Interspecific Aggression
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INTRODUCTION
The sea anemones Anthopleura elegantissma (Brandt,1835),
Metridium senile (Verrill, 1865), and Corynactis californica
(Carlgren, 1936) associate sympatrically on the pilings under
the Monterey wharf.
Anthopleura elegantissma is a west coast anemone. In-
tertidally, it is found from about -2.0 to +4.5 feet above
mean lower low water level. (This is the clonal variety)
Metridium senile is a circumpolar, temperate to cold
water anemone found on both coasts. It can be found as high
as +3.0 feet above mean lower low water level. It also
occurs well below tide marks.
Corynactis californica is found along the Pacific coast
north of the equator. It is most profuse at the mean lower
low water level and below.
On the pilings I have observed interspecific separation
lines approximately 1.5 inches wide between A. elegantissma
and M. senile. Interspecific separation lines about 1 inch
wide have also been found between A. elegantissma and C.
californica.
Childress (1969) has demonstrated that the inter-clonal
Interspecific Aggression
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separation lines (Francis, 1973a) found between A. elegantissma
clones are formed and maintained by intra-specific aggression.
Presented is a report on the interspecific aggressive behavior
of the three above mentioned sympatric sea anemones and its
relation to the observed interspecific separation lines.
MATERIALS AND METHODS
Collecting and field studies were carried out on the
first eight rows of pilings after the Marina boundary, at Pier
2, Monterey wharf. Field observations and studies were con¬
ducted using SCUBA gear. In the case of A. elegantissma and
M. senile a spatula was utilized to collect specimens (Francis, 1973a)
I used a hammer and chisel to chip offthe Balunus nubilus shells
and their attached Corynactis. This minimized damage and
provided experimental animals on a movable substrate.
The species of anemones were kept separately in tanks that
were supplied with flowing sea water at about 13°C. The
anemones were fed Tigropus califoricnus intermittently except
during experiments when food was withheld.
PROCEDURES, OBSERVATIONS AND RESULTS
Intra-specific Aggression
Francis (1973b) has shown intra-specific aggression between
different clones (Francis, 1973a) of A. elegantissma. This does
not seem to be the case with regard to M. senile and C. californica
In the lab, utilizing the procedure outlined below, I never
Interspecific Aggression
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observed aggression between colormorphs of these two species.
Furthermore, in the field, no intraspecific separation lines
were noted in either M. senile or C. californica.
Interspecific Aggression
To ascertain the dominance hierarchy order, the follow-
ing procedure was utilized (same procedure used in intraspecific
aggression).
Procedure: 1) An anemone was settled in a finger bowl and
its position marked on the outside of the bowl.
2) An interspecific antagonist, settled on a movable substratum
would be brought within contact distance.
3) After a day the outcome of the aggressive event could usually
be determined. Movement away from the confrontation and/or
visible necrosis were the factors used in assessing the outcome.
M. senile was the most sucessful interspecific aggressor,
followed by C. californica. A. elegantissma turned out to be
the least dominant aggressor. These findings and their stat¬
istical significance are listed in Table 1. e tadte i he
Aggression and the maintenance of interspecific separation lines
To determine whether aggression was a major factor main-
taining separation lines, the following experiments were performed.
Procedure: 1) Three dissecting pans (8x111.5 inches) were layered
with a white wax bottom.
2) Each pan was filled with a different binary combination
of the three species. The animals were pinned to the wax so
that the initial aggregation patterns were fixed.
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Interspecific Aggression
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3) The two different species were separated by a zig-zag
pattern boundary made up of microscope slides glued together.
The pattern maximized border anemone contact.
4)
After three days the pins and boundary were removed.
5) Dead or unattached anemones were removed to simulate
field conditions. The experiment was run for two weeks.
Net movement at the borders confirmed the results of
the individual interspecific aggressive confrontations.
M. senile pushed back the A. elegantissma border and killed
the C. californica border anemones. A. elegantissma retreated
from the Corynactis border. A stable border with no contact
between neighboring species was established by the two weeks.
Aggressive behavioral pattern of A. elegantissma
The interspecific aggressive pattern exhibited by A. el
egantissma is identical to its intraspecific behavioral
pattern described by Childress (1969). This behavior can be
separated into six stages: 1) Stimulation, which involves
repeated tentacle contact and withdrawal; 2) Inflation, when
the acrorhagi become turgid; 3) and 4) the Movement of Inflation
and Movement of Application, during which the acrorhagi move
towards the victim; 5) Application of Ectoderm, during which
acrorhagi are applied; and 6) Recovery, when the anemone
returns to its normal posture.
Aggressive behavioral pattern of C. californica
The term "aggressive" is misleading when used to describe
Interspecific Aggression
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C. californica. Defensive response would be the more correct
terminology. I have never observed C. californica initiate
interspecific aggression. When attacked, it will retract its
tentacles, shorten its column and contract the sphinctor at
the top of the column. Close examination will disclose a
minor extrusion of some mesenterial filaments thru the mouth.
If not attacked again, C. california will regain its normal
posture in a matter of hours. However, if attacked again,
it will fully extrude its mesenterial filaments onto the
aggressor. This behavior will usually cause the interspecific
antagonist to retract and/or retreat.
I have not seen the capitate tentacles actively used in
aggression by C. california.
Aggressive behavioral pattern of M. senile
Upon tentacular contact with an interspecific antagonist
M. senile partially retracts its tentacles in the local region
of stimulation. It then elongates its body column and bends
toward the victim. During this period, all the tentacles that
face the victim are extended and then applied to it. M. senile
does not withdraw after tentacle application. Usually it is
the victim that breaks contact and initiates avoidance behaviors
such as leaning away, retracting the column, or physically
moving away.
Approximately 70% of the time, M. senile will initiate
aggressive behavior before its interspecific antagonist.
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Receptor location
To find the location of the receptors that detect in¬
terspecific antagonists, the following experiments were
performed.
Procedure: 1) Tentacles were snipped off all three species
of anemones and used as probes to touch different parts of
an interspecific antagonist.
2) As a control, a clonemates tentacle was used to see if
the anemone would discriminate between self and non-self.
3) A positive response was assumed if the subject retracted
its tentacles and/or signs of aggressive response followed.
Childress (1969) found that an excised A. elegantissma
tentacle was still completely effective in eliciting aggress¬
ion after many (greater than 10) contacts. This observation
was found to be true also with M. senile and C. californica
tentacles.
Metridium and Corynactis did not respond to stimulus by
their own species, while Anthopleura would respond to a non-
clonemate. All three species of anemones also did not respond
to an interspecific tentacle if touched anywhere other than
the tentacles. If touched elsewhere and at the tentacles,
retraction occured. Furthermore, Anthopleura and Metridium
responded only if contact was at the tentacle tip. On the
other hand Corynactis responded only if contact was along the
tentacle sides. I would assume that the receptors that detect
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interspecific antagonists are located in these respective areas.
Light intensity and anemone distribution
Hand (1955) noted that the Corynactis colonies under the
Monterey wharf occurred on pilings where there was no direct
sunlight. I have found colonies in my study site exposed
to direct sunlight, but these colonies have an upper limit
approximately 3 feet lower than the Corynactis found under
H
the wharf in the shadows. (Figure 1) Light meter readings
taken May 28 ", 4:30 pm, show a four-fold decrease in
foot-candles from the outermost to the innermost pilings,
the latter being in constant shadows. Thus, there is a
positive correlation between decreasing light intensity
and higher Corynactis distribution. The same type of
correlation, but to a lesser degree, holds for Metridium.
On the other hand, regardless of piling position, the A.
elegantissma colonies in my study site always were spread
down to the upper limit of the Corynactis or Metridium
colony below it. It would seem then that light intensity
is not a major factor limiting Anthopleura distribution
on the pilings.
I have observed that within my study site, Metridium
and Corynactis occupy different microhabitats, i.e. they
do not occupy the same piling. I have observed two pilings
that have both Metridium and Corynactis settled on it, but
the interspecfic separation lines are over two feet wide.
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Interspecific aggression
Preliminary Observations
have observed that M. senile will secrete abnormal
amounts of mucus when disturbed, such as in aggression.
Tigropus califoricnus coated with Metridium mucus survive
the entire test period (10 seconds) when placed on the
tentacles of an A. elegantissma. In contrast, uncoated
Tigropus are instantly killed when placed in the same
situation.
Both the coatéd and uncoated Tigropus are instantly
killed when placed on the tentacles of a C. californica.
DISCUSSTON
As I have noted before (Intro.) the range of A. elegant-
issma can extend down to -2 feet below mean lower low water.
Furthermore, I have found solitary A. elegantissma living
at -52 feet. Yet on the wharf pilings, both varieties
are restricted to a narrow range delineated at the low-
er limit by interspecific borders shared with either
Metridium or Corynactis. This fact, coupled with the border
experiments and dominance hierarchy experiments, indicate
that the distribution of A. elegantissma has been limited
by interspecific aggression with Metridium and Corynactis.
In turn, there is evidence that environmental factors
such as light prevent Metridium and Corynactis from extending
their upper limit of distribution. Other environmental
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Interspecific Aggression
factors such as dessication, food supply and wave action
undoubtably have some influence on anemone distribution.
The reasons for the aggressive superiority of Metridium
are not immediately obvious. However, my preliminary
observations point out the possibility that the mucus
secreted by Metridium could protect it against Anthopleura.
This protection could be in the form of an anesthetic
inhibiting the firing of nematocysts. Another possibility
is that the mucus serves as a protective coating against
Anthopleura or Corynactis.
Corynactis
feeding" response
Lang (1971 and 1972) noted interspecific aggression in
Scleractinian coral, which are of the order Madreporaria.
She observed that when Scolymia lacera was brought into
contact with Scolymia cubensis, the former would extrude
its mesenterial filaments and digest the tissue of the
latter. Lang (1971) describes this act of aggression as
basically an extracoelenteric feeding response.
This behavior is remarkably similar to the interspecific
aggressive pattern of C. californica, a member of the Order
Corallimorpharia. In Hyman (1940) the Madrepores are reported
as readily derivable from Corallimorphidae. If so, this
would indicate that the receptors that detect interspecific
aggressors are linked to the normal extracoelenteric feeding
behavioral pattern in C. californica.
Interspecific aggression
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SUNARY
The sea anemones Anthopleura elegantissma, Metridium
senile, and Corynactis californica are observed to engage
in interspecific aggression. This aggressive behavior is
elicited only thru tentacle contact. The receptors that
detect interspecific antagonists are located only on the
tentacle tips in Anthopleura and Metridium, while Corynactis
detects from the sides of its tentacles.
Metridium and Corynactis were found to occupy different
microhabitats at the field site. I did not observe inter-
specific aggression between them in the field.
I did observe interspecific aggression in the field
between Anthopleura and Metridium, and Anthopleura and
Corynactis.
Interspecific separation lines can be set up by inter¬
specific aggression, in the lab.
Each species of anemone exhibits an aggressive behavioral
pattern peculiar to its own species. Anthopleura elegantissma
has morphological structures called acroraghi used specifically
for aggression. Corynactis californica extrudes mesenterial
filaments in a manner similar to its extracoelenteric feed-
ing response when attacked. Metridium senile may produce
mucus during aggression as a self-protective mechanism.
Interspecific aggression and environmental factors such
as light intensity influence the distribution of sea
anemones on the pilings under the Monterey wharf.
Interspecific aggression
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ACKNOWLEDGEMENTS
I wish to thank Nat Howe for his helpful advice.
I am indebted to all my diving buddies for getting
this project into the water. Special thanks go to
Chuck Baxter forhelpfully tearing my paper apart,
and making it better each time.
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Interspecific Aggression
LITERATURE CITED
Childress, L., 1969. Intra-specific aggression and its
relation to the distribution pattern of the clonal
sea anemone Anthopleura elegantissma. Ph. D. thesis,
Stanford University, 123 pg.
Francis, L., 1973a. Clone specific segregation in the sea
anemone Anthopleura elegantissma. Biol. Bull., 144:64-72
Francis, L., 1973b. Intraspecific aggression and its effect
on the distribution of Anthopleura elegantissma and
some related sea anemones. Biol. Bull., 144:73-92
Hand,
C., 1955. The sea anemones of central California,
Part I. The Corallimorpharian and Athenarian anemones.
Wasman J. Biol., 12:345-375
Hyman, L. H., 1940. The Invertebrates: Protozoa through
Ctenophora. First edition. McGraw-Hill Book Company,
Inc.
Lang, J., 1971. Interspecific aggression by Scleractinian
corals. 1. The rediscovery of Scolymia cubensis (Milne
Edwards and Haime). Bull. of Mar. Sci., 21(4):952-959
Lang, J., 1973. Interspecific aggression by Scleractinian
corals. 2. Why the race is not only to the swift.
Bull. of Mar. Sci., 23(2): 260-279
terspeific Aggression
TABLE
DOMINANT AGGRESSOR
TYPE OF
ENCOUNTET
C.c.
A.e. M.s.
A. elegantissma
vs.
M. senile
—
A. elegantissma
vs.
--
2
californica
californica
Vs.
M. senile

Draw
PæO.05
PO.01
Pæ0.025
5

5
5
3
+
+

5
2

—
5

3

S
2

r —
c
Interspecific aggression
Record of
able I
aggressive confrontations
ividual inte:
Page
0
Interspecific aggression
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FIGURE CAPTIONS
Figure L. Vertical distribution of the three major
sympatric sea anemones at the wharf, Anthopleura
elegantissma(A.e.), Metridium senile (M.s.), and
Corynactis californica (C.c.). Absolute depth was
obtained using the tidal guage kept by the Naval
Postgraduate School at Pier 2.