Rene Toolson
INTRODUCTION
Competition among benthic marine invertegrates revolves importantly
around finding a place to settle and, maintaining that space and acquiring
new space for growth during the life of the organism (Connell,1961;
Dayton, 1971; Jackson,1977; Paine, 1974). Buss and Jackson's examination
of associations and interactions of coral reef invertebrates (1979).
Connell's account of interspecific competition among barnacles, as well
as Dayton's study of community organization (1971) introduced exper¬
imental approaches to a previously descriptive field. Their research
has suggested that competition for space in the intertidal is complex,
involving definite adaptive strategies.
Colonial animals from such groups as sponges, coelenterates, bryozoa,
and tunicates are often found as the dominant animal cover on benthic
substrates. Jackson (1977) makes the case for the strategy of the
indeterminate growth and acquisition of space by asexual reproduction
of these colonial forms being one of the most effective methods of
space competition. The colony can survive damage and continue to
spread (Buss, 1979). Thus the abundant and diversified colonial ascidians
are an interesting group to examine for competitive interactions for
living space.
In the rocky intertidal regions of the mid-California coast, full
utilization of all available space by competing algae and invertebrates,
both solitary and colonial, shows the importance of space in the
community. That so many competitors here are colonial gives reason to
believe they are successful (Jackson 1977). The lower intertidal is
marked by the presence of colonial ascidians who occupy a significant
portion of the rocky substrate (Morris, Abbott and Haderlie, 1980).
p.2
Rene Toolson
The purpose of this study was to examine the competitive associations
between colonial ascidians at Cabrillo Point and evaluate competitive
strategies. Classification of species followed the categories of
growth forms in order to determine a relationship between structure and
competitive strategy. Finally, growth forms were evaluated according
to their competitive abilities.
METHODS
The study was conductedin the intertidal at Hopkins Marine Station,
Cabrillo Pt., Pacific Grove, California in April and May of 1981. The
study area was located northwest of Bird Rock and sites were chosen
for high tunicate density and variation in physical conditions of
exposure and tidal height. Thirteen undisturbed areas were photographed
weekly over a four week period of time to note any changes that occur
in the field over time. Other areas were observed for ongoing competitive
interactions and selected borders between ascidians were sectioned and
examined under a dissecting scope to observe the results of previous
competitive interactions.
Two follow-up experiments were performed to lead to an understanding
of growth strategies of individual species.
1. Entire colonies of different species were brought from the field
and tied down onto glass plates,alone and in species pairs. Responses
were observed, and each plate was photographed over time.
2. In the field, eight species were isolated by removing all veget¬
ation and invertebrates that were within a two-inch radius around the
colonies, and these were photographed over time.
In both experiments, colony sizes were traced from photos and compared
over time.
p.3
Rene Toolson
RESULTS
The results of the field study detailing competitive interactions and
the observations from sectioning interactive borders are summarized in
Table 1. The interactions are listed by pairs of species of ascidians,
They are classified by growth strategies (as introduced in the discussion),
and the relative proportion of times that a given response was observed
out of all contacts analyzed is listed. The results of the growth
responses observed from the laboratory experiment done on glass
plates are presented in Table 2 for comparison. The following is a
description of the common interactions, categorized by growth forms.
Encrusting Sheets Meeting Encrusting Sheets
Colonies of Aplidium californicum met frequently since the species
is abundant and occurs in a variety of habitats and tidal heights. It
was never observed to overgrow another colony of the same species in the
field,and the colonies would meet and maintain their borders, frequently
accumulating sand in the space between contiguous colonies. However,
when two colonies were paired on glass plates, overgrowth without fusing,
was observed.
In all observed contacts, A. Californicum grew over Archidistoma
psammion. When overgrown, A. psammion often had a thick test, encrusted
with sand and empty of zooids in the area of overgrowth, but it might
also have been unaffected. A. californicum colonies moved 8 mm and
overgrew colonies of A. psammion in seven days on glass plates.
Due to its large range, A. psammion met often in the field with
colonies of the same species, one-third of the time growing over it.
Whether they met or overlapped, they had thick, zooidless tests in the
area of contact that were often encrusted with sand. On glass plates
when two A. psammion colonies were paired, the colonies spread out but
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grew too slowly to note any response to neighboring colonies.
Encrusting Sheets Meet Encrusting Mounds
Interactive borders of A. psammion and Archidistoma molle were often
found. Both leaned and overgrew each other with the same frequency
while A. psammion zooids were occasionally found at the base of the tunic
of A. molle. The test of A. psammion, when underneath an A. molle
colony was thick and empty of zooids. When colonies were paired on
glass plates, A. molle projected its tunic over A. psammion and attached
to it.
When colonies of Aplidium solidum and A. psammion grew together
in the field, the outcome was quite variable with displacement, leaning
and mutual overgrowth all occurring. Both colonies responded to the
contact with thicker, sand-encrusted tests, and sand was found in between
the two colonies.
In the field, when colonies of A. californicum overgrew both
mound species, A. solidum and Archidistoma diaphanes, the mounds
showed empty and deteriorating tunics. A.solidum was also observed
to have a thick test, sometimes encrusted with sand. Colonies of
A. californicum and A. molle were never observed to meet in the field
but when paired on glass plates, colonies of A. californicum were seen
to spread out zooid systems and overgrow the adjacent colony of A. molle.
The same results were seen when A. californicum was paired with
A. solidum on glass plates.
Lobe-shaped Species Meet Encrusting Sheets
A. californicum was observedto overgrow the sandy-lobed species,
Synoicum parfustis, Aplidium propinquum, and Aplidium arenatum.
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Rene Toolson
A. psammion would displace, but rarely overgrow, Synoicum and A. propin-
quum lobes. Both of these sandy lobes, and colonies of Polyclimum planum,
were commonly observed to use A. psammion as substrate while A. arenatum
did not. Most often, A. arenatum was surrounded by A. psammion, or
the sandy lobe was observed leaning over the encrusting sheet - in
which case the test of Archidistoma psammion was thick and sand
encrusted.
Lobe-shaped Species with Encrusting Mounds
In general, the sandy lobes most commonly were found being leaned
upon and overgrown by the mound species, A. solidum,A. molle, or
A. diaphanes. The lobes were usually displaced,and the mound species
frequently had a thick layer of test at these borders. The mounds were
never used as substrate by the lobes.
Social Colonials, In General
A. californicum was the only species that was observed to overgrow
any of the social colonials, and this was Clavelina huntsmani. Since
its growth only extended around the base of the Clavelina colony,
no damage was observed. Frequently, both Clavelina and Euherdmania
claviformes attached stolons to colonies of A. psammion, using it
as substrate, but these social colonial species were more often displaced
when located at the borders of this encrusting sheet.
When in contact with species of encrusting mounds, both Clavelina
and Euherdmania either showed no response or were displaced. When
social colonials were in contact with lobes, they were generally tol¬
erant, and showed little competitive interaction. In the field, when
Euherdmania and Clavelina come in contact, they showed no visible
p.6
Rene Toolson
response to each other, but their stolons were all intermingled at the
base of the colonies.
Perophora annectens, (with the exception of A. molle) extended its
stolons out over the tunic of the neighbor colonies it encountered, and
the neighbor colony showed no response to the interaction.
All of the social colonial species exhibited budding in contact
situations and did no damage to the colony they bordered.
Of the colonies isolated in the field or placed singly on glass
plates, all increased their colony sizes within ten days, except
Synoicum parfustis, Archidistoma diaphanes, and Clavelina huntsmani.
Results are summarized in Table 3 with sample growth plates in the
Appendix.
A. californicum showed the fastest increase in size. A. californicum,
A. molle, and A. solidum showed the fasted repair to damage done by the
string used to tie down the colonies with. Repair consisted of
separating the tunic and zooids at the point of contact with the
string and moving the test up and around the string until it was
completely covered. The colony appeared normal after covering the
string.
Rearrangement of zooid systems was observed in both A. californicum
and A. solidum. In A.californicum, systems would spread out, and more
test was observed in between systems. The systems of A. solidum broke
apart into lobes.
Clavelina, when isolated on glass plates, was observed to have
grown buds that were two to seven millameters long within two weeks.
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Rene Toolson
All colonies flattened out onto the plates within one week, and
fusion to the glass plates occurred within two weeks. When groups
of colonies of A. molle or A. psammion were isolated in the field,
the colonies would move closer together when different competing
species were removed from between them.
DISCUSSION
In observing the variety of interactions involved in space competit¬
ion between the intertidal colonial ascidians, it became apparent that
there was a relationship between the growth formsof the ascidians and
the strategies empled. This led to classifying the colonies into
four categories: 1)encrusting sheets, 2) encrusting mounds, 3) lobe-shaped
colonies, and 4)social colonials. Encrusting sheets are compound
colonials that are often large, cortinuous colonies that sit low on
the surface of their substrate. Encrusting mounds are compound
species that are attached to the substrate and slightly "mushroom'
out beyond their attachment, usually growing taller than the sheets.
Lobes are attached to the substrate at one small point at their bases,
and the remainder of the colony is projected outwards. Social colonials
are groups of individuals that are attached at the base by stolons which
often include parts of the colony's circulatory system, in contrast to
compound ascidians that are composed of a group of individuals bound
together in a common test.
Each of these groups has a distinctive set of competitive strategies
though individual members of the set may be shared with other growth
forms. In addition, each strategy may be developed to different
degrees by species of the same or different groups. Table 4 summarizes
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Rene Toolson
these observations for the intertidal ascidians examined for this study.
The strategies are discussed below in relation to their effectiveness,
and the summary of their success is presented in Table 5 in the form
of a dominance matrix.
Sheets
Among the growth forms of ascidians, encrusting sheets show relatively
good competitive abilities (Buss, 1979), as evidenced by the large
anomt of space they occupy in the intertidal (Pulliam, 1981).
Encrusting sheets use the most direct techniques for obtaining space:
by creating new substrate out of living ascidians through overgrowth.
In the case of A. californicum, growth rate, on top of structure, is
an important strategy influencing competitive success. Both on glass
plates and in the field, this seasonal ascidian showed the fastest
increase in colony size of all the species examined. Though it has a
low stature on the substrate interms of height which would make it
easy for a mound species to lean over it, A. californicum was determined
to be the most dominant species because it grows faster than any other
species it was observed to come in contact with (Buss, 1979; Buss and
Jackson, 1979). No other species was ever observed to overgrow or
lean onto A. californicum.
On the other hand, A. psammion, the other very common encrusting
sheet, showed very slow growth rates in the field and on glass plates.
Its thick test is unsuitable for fast growing, so it is often suscept¬
ible to the leaning of the mound species who grow both taller and
faster. The growth-limiting tunic, though, possesses protective
features which also serve as stra egic advantages. The thick test
is hardened by sand encrustation and acts as a barrier to movement
by the softer species and as a protective shield for the zooids
p.9
Rene Toolson
beneath.
Munds
Encrusting mounds use the leaning strategy most often because
it conforms most usefully with their mushroom-like structure. They
are able to root down in an area, and then extend their tunic, with
zooids, beyond their point of attachment. The soft, jelly-like
composition of their tests (also like that of A. californicum) probably
facilitiates faster growth rates, so when they come into contact
with a dominant encrusting sheet like A. psammion, they are not as likely
to be overgrown because they grow taller and faster. The combined
use of these strategies also proves to be effective when in contact with
the less aggressive lobe and social colonial species, as is indicated by
the direction of the arrows on Table 5.
Lobes
Compound ascidians found in lobe forms are efficient with their use
of space due to their small point of attachment which is able to support
a large colony in very little space. Their only real aggressive
strategy is the ability to crowd out another species with a large
colony of lobes, though laboratory observations have not shown this
response to be as quick as the growth rate of encrusting sheets or
mounds. Lobes may also be successful when they use encrusting sheets
as substrate by settling on and embedding into them. When confronted
with an aggressive neighbor, the lobes will respond (except A. arenatum)
by leaning and growing away from the neighbor, a form of displacement.
The hard, sand-encrusted tests of the sandy lobe species, A. propinquum.
A. arenatum, and Synoicum, serves the same protective purposes as
it does for A. psammion.
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Rene Toolson
Social Colonials
The main strategy of the social colonial ascidian, who is structurally
much more fragile than any other growth form, is its ability to repro¬
duce asexually very quickly, as seen on glass plates in the laboratory.
This allows them to occupy alot of space in a short amount of time
evade overgrowth, and even crowd out another species on occasion. The
social colonials are also very succcessful, more so than the lobe species,
at using other growth forms (including lobes) for substrate. This
is a highly advantageous strategy for such easily damaged ascidians.
The overall diversity of the rocky intertidal can be attributed at
least in part to the variety of differen tially successful competitive
strategies used by the organisms within the community structure (Buss
and Jackson, 1979). The species' structure, composition, longevity,
and growth rate all contribute to its overall success against other
ascidians. By using these varying combinations of factors as well
as others not investigated, most species are equipped with the means
to survive even in the toughest neighborhoods. Diversity in the pattern
of distribution of intertidal tunicates may well be the result of
this variability of competitive strategies between species, between
their effectiveness in different habitats and the life history
strategies of growth rate, longevity and successful recruitment.
SUMMARY
1. Strategies for space competion between species of colonial
ascidians were studied in the rocky intertidal of Hopkins Marine
Station, Pacific Grove, California.
Rene Toolson
2. A general hierarchy of successful competitors based upon growth forms
can be presented (in order of greatest to least success):
Encrusting sheets, encrusting mounds, social colonials, and lobe¬
shaped species. Within these categories, however, there are exceptions,
so that growth forms alone cannot be considered as a universal
factor for determining competitive success.
3. Rate of asexual reproduction, or colony growth rate, based upon
a list of individual species, can also be used to produce a
hierarchy of successful competitors; however, this conclusion would
be even less accurate if applied to all ascidians, and thus
can only be considered as a secondary factor influencing competitive
success.
4.
Composition of the test, ranging from hardness to softness, because
it has an effect bo on otective and growth abilities, has a
correlation with competitive ability.
5. Protective as well as aggressive strategies are at work during
space competition between ascidians, leading to a complex community
structure.
6. The diversity of species present and the community and the complex
structure of their distribution is a result of the variety
of competitive tactics utilized by the individual ascidian.
ACKNOWLEDGEMENTS
I would like to thank eyeryone at HMS for the opportunity to do
this study, especially Dr. Abbott for his enthusiasm, Dr. Gilly for
his tolerance, and mostly Chuck Baxter for endless patience. I would
also like to express my appreciation for the indispensable help I
received from Keith Kohatsu and collaboration from Bill Pulliam.
p.12
Table
SHEETS + SHEETS
Aplidium californicum
Aplidium californicum
Aplidium californicum
Archidistoma psammion
Archidistoma psammion
Archidistoma psammion
SHEETS + MOUNDS
Aplidium californicum
Archidistoma molle
Aplidium californicum
Aplidium solidum
Aplidium californicum
Archidistoma diaphanes
Aplidium californicum
Archidistoma ritteri
Aplidium californicum
Distaplia occidentalis
Archidistoma psammion
Archidistoma molle
Archidistoma psammion
Aplidium solidum
Archidistoma psammion
Archidistoma diaphanes
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Rene Toolson
p. 13
arde
MOUNDS + MOUNDS
Archidistoma molle
Archidistoma molle
Archidistoma molle
Aplidium solidum
Archidistoma molle
Archidistoma diaphanes
Aplidium solidum
Aplidium solidum
Aplidium solidum
Archidistoma diaphanes
Archidistoma diaphanes
Archidistoma diaphanes
LOBES + SHEE
Polyclinum planum
Archidistoma psammion
Synoicum parfustis
Aplidium californicum
Synoicum parfustis
Archidistoma psammion
Aplidium propinquum
Aplidium californicum
Aplidium propinquum
Archidistoma psammion
19
10
1
3
2-3
—
a
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7
p.14
alle
Aplidium arenatum
Aplidium californicum
Aplidium arenatum
Archidistoma psammion
—
—
LOBES + MOUNDS
Synoicum parfustis
Archidistoma molle
Synoicum parfustis
Archidistoma diaphanes
Synoicum parfustis
Archidistoma ritteri
Aplidium propinguum
Archidistoma molle
Aplidium propinquum
Archidistoma diaphanes
Aplidium propinquum
Aplidium solidum
Aplidium arenatum
Archidistoma molle
Aplidium arenatum
Aplidium solidum
Aplidium arenatum
Archidistoma diaphanes
1 .


8
Rene Toolson
p.15
lable
SOCIAL COL + LOBES
Clavelina huntsmani
Synoicum parfustis
Clavelina huntsmani
Aplidium propinquum
Clavelina huntsmani
Aplidium arenatum
Euherdmania claviforme:
Synoicum parfustis
SOCIAL + SHEETS
Clavelina huntsmani
Aplidium californicum
Clavelina huntsmani
Archidistoma psammion
Euherdmanis claviformes
Archidistoma psammion
SOCIAL + MOUNDS
Clavelina huntsmani
Archidistoma molle
Clavelina huntsmani
Aplidium solidum
Clavelina huntsmani
Archidistoma diaphanes
Euherdmania claviforme:
Archidistoma molle
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p.16
Euherdmania claviformes
Aplidium solidum
-
Euherdmania claviformes
Archid
stoma diaphanes

IAL + SOCIAL
Clavelina huntsmani
Euherdmania claviforme
Perophora annectens
all, except A. molle
-
4
10

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Jable
Species Pairs
—
Aplidium californicum
Aplidium californicum

Aplidium californicum
Archidistoma psammion

Archidistoma psammion
Archidistoma psammion
Aplidium californicum
Archidistoma molle
Aplidium californicum
Aplidium solidum

Archidistoma psammion
Archidistoma molle
—
Archidistoma molle
Aplidium solidum
—
—
Synoicum parfustis
Aplidium californicum
Perophora annectens
Archidistoma psammion
8
—
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Talle
Field:
Glass
Plates
Isolated Species
Aplidium californicum
* Aplidium solidum
* Archidistoma psammion
* Archidistoma molle
Archidistoma diaphanes
Synoicum parfustis
Clavelina huntsmani
Aplidium californicum
Aplidium solidum
Archidistoma psammion
Archidistoma molle
Archidistoma diaphanes
Aplidium propinquum
Clavelina huntsmani
Increase ir
Colony Area
16%
15%
69
14%
12-19%
117
1%
-9%
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Observed
Rearrangement
Time
of Systems (days)
no
no
no
no
yes
(3 days)
yes
12
(7 days)
no
10
no
10
no
10
no
10
budding
10
labl

e



OO


a-
(5. 5.
(5. 5.

88
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80




88
5.
.

0


8
(lole 5

—
4.
4.

8
—



85
22.4.

—


+
—+—
O
+


O
4
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Aplidium californicum
Archidistoma psammion
—
Archidistoma molle
—
Aplidium solidum
Archidistoma diaphanes
—
Archidistoma ritteri
a-
Distaplia occidentalis
Synoicum parfustis
—1
Aplidium propinquum
Aplidium arenatun
Polyclinum planum

Perophora annectens
Clavelina huntsmani
—

Euherdmania claviformes
p.21
Rene Toolson
TABLE LEGENDS
Table 1:
Responses of species combinations of colonial ascidians based
upon field observations and sections of interactive borders. Numbers
indicate the proportion of a response occurring for that interaction.
Blanks indicate that the response was not observed. Thick test = given
species had a layer of thick test with a conspicuous lack of zooids at
the border of interaction; sand encrusted = bordering test was sand encrusted;
loose sand - sand in between the two colonies, not encrusted;
displacement = given specieswas displaced by neighbor species; projection -
given species, tunic and zooids, was leaning over neighbor species;
overgrowth = given species was growing over the other species, but
was also attaded to primary substrate; 2° substrate = given species was
using other species as substrate; other zooids = presence of zooids of
given species in the test of the other species; budding = buds were
attached to the species. Damage was rated on the following scale :
O = no damage; 1 = slight damage (lack of zooids, very thick test);
2 - significant damage (no zooids, deterioration of test); 3 = death.
No response = interaction had no visible effect on either colony.
*The tracings of these colonies appear in the appendix.
Table 2:
Responses of species combinations of colonial ascidians removed
from the field and tied to glass plates. + = given response was observed;
- = opposite of given response was observed; blank indicates that the
response was not observed. Expansion = size of the colony increased;
overgrowth = given species grew over neighbor species; projection =
given species leaned over other species; spreading = systems within the
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Rene Toolson
colony spread out, bec me less dense within the test. *The tracings
of these colonies appear in the appendix.
Table 3 :
A summary of the responses of colonial ascidians to isolation, both by
clearing borders in the field and tying colonies on glass plates. Colony
area was determined by tracing photographs and calculating the area
within the tracing. + = the change in colony size of these species was
positive, yet not determinable by tracing. *The tracings of these
isolated colonies appear in the appendix.
Table 4:
List of all species classified according to growth forms with compet¬
itive strategies summarized for each growth form.
Table 5:
A matrix of dominance of ascidian species observed in the field.
Dominance, indicated by the direction of the arrow, was determined
through observations and sectioning interactive borders.— - direction
of dominance;+ - both share dominance, but empty arrow less often
than blackened arrow;dominance is approximately equalrelation-
ship was only observedon glass plates;almost total dominance,
occasionally, the other species won;()-no observable effects of the
interaction; blank indicates that the interaction was not seen.
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LITERATURE CITED
Buss, L.W. 1979. Habitat selection, directional growth and spatial
refuges: why colonial animals have more hiding places. in B. Rosen
and G. Larwood, eds. Biology and Systematics of Colonial Animals.
Academic Press. London. 598pp.
Buss, L.W. and J.B.C. Jackson. 1979. Competitive networks: nontransitive
competitive relationships in cryptic coral reef environments. Am. Nat,
113:223-233
Connell, J.H. 1961. The influence of interspecific competition and
other factors on the distribution of the barnacle Cthamalus stellatus.
Ecology 42:710-723
Dayton, P.K. 1971. Competition, disturbance, and community organization:
the provision and subsequent utilization of space in a rocky intertidal
community. Ecol. Monogr. 41:351-389
Jackson, J.B.C. 1977. Competition on marine hard substrata: the adaptive
significance of solitary and colonial strategies. Am. Nat. 111:743-768
Jackson, J.B.C. 1979. Morphological strategies of sessile animals. in
B. Rosen and G. Larwood, eds. Biology and Systematics of Colonial Animals.
Academic Press. London. 598pp.
Morris, R.H., Donald P. Abbott and Eugene C. Haderlie. Intertidal Invert-
ebrates of California. Stanford University Press. Stanford, California.
1980. 69opp.
Paine, R.T. 1974. Intertidal community structure: experimental studies
on the relationship between a dominant competitor and its principal
predator. Oecologia. 15:93-120
APPENDIX:
A. californicum + A. propinquum
(5 days) - - - -

Ac
Ap

A. californicum + A. solidum
(5 days) - - - - -
Ac

(All colonies 1X)
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A. californicum + A. molle
— —
- (5 days) - - -

Ac
Am

A. psammion + A. molle
(10 days)- - -



p.2.
)

N

S









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A. solidum (2X)
(6 days)
A. psammion (2X)
(6 days)
alifornicum (1X)
(5 days)


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A. molle (2X)


(6 days)

A. propinquum (1X)
(10 days)



p.27