Subtidal Tegula distributions
Abstract
Abundance and distribution data were collected on three
species of turban snails (Tegula) in a temperate kelp forest, the
Hopkins Marine Life Refuge. Similar data at identical sites in
the same forest were collected by Watanabe (1984). Tegula
abundances from the present study and Watanabe (1984) were
compared. Overall Tegula density has increased significantly
since the earlier study. A bathymetric zonation of Tegula that
was documented in the earlier study was borne out in my study,
Tegula brunnea occurs in shallow water (0-6m), T. pulligo occurs
mostly in deep water (7-12m), and T. montereyi is the least
abundant everywhere, but present everywhere. T. montereyi has
been anecdotally observed to have increased in density in the
past 15 years, and this trend appears in my data, but is not
statistically significant.
Subtidal Tegula distributions
Introduction
Three species of archaeogastropod trochid snails in the
genus Tegula are abundant in subtidal kelp forests near Monterey:
Tegula brunnea, T. pulligo, and T. montereyi (Abbott and
Haderlie, 1980). They are closely related, but exhibit a
bathymetric zonation: T. pulligo is most prevalent between 7-
12m, T. brunnea occurs from the low intertidal zone to about 6
meters subtidally, and T. montereyi is less abundant, and
relatively evenly distributed between 2m and 12m subtidally
(Lowry et al., 1974; Riedman et al., 1981)
Tegula spp. are the most abundant grazer on the giant kelp
Macrocystis pyrifera (Lowry et al., 1974), and are an important
grazer on benthic algae (Schmitt, 1996). Pisaster giganteus and
Pycnopodia helianthoides, two species of sea star, are the most
common benthic predators on Tegula (Watanabe 1983). Enhydra
lutris, the sea otter, also preys on Tegula, but does not take as
many as the sea stars (Watanabe, 1984).
Tegula abundances in the Hopkins Marine Life Refuge were
recorded by Watanabe (1984). In this present study, I duplicated
his study sites and methods, to try produce a dataset comparable
to his, but at a later time. I will compare data from this study
with data from the previous study to examine temporal changes in
Tegula populations over the past 15 years.
Subtidal Tegula distributions
Materials and Methods
All observations were made using SCUBA in the Hopkins Marine
Life Refuge, off Cabrillo Point, California, USA (Fig. 1). Three
study sites located in the Hopkins Marine Life Refuge were used
in a study of Tegula conducted in 1978-1981 by Watanabe (1984),
All observations in this present study were carried out at those
study sites. All observations were made during daylight hours,
in April and May of 1996.
The study area consists of rocky (granitic) benches that
gradually give way to pinnacles of rock in deeper water. This
produces a substratum with both horizontal and vertical faces.
Intermixed with the area of rocky substrate are patches of sand
or shell fragment rubble. At the shallower site, there is a
dense understory of Chondracanthus (=Gigartina) corymbifera and
other genera of erect, small (less than .5 m frond length) algae.
At the middle site, this algal turf is replaced by more
encrusting algae and sessile invertebrates. At the deeper site,
the biotic cover becomes a thin, low-relief crust in places.
(For a more complete description of this area, see Riedman et
al., 1981.)
I gathered two different types of data, benthic and on
Macrocystis pyrifera.
To collect the benthic data, I used square 0.25m2 quadrats,
Subtidal Tegula distributions
placed in haphazard locations within each study site. I counted
individuals of each of the three species of Tegula present in the
quadrat. These numbers were multiplied by four to give densities
(snails per meter?) of each of the three species, in each of the
three depth zones.
To collect the data on Macrocystis plants, I first
haphazardly selected a plant within one study site. Then I
counted all Tegula present on the plant. I began with the bottom
1.5m of kelp, including the holdfast, and counted each subsequent
1.5m section separately. I also counted the number of fronds on
each kelp plant within each 1.5m depth interval. The number of
individual snails of each species was then normalized to number
of snails per frond (in each 1.5m section). To allow direct
comparison with benthic data, these data were then multiplied by
the average number of fronds per plant and average number of
plants per meter?, to yield average number of snails on
Macrocystis per meter? of substrate.
I obtained the raw data from the previous study of the area
by Watanabe (1984), and normalized it to number of snails per
meter?, as outlined above. I then performed a fully factorial,
four-way analysis of variance (ANOVA) on the numbers, to test for
significant differences between: time (the present study and the
prior one), the three different depths, habitat (benthic vs.
Macrocystis), and snail species. All four factors were
Subtidal Tegula distributions
considered fixed factors.
Results
The three species of Tegula are distinctly segregated by
depth. T. brunnea occurs more commonly in shallower depths,
closer to shore (subtidally to 6m). T. pulligo is more common
further from shore, deeper (7-12m). T. montereyi does not show a
strong depth zonation, but is less abundant than the other two
species at all study sites (Fig. 3, p«0.001). This is a change
from the situation in the 1930's, when T. montereyi was the most
abundant species (Rudolf Stohler, unpublished data)
The overall Tegula density in the kelp forest at the three
study sites has increased significantly since 1980 (Fig. 4,
p=0.020).
Tegula montereyi had been anecdotally observed to have
increased in numbers between 1978 and 1996. Although this trend
was shown by the data (the mean has doubled since 1978), it was
not statistically significant (p=0.5, see Fig. 4).
There were also some trends in the data I gathered on
Macrocystis plants. T. montereyi were more abundant higher up on
the plants than they were near the holdfast (Fig. 5). At some
places, T. montereyi were even the most abundant species within a
depth interval on a plant. These patterns of zonation within a
Subtidal Tegula distributions
plant are very transitory, since Tegula move so quickly along
kelps. However, the data show some trends of usual heights of
occurrence for Tegula of different species (Fig. 5).
Discussion
More research is needed to determine whether the increase in
Tegula density is a long-term effect or simply a spike in the
population, or a seasonal effect.
Watanabe (1984) examined some possible causes for the
observed habitat partitioning of the Tegula species. He found
that the bathymetric zonation was maintained by an interaction of
several factors. Larval settlement tends to set up the
bathymetric zonation, with T. pulligo larvae settling deep and T.
brunnea shallow. However, since the snails move very rapidly and
live for several years, a mechanism is needed to maintain the
distribution. Watanabe (1984) found in the same study that T.
brunnea is more likely than T. pulligo to travel between
Macrocystis plants on the substrate. T. pulligo travels between
plants mostly through the canopy. Since there is a dense cover
of erect benthic algae in shallower areas of the kelp forest, T.
brunnea traveling between kelps in the shallow study site are
more protected from benthic predation than they would be in
deeper areas, where algal cover is less. So different behaviors
Subtidal Tegula distributions
leading to differential predation might maintain the bathymetric
zonation that is set up by larval settlement.
The change in overall Tegula abundance might be linked to a
decrease in abundance of Pisaster giganteus, a sea star and
important predator on all three species of Tegula. P. giganteus
abundance is less now than it was when Watanabe's data was
collected, but the difference is not statistically significant,
(1.63 per 10 m2 in 1980, versus 1.15 per 10 m2 in 1996. Data
from Watanabe, 1984, and Lu, 1996, unpublished data.) These data
are only valid for the middle depth zone.
Another factor that influences adult populations is larval
settlement (eg. Lowry et al. 1974, Watanabe, 1984). If there has
been an increase in settlement rate recently, that could explain
the increase in Tegula populations with no reference to changing
predation. The same is true of a decrease in post-settlement
mortality; it could explain an increase in adult populations.
did not examine larval settlement due to time constraints on my
study.
Another factor that I would have liked to examine but did
not is the size structure of Tegula populations. Size structure
of adult populations would give clues to past patterns of larval
settlement (Lowry et al. 1974, Watanabe 1984). Modes (local
maxima in the size frequency histogram) indicate past years of
unusually successful recruitment (Lowry et al., 1974). If there
Subtidal Tegula distributions
were several very successful generations of snails in the recent
past, this would show up as several modes in the adult size
frequency distribution.
Tegula in Southern California also exhibit bathymetric
zonation (Schmitt, 1996), as do many other marine invertebrates
(eg. Connell, 1961; Grosberg, 1982).
This study should be re-done in several years, to determine
whether the observed changes in Tegula abundance are trends or
simply short-term anomalies.
Conclusions
Tegula exhibit a bathymetric zonation in this study area,
similar to that found by Watanabe (1984). Overall Tegula
abundance has increased significantly since Watanabe's 1984
study. Abundance of Tegula montereyi has also increased
noticeably, but the change is not statistically significant.
Subtidal Tegula distributions
Works Cited
Abbott, D.P. and Haderlie, E.C. (1980).
Prosobranchia: Marine
Snails. In Intertidal Invertebrates of California (R.H.
Morris, D.P. Abbott, and E.C. Haderlie), pp. 230-307.
Stanford, Calif: Stanford University Press.
Connell, J.H. (1961).
The influence of interspecific competition
and other factors on the distribution of the barnacle
Chthamalus stellatus. Ecology. 42. 710-723.
Grosberg, R.K. (1982). Intertidal zonation of barnacles: the
influence of planktonic zonation of larvae on vertical
distribution of adults. Ecology. 63. 894-899.
Hunt, D.E. (1977). Population dynamics of Tegula and Calliostoma
in Carmel Bay, with special reference to kelp harvesting.
M.A. thesis, San Francisco State University, San Francisco,
CA.
Lowry, L., Mcelroy, A.J., and Pearse, J.S. (1974). The
distribution of six species of gastropod molluscs in a
California kelp forest. Biol. Bull. 147. 386-396.
Riedman, M.L., Hines, A.H., and Pearse, J.S. (1981). Spatial
segregation of four species of turban snails (Gastropoda:
Tegula) in central California. The Veliger. 24 (2). 97-102.
Schmitt, R.J. (1996). Exploitation competition in mobile grazers:
tradeoffs in use of a limited resource. Ecology. 7
(2). 408-
425.
Watanabe, J.M. (1983). Anti-predator defenses of three species of
kelp forest gastropods: comparing adaptations of closely¬
related species. J. Expt. Marine Bio. and Ecology. 71. 257-
70.
Watanabe, J.M. (1984). The influence of recruitment, competition,
and benthic predation on spatial distributions of three
species of kelp forest gastropods (Trochidae: Tegula).
Ecology. 65 (3). 920-936.
10
Subtidal Tegula distributions
Tegula abundances on Macrocystis, tabulated by height (meters)
above the holdfast (N=5)
T.brunnea Shallow
Middle
Deep
mean
std error
mean
std error mean
std error
0-1.5
8.37
6.170
1.154
0.127
0.16
0.54
1.5-3
3.50
1.096
0.41
0.176
0.02
0.011
3-4.5
6.52
0.544
3.057
0.45
0.08
0.038
4.5-6
4.30
2.728
0.46
0.486
0.07
0.033
6-7.5
0.11
0.26
0.177
0.19
7.5-9
0.61
0.513
0.21
0.141
9-10.5
0.76
0.325
T.montere Shallow
Middle
Deep
std error
std error
mean
mean
mean
std error
0-1.
0.181
0.150
0.36
0.31
0.49
0.185
1.5-3
0.89
0.255
0.65
0.184
0.26
0.107
3-4.5
0.47
0.51
0.101
0.28
0.172
4.5-6
0.26
0.11
0.82
0.221
0.21
0.106
6-7.5
0.51
0.302
0.61
0.289
7.5-9
1.19
0.61
0.207
0.330
9-10.5
0.98
0.570
Deep
T.pulligo
Shallow
Middle
std error
mean
std error mean
std error
mean
0-1.5
1.141
1.750
2.02
1.29
0.473
2.76
1.5-3
0.53
0.231
1.00
0.360
0.91
0.275
3-4.5
0.599
0.30
1.78
0.132
0.62
0.282
4.5-6
0.342
0.00
0.000
0.52
0.191
1.10
6-7.5
1.07
0.94
0.012
0.435
7.5-9
0.000
0.45
0.25
0.335
9-10.5
0.024
0.05
11
Subtidal Tegula distributions
Table 2: Number of stipes in each depth interval, tabulated by
height (meters) above the holdfast (N=5)
Stipes
Shallow
Middle
Deep
mean
std error
std error
mean
mean
std error
0-1.5
19.60
9.373
10.198
25.80
35.00
9.324
1.5-3
18.40
5.784
27.40
6.615
7.926
23.40
14.80
4.954
3-4.5
5.943
19.20
20.40
5.490
4.5-6
4.000
12.00
18.40
6.831
17.80
4.748
6-7.5
4.324
15.40
6.743
14.00
7.5-9
7.60
2.638
14.20
6.717
9-10.5
13.00
2.828
12
Subtidal Tegula distributions
Table 3: Data from Watanabe (1984) and my data, as used for
ANOVA. All numbers represent individuals per square meter,
normalized as outlined in the text,
Means
Mid
Deep
Shallow
T.pulligo
Kelp
1.05
9.81
4.90
T. montereyi
1.79
0.59
1.06
T.brunnea
4.60
3.24
0.34
Timel
Shallow
Mid
Deep
Benthic T.pulligo
2.67
9.07
2.05
T.montereyi
1.80
2.40
0.82
T.brunnea
22.86
0.27
0.41
Shallow
Deer
Mid
T.pulligo
Kelr
3.62
0."
4.40
T. montereyi
Time2
1.19
2.52
2.20
T.brunnea
9.83
1.92
1.92
Shallow
Mid
Deep
Benthic T.pulligo
8.84
4.19
12.00
T.montereyi
5.20
3.40
1.00
T.brunnea
14.00
2.60
0.20
Standard
Deviations
Shallow
Mid
Deer
T.pulligo
4.34
Kelp
0.95
7.16
T. montereyi
Timel
0.73
1.59
1.18
0.7
2.93
T.brunnea
3.21
Shallow
Mid
Deep
Benthic T.pullige
4.84
3.65
23.23
T.montereyi
3.04
5.10
2.09
T.brunnea
43.
1.46
1.53
Shallow
Mid
Deer
Time2
T.pulligo
Kelr
0.47
2.68
2.98
T. montereyi
0.78
1.05
1.43
T.brunnea
1.55
7.78
1.08
Mid
Deep
Shallow
Benthic T.pulligo
4.64
11.82
15.63
T.montereyi
9.63
4.55
1.78
T.brunnea
12.14
7.14
0.89
Table 3, continued
Numbers of replicate
observations
Timel Kelp
T.pulligo
T.montereyi
T.brunnea
Benthic T.pulligo
T.montereyi
Time2
T.brunnea
T.pulligo
Kelp
T.montereyi
brunnea
Benthic T.pulligo
T.montereyi
T.brunnea
14
Shallow
Shallow
Shallow
Shallow
20
Subtidal Tegula distributions
Mid
Deep
Mid
Deep
30
39
30
Deep
Mid
Mid
Deep
20
20
20
Table 4: Analysis of variance results on data from Watanabe (1984) and
present study.
ANALYS
S OF VARIANC
SOURCE
SUM-OF-SQUARES
F-RATIO
DF MEAN-SQUARE
TIMES
4.999
4.999
5.525
0.019
DEPTHS
18.185
9.092
10.048
O.000
HABITAT
2.667
2.947
0.087
2.667
SPECIESS
5.736
6.339
11.472
0.002
TIMESDEPT
0.876
0.438
0.484
0.616
TIMES*HABITAT:
1.659
1.659
1.835
0.176
TIMESSPECIES:
0.249
0.125
0.138
0.871
DEPTHS*HABITAT
4.069
8.138
4.49
0.012
DEPTHS*SPECIESS
15.57
56.367
14.092
0.000
HABITATS*SPECIES
0.065
0.033
0.036
0.964
TIMES*DEPTHS
*HABITAT
2.241
0.291
1.120
1.238
TIMES*DEPTHS
*SPECIESS
2.808
0.776
0.541
0.702
TIMES*HABITATS
*SPECIESS
3.992
1.996
2.206
0.111
DEPTHS*HABITATS
*SPECIESS
1.439
0.360
0.397
0.811
TIMES*DEPTHS
HABITATSSPECIES
0.421
1.684
0.465
0.761
466.911 516
ERROR
0.905
Subtidal Tegula distributions
Figure 1: Map of study sites in the Hopkins Marine Life Refuge,
and a map of Central California showing their location,
Figure 2: Tegula abundances separated by time, species, and
habitat, graphed over depth zones.
Figure 3: Relative Tegula abundances at the three study sites in
1996
Figure 4: Overall Tegula abundance changes between 1978 and 1996
Figure 5: Tegula abundance, as a function of height above
holdfast on Macrocystis plants in 1996
15
San Francisco

km

Santa Cruz

Moss Londing

Pacific Grove

Monterey
Hopkins Marine Station
50m
Pacific Grove, CA
gpfpkel
phe
A
study

sites
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20
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SNAILS ON BOT TOM (No. m22)
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SNAILS ON KELP (m2 of Substr)

kka-

O
—
-

0

o
OO

—
1.2
Shallow
Relative Tegula abundances by depth
Mid
Deep
OT.pulligo
ET.montereyi
m T.brunnea
1
197

T. pulligo
T. brunnea

T. montereyi
1000
TE
10
Shallow relative frequencies

——
—---------

0.7 -———
—------------
0.6
------------------------
-----------------------
—---------------------- - -
0.2 —--


0.14
0-1.5
1.5-3
3-4.5
4.5-6
Height above holdfast
Middle relative frequencies
—--------- - - - -


- - —

0.4 +------—




e
---------
0.2 +----
—------------------— — — —
—------------——
3-4.5
0-1.5
1.5-3
4.5-6
6-7.5
7.5-9
Hieght above holdfast
Deep relative frequencies
0.9
08
—-------

0.7 +---------------------
- - - - —-

0.6
--- ----
0.5  -- - - - - ---------------------,----------
0.4 1-----------------------------

0.3

02
0.1
PP
4.5-6
0-1.5
1.5-3
6-7.5
3-4.5
7.5-9 9-10.5
Height above holdfast
—2—T. puligo
----T. montereyi
— A — T. brunnea
—2—T. pulligo
--T. montereyi
— +- T. brunnea
—2—T. puligo
----T. montereyi
— +- T. brunnea