MACROSCOPIC ALGAL FOODS OF
LITTORINA PLANAXIS AND L. SCUTULATA
by
Arthur Lyon Dahl
Hopkins Marine Station
Biology 175h
Dr. Blinks
May 30, 1964
MACROSCOPIC ALGAL FOODS OF
LITTORINA PLANAXIS AND L. SCUTULATA
The upper littoral periwinkles of the central
California coast are often found in association with
a number of the higher intertidal macroscopic algae.
This paper examines the place of these algae as a food
source for Littorina planaxis Philippi, one of the
highest intertidal animals, and for its somewhat lower
though overlapping relative L. scutulata Gould. The
studies and observations upon which this paper is based
were made on the Monterey Peninsula, on the central
California coast, especially at Mussel Point in the
vicinity of the Hopkins Marine Station, and near Pescadero
Point. The coastline in these areas consists principally
of granite boulders and outcroppings interspersed with
sandy beaches, and heavily overgrown with larger algae
through most of the littoral zone. Observations were
made during May, 1964, and therefore do not reflect any
seasonal fluctuations that might take place.
Despite the fact that both Littorina planaxis and
L. scutulata are a conspicuous part of the upper littoral
fauna over much of the Pacific coast of North America,
(2)
almost nothing has been published about them up to now,
and while the eastern U.S. and European littorines are
generally better known, information on food and feeding
patterns is very scarce. North (1954) analyzed size
distribution, erosive activities, and gross metabolic
efficiency of both L. planaxis and L. scutulata.
Castenholz (1961) used L. scutulata as well as Acmaea
spp. in his studies of grazing effects on diatom populations.
From his observations, he assumed "that the primary
food of these gastropods is diatom material," with
blue green algae the principal food in the "supra-littoral"
fringe. Ricketts and Calvin (1952) state that L. planaxis
feeds on detritus and microscopic plants scraped from
almost bare rock, and that "certain of the rockweeds
(Pelvetia or Fucus) serve the young periwinkles as a
sort of nursery, for it is on their fronds and stems
that the young will nearly always be found, "(p. 20),
an observation for which no evidence has been seen
during this study. Concerning other species, Newell
(1958) noted that Littorina littorea ate surface deposits
of diatoms and small algae, and also browsed on Ulva
and Enteromorpha.
The answers to three questions were sought in this
study. Can these snails eat macroscopic algae? Which
algae, if any, are normally part of their diet? How
important is macroscopic algae in their diets? A number
(3)
of approaches were used in the attempt to answer these
questions. Numerous field observations were made to
determine what algae the snails had access to, and if
they were ever in contact. Principal proof of feeding
was based on examinations of the stomach contents of
disected snails, from which ingested tissue fragments
could be compaired with scrapings from the algae on
which the snail was found, or on which it was believed
to have fed. In a few cases, feces were also examined
for recognizable algal fragments. Several types of
laboratory experiments were attempted to supplement
field observations. In a few instances it was sufficient
to place a winkle together with a dampened piece of
algae in a dish. Better results were obtained in an
aguarium the contents of which were kept wet but not
submerged by a fine sea water spray. An aquarium was
also rigged to duplicate tidal fluctuations, but in no
case were conditions similarenough to those in the field
to get normal responses for any length of time. North
(1954, p. 193) noted that food cycled completely through
the digestive tract in from 24 to 6 hours. Thus the
stomach contents of any snail kept for six hours with a
single type of algae, or starved for six hours and then
placed with an alga, could come only from that alga.
This provided a simple and reliable check for other
types of observations.
A number of difficulties were encountered in the
0

(4)
attempt to induce feeding in the laboratory. L. planaxis
has a strong tendency to crawl to the highest part of the
dish or aquarium, especially if placed in water.
Individuals of both species would frequently cease
activity if left undisturbed for more than 6 to 12 hours.
Natural conditions, especially with respect to tidal
and diurnal fluctuations, proved almost imposible to
duplicate, yet it would have been even more difficult
to keep track of individual snails in the field, or to
eliminate unwanted food sources there, especially the
ubiquitous GATGOR (Green Algae That Grows On Rocks, a
term applied to the microscopic flora. Work on this
aspect of littorine feeding has been done by Mr. Michael
Foster). Experimental animals had to be run singly in
experiments where starvation was a factor because of the
N
tendency of snails to feed on each others' backs.
Additional complications resulted from epiphytic algae
growing on the test specimins, and from the rapid decay
of certain types of algae, especially Iridophycus flaccidum
and Laminaria Andersonii, under conditions available
in the laboratory.
A number of criteria were set up by which feeding
on any particular alga could be substantiated with a
reasonable degree of certainty. At least two of these
three standards had to be met before a snail could be
considered to have "fed" on the alga in question. These
O
O
(5)
were as follows: (a) the snail was observed on the alga,
preferably actively moving, and with the radula active;
(b) whole pieces or fragments of tissue were found in
the stomach or feces exactly matching in cell size,
shape, structure, color, type of chloroplasts, etc.,
scrapings or known forms of the alga in question (usually
determined by direct comparison); (c) the animal was
starved (kept with no signs of feeding or no food) for
at least six hours prior to feeding or disection, and had
stomach contents not dissimilar to fragments of the
test alga.
Most of the observations of feeding on any particular
alga are based on only a small number of individuals
because of the time involved in making a positive
determination of feeding, and, in some cases, because
such feeding itself may occur only rarely. Some algae
are found over only a small part of the range of L.
planaxis or L. scutulata, or snails may feed on them
only under particularly favorable conditions. It was
assumed, however, that positive proof of feeding on an
alga, even if based on only one individual, would indicate
a species capability if not a preference, and that once
such a capability was demonstrated, snails observed on
such an alga were probably feeding on it.
Littorina planaxis is generally found in the upper
regions of the intertidal zone, on "bare" rocks normally
66
O
(6)
moistened only by splash. The height of this region
varies considerably with the amount of exposure to
surf. Littorina scutulata occurs somewhat lower, and
in more protected areas, ranging from about +2 feet up
to about +6 feet in some areas. It is frequently found
in moderate to heavy growths of algae, and smaller individuals
occur in large numbers in some horizontal Balanus beds.
The ranges of the two species of littorines overlap
considerably in some areas.
Both species are known to depend on microscopic
algae (diatoms, greens, and some blue greens) as a
principal food source (Castenholz, 1961; Foster, unpublished),
especially where macroscopic algae is scarce or absent,
as it is over much of the range of L. planaxis. However,

there are areas in which considerable macroscopic algae
is available, and it is with these areas that this study
is primarily concerned. A number of common algae occur
in the vicinity of littorine populations, especially
Cladophora trichotoma (C.A. Agardh) Kützing, Endocladia
muricata (Postels and Ruprecht) J.G. Agardh, Fucus
furcatus C.A. Agardh, Gigartina Agardhii Setchell and
Gardner, Gigartina cristata (Setchell) Setchell and
Gardner, Iridophycus flaccidum Setchell and Gardner,
Pelvetia fastigiata (J.G. Agardh) DeToni, Porphyra
perforata J.G. Agardh, Rhodoglossum affine (Harvey)
Cladophora is generally found
Kylin, and Ulva spp.
1. Nomenclature of algae follows G.M. Smith, Marine
Algae of the Monterey Peninsula California.
a?
(7)
in cracks, crevasses, and tidepools, on horizontal
surfaces, sometimes together with Ulva; Endocladia,
Rhodoglossum, and the gigartinas are frequently associated
together in crevasses and on rock surfaces, as well as
in scattered individual clusters; Pelvetia, Fucus, and
lridophycus are commonest on sloping surfaces at slightly
lower levels; Porphyra often covers the tops of boulders
and outcroppings.
One area, in which many of the field observations
included in this study were made, includes all of the
above species except Fucus, and in addition has large
populations of both L. planaxis and L. scutulata. It
is situated on a granite shelf west of Pescadero Point.
at a height of about +4 feet, and was divided for survey
purposes into two sections, each of approximately 2
square meters in area, The first is largely horizontal,
with a number of crevasses up to 10 cm deep, and a small
tidepool. Cladophora is the dominant alga, with some
Endocladia and Rhodoglossum also present: the algae
aree largely confined to the crevasses and around the
tidepool, leaving the elevated rock areas bare. The
second consists of the verticle west face of the shelf
and some of the contiguous horizontal top, about 2 meters
from the first area, and at the same level. It is almost
entirely covered with algae, with Endocladia, Rhodoglossum,
Pelvetia, and Gigartina cristata the most common species,
(8)
and the others present in lesser quantities. One survey
showed that the first area contained 147 L. planaxis
and 329 L. scutulata. Surveys were generally made in
the morning while the snails were still active from their
previous nights wetting, yet had had enough time to feed
adequately on the algae.
The feeding behavior of Littorina planaxis and
L. scutulata seems to be a combination of random activity
with preferential movement. In the laboratory, snails
are frequently observed scraping their radula along a clean
glass surface, and have been known to consume considerable
quantities of paraffin and powdered carbon. In the field,
the stomach contents of snails have revealed large
qunatities of sand and rock particles, detritus, any
number of algal types, and occasional small animals,
often still alive and active. There seems, therefore,
to be little or no selectivity in what is ingested from
any given surface. However, field studies to be discussed
later suggest that each species has, at least under
some conditions, definite preferences for certain types
of substrates or algae.
The radula of littorines seems capable of tearing
off quite large pieces of substrate material, and particles
of considerable size have been found in the stomachs.
Some of the smaller algae such as the filamentous
Cladophora and the small monostromatous Prasiola meridionalis
(9)
are frequently found almost entire on the stomach of
L. planaxis. The structure of the radula and other
mouth parts seems to be similar for both specias, and,
superficially at least, matched that of Littorina littorea
as described by Fretter and Graham (1962).
What the littorines digest out of what they take in
is still uncertain. Undigested cells and tissue fragments
are frequently found in the feces, supporting North (1954)
in his conclusion that their digestive efficiency is
low, with organic matter assimilation about 7 percent
of consumption. Cell wall material seems especially
conspicuous, at least in the stomach, and was often
useful in identifying the algae on which the snail had
fed.
Littorina planaxis does not frequently come into
contact with macroscopic algae; most of its range is
well above the limits of algal growth. Thereeare areas,
however, where it is found with all of the algae listed
previously. In such areas, it shows a decided preference
for a rock substrate (For the purposes of this study
the term "rock substrate" includes both microscopic
and encrusting algal forms; "algal substrate" refers
to macroscopic algae with other than an encrusting
thallus. The substrate is the surface to which the
snail is attached, and on which, presumably, it is
feeding). One survey of the area near Pescadero Point
(10)
showed only 2% of the L. planaxis on algal substrates;
for another check under more favorable conditions
(overcast sky), about 10% were observed on Cladophora.
One individual collected and disected had a considerable
quantity of Cladophora in its stomach and a filament
hanging out of its mouth, so feeding was almost certainly
taking place in these cases. L. planaxis has also been
collected in a few instances on the thallus of what is
either a high form of Rhodoglossum affine (the name used
in this paper), or a juvenile form of Gigartina cristata.
The stomachs of these snails contained a few fragments
of tissue closely resembling scrapings from the alga, but
in such small quantities as to be an insignificant part
of its diet. Snails collected from patches of Prasiola
meridionalis Setchell and Gardner on rocks covered with
bird guano at Mussel Point proved to have large quantities
of this alga, often whole thalli, in their stomachs and
feces, in addition to quantities of a microscopic
chlamydomonad. These are the only instances in the
present study in which L. planaxis has been found to eat
macroscopic algae under natural conditions. A number of
laboratory experiments suggest reasons for this behavior.
In one experiment, thalli of Endocladia, Rhodoglossum,
and Porphyra were finely chopped with a razor blade
and spread on the wet bottoms of petri dishes; dishes
with whole thalli, and a dish with a growth of GATGOR
(11)
served as controls. A single freshly-collected L.
planaxis was placed in each dish for 5 hours and then
disected. The snails consumed large quantities of
the chopped pieces whole, and also the GATGOR, while
the stomachs of the control winkles were empty, even
whentthey were attached to the algae. In an algal
preference test run in an aquarium kept damp by spray,
the few L. planaxis that did not crawl to the top of the
tank fed either on rock samples or on soft, decaying
pieces of Iridophycus flaccidum. There
a
There are several possible reasons why L. planaxis
does not normally eat much macroscopic algae: (a) it is
unable to "bite off" pieces of any but the softest of
most finely divided algae; (b) it for some reason will
not crawl on the "unstable" substrates provided by algae;
(c) most macroscopic algae are somehow chemically
disagreeable to it. Item (c) can be safely discarded,
since L. planaxis showed no aversion to chopped algae.
There is at present no evidence for or against reasons
(a) and (b), although a brief examination of the mouth
and radula showed no obvious differences from those of
L. scutulata, which feeds much more on macroscopic algae.
In contrast with L. planaxis, Littorina scutulata
both encounters more macroscopic algae and eats more of
it. For'instance)oinrthe survey near Pescadero Point
cited above, 47% ofthe L. scutulata in both areas were
(12)
on algal substrates. There were marked preferences
for certain species of algae. Of the 153 snails on
algae, 72 were on Cladophora, 46 on Pelvetia, 20 on
Rhodoglossum, and no more than 5 on any of the other
species. Nor is this entirely a reflection of the
relative abundance of the algae; Endocladia, the most
common, had only 3 L. scutulata on it, and some of the
others had fewer than their relative areas would suggest.
Also, observations under more favorable conditions than
those reflected in the above survey suggest that all
these figures, especially for Pelvetia, are at times
considerably higher. In other areas, L. scutulata is
also found on Fucus and Porphyra. In addition, tidepool
populations of L. scutulata have been observed scavenging
on decaying algae such as Gigartina corymbifera and
Prionitus lanceolata that had been left in the pools
by very high tides. In these particular instances, the
stomachs of snails collected from the algae were stuffed
with partially decomposed algal material.
Laboratory studies confirmed that L. scutulata
will readily eat a variety of macroscopic algae. In
addition to the Cladophora and Pelvetia mentioned above
in which feeding was definitely observed, snails readily
attacked Ulva spp., leaving only tattered shreads. In
the preference tank previously mentioned, L. scutulata
was observed on all algae in the tank (Pelvetia, Cladophora,
(13)
Gigartina cristata, Endocladia, Rhodoglossum, Porphyra,
Ulva, Iridophycus, Laminaria Andersonii), and all
individuals disected had macroscopic algal fragments
ase
in their stomachs (these were identified for Porphyra,
Laminaria, and Ulva).
Concerning the effects of littorine feeding on
macroscopic algae, the situation seems to be similar
to that noted by Castenholz (1961) for the effects of
L. scutulata on diatom populations; the snails are able
to prevent sparse populations from increasing, but do
not have much effect on well established colonies. The
completely bare areas between algal growths in which
littorines are so frequently found (Castenholz, 1961)
are probably kept bare by continual grazing, and the
occasional presence of very young thalli in littorine
stomachs supports this. In the study area near Pescadero
Point, the Pelvetia is worn and indented, and in some
places only the stubby remains of thalli can be found
in the bottom of cracks, too tightly packed for the snails
to reach. Even the Endocladia is undercut and trimmed
back for about 1 cm above the rock surface in some areas
bordering on bare rock. Whether these effects are due to
the feeding activity of littorines is not possible to
say without lengthy observations and measurements, but
considering the quantities of cell material found in
stomachs, the depredations observed intthe laboratory,
(14)
and the number of snails involved, the effects of
feeding are by no means insignificant. However, well
established algal growths seem to be able to hold their
own successfully, at least for short periods. Two
areas, one with Porphyra, the other with Cladophora
and Ulva were caged with fiberglass netting and + inch
mesh galvanized screen into two compartments, one with
25 L. scutulata, the other empty as a control (a third
compartment with 25 L. planaxis was added for the Ulva,
Cladophora test). After two weeks, no effects were
observed that could be directly attributed to the snails,
although measurements were not very accurate. The effects
on different algae probably vary considerably, judging
from the varying quantities of algal material in the
stomachs of snails feeding on different plants. Porphyra
and Rhodoglossum, for example, were generally found in
much smaller quantities than the softer Pelvetia and
decaying algae, or the more easily ingested Cladophora.
This study was of course) limited in its scope
because of the short duration of the observations.
More statistical surveys and field observations on
preferences are needed, covering a larger number of areas;
the effects of littorine feeding on algae should be
measured quantitatively. Much more has yet to be learned
about the mechanisms behind preferences, and the differences
in feeding habits of the two species when in the same
(15)
area under the same conditions. A number of possible
sources of error need also to be considered and remedied
if possible: in almost all cases the samples were small;
study areas in the field were selected because of the
prevalence of littorines feeding on algae; no attempts
were made in the laboratory to take into account the
previous conditioning of the snails in the field with
regard to food; laboratory conditions were far from
duplicating natural situations (in the most successful
attempt, the spray-doused preference tank, the three
algae on which the least feeding was observed were
Pelvetia, Cladophora, and Porphyra).
In summary, Littorina planaxis generally eats only
microscopic foods and those macroscopic algae small
enough to be consumed entire or nearly so. The reasons
for this are not clear, but may be related to the structure
of the mouth, or the preference for certain types of
substrates. Littorina scutulata is frequently found
eating macroscopic algae, especially Cladophora, Pelvetia,
and other more easily ingested forms. The effects of
this feeding are probably important in limiting the
spread of new plants, and the effects on established
thalli, while possibly considerable in some instances,
appear to vary greatly with the species of algae.
(16)
WORKS CITED
Castenholz, Richard W. 1961. The effect of grazing on
marine littoral diatom populations. Ecology, 42:
783-794.
Fretter, Vera and Alastair Graham, 1962. British
Prosobranch Molluscs, Their Functional Anatomy
and Ecology. London, The Ray Society, 1962, pp. 26-27.
Newell, G. E., 1958. The behavior of Littorina littorea (L.)
under natural conditions and its relation to position
in the shore. J. Mar. Biol. Ass. U. K., 37: 229-239.
North, Wheeler J., 1954. Size distribution, erosive
activities, and gross metabolic efficiency of the
marine intertidal snails, Littorina planaxis and
L. scutulata. Biol. Bull., 106: 185-197.
Ricketts, Edward F. and Jack Calvin, 1952. Between
Pacific Tides. Third Edition, revised by Joel W.
Hedgpeth. Stanford, Stanford University Press, 1952,
pp. 12-20.
Smith, Gilbert M., 1944. Marine Algae of the Monterey
Peninsula California. Stanford, Stanford University
Press, 1944.