Differences in Aggressiveness
Introduction
The sea anemone Anthopleura elegantissima is typically found in
dense intertidal beds which may be subdivided by anemone-free trails
(Hand, 1955). Investigations by Francis (1973a) indicate that these
trails separate genetically distinct aggregations (clones) of asex-
ually reproducing anemones. Francis (1973b) has also reported anemone-
free zones separating large, solitary Anthopleura elegantissima from
adjacent clonal A. elegantissima.
An elaborate behavior pattern resulting from contact between gene-
tically distinct A. elegantissima has been studied by Francis (1973b).
She noted that a single, well settled anemone from her "Clone 2'
placed in the conter of a bowl containing several "Clone 1" A. elegan
tissima would be continually attacked and killed within ten days if not
removed. Francis (Childress, 1970) was also able to demonstrate the for-
mation of separation lines between two clones in the laboratory. She
noticed that only animals from one of the clones detached as a result
of the interclonal interaction. Such observations inspired this inves-
tigation of possible differences in aggressiveness among distinct
Anthopleura elegantissima genotypes.
Materials and Methods
General procedures
Clonal Anthopleura elegantissima specimens were collected from four
locations along the central California coast: Clone A and Clone E from
Point Pinos at Pacific Grove; Clone B from the intertidal region at
Hopkins Marine Station, Pacific Grove; Clone C from a rocky area just
ressiveness
Differences in Agg
outside of Hovden Cannery, adjacent to Hopkins Marine Station; Clone D
from a piling underneith Wharf Number 2 in Monterey Harbor, Monterey.
The animals were collected from the approximate centers of dense clones
at least 0.5 square meter in area. Clones A, B, C, and E were carefully
pried off their granite substrata with a thin spatula. Clone D specimens
were gently peeled off of the wooden piling with a fingernail. Animals of
Clones A, B. C, and D had expanded oral disc diameters of 1.5 to 3 cm and
had dry weights of 0.05 to 0.30 gm; Clone E anemones had diameters of
5 to 8 cm and weighed 0.3 to 1.0 gm. All clones were located from +O.15
to +0.35 m above zero tide level. The areas where Clones A, B, C, and E
were collected were chosen for similar exposure to waveshock and sunlight.
Clone D anemones were more protected from strong light and wave action.
Specimens were maintained in glass bowls supplied with flowing
seawater at 13 C in the laboratory for at least one week prior to exper-
imentation. Continuous illumination of 3500 Lux was provided by over-
head fluorescent lights. Anemones were not fed prior to or during the
experiments. Only specimens that were attached to the glass bowls and
fully open were chosen for experimentation.
For the first experiment six 35x22x4 cm Pyrex baking dishes were ruled
with a 2 x 2 cm grid. Three thin Plexiglass dividers spaced 2 cm apart
were placed in the center of each dish. Ten members of each clone of
two clones of approximately equal size were placed on each side of the cen¬
tral divider as follows:
Differences in Aggressiveness
Dish 1
Clone A vs. Clone B
Dish 2 Clone A vs. Clone C
Dish 3
Clone A vs. Clone D
Dish 4 Clone B vs. Clone C
Dish 5 Clone B vs. Clone D
Dish 6 Clone Cvs. Clone D
Dividers were lightly greased with petroleum jelly to restrict settling
to the dish.
In the second experiment single large anemones from Clone E were
settled in the center of similar Pyrex dishes with the aid of plastic
collars. Varied numbers of small Clone A animals were settled adjacent
to the larger Clone E individual. Experiments were repeated with Clone E
individuals of different size from those of the first run.
Dish 1 1x Clone A vs. 1x Clone E
Dish 2 1 x Clone A vs. 1x Clone E
Dish 3 2 x Clone A vs. 1 x Clone E
Dish 4 2 x Clone A vs. 1 x Clone E
Dish 5 8 x Clone A vs. 1 x Clone E
For both experiments the dishes were supplied continuously with
flowing 13°C seawater.
Scoring procedures
Behavior was scored according to an arbitrary scale based on many
observations of interclonal interactions in the laboratory and in the
field (see Table I). Behaviors judged "aggressive" were ranked in order
of their presumed potential for inflicting damage on nonclonemates and
Differences in Aggressiveness
were assigned scores positive in sign. Movements of application
(Francis, 1973b) were given a point value of +5. Displacement of an
anemone's pedal disc toward a nonclonemate was assigned a score of +4.
Swelling of acrorhagi and rapid tentacle retractions after contact with
a nonclonemate (Francis, 1973b), preliminary steps toward the movement
of application, were ascribed scores of +3 and +1 respectively.
"Submissive" responses were ranked in order of the degree of
damage that they reflect and were assigned negative values. Severely
damaged animals tend to detach their pedal discs from the substrate.
Such detachment was assigned a score of -5. Tentacle retraction is a
common response of the victim to application of ectoderm. Various degrees
of tentacle retraction were assigned the scores listed in Table I.
Moderately damaged animals often retreated from the aggressor. Dis-
placement of the pedal disc away from a nonclonemate during the scoring
period was awarded a score of —4. A score of O was assigned to anemones
that exhibited no response after contact with a nonclonemate. A notation
of noncontact was made for animals that did not touch animals from the
other clone. Data was collected only from animals that contacted non-
clonemates.
Twenty-four hours after placement in the dish was allowed for the
anemones to settle. Restraints separating nonclonemates were then lifted,
and the animals' behavior was observed continuously for one hour.
Cumulative aggressiveness scores were recorded at five minute intervals
for each individual of each clone.
At the end of the one hour scoring period the displacement of each
Differences in Aggressiveness
anemone's pedal disc was measured. Subsequent displacement was recorded
at various intervals for twenty-four hours. The water source for each
dish was moved periodically to prevent significant movement upstream
(Buchsbaum, 1968).
Forty-eight hours after the one hour scoring period began the
anemones were removed from the dishes for measurement of tentacle length,
scarring, and weight. Anemones were placed in 500 ml bowls filled with
seawater and a 1 ml dose of 0.25% (V/V in seawater) squid extract.
Anemones thus treated completely extended their tentacles. Several
relaxed tentacles were measured at random on each animal. Scars (Francis,
1973b) on the column, tentacles and acrorhagi were counted under a dissect-
ing scope. The weight of each anemone was determined either by weighing
it under seawater ("reduced weight") or measuring its dry weight.
Results
Interclonal confrontation experiment
Table II outlines a dominance hierarchy among four experimental
clones: in all its confrontations Clone A had a higher percentage of
fully open animals at the conclusion of the one hour scoring period
than did its opponents. Clone B had greater proportions of open in-
dividuals than its opponents in interactions with Clones C and D but
not when confronting Clone A. Clone C showed a higher percentage of
open animals than opponents only against Clone D, which in all cases
exhibited fewer open animals than its opponents.
Average aggressiveness scores per animal per five minute inter-
val (over twelve intervals) of the two clones in each dish (listed in
Differences in Aggressiv
ess
Table III) were found to be significantly different (p 0.05, Mann-
Whitney U-test). These scores closely follow the dominance hierarchy
from Table I. The average points scored decrease consistently as one
descends the hierarchy from Clone A to Clone D. Table III also indi-
cates that the aggressiveness scores of the opponents steadily decrease
down the hierarchy. As the distance between two clones in the hierarchy
increases the difference in the aggressiveness scores per animal per
five minute interval (over twelve five-minute intervals) of the clones
increases linearly (r = 0.93, p£0.01 ; see Figure 1).
Scarring counts, a more objective measure of aggressive behavior,
also correspond well with the dominance hierarchy. Table IV indicates
that the number of scars inflicted by each clone on its opponents de¬
creases consistently from Clone A to Clone D. Fewer scars are inflicted
on Clone A than on the lower clones, but there is no regular decrease
as the hierarchy is descended.
The total number of rapid tentacle retractions (Francis, 1973b)
observed in the one hour scoring period for each clone in each con-
frontation did not consistently follow the dominance hierarchy. A modifi-
cation of the aggressive behavior pattern was observed in a few individuals
from Clone A. These animals tended to extend their columns, bend over
and sweep the surroundings with their tentacles.
Dry weights of individual anemones in the two opposing clones
were not significantly different in any of the confrontations; differ-
ences in tentacle length were similarly insignificant. Additionally,
a plot of aggressiveness scores per animal per five minute interval
Differences in Aggressivenes:
(over the one hour scoring period) was not significantly correlated
with weight.
Differential size experiment
Figure 2 shows that as the reduced weight ratio of each Clone E
individual to its average surrounding Clone A anemone increases, the
difference between the aggressiveness scores of the Clone E and its
average Clone A increases linearly (r = 0.82, p £ 0.05).
Table V indicates that Clone E individuals in each case exhibited
more aggressive behavior than the average Clone A adjacent to that Clone
E animal.
Discussion
Consistent differences in aggressive behavior among four clones of
Anthopleura elegantissima were identified in the first experiment.
Several factors could explain these differences, including variations
in morphology, experience, behavior, and physiology. Significant vari-
ation in weight or tentacle length was not observed, however. Dissim-
ilarity of recent past histories was not explored since the anemones were
collected from the centers of large clones where animals were unlikely to
have had recent contact with nonclonemates.
Behavioral modifications which increase the probability of an
aggressive response developing might be expected in the more aggressive
clones. The occasional sweeping behavior observed exclusively in a few
Clone A individuals would increase the chances of contacting a nonclone¬
mate and may represent such a behavioral modification. It might be ex-
pected that the more aggressive genotypes would exhibit more rapid tentacle
Differences in Aggressiveness
retractions over the scoring period, signifying initiation of aggressive
behavior. Rapid retractions, however, did not appear to be a reliable
indicator for the aggressive response.
Physiological modifications such as faster neural machinery or greater
density of nonclonemate recognition sites could contribute to the more
aggressive behavior exhibited by the Clone A genotype.
That individual large A. elegantissima are more effective than
individual small animals in defending their space against several small
nonclonemates could explain how large, solitary A. elegantissima are able
to maintain positions in the middle of dense aggregations of smaller,
clonal anemones, whereas small nonclonemates are either displaced or
destroyed.
Summary
Consistent differences in aggressive behavior were found among four
distinct genotypes of Anthopleura elegantissima. These differences
could not be attributed to dissimilar weight, tentacle length, or recent
past history.
Experimental observations indicate that one large Anthopleura
elegantissima is more effective in defending its space against small
nonclonemates than a single Anthopleura elegantissima of a size similar
to its opponents.
I thank Nathan Howe for his encouragement and for reading this manu-
script, and the students and staff of Hopkins Marine Station for their
valuable assistance during the research.
Differences in Aggressiveness
Literature cited
Buchsbaum, V., 1968. Behavioral and physiological responses to light
by the sea anemone Anthopleura elegantissima as related to
its algal symbionts. Ph.D. thesis, Stanford University, 123pp.
Childress, L., 1970. Intra-specific aggression and its relation to the
the distribution pattern of the clonal sea anemone, Antho-
pleura elegantissima. Ph.D. thesis, Stanford University, 123pp.
Francis, L., 1973a. Clone specific segregation in the sea anemone
Anthopleura elegantissima. Biol. Bull., 114: 64-72
Francis, L., 1973b. Intraspecific aggression and its effect on the
distribution of Anthopleura elegantissima and some related
sea anemones. Biol. Bull., 144: 73-92.
Hand, C., 1955. The sea anemones of central California, Part II.
The Endomyarian and Mesomyarian anemones. Wasman J. Biol.
13: 37-99.
10
Points Assigned
45
+4
Table I.
Behavior Observed
Movement of application
Displacement of pedal disc toward a
nonclonemate
Swelling of acrorhagi*
Rapid retraction of tentacles
No response when in contact with a
nonclonemate
Retraction of 1/8 - 1/4 of tentacles
Retraction of 1/4 -1/2 of tentacles
Retraction of 1/2 - 3/4 of tentacles
Retraction of 3/4 - 4/4 of tentacles
Displacement of pecal dise away from
a nonclonemate
Detachment of pedal disc
e
Behavior Observed
% fully open
animals after
1 hour
Average
displacement per
animal in 24 hours
Total number of
quick retractions
in 1 hour
Table II.
Dish Clone A Clone B
138
0%
1008
100
——
678
100
——
-4.94
-3.17
——
-0.25
+0.20
+0.17
+0.92
——
——
14
——
——
39
Clone C Clone D
——
0%
0
——
408
——
208
100%
128
——
-2.06 --
——
-1.50
-0.63 --
——
-1.58
-0.40 -0.81
——
—
13
——
C
Table III.
Opponent Clone
——
12.5
+2.8
——
+1.0
+1.8
+1.0
0
-1.3
-3.5
-2.3
-2.5
+3.3
Points Scored Against Subject
(colum sums)
+2.6
+1.0
+1.1
——
14.7
Foints Scored by
Subjects
(row sums)
17.9
+3.8
+2.1
-7.1
e
Table IV.
Opponent Clone
——
2.7
1.0
O
——
8.2
0.6 0
3.8
4.2 —
0.6
——
1.8 3.1
3.0
4.7
15.0
8.7
0.6
Scars Inflicted by Subject
(column sums)
Scars Inflicted by
Opponents
(row sums)
3.7
8.8
8.6
Number of Clone A
animals
Table V.
Clone E score per
five minute interval
+4.08
+4.42
+3.17
+4.25
+4.17
Clone A score per
animal per five minute interval
+1.17
-0.17
+2.25
+0.58
+1.73
e
e
FIGURE 1
+7
DISTANCE IN HIERARCHY
3
SOS SS
o
Differences in Aggressiveness
Table captions
Table I : Scoring of aggressive behavior. +5 to -5 points were
assigned to each behavior observed for each individual over a
one hour scoring period. *(Francis, 1973b)
Table II : Some behavioral observations on interclonal confrontations.
Displacement of the pedal disc was positive in sign if in the direction
of a nonclonemate, negative if away from a nonclonemate.
Table III : Aggressiveness scores per animal per five minute interval
(over the one hour scoring period) in interclonal confrontations.
Table IV : Number of scars per animal forty-eight hours after interclonal
confrontation began.
Table V : Aggressiveness scores per animal per five minute interval
(over the one hour scoring period) of each Clone E individual and
its average Clone A opponent.
Differences in Aggressiveness
Figure captions
Figure 1 : The difference in average aggressiveness scores per animal
per five minute interval (over the one hour scoring period)
between each clone and its opponent clone as a function of the
distance between the two opposing clones in the dominance
hierarchy. A distance of 1 indicates that the clones are
adjacent to each other, while 3 indicates that the clones
are at opposite extremes of the hierarchy.
Figure 2 : Reduced weight ratio of each Clone E individual to its
average surrounding Clone A anemone as a function of the
difference in the aggressiveness scores (per animal per
five minute interval over the one hour scoring period).