TRANSITIONAL FORMS OF THE LIMPET COLLISELLA PELTA
AND DISRUPTIVE SELECTION
A Thesis Presented to the Faculty
of
Moss Landing Marine Laboratory
California State University, Hayward
In Partial Fulfillment
of the Requirements for the Degree
Master of Science in Marine Science
By
Fred Sorensen, Jr.
May, 1984 ABSTRACT
A transitional form of the limpet Collisella pelta was described and possible explanations of its ontogeny were presented using a discriminant analysis model and experimental growth controls. It appears that these limpets change the color and shape of their shells when they move from one substrate to another; but since the shell is permanent, this change makes the limpet more visable to predators. It was found in significantly larger ratios (p < .01} in the middens of the Black Oystercatcher, Haematopus backmani, than in surrounding control areas.
ii ACKNOWLEDGEMENTS
I would like to thank Drs. James Nybakken,
Mike Foster, and Bernd Wursig of Moss Landing Marine
Laboratories for their comments and encouragement.
Special thanks to Dr. David Lindberg, U.C. Berkeley, for his suggestions. Lynn McMasters did the illustrations and Ann Cambra the typing and guidance through the bureaucratic maze; my heartfelt thanks to both. This project was partially funded by a Packard
Foundation Grant. I'd like to thank the students and faculty of Moss Landing Marine Labs for their support and comradeship while I was a student there. Finally, thanks to my wife, Rosemary Avery, for her support and enthusiasm, without which some things become very tedious.
iv TABLE OF CONTENTS
Page
ABSTRACT . . . . ii
ACKNOWLEDGEMENTS iv
LIST OF TABLES . vi
LIST OF FIGURES vii
Chapter
INTRODUCTION 1
MATERIALS AND HETHODS 3
Study Sites 3
Limpet Forms 8
Growth Experiment 14
Predation Effects 17
RESULTS 25
Discriminant Analysis 25
Growth Experiments 43
Predation Effects 46
DISCUSSION . 49
LITERATURE CITED 58
v LIST OF TABLES
Table Page
1. Discrimination Coefficients for Separating the Collisella pelta Forms 26
2 . Group Centroids (Means) 27
3 • Classification of the Collisella Pelta Forms from the Discriminant Analysis 38
4. Areas Sampled and the Ratios of the Transitional Forms of Collisella Pelta 47
vi LIST OF FIGURES
Figure Page
1. Map of California with study sites located 4
2. Map of the Farallone Islands with nest collection sites marked 6
3. Photos of the different forms of the limpet Collisella pelta ...... 9
4. Map of Monterey Bay area with collection site and relocation site marked 15
5. Photo of multi-colored limpet with tag 18
6. Photo of relocation rocks at Hopkin's Marine Station ...... 20
7. Diagram of the laboratory growth experiment 22
8. Diagram of discriminant analysis territories and examples of plots 28
9. Scatter diagram for the Egregia form of Collisella pelta ...... 31
10. Scatter diagram for the Mussel form of Collisella pelta ...... 33
11. Scatter diagram for the Rock form of Colli sella pel ta ...... 35 12. Scatter diagram for the Apices of the Transitional forms of Collisella pelta 39
13. Scatter diagram for the Bases of the Transitional forms of Collisella pelta 41
14. Survivorship curves for the laboratory growth experimental limpets ...... 44
vii INTRODUCTION
Shorebirds are visual predators and their effects on populations of limpets have been discussed by a number of authors (Test, 1945; Giesel, 1970;
Hartwick, 1976, 1978, 1981; Frank, 1982). Giesel believed that shell coloration in Collisella digitalis was genetic and that the limpets sought out the right colored substrate. Disruptive selection of intermediate forms by predators resulted in the two distinct color morphs found in his study. Hartwick (1981), in his studies of the food of the Black Oystercatcher
Haematopus backrnani, provided further evidence of the role of predation on the color polymorphism in
C. digitalis. Collisella pelta, another major food item in the diet of the Black Oystercatcher exhibits a similar color polymorphism (Jobe, 1968; Lindberg, 1982).
During a study of the food items brought to the nest sites by Black Oystercatchers it was discovered that a transitional color form of ~- pelta was present in higher numbers than previously observed in the area.
These forms appeared to have apices and bases of different morphology and coloration, and have previously been described by McLean (1966) and Lindberg (1981).
1 2
This paper presents the results of a study to quantify the shell shapes of ~· pelta from different substrates, identify the habitats in the transitional morphs' ontogeny, and test whether Black Oystercatchers are preferentially selecting these transitional morphs as prey. MATERIALS AND METHODS
Study Sites
Specimens of the limpet ~· pelta were collected
from Black Oystercatcher nest sites at two locations.
The first location involved one nest at Point Piedras
Blancas, California (35°40 1 N 121° 17' W) (Figure 1).
The nest was located on a large rock that was connected
to the mainland by a small spit that was submerged at high tide. This nest site has been in use for at
least five years (1978-1982) by presumably the same
pair of Black Oystercatchers. All hard remains of prey
items brought to the nest site were collected
approximately weekly during nesting season. A total
of twenty samples were collected over two nesting
seasons (1979-1980) and C. pelta was present in 14 of
these samples.
The second location involved 10 nest sites on the
Farallone Islands off the coast of California (37°42' N
123°00' W) (Figure 1). These nest samples were from
various sites all on the main island (Figure 2) .
Collections were similar to those made at Point Piedras
Blancas except that only one collection was made per
3 4
Figure 1. Map of California with study sites located. 5
••
Francisco FaralloneG Islands
Monterey
Pt. Piedras Blancas ·.· .. "'
Figure 2. Map of the Farallone Islands with nest collection sites marked. 7
•. ••• .f .: : .."' "0 "C : .<: c ell .<: 0 -
~
••• G) .<: " ;:::"' 8
nesting season over the years 1977-1981. A total of
13 collections were made in which C. pelta was present in all samples.
Limpet Forms
Three major forms of the limpet ~· pelta were used in comparison with the transitional forms from this study (Figure 3), though others have been described
(Lindberg, 1981). The first, that I have called the
Rock morph, is found on rock surfaces and is generally large, flat, and lightly colored or banded. The second, designated the Mussel morph, is found on the surfaces of the mussel Mytilus californianus, and is usually smaller, taller, and generally black or dark brown in color. The third major type, the Egregia morph, is found on the stipes and holdfasts of the brown alga Egregia menziesii, and is similar to the Mussel morph in that it is usually tall, small, and dark in color. Because the apical portion of the transitional forms found in my samples appeared to be similar to both the Mussel and Egregia morphs, specimens were collected from these habitats for statistical comparison with the transitional forms. Specimens of the Rock morphs were also collected to compare with the similar base portion of the transitional forms. 9
Figure 3. Photos of the different forms of the limpet Collisella pelta. 10
C. pelta Mussel morph 11
C. pelta 'Eg regia morph 12
To collect specimens of the Mussel morph, mussels 2 were cleared in four 1/16 m quadrats on two transects running from +2.0 to~ 0.0 m MLLW and specimens of £· pelta were removed from these mussels. Only limpets greater than 5mm in length were collected since smaller limpets are difficult to identify to species. Ten specimens were collected that could be identified as Mussel forms of £· pelta. Holdfasts and stipes of the alga Egregia were examined for limpets and a total of
125 were collected. Of these, 9 specimens were C. pelta and the rest were Notacmeae incessa. Rock forms were 2 collected from three, 1 m quadrats randomly placed on rock surfaces. A total of 21 Rock specimens were collected.
Limpets were measured with vernier calipers to the nearest O.lmm. Six measurements were taken: (1) length,
(2) the distance of the apex from the anterior edge of the aperture (apexpos), (3) height, (4) width at 1/4 of the length from the anterior edge of the shell
(widfron), (5) width at 1/2 the distance from the anterior edge (width), and (6) width at 3/4 the total length from the anterior edge (widend) (Lindberg, 1982).
When measuring the transitional forms, the apex was 13
measured starting at the transition zone of the two shapes and treating that portion upwards as an individual limpet. The base measurements were made using the entire limpet. This method biases the base measurement of height since it includes the height of the apex growth. All the other measurements, however, should be representative of the new growth.
The data were analyzed with the discriminant analysis program of the Statistical Package for the
Social Sciences (SPSS) (Nie et al., 1975). This program determines which variables are most important in distinguishing between predesignated groups. Using specimens of the three forms (Rock, Mussel, and
Egregia) the program calculates discriminant functions, then compares the original group assignment to the predicted assignment based on the individual discriminant scores. The separate measurements of the
apex and the base of the transitional forms were then
added as unknowns and the resulting cluster diagram gives the predicted membership of these forms to the previously defined groups.
Scoring for shell color was performed on all
limpet shells (Hartwick, 1981). Apex and base color were scored as follows: 0 for white, 1 for grey, 2 for
brown, and 3 for black. Striping was score 0 for plain 14
white, 1 for light with single stripe, 2 for mostly light with more than a single stripe, 3 for mostly dark but striping heavy, 4 for almost all brown with very little white, 5 for mottled brown-black, and 6 for black or dark grey-black. The three scores were then added except in the case of the transitional forms where only the apex or base was scored, not both.
The scores of the three groups were then compared using the Kruskal-Wallis Test for non-parametric analysis. The scores for the transitional forms were analyzed using a Wilcoxon's Signed-Ranks Test for Two
Groups as arranged as paired observations, comparing the apex with the base in order to quantify any differences.
Growth Experiment
In order to test whether individual limpets would change the color and shape of the new shell material if the substrate were different, relocation and growth experiments were done. Specimens of the
Egregia form were chosen and collected for this purpose because of their higher availability. A total of 172 specimens were collected from the Carmel Point area of
Monterey Bay, California (36°32' N 121°56' W) (Figure 4).
All specimens were tagged using small 5mm orange
Temple Tags with three-digit numbers printed on them for 15
Figure 4. Map of Monterey Bay area with collection site and relocation site marked. 16
Monterey Bay
Hopkins Marine Station ·.
~ ... -...... - . Pacific ... : .. Monterey Ocean
Peninsula
...... ·. : : ., ... ·- ...... : .
Carmel Point
Carmel Bay
.. 17
individual identification (Figure 5). These tags were glued on the dry shell using Super Glue (T) (Super
Glue Corporation, Ridgewood, New York).
Seventy-five of these limpets were then relocated on the vertical faces of large intertidal rocks off Hopkins Marine Station, Monterey, California
(36°37' N 121°55' W) (Figures 4 and 6). These limpets were monitered over the next 15 weeks for any new growth. Another 84 specimens were placed onto various rocks contained in an outside laboratory water table
(aquarium) at Hopkins Marine Station where a "tidal" cycle of~ 2.5 hours was maintained (Figure 7). A control sample of 13 limpets was replaced on Egregia stipes in the lab experiments but otherwise treated in the same manner as the other lab animals. Growth of these limpets was monitored for 14 weeks and new growth recorded.
Predation Effects
To test whether Black Oystercatchers were selecting the transitional forms preferentially, comparisons were made of the ratio of these forms in the Oystercatchers' nest samples with samples from the surrounding habitat. The number and type of C. pelta present in the study area (Point Piedras Blancas) was 18
Figure 5. Photo of multi-colored limpet with tag. 19 20
Figure 6. Photo of relocation rocks at Hopkin's Marine Station. 21 22
Figure 7. Diagram of the laboratory growth experiment. 23
I l L water .1npu
brick brick brick I b-rick ~Egregoa brlc k I brick brick brick vacume I I C) siph.on e dra1n I brick I brick brick brick I e e e e e e 24
2 determined from 1/4 m quadrats stratified by three habitat types based on accessibility to Black
Oystercatcher predation. Because predation by
Black Oystercatchers may have selectively removed one or more forms of the limpets, counts were made along vertical rock faces that were inaccessible to
Oystercatchers. A total of 9 quadrats were sampled.
Additional samples were taken in a nearby intertidal boulder field by stretching a transect line across the boulders and sampling random boulders for C. pelta.
This area represented a heterogenous habitat accessible to predation but also some escape for different limpet forms. Finally, samples were taken on a homogenously flat intertidal bench. A total of
8 random quadrats were sampled along a transect line in this area. A x2 test was used to compare the number of transitional forms found at the Black Oystercatcher nest sites with the intertidal data to determine if there was any difference between their abundance in the habitat and their representation in the Black
Oystercatcher nest site samples. RESULTS
Discriminant Analysis
Using the morphometric measurements of the sample limpets from known environments (Rock, Mussel, Egregia) a discriminant analysis was done in order to be able to predict the "ecotype" of unknown samples. In this case the unknown samples were the apex and base of the transitional forms. The first step was to find the most discriminating of the possible measurements and eliminate those that were not that powerful. In this way length and width were eliminated because of their low discriminant power. The remaining four measurements were then used to calculate linear functions that weighted the measurements in order to maximize the discrimination. Two equations, function 1 (D ) and 1 function 2 (D ) resulted, and the coefficients are 2 given in Table 1. The means of the different measure ments for each group (Rock, Mussel, Egregia) were then calculated and centroids or mean n and D scores 1 2 were found (Table 2). These were then plotted and territories defined (Figure 8). The territory boundaries represent lines that are equidistant from the nearest two centroids.
25 Table 1 Discrimination Coefficients for Separating the Collisella pelta Forms
Function Height Widfron Widend Apexpos
1 (D l = 0.36169(height) + 0.88962(widfron) -0.49783(widend) + 0.33386(apexpos) 1
2 (D ) = + O.B2050(widfron) + -1.67917(widend) - 0.54044(apexpos) 2 l.ll548(height)
"' 27
Table 2
Group Centroids (Means)
Group Function 1 Function 2
Rock 1. 41011 0. 05118
Mussel -1.79544 0.58138
Egregia -1.29532 -0.76539 28
Figure 8. Diagram of discriminant analysis territories and examples of plots. ( * centroids) 29
6
MUSSEL : ROCK 4 I I I I 2 c------Ill I C\J z 0 ~ I - 0 I I I f- lA () I * z 18 I ::J :c*I LL -2 \I I ! I i l I I I -4 I I -! I I I I ------1- --- -ill I I -6 Egregia J I I
-4 -2 0 2 4 E 30
Individual limpets can then be located by first using the equation for function 1 which separates the
Rock form from the other two possibilities. For example if n = +3.0, then plotting on Figure 8 (A) 1 it would be a Rock form regardless of the D score. 2 However, if D = +1.0 (B in Figure 8), it could be 1 either a rock or Egregia form. In this case you must go on to calculate the n score. If n = -5.0 then B 2 2 would be an Egregia form (Figure 8). If n -2.0 it 1 = could be either an Egregia or Mussel form (C in Figure 8).
Given a n score of +2.0 then the limpet would be in the 2 Mussel group.
All the individual limpets that were used in the analysis were then reentered into the program to verify group membership and identify any overlap.
Diagrams were made of their locations relative to the centroids calculated earlier and the territorial boundaries (Figures 9 through 11) . Table 3 gives the classifications for the different forms and their predicted group membership. The predeterminations of the Rock forms and Mussel forms agreed well with their
Subsequent group assignments (90.5% and 100%), while only half of the Egregia forms (55.6%) were distinct; the rest were divided beb,reen the Rock and Mussel forms. 31
Figure 9. Scatter diagram for the Egregia form of Collisella pelta. 32
EGREGIA MORPH
MUSSEL ROCK 4
2 z . Q 0 0• 1- uz ~ 0 IJ.. -2
-4
£GREG/A -6
-6 -4 -2 0 2 4 6 FUNCTION I 33
Figure 10. Scatter diagram for the Mussel form of Collisella pelta. 34 MUSSEL MORPH
6
MUSSEL ROCK 4
2 0 C\1 0
2 40 0 Q 0 co 1- u 2 ::;) IJ... -2
-4
£GREG/A -6
-6 -4 -2 0 2 4 6 FUNCTION I 35
Figure 11. Scatter diagram for the Rock form of Co11ise11a pe1ta. 36 ROCK MORPH
6
MUSSE'L ROCK 4
2 0 N 0 z 0 0 0 Q 0 • 0 f-. 0 0 u 0 z ::::> 0 0 0 1.1.. -2
-4
EGREG!A -6
-6 -4 -2 0 2 4 6 FUNCTION I 37
The measurements from the unclassified apices and bases of the transitional forms were added and classified (Table 3; Figures 12 through 13). The transitional apices were grouped primarily with the
Mussel forms (85%) and a few with the Egregia group
(15%). None were grouped with the Rock forms. The bases, however, were primarily grouped with the Rock forms (70%) and the remaining bases divided between the Mussel and Egregia groups. One cause for this division in the base grouping may be that in taking measurements for the base (described in methods section), height included the apex height since separate base height could not be made. As a result, particularly pronounced differences in the height of the apex (i.e., a tall Mussel apex type) would bias the base measurement towards a Mussel type form.
Results of the Kruskal-Wallis Test show that there was a significant difference among the three groups, with the Rock forms scoring much less than the other two groups (adjusted H = 24, x2 critical, a .005 = 10.6). The Mussel group scored highest (darker without striping). The results of the Wilcoxon's Signed-Ranks
Test on the apex and base of the transitional forms
showed that there was a significant difference between 38
Table 3
Classification of the CollisellaPelta Forms from the Discriminant Analysis
Predicted Group Membership
Form No. Cases Rock Mussel Egregia
Rock 21 19 (90.5%) 1 (4.8%) 1 (4.8%)
Mussel 10 0 10 (100%) 0
Egregia 9 2 (22.2%) 2 (22.2%) 5 (55.6%) Transitional
Apex 20 0 17 ( 8 5%) 3 (15%)
Base 20 14 (70%) 3 (15%) 3 (15%) 39
Figure 12. Scatter diagram for the Apices of the Transitional forms of Collisella pelta. 40 APEX- TRANSITIONAL MORPH
6
MUSSEL ROCK 4
2 C\1 0 0 z 0 • Q 0 co 0 1- coo (.)z ::> u.. -2
-4
£GREG/A -6
-6 -4 -2 0 2 4 6 FUNCTION I 41
Figure 13. Scatter diagram for the Bases of the Transitional forms of Collisella • pelta. 42
BASE- TRANSITIONAL MORPH
6
MUSSEL ROCK 4
2 0 N 0 0 0 z 0 0 0 Q 0 0$0 1- 0 u $ z ;:) u.. -2
-4
EGREG!A -6
-6 -4 -2 0 2 4 6 FUNCTION I 43
the scores of the apices and the bases, with the apex scores higher (darker) than the base scores (T = 15; critical at n = 20, a .005 is between 37 and 38).
Growth Experiments
The translocation of limpets from Egregia to the natural rocks at Hopkins was not successful. Of the 75 limpets relocated, only 13 were located two weeks later. Only five remained after five weeks, and only
three survived through the experimental period (108 days). Five possible reasons for the losses are: mortality (capture techniques and natural), competition, movement out of the study area, hiding, and loss of tags.
It was noticed that some untagged limpets, similar in appearance to the transplanted ones, were present on the site. In fact, many of these showed a changing color pattern similar to the five remaining tagged limpets.
However, since they were no longer identifiable they were no longer considered in the experiment. Of the
five remaining tagged.limpets after five weeks, all
showed a lighter, banded shell growth. This number was too low for statistical analysis.
In the laboratory growth experiments, 30% (28)
survived the entire experimental period. Figure 14
shows the survivorship curve for the lab animals. Two 44
Figure 14. Survivorship curves for the laboratory growth experimental limpets. 45
50 IIIII- Group 1 (N:42) 95 days en o- Group 2 (N:51) 79 days -(\! ::l --~~~~~, '0 40 \ 11111-111!1 > \ ·-'U c: \ 30 \ Ill -0 lo.. (!) ..a 20 E ::l -IIIHIII 2 \ 10 \ \ \ \ II 0 0 5 10 15
Days 46 translocations occurred approximately one week apart.
The first contained 46 animals, the second 51. Four of the initial set were missing after one week and were no longer considered in the experiment. Limpets found dead in the laboratory were collected, measured, and their growth scored as described in the discriminant analysis section above. New growth did not appear in animals which died in less than approximately 12 days. Growth rates, estimated by dividing the new growth (rnrn) by number of experimental days, showed no pattern for different sizes or over the experimental period and ranged from 0.006rnrn/day to over 0.08rnrn/day.
Results of Wilcoxon's Signed-Ranks Test between the old and new growth coloration for limpets placed on rock surfaces showed significant lighter shell coloration and banding in their new shell material than in their old (Ts = 0; critical n =57, a .005 > 374). Control limpets replaced on Egregia showed no significant difference in shell coloration (T~ = 37; critical at s n = 13, a .005 is between 21 and 22).
Predation Effects
Table 4 gives the results of the x2 test comparing the ratios of transitional forms in the different areas sampled. Mussel beds and Egregia 47
Table 4
Areas Sampled and the Ratios of the Transitional Forms of Collisella Pelta
Area (No. Samples) No. c. pelta No. Transitional (%)
Mytilus Beds (4) 10 0
Egregia (10) 9 0
Rock
Vertical (9) 72 5 ( 6. 9)
Boulder (7) 132 9 ( 6. 9)
Flats ( 8) 33 2 ( 6 .1)
Oystercatcher Nests
Pt. Piedras Blanc as (14) 1005 188 (18.7)*
Farallone Is. (13) 965 205 (21.2)*
* samples had no transitional forms. Rock samples With different vulnerabilities to Black oystercatchers had ~ 6-7% transitional forms. Black oystercatcher nest site samples had significantly more transitional forms (18-21%; p < .055) than the other sites. DISCUSSION Shell coloration in Archaeogastropods is usually a result of disposal of undigestable pigments from food or unmanageable metabolic residues (Comfort, 1951). Effects of diet on shell coloration in abalones (Haliotus spp.) has shown that red and brown algae in the diet causes brown or dark shell coloration (Leighton, 1961; Olsen l968a,b). A diet of green alga causes a light blue or white pigment to be incorporated into abalone shells. In this study of Collisella pelta, shell coloration also appears to be a result of diet. When the limpets are on the brown alga Egregia they appear to incorporate the brown pigments of the kelp into the shell. When on rocks they have a lighter coloration. In the laboratory, growth experiments where only a diatom film and the green alga Enteromorpha sp. were allowed to grow, differences in the color of new growth were significant between the experimental animals and controls which were kept on Egregia. Since all were treated the same except for diet, it appears that diet controls the pigmentation of the shell in C. pelta. The diet of ~- pelta appears to depend on the substrate where the animal is located. Limpets located 49 50 on the stipes and holdfasts of Egregia have a ready source of food so their diet is usually exclusively the brown alga Egregia. Limpets on Rock have access to different arrays of alga but generally the most common type at the higher tide levels are the greens and diatom films. This variable diet gives them the lighter and banded coloration found in the Rock forms. The dark coloration found in the Mussel forms is harder to explain, but may be the result of incorporation of pigments from the shell of the mussel. Mytilus Californianus is dark in color and the scraping of the shell by the limpet radula would result in the digestion of shell materials. In all three cases the coloration of the shell of f· pelta appears to be habitat induced rather than a result of genetic variability. Collisella pelta has been observed to be in the mid- to low-intertidal region (Shotwell, 1950b). This area is usually bordered by mussel beds above and Egregia spp. holdfasts loosely bordering the bottom. Collisella pelta have been found living on mussels, rocks, and on the stipes and holdfasts of the alga. Each habitat has limpets but generally smaller ones are present on the mussels and alga and larger ones on the rocks. Young f· pelta apparently settle in all areas but most often occur lower in the intertidal (Test, 1945, 1946). 51 I have found newly settled limpets on the holdfasts of Egregia and fewer numbers on Mytilus. Lindberg (personal communication) suggests that a good number also settle in the low coralline alga zone. If most of the larvae settle in the lower zones, then some migration is to be expected since larger individuals are found farther up in the intertidal zone on the rock surfaces. If this migration occurs, then the limpets would have to move through first the coralline zone then brown alga and finally to the rock. At each zone the pigments incorporated into the shell would differ. Some of the limpets in this study were found to have multicolored shells with different rows of color suggesting just this type of migration pattern (see Figure 5). This type of migration, from settling young through adults, would also be unidirectional. The colors laid down would start at a lighter coralline to a brown/black to a lighter banded color as the limpet moved up the intertidal zone. As can be- seen in Table 4, no transitional forms were found on mussels or on Egregia in the samples taken from those areas. If migration occurred, these areas would be starting areas or settling areas whereas the rocks, representing final resting areas, have a number of transitional forms present. Those settling originally on rocks, a more stable substrate, would be less likely 52 to move and therefore be "pure" Rock forms. Egregia undergoes damage when storm waves hit the intertidal zone. This is especially true for plants that have limpets living on them (Black, 1976). Even Notacmea incessa, a limpet almost exclusively occurring on Egregia, has been found on mussels after storm damage to the kelp (Lindberg, personal communication). Collisella pelta may survive better on Egregia because its food supply is directly underneath it, and it can grow taller, thereby increasing its volume to prevent desiccation. However, because of the instability of Egregia, especially during storms or high wave action, they must move. It is easy to move from specialized, unstable environments such as Egregia to stable, predictable environments such as rocks. In Mytilus beds the major factors would be food availability and limits to the size which a limpet can attain in such crowded conditions. Collisella pelta does not have a homing behavior (Villee & Groody, 1949) and therefore once it has moved and possibly found better conditions, as from mussels to rock, it would stay. The result of incorporating pigments from their substrate as £· pelta does, suggests positive adaptive value since it would result in the shell coloration 53 matching the substrate on which the limpet rests and thereby making it invisible to visual predators. However, because the limpet cannot change older shell coloration, when it moves this older shell material may no longer match the new substrate. It can and does make new shell that matches but,as can be seen in the transitional forms, this often enhances the contrast between the substrate and the old shell. This causes a "bulls eye" effect that may make the limpet more obvious to visual predators such as the Black Oystercatcher. The higher proportion of these transitional forms in the Black Oystercatcher nest samples suggest that this is in fact what may be happening. This situation is similar to disruptive selection as described by Pianka (1983) . The transitional forms of ~· pelta represent intermediate phenotypes and have a lower fitness (against predators) than the other "pure" forms in a patchy heterogenous environment. However, ~· pelta transitional forms start out as "pure" forms and it is only after moving from less stable specialized environments to stable ones that the individual limpet becomes less fit due to predation. It appears that an adaptation to fit the environment, incorporating pigments, can work against the individual that has to move later in life. 54 Since the coloration in the shell material in C. pelta does not appear to be genetic as Giesel (19?0) suggested, the color forms are not transmittable and therefore do not alter a population's gene pool. The distribution of the color forms (or morphs) are determined by the amount of heterogeneity in the environment and the presence of visual predators such as the Black Oystercatcher. Being a eurytopic species, it does not appear that larvae of C. pelta select preferred sites (Test, 1945), a behavior that could be transmitted genetically. The larvae of ~- pelta settle throughout the intertidal (Test, 1945, 1946). If the kelp Egregia is not present then the Egregia forms of ~- pelta will not be present. If mussels dominate the intertidal as on the Farallones then these forms will be present in high numbers. If Black Oystercatchers are present, limpets that move to other substrates are likely to be removed through predation. Over time there will be a steady number of these limpets produced since it is presumed that the environmental factors that cause the move would stay the same. However, as each limpet reaches a certain level of contrast between old and new shell color the probability of being observed and taken by the Black Oystercatcher increases. No removal of these "movers" from the population will occur since this 55 is not determined genetically. In areas without Black Oystercatchers the number or proportion of these transitional forms will build up over time, since predation pressure would not be present. Human disturbance is a major factor determining the distribution of Black Oystercatchers (Ainley & Lewis, 1974; Nyswander, 1977). They are easily disturbed and will leave an area if disturbance is frequent. Prior to 1974 the site at Point Piedras Blancas was manned by Coast Guard personnel. During this time it is unlikely that Black Oystercatchers were able to nest at the site. In 1974 the lighthouse facility at the Point became automatic and the personnel were removed. This study started in 1977 but U.S. Fish and Wildlife personnel had been observing the site off and on since 1975. The first pair of Black Oystercatchers nested in summer 1977 and large numbers of transitional forms of ~· pelta appeared in the nest samples. This buildup of transitional forms probably occurred during the period when human disturbance kept Black Oyster catchers from the site. Once the disturbance was removed and following a wait and see period for the birds, predation pressure on the transitional forms returned. The same type of events may have occurred on the Farallones, however the number of years since 56 disturbance has been greater (Ainley & Lewis, 1974). Descriptions of ~· pelta in the literature rarely include these transitional forms (Jobe, 1968; Lindberg, 1981). Jobe (1968) considered these forms a separate morph, the green morph. This study has shown that these forms are actually the composite of other forms and that individuals can change during their lifetimes. Since these transitional forms are conspicuous it would seem likely that collectors would notice them and descriptions would be plentiful. However, this is not the case. One possibility is that Black Oystercatchers were present at collection localities and therefore transitional forms absent. It is interesting to contemplate whether the presence or absence of transitional forms of ~· pelta could be used as an index to distribution patterns of Black Oystercatchers. In summary, it appears that human disturbance determines the distribution of Black Oystercatchers, which in turn determine the presence and distribution of transitional forms of C. pelta. 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