J. T. Jensen/2
Two snails abundant on rocky shores in the midtide zone along
the California coast are Tegula funebralis A. Adams 1853 and Littorina
scutulata Gould 1849. Their large size, numbers, and easy avail-
ability have made the adults of T. funebralis attractive to research-
ers, and much work has been done on them (see various papers in The
Veliger, volume 6 (supplement) 1964, and Abbott and Haderlie, 1979).
The juveniles of T. funebralis have received comparatively little
study. Littorina scutulata has also received attention from previ-
ous researchers (see Abbott and Haderlie, 1979), and more is known
of the earlier developmental stages here, but many aspects of the
natural history are still incompletely known.
No previous studies have focused primarily on the juveniles
of either species, particularily on their relationship to one an¬
other in the middle intertidal zone community dominated by the red
algae Gigartina papillata (C./Agardh) J. Agardh 1846 and Rhodoglossum
affine (Harvey) Kylin 1928. The food web of this midtide "forest
is complex, and the competitive interactions between two species
of herbivorous gastropods are difficult to demonstrate. However,
a study of the distribution, activity patterns, and food habits of
the juveniles of Tegula funebralis and Littorina scutulata does re¬
veal a great deal concerning the manner in which resources are par¬
titioned between them.
Distribution and Movements
Juvenile snails of both species were more or less arbitrarily
defined as those individuals measuring 4 mm or less in maximum
J. T. Jensen/3
linear shell dimension. Juveniles, particularily those of Tegula
funebralis, differ from the adults in more ways than mere size, but
when specimens are arranged in a series of graded shell size identi¬
fication presents no problems.
All studies were conducted on Mussel Point, in Pacific Grove,
California, adjacent to the Hopkins Marine Station, in a region
protected from strong wave action. Sites chosen displayed a moder¬
ate (approximately 45 degree) slope showing marked vertical zona¬
tion and a diversity of habitats including bare rock, rock covered
with barnacles or algae, and a number of small pools and cracks
where sand could accumulate. Atransect was extended from the water
to the highest point above the water in each of four study areas.
Two 25 cm“ samples were taken at each 0.5 foot increment in height
above mean lower low water (MLLW), beginning at approximately the
0.0 foot level. Absolute height was determined by comparison to
an existing surveyed benchmark nearby. To assure random sampling,
the 25 cme quadrat was placed a variable distance from the midline
of the transect on the basis of random numbers. All Littorina and
Tegula were inspected in each quadrat. Holdfasts and fronds of al¬
gae were removed from the quadrat and examined separately. Field
records were made of the habitats occupied by animals as they were
collected. With the exception of bare surfaces, and other situations
where accurate counts could be made in the field, samples were bag¬
ged and examined in the laboratory with the aid of a dissecting
microscope.
J. T. Jensen/4
The areas sampled showed consistent trends, justifying the pool¬
ing of results. Data from all four transects are shown together in
Figure 1. With respect to vertical zonation, juvenile Tegula funebralis
occured predominantly below the 4 foot level above MLLW, while juv¬
enile Littorina scutulata were found mainly above the 3 foot level.
The greatest overlap of the two populations occured 3-4 feet above
MLLW. Plotting the distribution according to nature of the specific
habitat and substrate where the snails were found reveals another
important trend..Young Tegula funebralis occur chiefly in sandy hold¬
fasts of the common macroalgae and in rocky crevices, while Littorina
scutulata are more frequently on the fronds of macroalgae and in
empty barnacle shells.
The data on small Tegula funebralis complement existing inform¬
ation on the distribution of the species. Wara and Wright (1964),
working with the occurence at lowtide of specimens over 13 mm in
length, found that the population density decreased as the amount
of algal cover on the substrate increased. A decrease in the mean
size of the animals with increasing tidal height was also noted.
In the present study the adults were found lower down and on relativ¬
ely bare rock surfaces, whereas the juveniles occured in the sandy
holdfasts of macroalgae. This seperation should tend to reduce the
level of competitive interactions between different stages in the
life history, probably important in a species in which adults may
live as long as 30 years (Darby, 1964).
Qualitative observations made during the present study revealed
J. T. Jensen/5
additional information. Juvenile Tegula funebralis were often found
in tidepools along with adults, but they were commonly on the under¬
sides of rocks in the pools — areas where the larger adults cuuld
not take refuge. As Tegula grow larger they assume the adult habit,
and specimens over 6 mm routinely occur in the same crevices as the
larger animals.
The studies of Chow (1975), on the vertical distribution of the
Littorina scutulata population at Bodega Head showed that individual
size tends to increase with increasing vertical height in the inter¬
tidal zone. Juvenile (1-4 mm) snails, however, were noted to occur
over the entire vertical range. Both the vertical range of the pop¬
ulation (3.0-12.0 feet above MLLW) and vertical position of the peak
in population density (6.2 feet) in Chow's study site were higher
than those observed at Mussel Point. The difference in distribution
is probably related to differences in degree of wave action; the
earlier work reflects distribution on an exposed coast where wave
action extends zonation vertically, whereas the study sites selected
at Mussel Point were relatively protected from strong surf.
Even where Tegula funebralis and Littorina scutulata juveniles
occur in the same area, the observed substrate preferences tend to
place them in distinct subhabitats. For example, in the present
study, on any particular Rhodoglossum affine plant the small L.
scutulata are more commonly found on the fronds, while young T.
funebralis more often inhabit the sandy holdfast. Thus, subhabitat
partitioning and zonation sharply limit the possible interactions
between juveniles of the two species.
Activity
J. T. Jensen/6
Activity
Although it was not difficult to establish the subhabitat
partitioning in the low tide distribution of juvenile Tegula funebralis
and Littorina scutulata, it was desirable to supplement this with
information on activity and movements of the animals, and on their
distribution at high tide. Even in a region of moderate surf, it
proved difficult to observe the positions and movements of 4 mm snails
in the field over complete tidal cycles.
The following experiments were peformed under laboratory con¬
ditions. In an aquarium provided with a constant inflow of seawater
at 14-15° C, a clock was used to control water level. The tip of
the outflow hose of the aquarium was attached to a rod extending
from the hour hand of the clock (Figure 2). The twelve hour period
of the clock gave two "high", and two "low" tides in 24 hours, ap¬
proximating the natural tidal cycle.
To simulate the natural habitat, a rock was collected from the
intertidalin an area common to both species, and placed in the
aquarium. This rock could be visually divided into three zones
(high, medium, and low) each forming a belt 5 cm wide and each with
approximately the same proportion of Gigartina papillata and bare
surface. A vertical crevice ran down one side of the rock. The
connection of the clock to the outflow hose was adjusted so that the
artificial tide covered the rock completely at high water and exposed
it completely at low water. A skylight in the laboratory provided
a natural lighting regime.
J. T. Jensen/7
To increase contrast between snails and background, the exper-
imental animals were marked with fingernail polish, a different
color being used for each species. Twenty marked juveniles of each
species were selected, placed on top of the rock in the aquarium,
and allowed to move about and acclimate for 24 hours. Thereafter,
observations were made at two hour intervals over a 24 hour tidal
cycle. The number of snails in each vertical zone on the rock (high,
medium, low) and on each type of substrate (Gigartina papillata
fronds, the lower frond/holdfast area, the rock surface, or the
rock crevice) were recorded for each species at every reading. To
avoid disturbing the animals, a dim red light was used to make ob¬
servations at night. Later, the same experiment was repeated, but
with the tidal clock shifted by six hours, and the tidal regime start¬
ed 24 hours before records of movement began. This yielded data
under a different relationship of the diurnal and tidal cycles.
The results are shown in Figure 3. As regards vertical zo¬
nation on the rock, the Littorina scutulata population was found
to occur higher than the Tegula funebralis population under all
conditions of light and tide. The greatest overlap between the
populations occurred on the middle region of the rock. The
T. funebralis juveniles, however, displayed greater vertical
mobility. Individuals often moved to the high surface of the
rock as the water rose, and returned to the lower regions as it
fell. When the receding tide occurred at night, fewer snails
moved down, and as a result were left high on the rock at the mor-
ning low tide. The large number of Tegula low on the rock at
J. T. Jensen/8
high tide in the dark is contrary to the general trend. Most of
these snails, however, were active on the rock surface, and were
observed moving up the rock. The L. scutulata juveniles showed
a slight tendency to move downward on the rock during daylight
falling tides. Snails of both species were observed moving away
from direct sunlight when completely submerged during a daylight
high tide.
Examination of the data with respect to the specific sub¬
strates on which the animals were found showed other significant
relationships. Littorina scutulata juveniles tended to remain
on Gigartina papillata, moving to the tips of fronds when covered
by water, while young Tegula funebralis were observed predomi-
nantly on the rock surface, retreating to the crevice, rock bot-
tom and G. papillata holdfasts at low tide.
The separation observed under laboratory conditions corre¬
lates well with data on zonation and subhabitat partitioning pre¬
sented in Figure 1. The trend towards occupation of a higher zone
by young Littorina scutulata and a lower zone by Tegula fune¬
bralis juveniles remains despite compression of the six foot ver¬
tical range observed in the field into a 15 cm range in the lab¬
oratory tank.
The differences in the habitats of the two species observed
in the field at low tide persist through the tidal cycle; Tegula
funebralis juveniles continue to be found mainly on the rock sur-
face and Gigartina papillata holdfasts, and young Littorina scu¬
tulata predominate on the G. papillata fronds. This continued
occupancy of distinct subhabitats supports the idea of a real re¬
J. T. Jensen/9
source partitioning between juveniles of the two species.
Daniels (1978) studied the activity patterns and movements of
adult Tegula funebralis under field and laboratory conditions. He
found that population movement corresponded to the tidal cycle.
However, in the absence of tidal fluctuations (as in outdoor aquaria
kept continuously full of water) movement occured according to a
diel cycle, with the animals moving up rock surfaces at night, and
down to shaded areas in the day. He also observed adult T. funebralis
occasionally left high and dry in the field after a receeding tide
at night, and concluded that light somehow reinforces the downward
response of the animals. The results of the present study indicate
that juvenile T. funebralis generally show these same tendencies,
but the phototaxic responses of both adults and juveniles need fur¬
ther investigation.
Food Habit
Juvenile snails of both species were collected from the field.
The substrate on which they were found was noted, and the animals
were immediately preserved in 102formalin. Most collections were
made in the early morning hours, with the animals still wet from
the previous high tide. The stomachs of the animals were full.
The shells were removed, and the stomachs excised under a dis¬
secting microscope. The gut contents were sepérated from tissue,
mounted on glass slides in glycerol, and examined at 400 X. Dr.
Isabella A. Abbott and Mr. William Magruder, both of the Hopkins
Marine Station, assisted in the identification of possible food items.
Gut contents were grouped into five general catogories; detritus
J. T. Jensen/10
(defined as organic material of unidentifiable origin), green algae,
dinoflagellates, and other (miscellaneous).
After proficiency in the recognition of these catogories was
established, the stomach contents of ten individuals of each species
were scanned critically. Relative abundance of the different con¬
stituents, as a percentage of the total biomass of the contents,
was visually estimated.
Figure 4 shows the means and ranges of the catogories for the
gut contents of the two species. Both contained a high percentage
of detritus. Littorina scutulata juveniles contained more green
algae and diatoms than did juvenile Tegula funebralis, and the latter
showed many small brown dinoflagellates lacking in the gut of L.
scutulata. A few cells from encrusting red algae, and an occasional
foraminiferan were found in the stomachs of small T. funebralis.
The high concentration of detritus in the stomachs contents of
both species suggests that both feed in a similar and relatively
non-selective manner. The higher percentage of green algae and diatoms
consumed by young Littorina scutulata may possibly represent inci¬
dental ingestion of epiphytes, for these are found on the macroalgal
fronds frequented by the snails. Isaw no evidence of feeding on
any of the larger macroalgae, such as Gigartina papillata or Rhodoglossum
affine.
Comparison of my results with the findings of Best (1964) on
the food habits of adult Tegula funebralis indicate that the diets
of adults and juveniles are quite different. Best found the adults
fed on a variety of macroalgae and microalgae, with detritus contrib¬
J. T. Jensen/11
uting little bulk. The amount of detritus consumed by juveniles in
the present study again suggests a partitioning of resources between
different stages in the life history.
The investigations of Dahl (1964) and Foster (1964) on the
microscopic and macroscopic food sources of Littorina scutulata
show that macroalgae, particularily Pelvetia fastigiata Setchell
and Gardner 1917 and Cladaphora trichotoma (C. A. Agardh) Kützing,
were prefered food sources of snails in the laboratory, although
green algae and diatoms contribute significantly. No indication of
feeding on detritus was noted, however, the stomach contents of
animals freshly collected from the field were not examined. Just
as in Tegula funebralis, this difference in food habits between juv-
eniles and adults may enable larger populations to be supported.
Despite the similarity of the gut contents in juveniles of
Tegula funebralis and Littorina scutulata, it appears likely that
there is little competition for food resources between the two species.
Both field and laboratory studies indicate that they forage in rel¬
atively distinct subhabitats.
Summary
Field studies of distribution of juveniles (4 mm or less in
greatest shell dimension) of Tegula funebralis and Littorina scutulata
indicate they occupy largely sepérate subhabitats. T. funebralis
juveniles occur predominantly in the sandy holdfasts of macroalgae
and in rock crevices, less than 4.0 feet above mean lower low water.
Littorina scutulata juveniles are found mainly on the fronds of macro¬
algae and in empty barnacle shells more than 3.0 feet above MLLW.
J. T. Jensen/12
The greatest overlap between the populations occurs between the 3.0
and 4.0 foot tidal levels. Laboratory experiments in aquaria with
artificial tides demonstrate a similar partitioning.
Juveniles of both species appear to be mainly relatively non¬
selective detritivores. Young Littorina scutulata consume more green
algae and diatoms, but this may reflect incidental ingestion of
epiphytes. While the two species consume roughly the same food,
they obtain it in different places, somcompetition appears slight.
Acknowledgments
I wish to thank the entire instructional staff of the Hopkins
Marine Station for their help and encouragement throughout the course
of this study. I am particularily indebted to Dr. Donald P. Abbott
for helpful advice and guidance, and for the careful critisism of
this manuscript.
J. T. Jensen/13
Literature Cited
Abbott, Donald Putnam, and Eugene Clinton Haderlie
1979. Prosobranchia:
Marine Snails, in Morris, R., D.
P. Abbott, and E.CC. Haderlie (eds.), Intertidal Invertebrates
of the California Coast. Stanford, Ca., Stanford University Press:
(in press)
Best, Barbara A.
1964. Feeding Activities of Tegula funebalis (Mollusca: Gastropda)
The Veliger 6 (supplement): 42-45
(November 15, 1964)
Victor
Chow,
1975. The Importance of Size in the Intertidal Distribution of
Littorina scutulata (Gastropoda: Prosobranchia)
The Veliger 18 (1): 69-77
(July 1, 1975)
Dahl, Arthur L.
1964. Macroscopic Foods of Littorina planaxis Philippi and Littorina
scutulata Gould (Gastropoda: Prosobranchia)
The Veliger 7 (2): 139-143
(October 1, 1964)
Daniels, Matthew
1978 (spring). Rhythmic Movement of the Marine Snail Tegula funebralis
(Prosobranchia: Trochacea) On Intertidal Rocks in Pelation to
Tidal and Diel Cycles. Unpuplished student report on file at
the Hopkins Marine Station Library, 12 pages
Darby, Richard L.
1964. On Growth and Longevity in Tegula funebralis
(Mollusca: Gastropoda)
The Veliger 6 (supplement): 6-7
(November 15, 1964)
Foster, Michael S.
1964. Microscopic Algal Food of Littorina planaxis Philippi and
Littorina scutulata Gould (Gastropoda: Prosobranchia)
The Veliger 7 (2): 149-152
(October 1,1964)
Wara, William M. and Benjamin B. Wright
1964. The Distribution and Movement of Tegula funebralis in the
Intertidal Region, Monterey Bay, California (Mollusca: Gastropoda)
The Veliger 6 (supplement): 30-37
(November 15, 1964)
J. T. Jensen/14
Figure Captions
Figure 1: Low tide distribution of juvenile Tegula funebralis (white
bars) and Littorina scutulata (black bars). Letters refer to the
following substrates:
A. Sandy holdfasts of Rhodoglossum affine
B. Sandy holdfasts of Gigartina papillata
C. Rock crevices
D. Sandy holdfasts of Gigartina agardhii Setchell and Gardner 1933
E. Crustose red algae
F. Sand
G. Sandy holdfasts of Endocladia muricata (Post and Rupr.) J. Agardh 1847
H. Sandy holdfasts of Gelidium sp.
I. Rocky holdfasts of Gigartina papillata
J. Sandy holdfasts of Gigartina leptorhynchos J. Agardh 1885
K. Holdfasts of Iridaea sp.
L. Tidepools
M. Crustose coralline algae
N. Fronds of Iridaea sp.
0. Fronds of Rhodoglossum affine
P. Pelvetia fastigiata
0. Rocky holdfasts of Endocladia muricata
R. Fronds of Gigartina agardhii
S. Fronds of Endocladia muricata
T. Bare Rock
U. Empty barnåcle shells
V. Fronds of Gigartina papillata
J. T. Jensen/15
Figure 2: The setup used to simulate tidal conditions inthe lab.
Hour hand of clock (A) raises and lowers outflow hose of aquarium (B)
as the time changes. Seawater inflow into tank (C) is constant.
Rock simulating natural habitat (D) and its supporting stand (E).
Figure 3: Vertical position and substrate of juvenile Tegula funebralis
and Littorina scutulata under different conditions of light and tide
in laboratory aquaria. The percent of the populatios observed on
the different substrates and positions of snails on the rock are in¬
dicated in the bars. Horizontal rows are arranged in order of great¬
est number of snails on high zone of rock at high tide. The arrows
refer to the predominant movement of the populations during the two
readings previous to each condition, the left hand arrow in each
righ
column refering to T. funebralis, the left to L. scutulata. Where
no arrow appears, vertical shifts in the populations of snails ap¬
proached zero. With respect to vertical position, the midline rep¬
resents the middle of the rock The midline in the substrate columns
divides snails on Gigartina papillata (above the line) from those
on rock substrates.
Figure 4: Diet of juvenile Tegula funebralis and Littorina scutulata
as percentage of the total biomass of stomach contents. Results shown
are averages based on ten snails, with the ranges given in parentheses.
Hort
oo
o
Figu 1.
NUMBER OF SNAILS
2 o 9
i


E


L
*
oL
I
SE
71
+

—
o

t

5

VE
1
5
L

oo
oooo
oo
oo
oo



oo
MTM HAOSV LHDIAH
V.Feesen
FIGURE 2
—
-



—
Fp.
2

2-E







B
I I. Jensen
p.17
DUSK
DAY
DAWN
DARK
FIGURE 3
HIGH TIDE
Position
Substrate
T.f. L.s.
T.f. L.s.

4
%
39
Z

12
N240
23
21
N240

N23
N=38
70
12
Zo
E
30
42
33 N=17
V277

37
V=19
N211
75
65
77
tag.
Aa
33
50
Vzzo
46
Nzo

22
N=18
N218
58
63
20

10 31
4
30 w-1
N-9

N=20
N=10
Vertical
KEY
Position on
Rock
high
medium
low

LOW TIDE
Position
T.f. L.s.
70


N220
Nels
L3
Vg

N=19
N220
61
20
215.
210
45
No
N240
75
3S
50 =20

N20
rocl
surface
J. 7. Jensen
p. 18
Substrate
T.f. L.s.
7o
40
0.
7.
2.5
60
V210

N212
47

19

32.
15
V219
L
N-20
38
10

42
35
35
V240
N240
—
57
5

26

20
45 N=20

N220
Substrate
Gigartina papillata fronds
lower fronds/holdfast
rock crevice/bottom
FIGURE 4
Tegula funebralis

Detritus 707
(60-75)
Littorina scutulata
Detritus 607
(55-70)
J. T.
Other 107
Green Algae 107
(5-20)
Dinoflagellates 102
(5-30)
Other 32
Diatoms 122
(5-30)
Green algae 257
(15-40)
Jensen
.14