cost (Denny, 1980, Calow, 1974). Finally, while Lindberg ≈ Duyer fumpubliehed) document the sottening and C mussequent scraping of the area beneath the C. scabra in order to create the homescar, the resulting spot of thinner shell is unlikely te affect the overall strength of the shell because, as Falmer (1980) implies, the shell structure as a factor in strength can outweigh a few local changes in shell thickness. Also, these spots of decressed shell thickness are usually covered by the Sa scabma which created the scar and are, thus, uzually not expmsed long enough for forres to take advantage o these weak spt. CONCLUSIONS 1.) The spacial density of C. scabra is ten times less on L. digantea than in the surrounding area, possibly due to limited foraging arsa on the Li gigantea. 2.) In areas of increazed wave action the density of ridems om the Ly gigantea increases, possibly dus to corresponding increase in rate of algal growth caused by increased ambient water flow. 3.) Hider Ca sgabra have a higher ratio of shell mass per hody mase than de nonriders, possibly due to either a.) a slower rate of body growth necessitating that the shell be laid down thickem, ar b.) the body mass being limited by the smallem internal volume available created by the shell being laid down thicker. A i LITERATURE CITED Abpan, E. B. & Farrow, G. E., 1985. Marine Geology, 67, 139-150. Branch, G. M., 1971. Zool. Afr., o, 1-38. Eranch, G. M. 2 Newell,, R. C., 1978. Mar. Biol., 47, J51-301. Eranch, G. M., 1981. Oceanogr. Mar. Biol. Ann. Rev., 19, 225-180, Breen, P. A., 1971. Veliger, 14, 177-187. Calow, P., 1974. Oecologia (Eerl.) 16, 149-161. CHEEE, H., 1978. Fh.D. thesis, University of Sydney, N.S.W., Australia, 375 pp. Dayiez, P. S., 1947. J. mar. biol. Ass. U.K., 47, o1-74. Denny, M., 1980. Scienme, 208, 1288-1270. Eikenberry, A. B. J. 2 Wickizer, D. E., 1944. Veliger, o, Suppl., 66-70. Hawatt, W. G., 1940. Am. Midl. Nat., 24, 205-208. Jesme, W. F., 1768. Veliger, 11, Suppl., 52-55. Morriseta a Stanford Univ. Fress, Stanford, CA. Mewell, R. C., 1977. Eiclogy of Intertidal Animals. Narine Ecalsgical Suryeys, Kænt, 781 pp. Palmer, A R., 1780. Ph.D. thesis, University of Washington, Seattle, Wo, O pp. Parmy, G. D., 1977. Fh.D. thesis, University of Melbourne, Victoria, Oustralia, 17 pp. Papry, G. D., 1978. J.anim. Ecol., 47, 351-368. aataaakataaat- Misl. Mat. 24. Melle, R., 1780. J. exp. mar. Biol. Ecol., 48, i5i-168. Wricht, W. S., 1977. West. Soc. Naturalist, 1777, 50. Voung, H. E. 8; Melson, C. S., 1785). Marine Geolagy, S, 3-45, Fig. la Lottia Gigantea Sizes Varying With Intertidal Ht. 1.2 - 14 0.9 - 0.8 - 0.7 0.6 0.5 0.4 - 0.3 0.2 - 0.1 - 20 100 120 Height Above Lowest Transect (cm) Fig. 1b Collisella Scabra Sizes Varying With Intertidal Ht. 63 43 - 46 - 44 - 42 - 40 - 38 36 - 34 - 22- 30 24+ 80 120 100 60 Height Prom Lowest Transect (cm) Fig. 2 SCABRA vs. LOTTIA Areas 150 - 140 - 130 - 120 - 110 - 100 90 80 70 0 50 + + + 40 - 30 20 10 0 + 2.2 2.8 14 1.4 0.8 0.2 (Thousonda) Lottio Giganteo Areo (em squored) Fig. 3 TOTAL SCABRA RIDERS vs. LOTTIA Areos 220 - 200 180 160 140 20 - 100 - 80 60 O 40 - 20 —kk t- 1.5 1.7 0.1 0.3 0,5 0.7 0.9 (Thousands) Lottio Oiganteo Ared (em squored) 3 1.9 8 — FiS. 4 TOTALRDERS STZES ve. AVALABLELOTTI GOATEAARA 1 3 o seeae a2 u AVALABIELOTRA GGANEA AREA CNT2) D — 22 28 E 0 00 — A + + L — (qouj bs/Bn) VZHV/TXHdOHOIHO 0 3— 2.9 — 2.B - 2.7 - 2.6 - 2.5 2.4 2.3 - 2.2 2.1 1.9 1.8 1.7 - 1.6 1.5 - 1.4 - 1.3 1.2 1.1 0.9 - + No Agoe Coot Fig. 6 LOTTIA SHELL THICKNESS With end Wiout Ag with aae ithout algae o — Lotto Oigonteo Sufoce Aeo (sq. Inch) — No Agoe Algoe Regresson a 20. 0 T mat I t 4 ti - ta ta t t. aa- dte kta- atatatataaaaa- ED S EIGUEE LECEMD Fig. 1aTheaerage areas of a concurrentlyadecrease in the same, successivly higher intertidal -91 , respectively) ttaa- Fig. 2 The area covered hy the C scabma is insignificantly oelatdwttt Fig. 3 The total area covered by all the D sabma miders on Each La didantea morrelates somewhat with the area of the Li diaatiaatiaa- Fig.4htat atttag -.51 - 45) (Extluding the ianta withi Fid. 5 Concentrations of chlorophyll a and chlomephyll b, usem sitta the humber of rider C. scabma increases. With two or more ridere on a Le didantea, the algal amount present is approaching sero.(n — ) Fid. 6 The presence of an algae coat (i.e. no riders) or lack of an algae coat (i.e, with riders) made no difference in how thick the Li dinantea makes its shell. Fig. 7 Co scabra riders have a higher shell mass per body mass than nepriders. This increased amount of shell mategial forme thicker shells for the riders than the nenridess. (g - .47, a ACKNDWLEDGEMENTS Ibis projemt would never have beem completed without the anvice and encouragement of my advisor, Hark Demny. Had it nat Laen for his ability to make the dreamy and often hazardous intertidal work seem "neat", I would have given up on this prejest leng ago. I would alze like to thank the rest of the faculty and staff at Hepkins Mamine Station fom imparting on me many of the ideas and technical knowledge I used in my work. Finally. I would like to thank my fellow students, especially those pesiding at 100 Forest Ave.. fom listening to all the thials and tribulations of working with C. kaalaaaiaaataat- in which I'm sure they held no previous interest.