Abstract. This study examined the diet of the generalized
predatory sea star Pisaster giganteus, comparing proportions of
various species in the diet to abundance in the habitat.
Comparisons to assess temporal variation were made to similar data
collected in the same site in 1981 (Harrold 1981). Relative
frequencies of prey species in the diet and in the habitat were
tabulated. The past and present diets of sea stars overlap
considerably, with a Morista overlap index of 0.667 and nearly
identical Shannon-Weiner diversity indices (2.035 in 1981 compared
to 2.029 now). However, a Chi-square analysis reveals several
major differences: (1) the absence of the vermetid gastropod
Petaloconchus montereyensis, and (2) an increase in the frequency
of two other sessile prey species, the barnacle Balanus spp. and
another vermetid, Serpulorbis squamigerus.
The absence of
Petaloconchus in the diet was due to its low abundance in the
habitat. The increased feeding on Balanus and Serpulorbis cannot
be explained by changes in abundance, which were not statistically
significant.
Rather. I hypothesize that the absence of
Petaloconchus, previously a reliable, easily obtainable source of
energy, led to increased feeding on these other sessile prey
species. Further study is needed to explore the impact of other
factors, such as individual sea star variation.
INTRODUCTION
The diets of various asteroids have been well studied (e.g.
Mauzey et. al 1968, Menge 1972, Nakashima 1974, Harrold 1981,
Herrlinger 1983). Observed prey choices and proportions in the
diets have been examined for several generalist sea stars.
Pisaster giganteus is a subtidal sea star which feeds by everting
its cardiac stomach on prey and digesting externally. Harrold
(1981) analyzed various aspects of its feeding ecology by
comparing the abundance of prey in the diet to availability
(factoring in numerical abundance, ease of capture and presence in
the same microhabitat), caloric yield and predator choice.
However, since then, little work has been done to follow up
this study and record or explain any temporal variation. Menge
(1972) reported temporal variation in prey choice by L. hexactis
prey, both daily (with the tides) and seasonally, as the
availability or ease of capture of different prey species varied.
Seasonal changes followed abundance patterns of calorie-rich or
calorie-poor species. However, studies have not been done to
explore variation in feeding ecology from year to year and make
comparisons to environmental changes such as prey abundance.
This study compared the diet of Pisaster giganteus to Harrold
(1981), with two main goals: (1) to record differences in diet
composition and prey abundance, and (2) to infer an explanation
from these results. The results showed several major dietary
changes correlated with the disappearance of a frequently eaten
species in the habitat. Analysis of these changes confirms
previous speculation on Pisaster foraging strategy.
MATERIALS AND METHODS
This study was conducted in the kelp forest of the Hopkins
Marine Life Refuge (HMLR), Pacific Grove, California, within an
approximately 40 mx 40 m patch of kelp forest, centered on a
black cable that runs north-south through the kelp (Fig. 1). The
bottom consists mostly of rocky substrate, with some sandy areas,
I limited all data collection to rocky substrate, the preferred
habitat of Pisaster giganteus. Studies were conducted using
SCUBA, in 32 dives between April 23 and May 23, 1996.
The mean density of the sea stars was calculated from 47 10
m' circular quadrats. I placed these quadrats at random locations
by choosing random compass bearings and numbers of kicks (to a
limit of 15) from the center of the study site. I moved quadrats
to the next random location if more than half of the area fell on
sandy substrate. For each sea star I encountered in the quadrat,
I recorded size (length from the tip of the longest arm to the
opposite interradius) and overturned it to determine whether it
was feeding and the number and identity of any prey species. From
this, the proportion of feeding sea stars and the percent of each
prey species in the diet were calculated. Sea stars were returned
to approximately their original position after measurements were
taken.
The main criterion for a feeding observation was an everted
cardiac stomach. Some sea stars had prey items pressed against
the stomach, and others simply had an everted stomach, e.g. if the
prey was attached to the substrate. Several sea stars were in
crevices and inaccessible, and could not be overturned,
tabulated these sea stars in a separate category since their
feeding behavior could not be determined.
I collected additional feeding data by overturning and
recording any encountered sea stars in 20 m swaths perpendicular
to the cable. These swaths were examined as closely as the
quadrats so that the data would not be skewed to favor more
visible sea stars. The same sea stars may have been tabulated
more than once during the study, but repetition was most likely
uncommon and thus had little effect.
I compared the observed diet to Harrold (1981) by using the
Shannon-Weiner diversity index, the Morista overlap index and the
electivity index of Ivlev (1961). The Shannon-Weiner index was
calculated as H' - -Ep, ln p,, where p, is the proportion of the it
species.
The Morista overlap index was calculated as
22xy,/(Exj'+Ly2), where x, and y, are the proportions of the i
species in the diet in 1996 and 1981 respectively. The electivity
index of Ivlev (1961) was calculated as E' - (D-H)/(D4H), where D
= relative abundance of a given prey species in the diet, and H =
relative abundance of that species in the habitat.
I calculated the mean density of various prey items by
plaging quadrats at random compass bearings and kicks from the
center of the study site. More abundant snails (Tegula spp. and
Calliostoma ligatum) were counted within 0.25 m2 quadrats, 20 for
Tegula and 19 for Calliostoma. Tegula data were supplied by Hunt
(1996). I counted 21 10 m’ circular quadrats for less abundant
species (Astraea gibberosa, Mitra idae, Serpulorbis squamigerus,
Ceratostoma foliatum and Conus californica). The abundance of
sessile species that aggregate on rocky substrate (Petaloconchus
montereyensis, Balanus spp., and polychaete worms) were measured
as percent cover, using 15 0.25 m quadrats divided into grids of
25 10 cm x 10 cm squares. The proportion of grid intersections
falling on a given species was used to estimate percent cover.
grouped polychaetes together since I initially did not distinguish
them in the diet. Balanus spp. were rare and thus also counted
individually within 1 m quadrats. Percent cover measurements
were used as estimates of relative abundance for Petaloconchus and
polychaetes, and relative abundance of individually counted prey
species was calculated separately as the density of a prey species
divided by the total density of all prey species in 1 m'.
To eliminate the effects of seasonal variations, only data
from Harrold (1981) taken during spring (late March through June)
were used. Calliostoma abundance numbers were compared to
measurements by Sellers (1977), taken during the same period and
in the same area as Harrold (1981).
RESULTS
During the study, I sampled 130 sea stars, of which 61
(46.93) were feeding. Eleven sea stars (8.53) were lodged in
crevices and inaccessible. The density of Pisaster giganteus was
1.15 + 1.47 per 10 m’ (mean + S.D.), not significantly different
from Watanabe (1984), t.=1.159. Based on size measurements of 124
sea stars, the mean size (+ S.D.) was 13.4 + 3.1 cm.
The most frequently found prey items were the barnacle
Balanus and the vermetid gastropod Serpulorbis, with relative
frequencies of 28.793 and 22.723 respectively (Fig. 2). No sea
stars were observed feeding on Petaloconchus. The species found
in the diet, in decreasing order of frequency, were Balanus spp.,
Serpulorbis, polychaetes, Tegula pulligo, Calliostoma, Tegula
montereyi, Astraea and Ceratostoma (tie), and Mitra, Nassarius
mendicus and an unidentified mussel (tie).
The diet composition was very similar to Harrold (1981). The
Shannon-Weiner diversity index was 2.029 compared to 2.035
(Harrold, unpublished data). These diets, in terms of actual
species and proportions, also overlapped quite substantially
(Morista index - 0.667). A Chi-square analysis, however, showed a
statistically significant difference between the diets (y=28.3,
po0.005). Qualitatively, the largest differences in the diet
between Harrold (1981) and my study were in the frequencies of
Petaloconchus, Serpulorbis, Balanus and polychaete worms (Fig. 3).
The most abundant individually counted prey species was
Calliostoma, followed by T. pulligo, T. montereyi, T. brunnea,
Serpulorbis, Balanus spp., Mitra, Ceratostoma, Astraea and Conus
(Table 1). No Petaloconchus was encountered while recording
abundance data. Polychaetes had a 6.253 cover.
Only Petaloconchus had a significantly different abundance
from previous measurements (p.0.01, Table 1). No Petaloconchus
was observed in the quadrats, although it was the most abundant
prey species (18.83 cover) in Harrold (1981). A qualitatively
large increase in Serpulorbis abundance was statistically
insignificant.
Prey species with the greatest changes in the diet were
compared to changes in relative abundance (Fig. 4). Balanus and
Serpulorbis both increased in the diet more than in the habitat
(Balanus abundance actually decreased).
The increase of
polychaetes in the diet was approximately the same as the increase
in abundance, and the disappearance of Petaloconchus was likewise
reflected in both the diet and the habitat. The relative
proportion of prey species in the diet and abundance in the
habitat were used to calculate the electivity indices for each of
the species now (Table 2) and in Harrold (1981) (Table 3). For
the species analyzed in Fig. 4, the electivity indices were
compared (Fig. 5). These indices increased for both Balanus and
Serpulorbis and decreased for polychaetes.
DISCUSSION
The relative frequency of polychaetes in the diet increased
by approximately the same amount as their relative abundance. The
dietary increase of polychaetes can thus be explained by the sea
stars' taking more worms in proportion to their higher
availability, rather than making a choice or adjustment to their
diet. However, the increased relative frequency of Balanus and
Serpulorbis in the diet was larger than any abundance changes,
which suggests another cause.
According to foraging theory, a predator must make choices to
maximize energy input while minimizing effort and difficulty to
gain that energy (Stephens and Krebs 1986). Emlen (1966)
demonstrated this with two species of carnivorous gastropods,
which optimized foraging with respect to time and energy
conservation. Menge (1974) showed how a predaceous snail made
foraging choices to maximize biomass intake per time and decrease
search time. Unpredictability of prey species has also been
linked to generalization of diets and prey choice (Menge 1972).
Harrold (1981) found that regardless of the relative
availability of T. pulligo (high caloric yield) and Petaloconchus
(low caloric yield), some Petaloconchus was always taken. He
hypothesized that this was due to the uncertainty of capturing T.
pulligo, which exhibits an escape response and is often found in a
different microhabitat than Pisaster. The strategy was thus to
always take some abundant but low-energy prey since even abundant
high-energy prey might not be reliably available.
Both species that increased the most in the diet, Balanus and
Serpulorbis, are sessile species, and thus cannot escape a
foraging sea star. Algae and other encrusting organisms found on
the shell of Serpulorbis render it less recognizable (Harrold
1981), but this defensive response is not as effective as that of
mobile snails (such as Calliostoma or T. pulligo). Although these
sessile prey are not now as abundant as Petaloconchus was in 1981,
they are easily captured upon encounter. Thus, the increased
frequency of Balanus and Serpulorbis may reflect an adjustment to
replace Petaloconchus as a predictable energy source.
These results call for further study and more data. Since
the sample size of this study was small, the resolution of
abundance and dietary changes is fairly low. Small frequency
changes could not be detected, and some apparent changes may have
been partly due to sampling error. More feeding data could
correct any such effects and yield more conclusive results.
The role of the sessile polychaetes in Pisaster's diet should
be further explored. Although their increase in the sea star's
diet may be explained by an increase in abundance, more data may
indicate a more precise change which may also stem from the
absence of Petaloconchus. Another unresolved question is the role
of individual variation in the observed differences, as has been
displayed in other predators (West 1986, 1988).
Other methods of characterizing prey availability could be
explored, to see whether the results are comparable. Harrold
(1981) used two distinct methods of measuring abundance, numerical
abundance, counting the number of each species, and encounter
abundance, which factored in the length of each encounter with a
prey species. Encounter abundance would theoretically account for
effects of prey avoiding predation by living in different
microhabitats or other adaptations, which would render these prey
less available than their numerical abundance might suggest. This
method should differentiate between a choice made by a predator
and a simple inability to find and capture prey. However, this
method seems to undersample rare events, since the limitations of
SCUBA prevent long-term observation of sea stars.
Harrold (1981) notes that for many sessile species which do
not exhibit escape behaviors, numerical abundance is an adequate
measure of availability. He noted a small difference in
electivity indices of Serpulorbis based on encounter abundance
rather than numerical abundance, but a smaller difference than for
mobile snails such as T. pulligo and Calliostoma. Thus, for the
species highlighted in my study (Balanus, Serpulorbis, and
Petaloconchus), numerical abundance should suffice. However,
using encounter abundance may show whether mobile species also
played a role in replacing Petaloconchus in the diet.
Menge (1972) defined prey availability as the ratio of
calories consumed from a certain prey to calories encountered.
However, since this uses the actual diet numbers, it includes
predator choice, making prey availability a function of the
predator, not simply the prey and environment, unless one assumes
that the diet follows actual availability. My study focused on
response to abundance changes, and caloric data were not
estimated, so this method was not used.
Given the time and methodological limitations of this study,
though, preliminary conclusions suggest that Pisaster giganteus
has compensated for the absence of Petaloconchus by feeding more
frequently on other sessile prey, keeping with the strategy of
maintaining a dependable energy source to offset the risk of not
capturing calorically more attractive prey. Further study should
support these findings.
LITERATURE CITED
Emlen, J.M. 1966. Time, energy, and risk in two species of
carnivorous gastropods. Doctoral thesis, University of
Washington, Seattle, WA.
Harrold, C. 1981. Feeding ecology of the asteroid Pisaster
giganteus in a kelp forest system: prey selection,
predator-prey interaction, and energetics. Doctoral thesis,
University of California, Santa Cruz, CA.
Herrlinger, T. 1983. The diet and predator-prey relationships of
the sea star Pycnopodia helianthoides from a central
California kelp forest. Doctoral thesis, San Jose State
University, San Jose, CA.
B. 1996. Bathymetric distribution of three species of kelp
Hunt,
forest gastropods (Trochidae: Tegula) in 1978 and 1996.
Unpublished student paper, Hopkins Marine Station, Pacific
Grove, CA.
V.S. 1961. Experimental ecology of the feeding of fishes,
Ivlev,
Yale University Press, New Haven, Connecticut, 302 pp.
Mauzey, K.P., C. Birkeland and P.K. Dayton. 1968. Feeding
behavior of the asteroids and escape responses of their
prey in the Puget Sound region. Ecology, 49:603-619.
Menge, B. 1972. Foraging strategy of a starfish in relation to
actual prey availability and environmental predictability.
Ecological Monographs, 42:25-50.
Menge, J. 1974. Prey selection and foraging period of the
predaceous rocky intertidal snail, Acanthina punctulata.
Oecologia (Berl.) 17, 293-316.
Nakashima, R. 1974. Asteroid predation in Monterey Bay. Senior
thesis, University of California, Santa Cruz, CA.
Sellers, R.G. 1977. The diets of four species of Calliostoma
(Gastropoda, Trochidae) and some aspects of their
distribution within a kelp bed. Master's thesis, Stanford
University, Stanford, CA.
Stephens, D.W. and J.R. Krebs. 1986. Foraging Theory. Princeton
University Press, Princeton, New Jersey, 247 pp.
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.
West,
L. 1986. Interindividual variation in prey selection by the
snail Nucella (Thais) emarginata. Ecology, 67(3):798-809.
1988. Prey selection by the tropical snail Thais melones; a
study of interindividual variation. Ecology, 69 (6):1839-
1854.
Table 1: Abundance of prey species compared to Harrold (1981).
Numbers given as mean i standard error. Calliostoma numbers from
Sellers (1977).
All numbers normalized to 1 m?. Previous numbers
based on N=21 for
Tegula, N=ll for Serpulorbis, N=13 for Balanus,
N=100 for Calliostoma, N=14 for polychaetes and Petaloconchus, and
N=15 for others.
NS-not significant. See Materials and Methods
for 1996 N values.
1996 (per m)
Prey Species
1981 (per m
t-statistic
8.8 + 2.6
T. pulligo
9.2 1
5.4
U.0658 NS

T. montereyi
3.4 + 1.O
2.84 1 1.
U.327 NS
T. brunnea
2.6 + 1.6
1.52 + U.8
O.614 NS
O.057 + O.0I3
Astraea
39 NS
0.09 + 0.05
0.
Mitra
0.348 + 0.04I
0.42 + 0.09
0.802 NS
Balanus spp.
0.67 + 0.32
0.64 + 0.44
O.O55 NS
Serpulorbis
2.148 + 0.376
1.17 + 0.22
1.765 NS
Polychaete worms
6.255 + 1.55
1.59 NS
2.55 + 1.795

c. ligatum
1.I5 NS
14.52 + 3.2
IU.12 + 1.64

Ceratostoma
0.119 + 0.045
811 NS
0.02 + 0.0
Conus
0.014 + 0.0076
972 N
0.04 + 0.03
Petaloconchus
18.83 1 6.35
J.081 (p80.0I)
Table 2: Relative abundance of prey species in the diet and in the
habitat. Petaloconchus and polychaete percent cover measurements
were used for relative
abundance.
Relative abundance of
individually counted species was calculated separately. Nassarius
abundance numbers were not available. Numbers in parentheses are
actual numbers of observations,
Prey Species
Diet
Habitat
Tegula pulligo
7.585 (5)
-0.559
26.8.
eula montereyi
10.
4.555 [3
-0.390
7.93
Tegula brunnea
-1.0
3.035 [2)
Astraea gibberosa
U.17
0.894
Mitra ae
5 (I)
1.065
O.I7
28.795 (19)
Balanus spp.
2.045
U.868

22.14
25 (15
6.5
Serpulorbis squamigerus
U.552
153 (I0)
Polychaete worms
6.2
0.416
44.2
Calliostoma ligatum
6.065 (4)
-0.759
Ceratostoma toliatum
3.035 [2)
0.365
0.788
onus calornica
0.435
-1.0
Petaloconchus montereyensis
05
N/A
Nassarius mendicus
1.515 (I)
N/A
N/A

Bivalve
1.515 (I)
N/A
N/A
2 137
Undentified
N/A
N/A
Table 3: Relative abundance of prey in the diet and in the habitat
at the time of the previous study (Harrold, unpublished data).
Nassarius mendicus was not included in the proportional abundance
in the habitat for better comparison with recent data, since
Nassarius abundance was not measured in the present study,
Prey Species
Diet
Habitat

5.775 (I5)
Tegula pulligo
34.775
-0.71
10.7
Tegula montereyi
-0.512
3.465 191
ula brunnea
15 6)
5.90
2.
-0.4.

Astiea gibberosa
2.318 16)
U.345
0.743
Mitra dae
1.593
5 (I)
-0.610
Balanus spp.
14.65 (38)
2.42
0.716
serpulorbis squamigerus
10.85 [28)
4.425
0.419
Polychaete worms
11.165 (29)
2.55
U.634
Calliostoma ligatum
2.695 77
38.2
-0.869
Ceratostoma toliatum
-0.610
3.085 (8)
O.085
Conus caliornca
1.155 (3
-0.135
0.705
Petaloconchus montereyensis
27.75 72)
0.I
18.85

4.23
(II
Nassarius mendicus
N/A
N/A
other
10.45 (277
N7A
N/A
Figure 1: The location of the study site at Hopkins Marine
Station. The hatched area marks the kelp canopy, and the dark
square marks the study site.
Figure 2: Relative frequencies of prey species in the diet of
Pisaster giganteus.
The most frequently eaten prey were Balanus
spp., Serpulorbis squamigerus and polychaete worms.
Percentages
rounded to nearest whole number.
Other" includes Nassarius
mendicus and a mussel.
Figure 3: Changes in diet composition, shown as relative frequengy
in 1996 minus relative frequency in Harrold (1981). The largest
differences in the diet between Harrold (1981) and the results of
my study were in the proportions of Petaloconchus, Serpulorbis,
Balanus and polychaete worms.
Figure 4: Changes in diet compared to changes in abundance for
species with the largest observed dietary change,
Changes in
abundance calculated as relative abundance in 1996 minus relative
abundance in Harrold (1981).
Serpulorbis and Balanus increased
more in the diet than in the habitat, while the increases of
polychaetes and Petaloconchus matched the abundance changes,
Figure 5: Electivity index comparison of several species (Harrold,
unpublished data).
Balanus and Serpulorbis electivity indices
increased, as expected from data in Figure 4.
4

Son Francisco
L
L
Santo Cruz
PMoss Londing

Pacific Grove

Monterey
Hopkins Marine Station
50m,
Pacific Grove, CA
cable
g

kelp
eee
study
A
111
00
site
a

gs






8


—o


19


.



e

1
Figur
Unidentified
Other
Ceratostoma
foliatum
3%
Calliostoma
ligatum
6%
Polychaetes
15%
Serpulorbis
squamigerus
22%
T. pulligo
8%

Figure 2
T. montereyi
Astraea gibberosa
3%
Mitra idae
2%
Balanus spp.
28%
19
ane
Nastarius mendicus
Ceratquetoma foliatum
Callipstoma ligatum
Polychaetes
Serpulorbis
squamigerus
Balanus spp.
Mitra idae
Astaea gibberosa
T. montereyi
T. brunnda
T. pulligo
-10
Frequency Change (%)
Figure 3
20
15
5
10
-15
-20
-30 1


2
Prey Species
Figure 4

Diet
c Habitat
0.8
0.6
0.4
0.2
02
-0.4
-0.6
-0.8
1
1




Prey Species
Figure 5
Q1981
21996
22