Mita, R.T.
p. 2
INTRODUCTION
Acanthina punctulata (Sowerby,1825) is a predatory gastropod of the mid to
upper rocky intertidal. In Monterey Bay it is reported to feed on Tegula
funebralis (A. Adams, 1855), Littorina planaxis (Philippi, 1847), L. scutulata
(Gould, 1849), Balanus glandula (Darwin, 1854), Chthamalus dalli (Pilsbry, 1916),
C. fissus (Darwin, 1854), and small Nucella emarginata (Deshayes, 1839). However,
the extent to which it may affect prey populations in this region has been largely
unresearched.
Glynn (1965) lists A. spirata as a "transient, moving in and out with
tidal, daily, and seasonal change". Dayton (1971) and Connell (1970), in eluci¬
dating the extent to which Nucella species in the Olympic Penninsula Region
affect populations of barnacles, also remark on the movement of predatory gas-
tropods with the tide.
Yet casual observation seemed to indicate that there were qualitative
and quantitative differences between the predatory snail populations of Pacific
Grove and those of Washington (Dayton,1971 and Connell, 1970), Millport (Connell,
1961), and the Santa Cruz Islands (Menge, 1974). In fact, it was not immediately
clear what exactly was the situation at hand at all. It had not been the focus
of previous research at Hopkins Marine Station to illuminate population charac-
teristics of predatory snails, although anecdotal observations along these lines
were recorded. (Glynn, 1965 and Sleder, 1981). Thus it was in an investigative
spirit, clipboard and wetsuit in hand, that this study was undertaken to charac-
terize the spatial and temporal distributions of Acanthina; specifically, to
determine in what habitats, engaged in which activities, in what numbers, and
1,
Mita, R.T.
when these snails occur.
p. 3
P. 4
Mita, R.T.
MATERIALS AND METHODS
A study site of approximately 49 m2 was established in a moderately pro¬
tected area which was felt to be representative of the mid- to upper-intertidal
in terms of substrate relief, orientation, and organic cover. A tape measure was
pulled taut in an east-west direction through the center of the study site.
Seven surface-conforming transect lines were centered at one-meter intervals
along the tape and were run perpendicularly to it on either side. Surface-con¬
forming transect lines were used because they reduced the tendency to over-sam¬
ple horizontal surfaces, a problem with taut lines in areas of high relief.
Subsequent checking of the placement of the transects indicated that they were
roughly parallel to each other, all being within 4.50 £ 0.50 to 10.5° + 0.30
west of true north. The transect line itself consisted of a light metal alloy
chain along which seven tags were spaced one meter apart, thus enabling a sam-
pling density of at most one quadrant per square meter of surface area. The
center of a permanent 25 cm by 25 cm quadrat was uniquely marked at each of the
tags using small plastic labels mounted onto the surface with marine epoxy. The
corners of the quadrats were marked on the substrate with small dots of nail
polish. The heights of the quadrats were determined using a stadia rod, a
transit, and local reference pegs whose elevations had been professionally
surveyed.
The percent cover of sessile space occupants, sand, and bare rock (here-
after referred to as free space) within each quadrat was estimated using a
flexible plastic sheet upon which was marked a regular grid of 100 points. With
p. 5
Mita, R
the sheet held in place on the substrate, whatever sessile space occupant, sand,
or free space was directly underneath each point was observed, as well as whether
or not the point was in a crevice or on a rock face. The observations were tal-
lied and percent cover estimates obtained forthwith. The slope of the surface
was estimated to the nearest ten degrees; the orientation of the surface was
taken as the horizontal projection of the line normal to the surface.
To enable sampling at high tide, a length of one-half inch nylon rope was
bolted to the substrate in strategic locations.
Observations of snail activity and distribution were made continually over
a period of four and one-half days. The quadrats were visited on th e low tides,
the high tides, the mid-tides falling, and the mid-tides rising, except when
wave action, accidental sleep, or, on one occasion, a class meeting prevented
sampling. If snails were present in a quadrat, information regarding their dis-
tribution and activity within the quadrat was recorded. Snails were marked as
feeding if they were positioned such that the front portion of the foot was
placed on a prey item with the rear portion of the foot on the substrate and the
snail was stationary. Otherwise, snails were marked as "moving" if the tentacles
were extended with the shell raised off of the substrate, or marked as "sitting'
if the tentacles were withdrawn with either the shell held close to the substrate
or with the foot withdrawn into the shell. Additionally, the microhabitat in
which this activity occurred was recorded--that is, whether the snail was exposed,
as on the "face" of a rock or snuggled among algal thalii, the anemone Anthopleura
elegantissima (Brandt, 1835), sand, or within a crevice.
To obtain a rough estimate of rock face residency and turnover, 88 indi-
viduals on a barnacle-encrusted face were marked with a spot of white fingernail
polish, and observed at two successive low tides.
A quick survey was conducted along 100 m of coastline to ascertain field
prey preferences. All snails found feeding and the objects of their endeavors
were tallied
p. 6
Mita, R.T.
RESULTS
The mean densities of Acanthina with respect to microhabitat, activity,
and height zone, averaged over all tides, is shown in figure 1. Overall, thne
average density of snails in the study area was 4.62 m"4. The snails were
densest from 0.8 mto l.Om (7.61 m-2) and 1.2 m to 1.4 m (6.76“2), but reached
a density of only 3.06 m"2 in the height zone between these two (1.0 m to 1.2 m).
Acanthina were never observed in quadrats above 1.4 m. As shown in figure 2,
however, the population was distributed such that the bulk of the snails (63.857)
was to be found in the lowermost zone, the fewest snails (14.657) were in the
uppermost portion of their range, and an intermediate but disproportionately
small fraction (21.39%) for the amount of habitat available (figure 2B) was found
in between.
It is evident in both figures 1 and 2 that within any given height zone
the activity of choice of the majority of snails was to sit still in a refuge.
In the lowermost zone a fair proportion of the population (22.237), one-third of
thne snails within that zone, could also be found on rock faces, engaged in
sitting still--once again the most common activity--feeding, or moving. 907 of
the snails in height zone 2 (18.86% of all snails) were sitting still in refuges,
with only 2.25% of the entire population situated on rock faces, most of which
were stationary. On rock faces in zone 3, more snails were feeding or moving
than sitting still, a reverse pattern of that seen on rock faces in lower zones.
Feeding activity in zone 3 seemed restricted to faces, and was not observed in
refuges. Looking at all height zones, 677 of all snails were sitting still in
P. 7
Mita,
refuges, 14% were sitting on rock faces, 77 were moving on rock faces, and 2.57
were feeding and moving in refuges.
In order to discern possible effects of tide on density and distribution,
only high and low tide observations were examined; mid-tide observations were
excluded (figures 3 and 4). The average overall density seemed to be somewhat
higher at low tide (4.99 m"2) than at high tide (3.85 m"2), a difference which
was significant at the et-O.1057 level. Curiously enough, snail densities in¬
creased in the lowest two zones at low tide,, but remained unchanged in zone 3.
However, it was in the lowest zone that the proportion of the snail population
did not fluctuate between tides; in zone 2, there tended to be a greater fraction
of the population at low tide, whereas thne opposite was true for zone 3.
As for snail activity, higher percentages of the population were (1) sit-
ting still at low tide, (2) moving at high tide, and (3) feeding at high tide.
This difference was particularly marked in zone 3 (1.2 m to 1.4 m), where on rock
faces at high tide feeding and moving occurred, but not sitting still--which at
this time was seen only in refuges--whereas sitting still on faces and in refuges
was the only activity observed at low tide.
Acanthina did not seem overly slective in their choice of habitat. Snails
were found for the most part among different types of surface cover, slope, and
orientation in proportion to the abundance of these surface types. There was,
however, a tendency for the snails to be situated in areas of somewhat higher
refuge-to-face ratios, particularly in the lower zones. Whereas factors such
as orientation, slope, and surface cover apparently had little effect on population
distribution, refuge availability and vertical height may have had limited
effects (figures 5,6,7).
It was found that of 88 individuals on a rock face at one low tide, 32
snails or. 36.36% remained at the next low tide, giving a turnover rate of 63.647
from one low tide to the next. If one-third of the population was on rock faces
Mita, R.T.
at any given time and about two-thirds of these turned over from one low tide to
the next, then the residency on faces was 0.75 day and the time spent in refuges
was 1.5 days. However, a group of 5 snails was once observed to spend 19 days
semi-buried beneath sand in a crevice, so the stay can evidently be quite variable.
A quick survey of feeding Acanthina revealed that the vast majority were
preying on Balanus glandula. Far fewer numbered those eating Chthamalus dalli,
with snails feeding on Tetraclita rubescens (Darwin, 1854) and Littorina scutulata
very infrequently encountered. (figure 8)
p. 9
Mita, R.T.
DISCUSSION
Differences between high and low tide snail activity exist. Snails
which are not in refuges are more active and more numerous at high tide. This
is most noticeable at the upper range of their distribution, where all individuals
on rock faces are either feeding or moving at high tide, whereas at low tide such
individuals are only sitting still. If the aerial extension and use of the loco¬
motory and feeding organs entails an evaporative loss, then the selective advan-
tage in restricting such activity to periods of reduced dessicati on potential
is apparent. Another possible and/or contributing explanation for this pattern
may be that if water-borne chemosensory cues are used by Acanthina in the location
of prey items, then foraging would be facilitated and thnerefore more apt to
occur during periods of submersion. While it is premature to make definitive
claims as to the reason, it is evident that sampling only at low tides cannot in
itself provide a representative view of the population, yet most intertidal work
is based on low tide observations.
One might make an indirect calculation of prey handling times in the field
from this data. Since all prey caught in the uppermost level were barnacles and
no feeding was ever recorded at this height during a low tide, the maximum time
required to process a barnacle under these conditions was less than about twelve
hours (the length of time between successive low tides). Insofar as the minimum
handling time is concerned, an approximate upper limit can be obtained from noting
that some individuals were seen feeding in only one survey, but not in those
immediately precedingor following. Surveys were made at tides and mid-tides, so
Mita, R.T
p. 10
a maximum minimum estimate of barnacle handling time is six hours. Actual handling
times are probably much less than these theoretical limits and more accurately
obtained directly by a research design intended to do so.
Another aspect of the snail population that may change with the tides is
the density distribution. There is a tendency for densities at lower levels to
be higher at low tide. This is very difficult to reconcile with what has pre-
viously been described for snail behaviour since their density in the uppermost
level remains unchanged between tides. If anything, the snails are not shifting
their distribution upward at high tide. What, then could possibly explain this
phenomenon? First, and most facile, is the hypothesis that such differences
are merely artifacts doe to experimental error. The surge and presence of water
at high tide does tend to complicate sampling somewhat, and may result in under¬
counting. Perhaps, also, the snails are moving to parts of the habitat matrix
which are undersampled due to an inappropriate sampling method.
In either case the densities may not be changing at all in any of the
levels. That is not to say that the population remains static in any given
height range--almost certainly, snails traverse up and down, but the net movement
of the populati on as a whole may be zero. This means that the snails maintain
a steady-state height distribution, although their choice of microhabitat and
activity depend to an extent on the tides.
On the other hand, the observed differences may accurately reflect reality.
in which case an explanation is not readily forthcoming. Since the influx of
snails is not from above, they could be entering the study area from the surrounding
depths. Why there should be a small net movement upward at low tide and downward
at high tide at the lower range of distribution, however, is not immediately
obvious,
The emrgent picture of predatory activity is one which depicts the snails
p. 11
Mita, R.T.
not as obligate foragers at each high tide, but rather as intermittent hunters
among their invertebrate prey. In lieu of en masse bicircadial migrations between
refuge and prey or between low and high intertidal with every ebb and flow of the
tide, the population distribution is maintained in a sort of dynamic equilibrium
in which the flux of organisms is in reality quite small. Individuals apparently
are not compelled to forage with every incoming tide. Indications from obser¬
vations of marked individuals are that a given snail may spend on the order of
about 3/4 of a day exposed on rocky surfaces in the pursuit and ingestion of prey.
The typical snail will subsequently retreat to an appropriate refuge where it
may remain inactive for approximately a day and a half. At any given time fully
two-thirds of the population can thus be found tucked away in refuges, caught up
in an activity that for most practical puposes resembles sleep. Other than to
make minor shifts in position or orientation, snails in this attitude sustain
no translocative or feeding behaviour. In contrast, only 127 of all snails are
actually feeding, on the average. Interesting avenues of insight into the
physiological ecology of the species may be opened by an investigation into the
length of the stay as it relates to the microclimatological features of the refuge,
calories of prey consumed, the energetic cost of foraging excursions, and the
basal metabolism, to name a few.
One may speculate of the selective advantage this strategy might have.
By having a relatively long cryptic phase and sporadic, asynchronous feeding
patterns, individuals may minimize exposure to those biotic and abiotic factors
which tend to reduce reproductive success (e.g., predation, debilitating wave
action, dessication, etc.). At the population level, this becomes a means of
hedging one's bets: in the event of a wide-latitude, density-independent catas¬
trophe (displacement from substrate by waves or as when harbor seals (Phoca
vitulina) choose barnacle- and snail-covered rocks upon which to sleep), only
33% of the population will be susceptible. The remaining 677, being in refuges,
letely obli
sth
ven com
e
will less lik
P. 12
Mit
had happened.
Evidently, the snails are efficient enough at procuring prey and their metabolism
is such that constant feeding is not a necessity. Further, prey must occur in
sufficient numbers to enable this lifestyle; otherwise, a large proportion of
time would be spent simply seeking prey. However, it is not altogether clear
what the quantitative relationshipa of this predator-prey interaction might be
over a long period of time. A priori, it would be difficult to tell whether or
not the Acanthina population at Hopkins Marine Station is of sufficient size to
"control" the prey populations of, for example, the barnacles. Certainly the
densities of Acanthina are far lower at Hopkins than those of Thais in either
the Olympic Penninsula Region (Connell, 1970, and Dayton, 1971) or in Millport
(Connell, 1961). However, the barnacles themselves seem to be far less abundant
at Hopkins, depending on the season (initial settlement seems far less, but the
suvivorship seems higher); the moderately protected intertidal community in this
area is one of incredible complexity and does not appear at this level of inves¬
tigation to be dominated by any particular organism.
This study is by no means comprehensive; on the contrary, it represents
almost an initial look into an Acanthina population on Monterey Bay, and as such,
was purely observational in nature. The sampling program was developed as the
result of several attempt to improve upon the haphazardly placed quadrats and
almost exclusively low tide observations of previous research. In having to cope
with a surface of considerable rugosity and highly heterogeneous biota, one
cannot help but envy thosewho worked with the flat surface of Cantilever Pier,
or empathize with early intertidal ecologists who demonstrated a particular
reluctance to sample at high tide. Much in the way of experimental and long
term examination is required to clear the issues of why these patterns were
observed, do they vary with time, and ultimately, how does Acanthina punctulata
fit into the structure and function of the intertidal ecosystem?
Mita, R.T.
p. 13
ACKNOWLEDGEMENTS
This paper would be lining a round file were it not for some very special
folks.
Sincere thanks are due Chuck Baxter, who earned it. I would like to
express gratitude also to Jin Watanabe, Neil and Diane Allen, John Kono, and
Paul Hutchins.
Thank you !!
p. 14
Mita, R.T.
BIBLIOGRAPH
Abbott, D.P., and E.C. Haderlie. 1980. Prosobranchia: Marine Snails, pp. 230-
307 in R.H. Morris, D.P. Abbott, and E.C. Haderlie, eds., Intertidal
invertebrates of California. Stanford, Stanford University Press. 690 pp.
Connell, J.H. 1961. The influence of interspecific competition and other
factors on the distribution of the barnacle Chthamalus stellatus. Ecology
42(4):710-723.
------------.
19/0. A predator-prey system in the marine intertidal region.
1. Balanus glandula and several predatory species of Thais. Ecol.
Monogr. 40:49-78.
Dayton, P.K. 1971. Competition, disturbance, and community organization: The
provision and subsequent utilization of space in a rocky intertidal com¬
munity. Ecol. Monogr. 41:351-89.
Glynn, P.W. 1965. Community composition, structure, and interrelationships
in the marine intertidal Endocladia muricata-Balanus glandula association
in Monterey Bay, California. Beaufortia 12 (148):1-97.
Hewatt, W.G. 1934. Ecological studies on selected marine intertidal commu¬
nities of Monterey Bay. Ph.D. Thesis, Biology, Stanford University. 150 pp.
Johnson, W.S. 1976. Biology and population dynamics of the intertidal iso¬
pod Cirolana hartfordi. Marine Bio. 36:340-350.
McLean, J.H. 1969. Marine shells of southern California. Los Angeles County
Museum, Nat. Hist. Sci. Service 24, Zool. 11:104pp.
Menge, J.L. 1974. Prey selection and foraging period of the predaceous rocky
1
intertidal snail, Acanthina punctulata. Oecologia 17:293-316.
Sleder, J. 1980. Acanthina punctulata. Its distribution, activity, diet, and
predatory behavior. Veliger 24(2):172-180.
Spight, T.M. 1981. Risk, reward, and the duration of feeding excursions by a
marine snail. Veliger 24(4):302-308.
-----------. 1974. Sizes of population of a marine snail. Ecology 55:712-729.
C
0
p. 15
Mita, R.T.
gure
Density distributions of Acanthina with respect to microhabitat, activity,
and height zone.
Mita, R.T.
1.50
8 1.00
1.00
0.50
5.00
4.00
3.00
2.00
1.00
0.50
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
Feed
Sit
Move
Feed

Sit
Move
All snails
zone 1


o
ka



t
L
pooled
zone 4
zone 3
zone 2
HEIGHT CATEGORY

p. 16
Mita, R.T.
Figure 2
Distribution of Acanthina with respect to microhabitat, activity, and
height zone.
p. 17
Mita, R.T.
10
Feed
0

k

sit

Move


Feed



Sit

Move

—
pooled
All snails
zone 4
zone 2 zone 3
zone 1
HEIGHT CATEGORY
—1
p. 18
Mita, R.T.
Figure 2b
Height distribution of the study area.
p. 19
O
Mita, R.T.
p. 20
oL
—
zone 1

zone 3
zone 2
HEIGHT CATEGORY
zone 4
p. 21
Mita, R.T.
Figure 3
Density distribution of Acanthina at high and low tides with respect to
microhabitat, activity, and height zone.
Mita, R.T.
1.00 -Feed
Sit
2.00
1.00
Move
2.00
1.00
Feed
0.50
Sit
5.00
4.00
3.00
2.00
1.00
0.50
-Moyg
All snails
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.0
Tide
hi
zone 1


Oo

0 C
—

0
O 0
000
00
10
hilo
10
zone 4
zone
zone 2
GORY
CAT
pooled
p. 22
..
p. 23
Mita, R.T.
Figure 4
Distribution of Acanthina at high and low tides with respect to micro-
habitat, activity, and height zone.
Mita, R.T.
10 FFeed

Sit
20
10
Move
Feed

Sit
60
10
Move
0
All snails
hi 10
zone 1
10
0

L

0 0 0
00
OO 0
0

0

O 0
0 (
hi 10
pooled
0
hi 10
hi 10
hi 10
zone 4
zone 3
zone 2
HEIGHT CATEGORY
p. 24
Mita, R.T.
Figure 5
uge/face composition of quadrats with, without, and all quadrats
Ref
regardless of snail residency as a function of height.
P. 25
ae

0
Mita, R.T.
100
80
60
40
100
40
100
80
60
40
Quadrats never visited by snails
Quadrats visited by snails
0
All quadrats
refuge face refuge face refuge face refuge face refuge face
zone 3
zone 4
pooled
zone 1
zone 2
HEIGHT CATEGORY
p. 26
1 .
Mita, R.T.
Figure 6
Surface cover of quadrats visited and never visited by snails, as
compared with total substrate cover available, as a function of height.
p. 2
p. 28
Mita, R.T.
50 L Quadrats never visited by snails
20
10

50  Quadrats visited by snails
20
10



50 r All quadrats
40
20
10



fc
facecrevice
free
crustose barnacles sand Anth.
erect
space
eleg.
algae
algae
SURFACE COVER
C

Mita, R.T.
Figure 7

Snail presence as a funtion of orientation, slope, and height of
substrate.
p. 29
7 -
W,NW, N
0
W,NW, N
no orientation
S,SE
0
4
S,SE
ht, zone
p. 30
O
4 ht, zone
NE, SW
OO 0
M. ht. 1one
Mita, R.T.
5
C
* .
Mita, R.T.
p. 31
Figure
Field prey preference.
Mita, R.T.
200
100
40
oL
p. 32
0
Tegula
funebralis
Balanus
glandula
Chthamalus
spp.

Tetraclita
Littorina
scutulata
rubescens
PREY ITEM