Mary G. Mooers -
Diet and Reproductive Biology of the
Rocky Intertidal Prosobranch Gastropod
icolia pulloides
Although Tricolia pulloides (Carpenter, 1865) is a common inhabi¬
tant in the intertidal region on California rocky shores, it is often
overlooked, probably due to its small size. Very little is known of
its natural history. A related British species, Tricolia pullus (Linnaeus,
758) has been extensively studied. T. pullus feeds on diatoms (Fretter
& Graham, 1962); it is dioecious and sheds its eggs singly into water
where they develop into free-swimming trochophore larvae (Lebour, 1937;
Fretter, 1955).
The present study of Tricolia pulloides was conducted on the popu¬
lation in the midtide zone dominated by the red algae Rhodoglossum
gan
ine (Harv.) Kyl. and Gigaltina papillata (C. Ag.) J. Ag. at Mussel
Point, Pacific Grove, California, in the period April to June, 1979.
The objectives of the investigation were twofold. The first was to
characterize the diet of T. pulloides, answering the questions: What
is it eating? Where is it obtaining food? How is it obtaining food?
When is it eating? The second objective was to characterize the repro¬
ductive biology of T. pulloides by studying the adult population, the
egg masses deposited by females.
and the gross sequence of events in
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egg development.
Food Studies
Material
& Methods To determine what Tricolia pulloides was
eating and where the food was obtained, 5 snails were collected from
each of 5 species of red algae: Gigartina papillata, Gigartina lepto-
rhynchos J. Ag., Ga
troclonium coulteri (Harv.) Kyl., Rhodoglossum
affine, and
siphonia woodii (J. Ag.) J. Ag. All collections were
made at or below the 1.5 ft. tidal level. The larger, more conspicuous
snails were collected, without scrutinizing the algae for juvenile
individuals. Each sample was placed directly in 5% formalin. The
stomach of each snail was dissected out, and the contents placed on a
slide in a drop of 30% corn syrup under a cover slip for viewing under
a compound microscope. Stomach contents were identified with the
assistance of Dr. Isabella Abbott. The large pennate diatoms present
in the preparations were counted; the relative abundance of other materials
was estimated.
To determine when Tricolia pulloides was eating over the tidal
cycle, hourly samples, each of 5 snails, were collected from a hori¬
zontal rock face covered with Rhodoglossum affine, about 0.5 ft above
zero tide level. Samples were taken over a seven hour period. The
samples were preserved and gut contents analyzed as before.
Radulae were prepared by removing the buccal mass and teasing free
the radulæein sea water under the dissecting microscope. To remove any
excess tissue, radulae were dipped in 100% HCl for 5 - 15 seconds and
immediately placed in seawater, where any excess tissue was teased away.
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Some radulae were then dipped in 70% alcohol and allowed to dry thoroughly
for scanning electron microscopy; they were watched under a dissecting
microscope as they dried, for radulae tend to curl and fold as they
dry, minimizing good viewing areas. Other radulae were placed in a
drop of water on a clean glass slide, for viewing under a compound
microscope.
Results Between 90 and 95% of the contents of stomachs of snails
from all five types of algae consisted of pennate diatoms. Five to ten
percent of the gut contents consisted of small sponge spicules, uniden¬
tifiable organic matter termed detritus, and small pieces of epiphytic
algae including coccoid blue-greens, Dermocarpa sp., Collinsiella
tuberculata S. & G., and Florideophycids. No one of these catagories
comprised more than 1 - 2% of the total diet. The diatoms and other
algae in the gut were browsed as epiphytes on the surfaces of macro¬
algae. T. pulloides was never found feeding on encrusting algae, rocks,
or exposed surfaces.
Figure I shows the results of sampling the gut contents of the
ricolia population over a seven hour period centering on low tide.
Quantity of food in the gut is shown in terms of number of large diatoms
counted in squashes of the stomachs of the 5 snails sampled each hour.
The population showed great variability, and the differences between
sample means are not significant, but diatoms were most abundant in the
stomachs at low tide.
Scanning electron micrographs of the radulae of T
colia pulloides
confirm the observations of Tryon (1888) in the original species
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Mary G. Mooers
description. The central or rachidian tooth is curved, smooth, and
(Piates1,2)
without cusps, overlapping the base of the first medial tooth.A The
latter has a central cusp and denticles to both sides. This appears to
overlap and buttress the second medial. Similårly, the second medial
has a large central cusp with denticles to both sides. Third and fourth
medials have denticles to the lateral side of one large pointed cusp.
Thirty-seven lateral teeth extend out to the side; they are heavily
(Plate 3)
denticulated and overlapping.A The radulae of T. pulloides appears to
be a composite of the radulae of T. pullus and T. compta (Robertson,
1955).
Discussion The diet studies show that Tricolia pulloides skims
the surface of macroalgae taking very largely pennate diatoms. Chun
(1979) has noted large populations of sessile pennate diatoms on
ungrazed, distal areas of fronds of Rhodoglossum affine, and noted a
significant reduction in these populations through grazing by T. pul
loides and other small herbivorous gastropods. Further, when submerged
at high tide, T. pulloides tends to move out to these distal areas of
algal fronds (Foster, 1979).. The 5 - 10% of the diet not consisting of
diatoms can probably be considered incidental. Particulate matter and
epiphytic algae also occur in the areas browsed for diatoms and may be
rasped up unintentionally. No single component comprises a large
portion of the incidentals.
Although Figure 1 shows more food in the stomach of T. pulloides
at the lowest tidallevel, the differences in mean gut content shown are
not significant. Some food was found in the stomachs at each sampling
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time, showing at least that food is being kept in the gut over a period
when the snails are exposed, out of water.
Reproduc
ive Biology
Materials and Methods From the snails collected during diet
experiments, data were taken on (1) the shell length from tip of spire
to base of aperture, (2) the sex of the snail based on examination of
the gonads, and (3), if the snails were female, the number of mature
eggs within the ovary. Sexes are easily determined. In females the
large yellow eggs are borne in a translucent gonad, and in males the
gonads are mottled and white. In the smallest animals a piece of the
gonad was viewed under the compound microscope for presence of developing
sperm or oocytes.
The egg masses of Tricolia pulloideg were first identified as
belonging to this species by noting the great similarity between the
large eggs in the ovaries of ripe females and the large eggs in certain
egg masses common on plants inhabited by T. pulloides. Observations of
egg laying and hatching in the lab confirmed the identification.
Yellow egg clutches were collected in the field from areas of dense
ricolia pulloides populations. Data were kept on the location of the
egg masses on the algal frond, the type of alga bearing the egg mass,
the diameter of the egg mass, and the number of eggs contained within
the egg mass. The algal fronds bearing masses were then maintained in
5-inch fingerbowls of seawater at about 14'C. The water was changed twice
daily. The samples were checked daily under a dissecting microscope for
stage of development.
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Mary G. Mooers
To see if Tricolia pulloides would lay egg clutches consistently
under laboratory conditions, 2 fingerbowls of 25 snails each were kept
with 6 - 8 clumps of Phodoglossum affine free of any previous egg
clutches and other organisms, at about 14'C. Water was changed daily,
and the R. affine fronds were checked daily for eggs. All new egg
masses found were measured, and the eggs were counted. The egg clutches
were then trimmed from the algal frond, placed in a depression slide
with a cover slip and examined under a compound microscope for develop¬
mental stage. The time and stage of development were noted. These egg
masses were then put in fresh seawater in fingerbowls and kept at 14°C.
The seawater was changed twice daily, and five selected clutches were
checked microscopically twice daily for general developmental features.
Three other clutches were checked every half hour each day, from the
1 - 16 cell stage to hatching, to determine accurately the time scale
of development under laboratory conditions.
Hatchlings from these clutches were kept on fronds of Rhodoglossum
affine under the same conditions in the laboratory as those used for
observation of clutches. The diet of the hatchlings was determined by
lightly squashing the bodies of five-day-old snails in 1 drop of 30%
corn syrup between a slide and coverslip and viewing them under a com¬
pound microscope. Hatchlings were best transferred individually by a
pipette.
Results Figure 2 shows a distribution by size and sex of the 55
individuals sampled from both diet studies. Juvenile animals are not
adequately represented in the sample considered, but for animals over
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Mary G. Mooers
2 mm long the sample is considered representative of the population.
The ratio of males to female is 1:1.2 indicating roughly equal fre¬
quency of sexes in the sample. Female snails averaged significantly
larger than the males (p°.001); the mean shell length for females was
3.14 mm (n = 28), and for males was 2.62 mm (n = 25). The smallest
female, at 2.0 mm, had immature oocytes in its ovary, but moving sperm
was found in the testis of the male at the same size.
The number of eggs counted in thebvary of a female ranged from
O to 206 eggs, with a mean of 56.5 for 30 females (Fig. 3). The number
of eggs in an ovary tends to increase with the size of the female, but
the variability at any particular size is great.
Fifty snails kept in fingerbowls in the laboratory at 14 C laid
57 egg clutches over a 6 day period. The number of eggs per clutch
ranged from 11 to 104 with a mean of 32.5. These eggs in newly laid
clutches appeared identical to those in the ovaries of mature female
ricolia pulloides. These eggs are light yellow, opaque, show a
gradient in distribution of yolk, and each is enclosed in a colorless
transparent capsule. The eggs and egg capsules are approximately
150 um and 180 um in diameter, respectively. Generally, the eggs are
laid in a compact, sinuous ribbon which is coated with a clear jelly.
The completed egg clutches are disc-shaped, average about 2.0 x 2.5 mm
in size, and are attached by one flat surface to a plant. They are
most generally found on a concave furface at the bifurcation of a
frond on such red algal species as Rhodoglossum affine, Gig
tina
apillata, and Gig
tina leptorhynchos, but some were found on Cryp¬
tosiphonia woodii which does not have a concave surface.
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The time schedule of development, based on external morphology of
the larvae, is shown in Figure 4. The time of fertilization is unknown.
The first 6 cleavage stages each lasted from 0.5 - 1 hour. A few eggs
showed abortive development; they became white and took on a granulated
appearance after a few hours. By 24 hours the larval shell appeared
pesterior
complete. At about this time the tissues at the end of the shell began
to turn light green. The soft body parts pull away from the interior
of the shell, anteriodorsally by 1.8 days, and soon portions are seen
extending from the aperture of the shell. Pedal cilia and the oper¬
culium were formed by 2.3 days. Within two hours muscular contraction
of the foot was seen. By 3.1 days the tentacles were present, and,
shortly thereafter, black eyespots appeared. Contraction of the body
into the shell and closure of the operculum were seen at 4.3 days. Soon
the embryos begin occasional rotations, crawling on the interior of the
transparent egg capsule. Frequency of rotation increased just prior to
hatching, and the jelly coating began to dissolve away about this time.
At 7 days the embryos hatched as miniature, young snails with nearly
clear, planospiral shells, with light green pigment concentrated in the
area of the visceral hump. The maximum diameter of the shell at hatching
is 240 um.
After hatching, the hatchlings move out onto the algal frond where
they can cling tenaciously from the very start. They are not swept away
by a jet of water forcefully extruded from an eyedropper pipette. Snails
knocked over maintain position by a mucus thread extruded by the foot.
Whole mounts of 5 day old snails crushed under a cover slip show that
the young are eating primarily small, naviculoid diatoms. By five days
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Mary G. Mooers
after hatching, shell aperature has widened with new shell growth. The
body tissues are still relatively clear except for the posterior parts
of the visceral hump which are yellow and darker green. New shell is
more darkly tinted and transluscent.
Discussion The size discrepancy between males and females is of
real interest. If size is mainly a function of age, and the fact that
males are smaller than the females, this suggests the possibility that
Tricolia pulloides is protandric and changes sex from male to female at a
certain age. The existence of the 2.0 mm long female tends to mefute
this. Another possibility is that females do indeed grow larger
(perhaps faster) than males, and the immature males and females occur
inmore secluded habitats than do the adults which were collected.
Validity of these ideas must be substantiated by further study.
Since most of the females examined contined more eggs than the
number found in the average clutch (32), and a large variability exists
in the number of large eggs in females of a particular length, it seems
likely that a female does not necessarily deposit all her eggs in one
clutch. Instead, she may lay two or more clutches for a given group
of mature oocytes.
ricolia pulloides appears to have many features suiting it for
laboratory studies. Egg clutches are easily obtained in the laboratory.
No special conditions were required to stimulate egg-laying. The egg
is large and yolky, and early development is quick and easy to follow.
Development from eggglaying (fertilization?) to hatching takes only
about 7 days at 14'C. At room temperatures, about 23°C, many eggs failed
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Mary G. Mooers
to develop or developed abnormally. Under intense light and moderate
heat, embryos were seen to rotate much more frequently within the egg
membrane. At low tide on warm days the egg clutches may sometimes be
exposed to increased temperature, possibly affecting mortality and
rate of development.
Tricolia pulloides develops directly and lacks swimming trochophore
and veliger stages. In contrast, the British T. pullus develops via
a free-swimming trochophore larvae (Lebour, 1937). Direct development,
bypassing a pelagic stage, is comparatively uncommon among prosobranchs
(Webber, 1977). The occurrence of direct development here may help
account for the somewhat patchy distribution of T. pulloides in the
intertidal. Where dispersal is probably mostly by creeping, local
populations may develop. Further investigation of this would be
desirable.
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Mary G. Mooers
Sur
1. The diet and reproductive biology of Tricolia pulloides was
investigated during the period April - June, 1979 at Mussel Point,
Pacific Grove, California.
2. T. pulloides feeds primarily on pennate diatoms grazed as epiphytes
from the surfaces of algal fronds; small particles of detritus and
pieces of other epiphytes are probably picked incidentally.
3. T. pulloides contains food in its gut at both high and low tide.
4. T. rulloidesshows roughly a 1:l ratio of males and females. Females,
averaging 3.14 mm in length, are very significantly larger than the
males, averaging 2.62 mm in length, though the smallest females are
much smaller than the largest males.
5. Females carry up to 206 and average 56.5 yellow yolky oocytes.
Up to 104 eggs, averaging 32.5 eggs per clutch, are laid in a
sinuous ribbon, coated with a clear jelly. Egg clutches are
located on algal fronds.
6. Eggs cleave spirally and develop directly within 7 days into clear,
miniature snails that feed on naviculoid diatoms and cling tenaciously
to algal fronds with the aid of mucus threads.
Acknowledgments
I would like to thank William Magruder and the faculty and staff of
Hopkins Marine Station for their unfailing assistance and copious humor
throughout this project. My deepest thanks go to Dr. Isabella A. Abbott
for her patient aid and expertise in identifying gut contents and
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Mary G. Mooers
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expecially to Dr. Donald P. Abbott and Dr. Isabella A. Abbott, both,
for their meticulous advice and foresight. Never could my excitement
be channeled so constructively without their aid.
5
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Mary G. Mooers
Literature Cited
Chun, Stephen A.
1979. The Effects of Grazing By the Prosobranch Gastropods,
Tricolia pulloides and Barleeia haliotiphila, on Epiphytic Micro-
algae Growing on the Red Alga, Bhodoglossum affine. Unpiblished
report on file at Hopkins Marine Station Library, Pacific Grove,
California, 93950
Foster, William K.
1979. Movement of Prosobranch Gastropod Tricolia pulloides on
Gigartina papillata. Unpulished report on file at Hopkins Marine
Station Library, Pacific Grove, California 93950
Fretter, Vera
1955. Some Observations on Tricolia pullus (L.) and Margarites
cinus (Fabricius). Proc. malacol. Soc. Lond. 31: 159 - 162;
cim
fig.
Fretter, Vera and Alastair Graham
1962. British Prosobranch Molluscs, Their Functional Anatomy and
Ecology. London, Ray Soc. xvi + 755 pp.; 317 text figs.
Lebour, Marie V.
1937. The eggs and Larvae of the British Prosobranchs with
Special Reference to those Living in the Plankton. Journ.Mar.
Biol. Assoc. UK. 22:105 - 166; 4 figs.
Robertson, Robert
1958. The Family Phasianellidae in the Western Atlantic. Johnsonia.
3 (37): 245 - 284; plts 136 - 148.
yon Jr., George W.
1888. Manual of Conchology. vol. x.;323 pp.;69 plts. Author,
Philadelphia.
Webber, Herbert H.
1977. Gastropoda: Prosobranchia. 1 - 114 pp.; 18 figs. Arthur
C. Giese and John S. Pierce. Reproduction of Marine Invertebrates.
vol 3; xii + 369 pp. Academic Press, New York
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Mary G. Mooers
Captions
Figure 1: Mean number and range of large diatoms in stomach of Tri¬
pulloides with repect to tidal cycle.
colia
Figure 2:
Number of females and number of males of Tricolia pulloides
with respect to size class distribution.
Figure 3:
Number of large eggs in the ovary of Tric
olia pulloides
with repsect to the length of the shell.
Figure 4: Development of Tricolia pulloides at 14 C. Solid bars show
exact beginning and end of a stage. Dotted lines indicate
approximate time certain features occurred. Exact beginning
and end of these could not be accurately determined. Times
are representative, and do not reflect individual variation.
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Plate 1:
Radulae of T. pulloides.
folded, with central portion
to the left.
Plate 2:
Magnification of central
portion of radulae.
Left - rachidian tooth
Central - medial teeth
Right - lateral teeth
Plate 3: Denticles of a lateral tooth of
Tricolia pulloides