O
ATTRITION ON THE LITTORINA PLANAXIS POPULATION
Fric Bigler
for Dr. D. Abbott
Bio 175h
May 1964
INTRODUCTION
It is assumed that all Littorina planaxis eventually die;
and that there is a steady turnover of individuals in the pop-
ulation. The most obvious possible causes of accidental death
a
e being torn from the rocks and tumbled into thesea, and
being eaten by possible predators as gulls, plovers, crabs,
starfish and squirrels, though these animals were never seen
ransacking the L. planaxis colonies. Since the actual causes of
death in the L. planaxis are not obvious, it was necessary to
devise field studies and laboratory experiments which would
bring out as many of these subtle fatal conditions as possible
The following paper deals first with the structure of thepop-
as a whole; the second and third sections deal with the two
large causes of attrition: wave shock and predation.
MFASURING
To measure the size of Littorina planaxis the following method
was adopted. This method is better than the common spire
length system because L. planaxis are very susceptible to
erosion of their spires. The snail isoriented with the lip of
the shell opening as in the figure. Themeasure is taken by
placing one point of a caliper at the widest part of the great
whorl opposite the lip, the other at the fringe of the lip.
lst point of calipers
shell opening
- Columella

— Body whorl of shell
and point of calipers
For the following work, the measurements were taken to the closest
1/32 of an inch.
POPULATION
In order to gain an appreciation of thesize and distribution
of the snails comprising the Littorina planaxis population, it
was necessary to sample various ardas along theshore and from
these samples to try to construct a meaningful picture of the
structure of the population.
The sampling was accomplished by first selecting an area.
Since this was not to be an extensive sampling project and the
numbers of individuals would be relatively small, the areas
were selected to be as varied in amount of splash and wave shock
as possible. In all nine areas along the shore were sampled:
three populations from a protected shore (226), two from
heavy wave shock areas (2:34), one from a semi-protected beach
(2/), and three from an area where there was a great deal of
spray but little direct wave shock (42:5 ). These areas are
indicated on the map preceding this page.
Once an area was selected, the height of the lowest indivi-
dual snail in thepopulation to be tested was determined by meas-
uring his distance from the water line at that instant above the
0.O' tide. The figures for water height above 0.0' tide were
determined by reference to three Benchmarks of known height that
are positioned in the rocks on shore. Readings from the bench
marks to the water line were made at ten minute intervals during
the time that the heights of the various selected populations
were being determined.
Once the height of the lowest individual was established,
the upward range of the population was marked off in one-foot
intervals. The width of the grid was two feet; each vertical
foot therefore designated a two square foot grid.
After the grids had been established, all the snails within
each grid-quadrant were collected and then taken back to the
laboratory for measuring.
The results of these population surveys are summarized in
the two graphs following. Additional graphs are found in the
appendix.
From casual observation it would seem that the smallest
individuals in the L. planaxis population live the furthest
down on the rocks, because the veliger larvae probably begin
their attached existance at or below the "waterline". Its
asumed further that the snails move gradually up the rocks as
they grow. From the snails I sampled and observed, the relation
ship of size to vertical distribution seems to be influenced
by more than age. For instance, there seems to be a correlation
between theamount of spray andthe dominant size in a population.
Sidlg
The g2.24 and Fg26 have very little splash and small
snails; 2:3,4 where spray is grest and wave force is also high,
is composedpredominantly of middle sized snails (appendix);
the  where spray is high but wave force is low has the
largest snails (appendix). Apparently then conditions for optimum
growth are a large amount of spray and minimum amount of direct
wave shock.
Another factor affecting the vertical distribution of smaller
snails is the location of crevices in the sampled area. Smaller
snails are found in these crevices at heights where they would
not be expected. Graph I, 384 show that the size/frequency
distribution can be different for a crevice and a vertical face
adjacent to one another. In this particular example the incid-
ence of small snails is greater in thecrevice. From other
populations I have observed, this would seem to betherule rather
than an exception. All individuals found below 5/32" were im-
bedded in crevices and chinks in the rock; individuals between
5-8/32" were found in protected areas not exposed to heavy
surf. Snails below8/32" are not commen on vertical faces exposed
to heavy surf and neither are individuals greater than 18/32"
The extremes in sizes apparently must be sheltered from the
direct force of the waves.
The question of where the inchoate rock-dwelling snails
appear on a vertical index could not be answered by this study.
The smallest individual collected was 4/32" and was from Oco Fa2:2.
In summary then the following things are apparent: The
dominant size class of snails in May 1964 was between 11 and 14/32":
as a general rule the smaller snails are found lower on a verti-
cal index, but this rule is tempered by factors such as degree
of spray and wave action, and the frequency andposition of
crevices in a population range.
M
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G
POPULATION GRAPES
Graph Fg
tio
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ve
O
5
Height aove 0.0'Tide
3
GRAPH SHOUS NOS. OF SNAILS OF A PRRTICOLAR
S13E
THEIR VERTICRL POSITION EACH
CURSS PLOTTED AGAINST
TAREA
FOOT IN HEIGHT INDICOT
TWO SOORRE FE
Ha
ple fo 8128 a1
Sant

Tta sop5. 4%
Fo
Siges — 4782 in
Bdi s or e not aboradart  Mag
tound oule in Gevtein dtea Eg,12
ted da pot
e
Heuler heret ai he fod in Gevie
tal Pop 15.9%
es S8/
Vencohctu let o orgee
Ouo
Sai
ICidvices
O-35
4 e Tetal Pp30.H10
51285 8-10/873

SS 1H
domin s s clas A fak
-515
Hssite aelege eneg
4 folat Pop
Me rock song stad swall
14.4%
egt lo  tg o lare
a oace  awave ate 80
tdoiduol ik lodug sat ore
seled
S2e9 15 18/82
atetcoe id Bloregs
Lagonen
N243 8.7%
e bu de
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Siass 18/83104d
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ae
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22
assrms
sc
CAUSES OF DEATH
Wave Shock
Wave shock would seem to bethe greatest enemy on the pop-
ulation since displacement from the rocks would expose the
snail to a host of predators and adverse conditions not encountered
in thehigh intertidal. In order to determine the effect of
wave shock upon a population of L. planaxis the following field
studies and experiments were undertaken:
1. To determine relative clinging ability, Russel Peterson and
I took five classes of L. planaxis and subjected them to a
stream of water (delivery-12.8 liters/min thru 8.5 mm. diameter)
from thespigots in Agassiz Laboratory. The snails were submerged
to the height of their spires. The results were as follows:
Snail Size 4-6.9/32"
11 remained/25 tested
44%
7-9.9
44
10-11.9
25
44
23
12-13.9
50
46
14-17
50
54
We were looking for a correlation between this experiment
andthe observation that middle sized snails are themost successful
snails in regions of heavy surf. This experiment did not con-
firm this although it must be noted that we were testing more
their ability to maintain position in a current than their ability
to withstand wave-shock.
2. Seven marked L. planaxis of about 6/32" were placed on a
vertical rock face exposed to heavy waves. Seven control snails
(about 12/32") living in thearea were also marked. After two
days and two high tides, only three of the small snails were
found after an exhaustive search. All seven controls were
recovered. This might indicate tuut small smails are dislodged
more readily than larg
eine
ons
heavy surf
3. Because I did not see what happened in the above experiment
I repeated the test at a later date. I placed 30 snails ranging
in size from 6-18/32" on the same rock face and watched them
during the period of one high tide. None was dislodged, but the
waves were not so violent as previously.
4. To determine a snails ability to return to the rocks after
being dislodged over a sandy bottom, 100 marked snails were put
into the sea five yards from some rocks in region  The
next day and the days following not one snail was recovered.
The correlary experiment of dropping snails over a rocky Bottom(Fg?:2
was tried under similar conditions. The animals were dropped
ten feet down the side of a cliff into water about two feet deep
at mid-tide. The average size of the recovered snails was 10/32"
Rec overu was 30%
5. To determine the snails ability to return to rocks after
being kicked off (people and other clumsy animals are frequent
enough visitors to the high intertidal to present a hazard to
L. planaxis) 20 marked snails dislodged and allowed to tumble
onto the sand about 3' below ther original positiontawarn.
The following day 12 were recovered dispersed over a
wide area (about five square yards).
Although two ans
6. This experiment was repeated under same conditions of dis¬
4
persal but with overcast and little wind. Animals were kept under
constant observation. The animals at first opened their opercula
and extended their feet in typical righting movements. Those
who were successful in righting themselves remained extended for
about five minutes but did not try to crawl over the dry sand.
The unseccessful tried for about five minutes and then closed
their opercula. The righted snails closed also. As soon as
omplete l
op
they were wetted they became active, crawling as rapidly as
they could away from the open sea towards a shore rock. With
the exception of one individual who later changed his direction
all snails crawled away from the sea. This experiment was per-
formed on the rising tide. Snails crawling on a sandy substrate
cannot hold their position when hit by wave surge but tumble
with the current. All those I was watching in this experiment
were lost to me after the third such wave surge inundated them.
7. The failure of the snails to regain the rocks in the above
experiment suggests that the incoming tide presents too much of
a handicap to crawling over the sand due to the increased wave
surge over time. The countersituation--snails attempting to
regain rocks on a falling tide--was tried. Twenty-eight snails
ranging from 6-18/32" were released on a strip of sand where the
nearest boulder was 15" away from the populktion. The sand was
damp and as in the previous experiment all individuals immediately
popped out, and those not righted immediately began righting
movements. Once righted all began crawling in a straight line.
Two-thirds of the animals crawled towards the sea and nearest
rock, the rest moved further up the shore. Although two snails
came within two inches of the rock, a wave surge tumbled one so kh
that he could not right again. The other snail((6/32') gained the
rock and soon attached himself by mucus. Of the thirty, then.
only one was able to get to the rock. The others closed and
would be caught by the incoming tide.
One of the problems the snails encountered was sand-miring
of their feet. This was a hazard to larger snails (14/32" and
above) particularily because adhering sand grains prevented
complete closure of their opercula.
On the lst of May there was a very rough sea during the HHV
8.
at night. Alan Miyamoto and others reported significant losses
to populations under study, and I noticed that many of my snails
were gone although I had no census figures on them. On therock
I was using for my aerial predator experiment, an estimated
fifty percent of my animals disppeared during that night.
9. In some of the foregoing experiments it was noted that the
larger snajls tended to remain under water for longer periods
than the smaller snails.
In summary, wave shock has been found capable of removing
sizable numbers of snails from the population. Whether they
survive removal depends largely upon the nature of the substrate
upon which they land. Sand in apparently the kiss of death.
Falling down from high rocks to a rocky substrate is not quite
so fatal unless the snails happen to land in pools where various
predators present a hazard.
CAUSES OF DEATR
Predators
The following possible predators occur within or close to
the L. planaxis populations: California Ground Squirrel; Oyster
Catchers; Herring Gull; Plover; Sandpiper
Hemigrapsus nudus;
Pachygrapsus crassipes; Cancer antennarius; C. productus; Pagurus
spp.; Acanthina spirata; Thais emarginata; Patiria miniata:
Leptosterias pusilla; Pisaster ochraceous, Anthropleura elegan-
tissima and A. xanthogrammica.
Theses predators will be considered in order except for the
carnivorous whelk Acanthina spirata which, because of its sign-
ificance, will be considered later.
1. California Ground Squirrel: To find out if squirrels rec-
ognize planaxis as food two experiment s were tried. The first
consisted of placing a number of dishes containing known numbers
of L. planaxis around an area where ground squirrels were fre-
quent. In no instance was a L. planaxis eaten by squirrels
although the dishes were investigated.
The other experiment involved hand-feeding the squirrels at
Lovers and Lighthouse Points. Although the squirrels were always
curious and responded eagerly to what they assumed to be prof-
fered food, in no instance could a squirrel be induced to eat
a L. planaxis. They apparently did not recognize them as food.
2. The Oyster Catchers were never observed near L. planaxis
populations. They seemed to feed exclusively on the offshore
seaweed covered rocks.
Herring Gulls were negative also.
3.
Plovers and Sandpiperg were occasionally observed feeding
in theBalanus beds and could possibly feed on the L. scutulata
harbored there. The experiment
test for aerial predator
consisted of the daily census of an isolated population on top
of a rock accessible to birds. Some individuals were painted u
unnatural colors to test the chance that birds would eat the
snails if they could see them better. The experiment was dis-
continued at the end of thirty days with negative results.
Birds would perch on therock, but evidence of their eating the
snails was never found.
Hemigrapsus nudus and Pachygrapsus crassipes were prime
suspects because they frequent the L. planaxis range. Nether
of these crabs was observed feeding on L. planaxis in thefield
nor could they be induced to eat the snails in the laboratory.
Cancer antennarius consummed without hesitation great
quantities of L' planaxis under lab conditions. Since this
animal is a visitor to the upper mid tide zone at the flood
tide (determined by trapping a few individuals at the base of
L. planaxis covered cliffs), it should be considered a definite
hazard to the L. planaxis that are accidentally dislodged from
the rocks. C. productus is a rarer crab in this area. The one
speciment tested would not eat planaxis, but he would not eat
dead fish either. The question remains open as to his predator
potential.
7. The Hermit crabs of the genus Pagurus would not attack a
living L. planaxis, although they would remove the soft parts
of a dead snail and clean the shell for a futurehome site.
8.
Thais emarginata was never observed eating L. planaxis nor
could they be induced to eat them even after three weeks of
starvation. I would not write off thepossiblity of their being
a predator, but the incidence of
Thais attacks must be slight.
9. The Bat Star Patiria miniata
eats L. planaxis in the labor-
atory. So does Leptasterias
zu ttla.
Although none of these
12
tide, they represent a threat to the dislodged snails.
10. Anthropleura xanthogrammica would alwys take in any L.
planaxis dropped onto its fringe, but, as described in Ricketts
& Calvin, the snails would be disgorged alive later.
In summary then, the most significant predator on L. planaxis
in Acanthina spirata. The other predators —-Cancer antennarius,
Cancer productus, and the starfish— are encountered by the
snails only when they are under water.
Acanthina spirata
In the field, Acanthina attacks were frequently observed
but these attacks were encountered only on a special type of
shore: regions where there are may little tidepools with numbers
of L. planaxis around their surfaces. If the snails are dislod-
ged and fall into pools, or if they venture below the water line
persuing their own activities, they are susceptible to attack
by Acanthina. In no instance did I or any one else working with
Acanthina observe a capture that wasnt under water.
The table below is a tabulation of L. planaxis shells recovered
from Hermit crabs. The percentage of these shells drilled by
Acanthina is also given.
Size (in 1/32")
Number
Drilled of N
%drilling
26
10
28.
8
20.0
44
27.
30.6
68
9.8
27.1
23.-
40.0
otals 443, 89, 20.1%
etra
Ego
I determined the upward range of Acanthina in my populatior
grids by measuring the heights of individuals in the grid or
the closest, to the grid if none was present in the grid itself.
These measuremerts were made at mid-tide because it was too
inconvenient (and dangerous in some places) to make the measure-
ments at high water.
Theresults of these measurements were that Acanthina appar-
ently does not range much morethan two feet above the Balanus
beds except when found in tidepools as described above.
From what I and others (Karin Rohe and Ken Tittle) have
observed, the capture and feeding must take place under water.
A particularly successful stalk and capture by Acanthina is
illustrated in figure ?.
A seriesof experiments was performed to determine reactions
of Acanthina to L. planaxis and vice versa.
1. Five Acanthina and 100 L. planaxis were placed in an aquarium
andobserved daily for a week. Since the L. planaxis all fled the
water and crawled as far above the surface as possible, it was
necessary to knock them into the water occasionally so that the
Acanthina wouldhave a chance to capture them.
I observed four attacks by Acanthina during theweek. In
the first, I did not see the capture. The Acanthina was rocking
the L. planaxis, his attack being made against the columella.
Judging from otherattacks observed, this would seem to be the
preferred position, although it is definitely not the only
position. The Acanthina was joined at intervals by two other
Acanthina. The three of them all but completely covered the L.
planaxis. In six hours they hadquit the snail. Although all
three had apparently participated in devouring the snail, there
was only one drill hole.
S



20
595
25
60
t




5

2
9
2. In about 10 cases feeding Acanthina were removed from their
prey and then replaced near it again. About half of theindiv-
iduals tested resumed their attack immediately. Theother half
showed no interest in feeding and could not be induced to repeat
an attack even when fresh sea water and then a fresh snail were
substituted for water in which the capture took place and then
the captured snail.
3. From the snail shell table above it is apparent that the
heaviest incidence of drilling occurs on snails from 10/32-
13/32". To test to see if this is a preferred size for a special
reason, Acanthina were attached to the backs of various sized
snails. I found thatI could not make an Acanthina remain at-
tached to a snail below 8/32"; snails larger than 14/32" could
drag an Acanthina weighing 4.82 grams up out of thewater and
hang on until the Acanthina eventually dropped off. On the basis
of these two observations, the Acanthina apparently prefers
snails between 8 and 14/32" because they are large enough to
attach to and small enough to be too weak to escape by crawling
up out of the water.
In summary, from the observation of a successful attack by
an Acanthina on a snail measuring 12/32"
it would appear that
a L. planaxis iseasy prey for a truly hungry snail, however there
are conditions that must be met before an Acanthina can encounter
a periwinkle. The incidence of Acanthina predation is highest
in those areas where there are small tide pools below the litt-
orine covered rocks. For convenience and ease of capture snails
between 10 and 14/32" seem to be preferred, although it must
be pointed out that snails between 10 and 14/32" comprise the
bulk of the population and are therefore the size range most
Eintered by Acant'ina. Thetwenty per cent drilling
given in theabove table for those shells recovered from
gur
hermit crabs probably does not represent a true indication of
Acanthina predaton on the entirepopulation because Acanthina
attacks occur apparently only in splash pool regions and the
bulk of the. planaxis population does not live above these
splash pools.
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