INTRODUCT
Aggressive behavior characteristic of territoriality has
been reported for several polychaetes. The polychaete Nereis
pelagica exhibits a fighting response which is linked to tube
defense (Clark 1959). The polynoid Hesperonoe defends the
burrow it shares with Urechis against polychaete invaders.
Dimock (1974) found that as a result of aggressive interactions,
Arctonoe pulchra distributes itself so that there is only one
individual per host. To date, intraspecific aggression in
Arctonoe vittata has not been reported.
In California waters Arctonoe vittata occurs most commonly
in the pallial grooves of the keyhole limpet Diadora aspera,
the starfish Dermasterias imbricata, and the giant chiton
Gryptochition stelleri. The scent of the host is detected at a
distance by receptor organs on the prostomial antennae. Hosts
are recognized on contact by sense organs in the palps (Evans
1971). In this paper data are presented which show that adult
A. vittata have differing levels of aggression and that groups
of these worms, through aggressive interactions, constitute
hierarchies. It is seen that removal of the antennae drastic¬
ally curtails the worms' fighting behavior.
RIALS AND METHODS
MA
Arctonoe vittata were obtained from only the giant chiton
ptochiton stelleri. The chitons were collected subtidally
Aggression in Arctonoe vittata
James Weisberger
off of the Monterey peninsula, California. Each worm was
measured to the nearest millimeter.
A series of tests were performed to determine whether
Arctonoe has different levels of aggression. The pallial
groove of the host proved to be the best environment to con¬
sistently elicit the fighting response. In each test the
worms' behavior was observed and the number of bites exchanged
in a head-on encounter between two worms was counted. The
worms were provoked by gently nudging their posterior ends
with blunt forceps. As Arctonoe is sluggish, provocation was
necessary so that the worms would quickly move near each other,
Test periods for each encounter were five minutes. In the
first series of investigations groups of four or five Arctonee
were tested together. No individual was used in two different
groups. In the remaining studies selected worms were tested
against seven others.
The same C
ptochiton was used throughout the tests. It
had a total length of 25.5 cm., with its pallial groove meas-
uring 17 cm. long and 0.7 cm. wide. The (
ptochiton was lain
on its dorsal side so that its pallial grooves could be more
easily seen. Gently flowing seawater bathed the host during
tests, submerging the worms.
Aggression is quantified in this paper by the number of
bites, accomplished by rapid eversion of its armed proboscis,
that an Arctonoe delivers to conspecifics in a given time period.
James Weisberger
Aggression in Arctonoe vittata

LSUET
Worms were observed in a fingerbowl, between glass plates
separated by 8 mm., and in a 7 mm. diameter plastic tube for
twenty minute periods in both light and darkness. Only one
bite was observed. These same worms would readily attack each
other on a host. Thus the host possesses some unidentified
factor needed to release the aggressive response,
Bites tore off elytra and produced open wounds and torn
appendages. Bites almost always were directed at the prostomial
region. There was considerable interworm variability as to the
duration of bites, some lasting up to 5 seconds. The most
bites recorded in a five minute period by one worm was 4 bites,
never in rapid succession.
When two worms first contacted each other they frequently
jerked away. When nearing again, the palps were extended;
fighting was most likely to occur if the worms' prostomiums were
in contact. Worms "reared up" slightly before they attacked.
Conversely, worms that assumed a "head down" posture in which
they lowered their prostomial region relative to their opponents
almost never were bitten; the other worms just slid by. Only
once in 15 trials was a worm attacked when in this position,
xperiment 1: To see if the worms exhibited differing
levels of aggression, five worms were tested together. Each
worm in the group was tested against the others 4 times in the
Aggression in Arctonoe vittata
James Weisberger
space of 8 days. The number of times a worm attacked and the
number of times the worm was attacked by others was recorded.
Prior to testing, the worms had no injuries. After the tests,
the worms were checked for injuries. The results are in Table 1.
There is an inverse relationship between the number of times
a worm attacked conspecifics and the number of times a worm
was bit by them: the more a worm attacked, the less he was
attacked, and vice versa (Figure 1). The reason worm E (the
circled dot in Figure 1) does not follow the linear relation¬
ship may be because it was very small and would not attack the
larger worms. In a different series another 30 mm. long worm
exhibited the same pattern of response.
Three other groups of four worms apiece were tested for
evidence of different levels of aggression. The worms in each
group were tested against each other twice in two days. The
results are shown graphically in Figure 2. Linear relation¬
ships are evident.
Experiment 2: To test the effectiveness of the observed
"hierarchies", all five worms from Group 1 were put in the
pallial groove of one Cryptochiton. Four other Cryptochitons
without worms were placed in the tank. They were left over¬
night.
The most aggressive, worm C, remained in the pallial groove.
Worm E, the smallest one, was found in the groove of the mantle
of the host, apparently not large enough to evoke full aggression.
James Weisberger
Aggression in Arctonoe vittata
All others were found in new hosts. When this was repeated,
the most aggressive, worm C, was the only worm on the host.
All 4 others had found new hosts.
Arctonoe sometimes switches to new hosts spontaneously.
Throughout the six weeks of investigation, 7 out of 27 worms
changed hosts.
Experiment 3: Most of the injuries sustained during
fights were the removal of prostomial sensory appendages. To
see which sensory appendages contribute to Arctonoe's level
of aggression the following tests were done. In the first
test 4 new worms were tested against each other twice. The two
most aggressive ones were anesthetized in an isotonic solution
of MgClo and seawater and both palps were excised. After a
day's recovery, all four worms were tested against each other
twice. There was no change in the worms' behavior when the palps
were removed, (Figure 3). No control was done in the following
tests because the shock of anesthesia and the operation did
not seem to affect the two worms whose palps were removed (i, e.
reduce their aggression).
Next, four new worms were tested against each other once,
Under anesthesia, both palps, both sets of tentacular cirri,
and the three prostomial antennae were removed from the most
aggressive. After a day recovery the worms were tested once.
The most aggressive worm did not bite, (Figure 4).
Under anesthesia, the antennae were removed from the most
aggressive of four new worms. After a day recovery the worms
Aggression in Arctonoe vittata
James Weisberger
were tested twice. The antennae removal affected the worm's
aggression level, changing it from an average of 4.5 attacks
per test to an average of 0.5 attacks, (Figure 5). The an-
tennae seem instrumental in maintaining aggressive behavior.
These results were confirmed by testing the most aggressive
of 8 new worms against the other seven. After antennae removal
and recovery, it again was tested. The results in Table 2
indeed show that antennae removal results in significantly
reduced aggression. Using a 2X2 contingency table, a "P" value
of less than .005 was achieved.
ISCUSSION
Arctonoe vittata is an obligate commensal. Apparently,
being isolated from a host produces abnormal behavior, a
manifestation of this being the conspicuous lack of intra¬
specific aggression. This is consistent with Dimock (1969).
who found that Arctonoe would bite a sea star while on its host
but at no time did an isolated worm bite a sea star.
Intraspecific aggression seems to be a way to effect
uniform distribution of large Arctonoe on a host population.
This is shown in the experiment in which five worms were placed
on the same host. In once case only the most aggressive worm
was left. In another run, the most aggressive and the smallest
were left. It could be hypothesized that Arctonoe, being an
obligate commensal, defends the pallial groove of its host
Aggression in Arctonoe vittata
James Weisberger
from others, as the host provides both protection and food,
Perhaps the amount of food available to Arctonoe on the host
is not very abundant. If this is the case, then a high aggress¬
ive level would be of selective advantage to the worm in com¬
petition for this scarce resource. Dimock (1974) states that
intraspecific aggression within Arctonoe results in the density
which occurs in the field, namely, one large worm per host,
The word "large" is important here. As shown in Table 1
and Figure 1, the smallest worm does not follow the linear
relationship. This is because it would not attack the larger
worms of 60+ mm. length. Out of a total population of worms
in the study, worm E was the smallest in both length and width,
Dimock (1974) also states that very small worms of Arctono
pulchra were never observed to bite the largest worms. Un-
fortunately, he gives no measurements.
There is no consistent slope to the attack vs. attacked
lines of Figure 2. However, the number of worms available was
limited (27 total) and more samples would presumably have in¬
dicated if there is one standard slope of the lines. These
lines point to a "hierarchy" between worms in a group, much
like a pecking order, with one worm on top and others below him.
The fact that the most aggressive worm stayed on the host while
the others left in experiment 2 points to the aggressor's
dominance.
There is nothing ritualistic about Arctonoe's aggressive
behavior; injuries are serious and sometimes lead to death.
Aggression in Arctonoe vittata
James Weisberger
Table 1 shows the injuries sustained by the first group after
4 trials. In all cases one of the palps are missing, yet
clearly there are aggressive worms here. The most aggressive.
averaging 5 attacks per test, was missing both palps. This
data substantiates the results of the experiment in which the
palps were excised and no change in aggression was observed.
It is curious that the palps play no critical role in aggression
as they are conspicuously employed during worm interactions.
The least aggressive worm, averaging only 1 attack per test.
was missing an antenna. This supports the results of the exper¬
iment in which the antennae were removed and a significant change
in behavior was noted.
Perhaps the reason that worms usually attack the prostomial
region is to bite off the opponent's antennae, thereby reducing
chances of retaliation. Evans (1971) states that the antennae
are used in distance chemoreception to detect the host and that
the palps are used in contact chemoreception to recognize the
host. It could be hypothesized that an Arctonoe without antennae
cannot detect his opponent easily and therefore is not quickly
aroused to aggressive behavior. Exactly how the antennae
mediate aggression is not yet known.
Aggression in Arctonoe vittata
James Weisberger
SUMMARY
1. Adult Arctonoe vittata were tested in head-on aggressive
encounters in the pallial groove of a Cryptochiton host.
2. Arctonoe was found to have intraspecific differences
with respect to levels of aggression.
3. The most aggressive worm in a group successfully
defends a host against others, evicting conspecifics,
4. Removal of the palps had no effect of the worms
aggression, but antennae removal significantly reduces aggressive
behavior.
Aggression in Arcto
ACKNOWLEDGEMENT
I wish to thank Dr. Robin Burnett for his su
help in this research.
Aggression in Arctonoe vittata
James Weisberger
LITERATURE CITED
1. Clark, R. B. 1959. The tubicolous habit and the fighting
reactions of the polychaete Nereis pelagica. Animal
Behav. 7: 85-90.
2. Dimock, R. V. and J. C. Dimock 1969. A possible "defense"
response in a commensal polychaete. Velige
12: 65-68.
3. Dimock, R. V. 1974. Intraspecific aggression and the
distribution of a symbiotic polychaete on its hosts.
pp. 29-44 In Vernberg, W. B. (ed.).
ymbiosis in the
Sea. Univ. S. Carolina Press, Columbia, S. C.
4. Evans, S. M. 1971. Behavior in polychaetes. Quart.
Rev.
Biol. 16: 379-405.
5. MacGinitie, G. E., and N. MacGinitie 1968. Natural
listory of Marine Animals. McGraw-Hill, New York.
11
Aggression in Arctonoe
James Weisberger
ittata
TABLE 1: A ratio of the number of attacks to the numbe
a worm was attacked by others. Worm E is the smallest.
effect of its size upon aggression is discussed in the
Aggression in Arctonoe vittata
TABLE 1
F NINRI
E NOFIDI
ER
AVEl
AVERAGE NUMBER
OF TIMES
OF TIMES
WORM
WORM ATTACKE
WORM WAS AT
4.5
1.0
1.5
5.0
1.75
2.75
3.0
1.0
2.0
James Weisberger
JURIE
missing:
front elytra
1 palp, 1 set
of cirri, 1 an¬
tenna
missing:
front elytra
2 palps
1 set of cirri
missing:
front elytra
2 palps
missing:
front elytra
2 palps
1 set of cirri
missing:
front elytra
1 palp
13
LENC
ITH WIDT
65 mm 5.5 mm
62 mm
6.5 mm
50 mm
5.0 mm
47 mm
5 mn
Aggression in Arctonoe vittata
James Weisberger
TABLE 2: Antennae from the most aggressive worm were exci
It was tested against 7 other worms before and after r
Values show a significant decrease in aggression and an
accompanying rise in aggression toward it by other worms
Using a 2X2 contingency table, a "P" value of less than
was obtained.
Aggression in Arctonoe vittata
TABLE 2
BEF
James Weisber
Aggression in Arctonoe vittata
James Weisberger
FIGURE 1: Average times a worm attacked vs. th
attacked by others. The circled value is the smalli
6-
—.—


FIGURE 1

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——
AVERAGE NUMBER OF TIMES WORM WAS ATTACKED
Aggression in Arctonoe vittata
James Weisberger
FIGURE 2: Relationships between levels of aggression in
different groups. Group 1 (without worm E) -2
Group 2— O, Group 3 -X, and Group 1 -O.
No uniform slope is seen.
18
o

FIGURE 2
—
AVERAGE NUMBER OF TIMES WORM WAS AITACKED
Aggression in Arctonoe vittata
James Weisberger
FIGURE 3: Before and after relationships in the palps excision
experiment. ° — before palp removal; O - after removal.
The circled values represent before and after values for the
worms that underwent palp removal.
2
—


FIGURE 3
AVERAGE NUMBER OF TIMES WORM WAS ATTACKED
Aggression in Arctonoe vittata
James Weisberger
URE 4: Before and after values when the palps, cirri, and
antennae have been removed. —
before removal;
O — after removal. The values for the worm that had
the sensory appendages removed are circled.
22
FIGURE 4
0

AVERAGE NUMBER OF TIMES WORM WAS ATTACKED
Jmes Weisberger
Aggression in Arctonoe vittata
1 5: Before and after values when the antennae w
— before removal; O-
after removal. The
for the worm whose antennae were removed are circled.
9
FIGURE 5
AVERAGE NUMBER OF TIMES WORM WAS ATTACKED