tsdrtatze
The Lower Limits of Littorina Scutulata and Littorina Planaxis
Russell E. Peterson
Hopkins Marine Station
Pacific Grove
(2 text figures)
While most intertidal organisms display a distinct upper habita-
tional limit, Littorina scutulata Gould (1849) and Littorina planaxis
Philippi (1847) have a distinct and unique lower limit. This limit is
set by a combination of physical and biological factors.
Chemical variants do not affect the lower limits as both Littorina
are able to cope with exposure to sea water. Biological competition and
food supply are not problems as there is no recurring pattern of animals
or algae below the Littorina; and even when no other snails are present,
Littorina will not move into lower areas abundant with the algae they
eat.
Submersion in water is also not a lower limiting factor for L.
scutulata, many of which live in tide pools, or L. planaxis. One hundred
and eighty L. planaxis were tested for righting responses and climbing
a vertical glass wall and were then put in a filled, covered tank with
a balanced flow of sea water. After twenty-five days of submersion,
these snails were again tested for righting and climbing. A comparison
ofthe time curves showed no impairment of ability after the submersion.
There was a definite non-responsiveness in about 20% of the L. planaxis
after the exposure, but this is little more than the non-responsiveness
of a random selection of L. planaxis from the field.
An increase in pressure and submersion, again, were eliminated as
lower limit determinants by placing ten L. planaxis, wrapped in plastic
screening, at a depth of thirty-three feet below mean low water. After
two week's submersion, all were retrieved in a healthy state.
The lower limits of Littorina are not uniform in different areas.
A correlation of heights and the type of area shows that the height at
which the Littorina population begins varies with the turbulence of the
tidal flow in that area. In areas of very heavy surf, L. planaxis do
not venture below nine and one-half to ten and one-half feet above
mean low water. In very quiet areas, L. planaxis live as low as one and
one-half feet above mean low water. Heights for other areas vary be-
tween these two extremes, but always in relation to the water action in
the area. Figure ! was made from a typical area of rock with the outer
face exposed directly to waves, a middle inlet with mediwm turbulence,
and a back inlet with very little flow accompanying the rise and fall
of the tide. As the areas become quieter, the level of the densest
population of both L. planaxis and L. scutulata becomes lower, and even
moving over seven feet on the front face -- where lower rocks break up
the waves -- enables L. planaxis to live three feet lower.
Several transplantations of L. planaxis and L. scutulata to low
rocks one-half to one and one-half feet above mean low water -- sur-
rounded by sand to prevent the snails' escape - resulted in 10% to 50%
disappearance in twenty-four hours, 75% to 96% disappearance in fourty-
eight hours, and 97% to complete disappearance in four days. Within
two weeks, every one of three hundred Littorina put on the rocks was
gone. These rocks are naturally inhabited by Tegula funebralis;
Tegula brunnea and Calliostoma canaliculatum put on the same rocks
were still there after three weeks. These tests were run on a quiet
beach; on a rough beach, 100% loss occurred over night.
Physical conditions, and not predation, were mainly responsible for
the vanishing of the Littorina off the rocks. Most of the Littorina
found after the first day were on different rocks, and of twenty
L. planaxis tethered to one of the rocks, only one was eaten - probably
by a crab as indicated by a broken shell -- in three days. Predation
would take longer than several days to deplete a colony of Littorina at
this level.
On a relative basis, L. scutulata and L. planaxis are less able to
withstand water flow than Tegula brunnea and Calliostoma canaliculatum.
L. planaxis, but not L. scutulata, however, can better withstand water
flow than can Tegula funebralis. This was determined by subjecting
fifty of each snail to a stream of water with a flow of 12.2 liters per
minute and a tube diameter of .85 centimeters (figure 2). Tegula
funebralis manage to live at a lower level than L. planaxis because its
response to water flow is to go to the base of the rock and reduce flow
to a minimum. L. planaxis, with its negative geotaxis, goes to the top
of the rock where, when the rock is inundated, the shearing force is
greatest.
Among L. planaxis, the medium large size (12-15 millimeters across
the base of the shell) was observed to best withstand water flow. This
size L. planaxis makes up to 80% of the population in heavy surf areas
while contributing far less of the total population. This shows, again,
that wave action determines where Littorina can live.
These results seemingly present a paradox because, though L. scutu-
lata are less able than L. planaxis to cope with water flow, they live at
a lower level in most cases. As figure 1 shows, L. scutulata do not live
as low as L. planaxis in heavy surf areas; and in everyplace but the
quiet areas, the L. scutulata are wedged into crevices or among barn-
acles while the L. planaxis are on the open faces. L. scutulata must
live lower because of the macroscopic algae it eats and its inferior
ability to withstand dessication,2 but it must also protect itself more
carefully against being washed away.
When a Littorina descends below its lower limit, predation becomes
a factor. Though Acanthina spirata, to some extent, live among the
Littorina, they do not seem to make large inroads into the population.
Other predators are more voracious, but being well below Littorina, have
little contact with them. Cancer antennarius and Cancer productus eat
Littorina at a tremendous rate when in contact with them, and even young
Cancers rip apart the Littorina's shell like a monkey peeling a banana.
Patiria miniata also prey on Littorina when they are within reach, and
probably accounted for the deaths of 50% of ten L. planaxis tethered
two feet above mean low water (two feet below their normal height in
this area) on algae covered rocks in a semi-quiet area. The remains of
these snails were unmarked, empty shells identical to those of L.
planaxis eaten by Patiria in the lab. It is very unlikely that Pisaster
ochraceus was responsible for the predation attributed to Patiria as
no Pisaster has been observed to eat a L. planaxis and only.3% have
been observed to eat L. scutulata. It took two and one-half weeks for
all the tethered snails to be killed, and a migration of Littorina to a
slightly lower level would, thus, not be met by prohibitive predation.
When a Littorina is washed away, unless it manages to cling to a
rock on which it can leave the water, it will eventually contact a
Cancer or Patiria and be destroyed. Patiria are so abundant at lower
depths that a Littorina washed out of the intertidal zone would have
practically no chance to escape. To even further reduce the odds,
Cancers patrol up to the water level and would snatch up an occassional
loose snail.
Thus, the physical force of water and the biological force of pre-
dation combine to set a lower limit on the habitation of Littorina.
This limit facilitates their survival and functions through a mechanism
of negative geotaxis. It is possible that long ago, the ancestors of
these Littorina developed in two directions. One branch had a negative
geotaxis and climbed out of the water. The other branch may have had a
positive geotaxis or none. Those Littorina living below the water were
eventually eaten by crabs, starfish, and fish while those above the sea
have survived.
O
C
SRE
TO CTN¬
STND
WTER
Fo

LITTORN TEGO
LITTORINA
FUNEBRRUS
SCUTULRTA PLRNEXIS

—
HEIGT N FEET
Reet mest lou baree














+



t
tttt
- +


5
——
T

H

IINOMIGERIOE SAAIAS PE SOUREE EOOT HE
HHEIEU R
COTO
TEGU
BEUNNER CANRRICUTUM


PA
POTECT
LCOTOL
LPRAR
PROTEGTED
L.SCUTDERTA
L.PLANXS
FIGD
336
Kenneth M. Tittle
Hopkins Marine Station
Pacific Grove, California
June 6, 1964
CHEMICALLY STIMULATED ESCAPE RESPONSES OF
LITTORINA PLANAXIS AND LITTORINA SCUTULATA TO
THE CARNIVOROUS GASTROPOD ACANTHINA SPIRATA
The escape responses of certain gastropods when stimulated
by predatory sea stars has long been known. (BAUER, 1913;
WEBER, 1924; HOFFMAN, 1930; BULLOCK, 1953; FEDER, 1963; MARGOLIN,
1964.) CLARK (1958) has described flight responses by herbivorous
gastropods on contact with a variety of carnivorous gastropods.
HOFFMAN (1930), BULLOCK (1953), and FEDER (1963) present evidence
that the primary stimulus of the sea stars is chemical. Although
CLARK (1958), working with crude homogenates, suggested that
the response to carnivorous gastropods is also chemically
stimulated, little further work on the nature of the stimulus is
available.
The animals used in this study were the predacious Muricid
whelks, Acanthina spirata (Blainville) and Thais emarginata
(Deshayes), common to the Monterey coast, and the common peri¬
winkles of the rocks and pools in the high intertidal and splash
zones, Littorina planaxis Phillipi, and Littorina scutulata Gould.
Observations and Discussion
Field observations. In a number of cases in the field when
an Acanthina spirata was introduced into a quiet splash pool in
which no other whelks were found, he would soon right himself and
begin to move about slowly and hesitantly. Within 2-5 minutes
Littorina planaxis or L. scutulata within a centimeter or two
would begin agitated movement, extending their tentacles and
crawling across the substrate at a noticeably more rapid rate than
those periwinkles at a greater distance. In large pools this
movement generally subsided when the Littorina was some distance
away from the whelk, but if this first, apparently random movement
brought the periwinkle into contact with the soft parts of the
Acanthina, his characteristic response was to raise up slightly
on the back half of the foot, swing abruptly around, and move
off very rapidly in a straight line in the opposite direction for
several centimeters. When the Littorina came into physical contact
with Acanthina, and also in smaller pools even when no actual
physical contact was involved, the tendency noted by FEDER (1963)
of "fleeing" periwinkles to move upward on sloping surfaces until
they have left the water was clearly noticeable. In response to
strong stimulus the cephalic tentacles of L. scutulata often whip
up and down apparently independently of one another.
In larger pools the whelk's "sphere of influence" gradually
extended until after 20-40 minutes he was sitting or slowly
moving in an area as much as 30 centimeters across which was
conspicuously clear of Littorina. An Acanthina weighted or
anchored in a pool so that he cannot right is apparently ignored
by the Littorina after the first 3-5 minutes. This may correlate
with the findings on the mucus trail of Acanthina which are
discussed below.
The effectiveness of this response in reducing predation is
not clear. BIGLER (1964) at the Hopkins Marine Station suggests
that Acanthina spirata is the most significant predator on Littorina
planaxis, at least within the periwinkle's natural range, and all
attacks observed by him or by this writer have occurred underwater.
Acanthina are occasionally found in pools feeding on L. scutulata.
However, laboratory evidence suggests that both species of Littorina
quickly lose the response on repeated exposure to the stimulus.
(See Figure 1.) L. scutulata and Acanthina are sometimes found
in very close proximity in a pool, particularly if the whelk is
wedged inactively into a small crevice or partially burrowed
into coarse sand.
Laboratory observations. The increased tendency of the peri-
winkles to leave the water in the presence of Acanthina provided
a simple laboratory assay for investigating the stimulus. For
the original stimulus, two Acanthina spirata were placed in
100 ml. of sea water in a clean finger bowl for at least one
hour prior to the first tests. One hundred milliliters of sea
water was used as a control. In some cases two herbivorous black
turban snails (Tegula funebralis ) were placed in sea water in
another bowl as a second control. For the initial determinations
one of the two Acanthina or Tegula was left in the bowl during
the tests.
All animals were freshly gathered before each series of tests.
For more uniform results, Littorina gathered from the rocks were
placed in sea water until they had opened and attached before trials
were started.
iche
In each test 10-20 L. scutulata (7/32-12/32 from the lip of
33
the shell to the widest point of the basal whorl) or L. planaxis
(9/32-15/32 inches) were dropped into a finger bowl and the number
of responses recorded at one-minute intervals. A snail was
considered positively responding as long as the anterior portion
of his shell was at all above the water. All tests were run at
approximately 20 degrees Centigrade.
On the basis of this data a test interval was chosen for the
comparison of control and test responses. The optimum interval
varied from run to run and especially from day to day, but results
were generally comparable within the interval from 7 to 9 minutes.
Each group of animals was allowed to drain one minute between trials
and was tested at least once in the control water between exposures
to the Acanthina stimulus. A typical run consisted of three
exposures to the Acanthina stimulus and at least four control
trials. A new group of periwinkles was used for each run. Test
responses were compared statistically with the natural tendency
of the Littorina to leave the water under control conditions.
Both L. scutulata and L. planaxis showed a clear response
to Acanthina and no significant response to the herbivore Tegula
funebralis when compared with the sea water control. See Table 1.
Under similar conditions both Littorina species responded sig-
nificantly to the carnivorous whelk Thais emarginata, whose range
also occasionally overlaps their ranges. (Table 2.) In most
responses no physical contact with the whelks was necessary.
Ten to twelve milliliters of sea water in which several
Acanthina had been placed for at least four hours was then
3.
substituted as a stimulus in 100 ml. of sea water and yielded
a positive response, strongly implicating a diffusible chemical
as the stimulus. (Table 3.)
The above computations are based on only the first two
exposures to the stimulus for each animal used. As Figure 1
illustrates, the response of both species falls off significantly
on the third exposure to Acanthina stimulus. Fourth trials were
seldom run, and rarely did they yield significant increases over
the control responses.
As a first step toward localizing the stimulus, two Acanthina
were cut into three gross portions, with the kidneys and digestive
glands placed in one bowl, the feet and operculums placed in a
second bowl, and the head, mantle and medial sections in a
third bowl. After one hour, assays were run on each bowl as
above. Only the foot section yielded a significant response.
However, this response was much less distinct than the response
to an active Acanthina, and it was significant only for the first
one or two times test animals were exposed to the stimulus.
Thereafter even fresh periwinkles failed to respond. This
suggested that attachment to the substrate or actual movement was
a factor in the secretion of the active substance.
To check the mucus trails for activity, Acanthina were removed
from a bowl where they had been active for at least 90 minutes.
The bowl was emptied and gently rinsed in fresh circulating sea
water, and then refilled with 100 ml. of fresh sea water. Activity
was still demonstrable in the bowl. Five Acanthina were left in
35
a bowl for two hours, and activity persisted in the bowl even
after it was leeched by submersion in running fresh sea water for
four hours (14 degrees C.). Finally two Acanthina were allowed
to crawl about for two hours in a Petri plate submerged in a
finger bowl without being allowed to contact the bowl itself. The
whelks and the Petri dish were then removed and the Petri dish
rinsed and partly filled with fresh sea water. The test snails
did not respond to the bowl or the water in it, but the same snails
responded clearly to the Petri dish (when compared to their
response in a clean, partly filled Petri dish), indicating that
the diffusible active substance is closely associated with the
pedal mucus secretions.
Active homogenates of the foot of Acanthina were then prepared
by homogenizing a foot in 10 ml. of artificial sea water in an
ice bath, centrifuging for five minutes, and decanting. In
testing homogenates, approximately 0.16 ml. of the homogenate was
transferred with a dropping pipette to a dry, clean bowl and
100 ml. of sea water added. For more sensitive assays in working
with small concentrations of the active substance, it proved
helpful to work with 10-12 Littorina at a time in the dark to
eliminate phototactic orientation and minimize clumping of the
snails. The test interval was ten minutes for all dark run tests.
Table 4 is a tabulation of such assays for a series of homogenates.
The activity of the homogenate was destroyed by two minutes
in a boiling water bath (sufficient to coagulate the protein), and
no significant activity was found in the supernatant after the
protein was removed by acid precipitation and the supernatant
33
readjusted to pH 8.0-8.5.
The active substance does not appear to be macromolecular,
however, and a substance capable of eliciting the response was
demonstrated to be dialyzable at various points in the process:
as secreted by a live Acanthina spirata in 10 ml. of sea water
in a dialysis membrane overnight, as present in sea water in
which several Acanthina had been placed earlier, and finally, as
present in the homogenate of the foot of A. spirata. (Figure 2.)
After dialysis of the homogenate (10 ml. against 100 ml. of
sea water) the active substance can be extracted with ether from
the dialysate at pH 2.6 and redissolved in sea water. The
activity of the crude homogenate seems to decline more rapidly
than that of the dialysis purified extract, even under refrigeration.
Summary
The predacious Muricid whelk, Acanthina spirata, secretes
a substance which diffuses from its pedal mucus and to which
Littorina planaxis and Littorina scutulata respond by accelerated
motion and an increased tendency to leave the water. Homogenates
of the foot of A. spirata have been prepared capable of eliciting
a similar response, and the active substance in the homogenates
shown to be dialyzable and ether extractable from acidified solutions.
Both L. planaxis and L. scutulata show a similar response to
the Muricid whelk Thais emarginata.
336
33
qure
33,
Figure 1. Fatiguing in the response of Littorina planaxis and
Littorina scutulata to stimulus by Acanthina spirata. White bars
represent L. scutulata, black bars represent L. planaxis. C, rep-
resents the response of each group of animals on their first test
in the controlwåter, A, represents the response of each group of
animals on their first exposure to the bowl containing A. spirata,
; the response of the same animals on their second test in the
control water, etc. Both control and test bowls contained 100 ml.
of sea water. A snail was considered to be responding as long
as the anterior portion of his shell was above the air-water
interphase.
TABLE 1
Response to Acanthina spirata. All bowls contained 100 ml. of
sea water. 1-2 Acanthina spirata were placed in the test bowl
at least one hour prior to the beginning of tests. A snail was
considered to be responding as long as the anterior portion of
his shell was above the air-water interphase. No animal was
subjected more than twice to the Acanthina stimulus.
A. Response of Littorina scutulata to stimulus by Acanthina
spirata. The test interval was seven minutes from the intro-
duction of the Littorina into the water.
% responding
responding
total
100
72
Acanthina spirata
100
Tegula funebralis
100
sea water control
Chi-square = 50.64
PO.01
B. Response of Littorina planaxis to stimulus by Acanthina
spirata. The test interval was 5 minutes.
% responding
total
responding
100
65
Acanthina spirata
20
Tegula funebralis
100
sea water control
Chi-square = 39.54
PO.01
TABLE 2
Response to Thais emarginata. All bowls contained 100 ml. of
sea water. 1-2 Thais emarginata were placed in the test bowl at
least one hour prior to the beginning of tests. A snail was
considered to be responding as long as the anterior portion of
his shell was above the air-water interphase. No animal was
subjected more than twice to the Thais stimulus. The test
interval was 9 minutes from the introduction of the Littorina
into the water.
A. Response of Littorina scutulata to stimulus by Thais emarginata.
% responding
total
responding
64.4
Thais emarginata
38.1
42
sea water control
Chi-square = 4.611
0.02P80.05
B. Response of Littorina planaxis to stimulus by Thais emarginata.
% responding
total
responding
40
77.5
31
Thais emarginata
32.5
40
sea water control
13
Chi-square = 16.36
P0.01
TABLE 3
Response of Littorina planaxis to a diffusible chemical from
Acanthina spirata. All bowls contained 100 ml. of sea water.
To the test bowl was added 10-12 ml. of sea water in which several
Acanthina spirata had been placed at least four hours before
the tests. The test interval was 7 minutes from the introduction
of the Littorina into the water. A snail was considered to be
responding as long as the anterior portion of his shell was above
the air-water interphase. No animal was subjected more than
twice to the Acanthina stimulus.
total
% responding
responding
100
Acanthina stimulus
55.0
100
18.0
sea water control
Chi-square = 29.54
PO.O1
S
TABLE 4
Response of Littorina scutulata to an homogenate of the foot
of Acanthina spirata. All bowls contained 100 ml. of sea water.
To the test bowl was added approximately 0.16 ml. homogenate prepared
by homogenizing the foot of one Acanthina spirata in 10 ml. of
artificial sea water. The test interval was 10 minutes from the
introduction of the Littorina into the water, and all tests were
conducted in the dark. A snail was considered to be responding
as long as the anterior portion of his shell was above the air-
water interphase. No animal was subjected more than twice to
the test stimulus.
total
responding
% responding
232
103
44.4
homogenate
414
23.2
96
sea water control
Chi-square = 31.41
Po.01
4
20
qure
20
845
Figure 2. Dialysis of the response-eliciting secretion of Acanthina
spirata as measured by the response of Littorina scutulata. The
white bar represents the response in the control water; the black
bar represents the response to the water containing the stimulus
to be assayed. The figures above each column are the fraction of
snails responding in the test interval. The probabilities for a
random distribution in each case are computed by chi-square. All
bowls contained 100 ml. of sea water. A snail was considered to
be responding as long as the anterior portion of his shell was
above the air-water interphase.
A. Response of L. scutulata to 100 ml. of sea water against which
one A. spirata in 10 ml. sea water has dialyzed 10 hours. The test
interval was 8 minutes from the introduction of the Littorina
into the water.
B. Response of L. scutulata to 100 ml. of sea water against which
10 ml. of sea water taken from a jar full of A. spirata has
dialyzed 14 hours. The test interval was 9 minutes.
C. Response of L. scutulata to the dialysis residue of the
homogenate of the foot of A. spirata. The foot of 1 A. spirata
was homogenized in 10 ml. of sea water. 0.16 Milliters of the
homogenate in 10 ml. of sea water was dialyzed 12 hours against
100 ml. of sea water and the residue added to 100 ml. of sea water
in the test bowl. The test interval was 10 minutes and all tests
were conducted in the dark.
D. Response of L. scutulata to 100 ml. of sea water against which
O.16 ml. of homogenate of the foot of A. spirata had dialyzed for
12 hours. The test interval was 10 minutes, and all tests were
conducted in the dark.
. . .
34
LITERATURE CITED
BAUER, V., 1913. Notizen aus einem biologischen Laboratorium am
Mittelmeer. Int. Rev. Hydrobiol., 6: 31-37.
BIGLER, ERIC, 1964. Personal communication.
BULLOCK, T. H., 1953. Predator recognition and escape responses
of some intertidal gastropods in presence of starfish.
Behaviour, V 2: 130-140.
CLARK, W. C., 1958. Escape responses of herbivorous gastropods
when stimulated by carnivorous gastropods. Nature, 181: 137-138.
FEDER, H. M., 1963. Gastropod defensive responses and their
effectiveness in reducing predation by starfishes. Ecology,
44: 505-512.
HOFFMAN, H., 1930. Ueber den Fluchtreflex bei Nassa. Z. vergl.
Physiol., 2: 662-688.
MARGOLIN, A. S., 1964. A running response of Acmea to seastars.
Ecology, 45: 191-193.
WEBER, H., 1924. Ein Umdreh- und ein Fluchtreflex bei Nassa
mutabilis. Zool. Anz., B60: 261-269.