Limpets on Pollicipes
page 2
Introduction
The limpets Collisella digitalis (Eschscholtz, 1833) Collisella
pelta (Eschscholtz, 1833) and Acmaea strigatella (Fritchman, 1960)
are common grazers of micro algae in the mid-tide region of the
exposed rocky intertidal of central California. Examination of
plates of the gooseneck barnacle Pollicipes polymerus (Sowerby, 1833)
living in this range reveals an environmentally different habitat for
morphologically altered forms of these and possibly other species
of limpets.
Though the relationship of limpets on Pollicipes has been
reported and described, (Giesel 1969) little is known about the taxonomy,
morphology and behavior of these limpets. The following paper discusses
the problems of taxonomy, and the difference in morphology and behavior
of the limpets on Pollicipes, and those found on adjacent rocks.
All collection and field work was done during the Spring of
1980 at Point Pinos and at the Hopkins Marine Station. (Collections
excluded Collisella scabra (Eschscholtz, 1833) and limpets smaller
than 4 mm in length.)
Limpets on Pollicipes
page 3
Distribution in field
Methods
The ratio of Pollicipes to limpets was determined by counting
limpets on 61 randomly selected barnacle clusters from the midportion
of their tidal range at Point Pinos. Cluster sizes ranged from three
individuals to 400 per cluster with 60% of the clusters having at least
50 Pollicipes. The density of limpets on rocks within four feet of
Pollicipes clusters (R-limpets) was determined by counting limpets
inside a surveying square (.04 m2). Fifty-six determinations were
made, taking note of the fraction of each square covered by clusters
or algae. The density of limpets on Pollicipes (P-limpets) was
determined by measuring the plate surface area available to limpets
on Pollicipes, during dissection of Pollicipes. Pollicipes were
estimated to double the surface area available to limpets.
Results
The lowest ratio of Pollicipes to limpets was 2.67. The largest
cluster without limpets was 400 Pollicipes. The mean ratio was
29.73 and the standard deviation was 34.085. No limpets were on 188
of the clusters. For density values see table 1.
page 4
Limpets on Pollicipes
Areas with macroalgae are more densely populated by both the
P-limpets and the R-limpets. The density of P-limpets per Pollicipes
is less than that of R-limpets on rocks. A qualitative analysis of
the mass ratio shows it is smaller than that of the ratio of
individuals.
Morphology of P-limpets vs. R-limpets
Methods
Morphological traits of P-limpets and R-limpets were scored:
A. Color: "white", "black", and "patterned" if two or more
colors were present
B. Curvature of bottom of shell: "flat" if no more light
passed between the middle of the shell and a flat surface than any
other part of the shell bottom, "slight curve" if more light passed
through the middle, "large curve" if a .25 mm dissecting pin fit
between the shell and the surface.
C. Ribbing: "suggested" if seen only on outer surface and
"ribbed" if seen from underneath
D. Length, height, width, distance posterior end to apex:
measured using a micrometer and expressed in ratios.
E. Radular ribbons: Microscope observations using the
methods of Giesel (1969)
Limpets on Pollicipes
page 5
F. Shell color patterns: Direct observations based on
descriptions by Giesel (1969)
Results
The bottom edges of a greater portion of the R-limpet shells are
curved, than those of P-limpets. The results are summarized in table 2.
A majority of black limpets with a white apex and an owl-shaped spot in
the apex have a curve. Interestingly, 80% of C. digitalis collected
from above the intertidal on rock surfaces are curved, and all have an
owl-shaped spot in the apex.
The mean length to heigth and length to width ratios of
P-limpets are significantly larger than those of the R-limpets. (Table 3)
The ratio of length posterior to apex to total length of P-limpets
showed the white, black and patterned P-limpets are three different
groups. (Table 4) Black shells with white apex are not statistically
different from the rest of patterned shells. 30 percent of all
limpets had a ratio greater than .84 (forward apex), indicating
they may be C. digitalis or A. strigatella, rather than C. pelta.
A low mean was observed for the black shells with a white apex and
lacking ribs. Twenty four percent of P-limpets showed signs of ribbing,
but only 128 had ribs visible on the inside of the shell. Five percent
showed signs of ribbing in combination with a curved dorsal surface and
a far-forward apex, ratio greater than.84.
Limpets on Pollicipes
page 6
The typical color patterns described by Giesel (1969) for
C. digitalis were poorly represented in the P-limpets, while C. pelta
and A. strigatella features were prevalent. (Table 5)
A preliminary qualitative analysis of the apex position of 79
P-limpets indicated 68 possible C. digitalis and 11 possible C. pelta.
Of these, a high percentage of radulae had a high radular length to
width ratio and a low percentage of radulae without a shoulder.
A high percentage of radular central plates of limpets whose apex
was subcentral had central plate overlap. The central plate divergence
accounted for 538 of all P-limpets. (Figure 1)
Orientation and distance traveled
Methods
The positions of P-limpets were marked in the middle intertidal
zone on clusters larger than 70 Pollicipes with fingernail polish and
noted again the 2nd, 6th, 7th, 8th, and 13th day. Not all clusters
were checked each day, due to adverse weather conditions. The resulting
limpet sample sizes were 67, 18, 9, 18, and 8 respectively.
A total of 60 P-limpets were painted with fingernail polish on
seven clusters no smaller than 70 Pollicipes, in exposed and protected
areas. Eighty P-limpets were painted on a vertical wall with western
Limpets on Pollicipes
page 7
exposure and no protection from waves, on clusters of varying sizes in
the middle Pollicipes range. The 140 limpets were checked every six
hours for 72 hours for their presence on rocks.
Results
P-limpets tend to move on their clusters without leaving them.
A low percentage of limpets retain their orientation for a 24 hour
period. (Table 6)
On the average, P-limpets are not found more than two neighboring
Pollicipes away from the original after 24 hours. In two cases, two
limpets moved to the same Pollicipes, and remained there for more than
three days. (Figure 2 )
Feeding patterns
Methods
The gut contents of 60 P-limpets, 20 R-limpets and the surface of
20 Pollicipes was analysed for: Hildenbrandia sp., Ectocarpus sp.
blue green algae, red algae, fungi, Enteromorpha sp., and diatoms.
The rock surface was checked for Ectocarpus sp. within three feet
of the clusters.
Limpets on Pollicipes
page 8
Results
Two of the 140 P-limpets observed left the Pollicipes clusters
within 72 hours. One did not return to the clusters within the period,
and the other dissapeared. No other marked limpets were seen on
the rock surface.
The results of gut contents analysed are summarized in table /.
Red algae was not observed on Pollicipes plates, and Ectocarpus was
not seen in the scrapings of the rock surface. Red algae and
Ectocarpus were found in both the guts of the P-limpets and the
R-limpets. P-limpets and R-limpets both eat all the food types
surveyed.
Transplants
Methods
Painted R-limpets and P-limpets were transplanted to Pollicipes
clusters of size 50 Pollicipes or more. The limpets remaining on each
cluster were counted one day later, and an area of ! m radius was checked
for marked limpets. Where controls were used, equal numbers ofdtested
and control limpets were transplanted.
Limpets on Pollicipes
page 9
Results
P-limpets can adjust significantly better to new Pollicipes under
natural conditions than R-limpets. See table 8. Between 68 and 302
of R-limpets transplanted to each cluster were observed climbing onto
the surrounding rock, and moving upward. Only one R-limpet moved
downward on the rock.
Substrate Choice
Methods
A Pollicipes cluster attached to its granite rock and a separate
granite rock (both 30 cm diameter) were placed in an outdoor salt
water tank with 108 P-limpets and 108 R-limpets around their base.
Salt water spray kept the system moist and limpets were counted on
each surface the 2nd, 4th, 5th, and 7th day.
Results
Ihe results are summarized in Figure 3. P-limpets showed a greater
preference for the Pollicipes cluster than R-limpets. Fourteen percent
of R-limpets and 82 of P-limpets did not choose either the cluster or
the rock, and remained at the bottom of the tank.
Limpets on Pollicipes
page 10
Dessication Resistance
Methods
P-limpets and R-limpets were allowed to acclimatize for two days
on a fresh surface of granite and on Pollicipes plates cleaned with
NaOH and attached with Epoxy Glue to a 35 x 15 cm board. The limpets were
dried under room temperature conditions for two days and subsequently
sprayed with equally strong streams of water to dislodge dead and
weakened limpets. The remaining limpets were counted.
Results
A greater percentage of R-limpets remained on both surfaces.
See table 19.
Low salinity resistance (seal efficiency)
Methods
After two days of acclimatization under moist, laboratory
conditions, 33 R-limpets and 34 P-limpets on a granite rock and
89 R-limpets and 122 P-limpets on a live Pollicipes cluster were
submerged in fresh water. The limpets were counted after three hours
and sprayed as above after 1I hours. The remaining limpets on each
Limpets on Pollicipes
page 11
surface were analysed for signs of life such as a soft, contracting
foot, a retracting head, and moving tentacles.
Results
The survival rates were greatest for P-limpets on Pollicipes and
for R-limpets on the rock. See figure 4. The tight seal of limpets
to surface is not released immediately upon death. The remaining
P-limpets were qualitatively judged more difficult to remove from
the cluster than the R-limpets. Early during the experiment up to
44% of P-limpets were observed climbing upon each other's backs,
forming stacks up to 6 limpets high, while on rocks. The corresponding
behavior was observed for only 3% of R-limpets.
Discussion
The taxonomy of P-limpets presents an interesting challenge.
Though it seems that approximately 9% are C. digitalis, 40% C. pelta,
and the remaining 51% a combination of A. strigatella and others,
enough discrepancies exist between various morphological features
to preyent clearcut identifications of individuals. The features of
many white shells, particularly the position of the apex, indicate
c. digitalis is the prevalent species on Pollicipes, but the same
limpets lack the inner shell patterns and radular ribbon structure of
C. digitalis found on rocks. The 100% lack of curvature of white shell
bottoms suggests a separate group of limpets, with similar genetic
Limpets on Pollicipes
page 12
background. A close examination of the radular ribbons and shell color
patterns of P-limpets and those described by Fritchman, (1960) clearly
shows that standard morphological features are not enough for the
identification of P-limpets.
P-limpets have shell adaptations for living on the Pollicipes
plates. The identical color patterns of shells and Pollicipes,
especially noted in the upper part of the mid-tidal range serve well
as protection against preditors such as Oyestercatchers, (Giesel, 1969).
The narrower and higher profile of P-limpets than that of R-limpets,
indicates an adaptation for living on Pollicipes plates. A narrower
base may be of advantage during stress periods. A wide base would
overlap the edges of Pollicipes, and cause dessication. A narrow
base may further facilitate feeding from the surface of Pollicipes
since less surface is covered by the limpet's foot.
While individual P-limpets have adjusted to living on small
surfaces, they enjoy two to fourteen as much space as the corresponding
R-limpets. Pollicipes clusters act as protective living quarters,
absorbing the force of waves through flexibility, retaining moisture,
and providing shade. Without traveling more than 1 cm, a P-limpet
finds another Pollicipes, possibly covered with microalgae, in exactly
the same environmental conditions as the previous Pollicipes. The
crossing between two individual Pollicipes is dangerous, because the
limpet needs to extend its foot through space, and may account for
the recorded loss of limpets.
page 1 3
Limpets on Pollicipes
The greater density of P-limpets on clusters near macroalgae
suggests a relationship of P-limpets to rock cover. Giesel (1969)
describes the transition of limpets 4.5 mm or larger to Pollicipes
clusters. A higher initial density of R-limpets on the rock, acting
as a source for P-limpets may explain the higher P-limpet density on
Pollicipes near algae. Direct observations that P-limpets remain
on clusters and do not leave to feed on rocks agree with most of
those of Giesel (1969). Over several months, he observed P-limpets
leaving the clusters only twice during cool and moist conditions.
The major food types for limpets grow on the surface of Pollicipes,
but are abundant only in the grooves between plates. Since limpets
were observed scraping the plates both in the field, and in the
laboratory, they possibly act as surface cleaners of Pollicipes plates.
The preference of P-limpets for Pollicipes and their greater
survival on Pollicipes during transplants and stress tests than that
of R-limpets suggests P-limpets are adapted for the life on Pollicipes
and benefit from it. Their low rate of survival off Pollicipes
suggests a strong dependence on the clusters. A study and comparison
(Conrad)
of limpets on mussels, Mytilus californicus
may answer more clearly the questions about predation, density, feeding
Mytilus lives within the
habbits and surface - limpet relationship.
range of Pollicipes, has a similar surface, yet offers a larger area
per individual Mytilus, and drastically differs in colors.
Limpets on Pollicipes
page 14
Summary
1. The density of individual limpets living on rocks within four
feet of Pollicipes clusters (R-limpets ) per surface of rock is
greater than that of the limpets living on Pollicipes (P-limpets)
per surface area of Pollicipes plates.
2. The morphology of P-limpets differs from that of R-limpets.
3. Many standard taxonomical descriptions of limpets do not
apply to P-limpets.
4. P-limpets move from an individual Pollicipes to the next,
but rarely leave the cluster.
5. P-limpets scrape food from the surface of Pollicipes.
6. P-limpets adjust better to new Pollicipes clusters and
R-limpets adjust better to new rocks.
7. P-limpets are attracted to clusters at a greater rate than
that of R-limpets.
8. P-limpets are better adopted to survive stresses on Pollicipes
than R-limpets, and R-limpets survive stresses better on rocks and
unnaturally arranged Pollicipes plates.
page 15
Limpets on Pollicipes
Acknowledgements
I am gratefull for the help received from Izzie Abbott, Don Abbott
and Chuck Baxter. Many thanks are due to all staff of the Hopkins
Marine Station. Above all, however, I thank Robin Burnett for his
advice and especially his patience during the last four days of
the spring quarter.
Limpets on Pollicipes
page 16
Literature Cited
Fritchman 1I, Dr. Harry K, 1960. Acmaea paradigitalis SP. NOV.
(Acmaeidae, Gastropoda ), The Veliger 2 (3): 53-57
Giesel, James T, 1969. On the Maintenance of a Shell Pattern and
Behavioral Polymorphism in Acmaea digitalis, a Limpet,
Evolution 24: 98-119 March, 1970
Limpets on Pollicipes
page 17
Table Legend
Table 1: Density values for limpets and the ratios of Pollicipes
to limpets on various surfaces at Point Pinos, in the intertidal.
Table 2: Curvature of limpet shell bottoms measured on flat surface
using light and a dissecting pin (.25 mm diameter).
No curve: no more light passes between the shell and the flat
surface in the center of shell, than towards the anterior and
posterior ends
Slight curve: more light passes through center
Large curve: dissecting pin fits between center of shell bottom
and flat surface.
Table 3: Mean and standard deviation values for limpet shell length
to heigth and length to width ratios. Values are significant
for p «.001 (Student's T-test).
Table 4: Mean and standard deviation values for ratios of distance
posterior end to apex over total P-limpet shell length. Pairwise
combinations of groups I, II, and III determined statistically
significant for p «.001 (Student's T-test)
page 18
Limpets on Pollicipes
Table 5: Percentages of 232 P-limpets showing particular color
patterns, and proposed species based on the color patterns
described by Giesel (1969).
Table 6: A summary of limpet movement and orientation changes in
terms of percent of each group observed a given number of days
after limpets were marked.
Table 7: Chart of type and relative amount of limpet food found in
the digestive track of limpets and on the surface of Pollicipes.
Table 8: Results of transplant experiments showing percent of
originally transplanted limpets remaining on each surface
24 hours after the transplant occured.
Table 9: Results of a dessication experiment, given in terms of
percent of limpets adhering to original surface after stress period.
Limpets on Pollicipes

I
2 0

9
page 19
2
00
otatakaa-
5
- —
*
Limpets on Pollicipes
page 20
Table 2
Curvature of shell bottom as seen on flat surface
P-limpets
R-limpets
AII
A
B C
578 1002
88 02
No Curve
102
138
08 298 08
Slight Curve
328
298
Large Curve
08 638 1008
579
127 56 51 10
Sample Size
105
A: All-white shells
B: Black shell, white apex, brown post-apical spot
C: Owl-shaped spot in the apex
Table 3
Ratios of shell dimentions
P-limpets
R-limpets
2.49
2.96
Length/heigth: mean
.38
.46
s.d.
1.38
1.34
Length/width: mean
.09
s.d.
208
149
Sample size
Limpets on Pollicipes
page 21
Table 4
Posterior end to apex over total P-limpet shell length ratio
Shell type
Sample size
Mean
S.D.
.06
1 White
.80
.08
11 Black
.76
.06
III Patterned
.72
IV Black with
.72
.06
white apex
230
.76
07
Total
* Shells with two or more colors in various patterns
"* Shells with distinct black-white boundary
Table 5
Shell marking of 232 P-limpet shells
Possible
Internal
External
color
8
Species
Apex
Color Apex
9 C. digitalis
Variable
Variable Owl-shaped spot
40 C. pelta
Variable Post-apical spot
Black White
4 A. strigatella
Variable
Blue-Wht. No Spot
Blue-Wht. Variable
34 A. strigatella
Variable
Variable ---------- Transitional -----
13 Any
Limpets on Pollicipes
page 22
Table 6
P-limpet movement and orientation change
Sample size + days % on other
Orientation
and type (after Ist) Pollicipes Zchanged %same glost
60 disturbed
10
77
60 not "
26
60 not "
10
18
60 not "
0
11
60 not "
12 12
60 not "
10 18
Table 7
Food type of limpets
Location (Gutaof limpets and surface of Pollicipes)
Food type
8 of 26
% of 60 % of 20
P-limpets R-limpets
Pollicipes plates
45
23
Hildenbrandia
Ectocarpus
Blue Green A.
Red Algae
38
Fungi
Enteromorpha
25
Diatoms
c
Limpets on Pollicipes
page 22
Table 8
The reaction of limpets to new Pollicipes clusters and rock surface
after transplants
f and origin of limpets
% of transplanted limpets present next day
on Pollicipes cluster on rock (control)
20 Flat cement wall
50 5 ft. Above cluster
50 6 ft. Above cluster
18 3 ft. Above cluster
36
17
50 Cluster level
(Immediate transplant)
16
50 Cluster level
(kept in lab ! day)
86
88 Other clusters
" No transplant performed
Limpets on Pollicipes
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Table 19
Dessication of P-limpets and R-limpets on Pollicipes and on Rocks
+ limpets before
Surface
limpets still
P-limpets R-limpets adhering after dessication
P-limpets R-limpets
Pollicipes
plates
Rock
Limpets on Pollicipes
page 24
Figure legend
Figure 1: Results of analysis of the radular ribbons of P-limpets
using techniques described by Fritchman (1960)
la: Plot showing divergence patterns of central plate
lb: Plot showing percentage of radulae with central plate overlap
Ic: Plot showing percentage of radulae with spatulum overlap
Id: Plot of radular length/width ratio
le: Plot showing percentage of radulae with shoulder
* Preliminary determination of possible species by qualitative
analysis of apex position and other features of shell
Figure 2: Results of limpet movement observation, showing percentage
of limpets that moved within given number of days over a given
number of individual Pollicipes.
Figure 3: Plot showing result of surface preference experiment
byrgiving the rate of increase and decrese of percentage of
P-limpets and R-limpets on:
a: Pollicipes clusters
b: rock
Figure 4: Results of the fresh water (seal efficiency) stress test,
given in percentage of limpets remaining and surviving the stress
over the given period of time and on the given surface.
Limpets on Pollicipes
total
C. digitalis
C. pelta
Percentages given by Fritchman (1960)
100
80
60
8 radulae
20
EN


: Parallel
P: Posterior
A: Anterior
Figure la
Page 25
Limpets on Pollicipes
100
80
8 radulae
40
20
100
80
60-
% radulae
40
20
—9—


Figure 1b
Figure 1c
page 26
Limpets on Pollicipes
100—e—
80
60-
% radulae
20
100
80
% radulae
-e


—

Figure 1d
—
:
Figure le
page 27
Limpets on Pollicipes
% limpets
page 28
Day1
50

S
Day 5
50
25
N
Day 6
50
25
Day 8
50
25
Day 13
50
25
S
2 3 4
+ neighboring Pollicipes from
original (in straight line)
Figure 2
off
Limpets on Pollicipes

katatatatatata-
page 29
—
1
Limpets on Pollicipes

ktatatataatatataa-
page 30
page 3
Limpets on Pollicipes
Limpets
90
rema ining
Limpets
80
remaining
70
60
8 limpets
50
40
Limpets
living
20
10


f Hours after start of experiment
R-limpets on rock
P-limpets on rock
R-limpets on Pollicipés
P-limpets on Pollicipes
Figure 4