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FUNCTION OF THE CEPHALIC TENTACLES IN
LITTORTNA PLANAXIS
Ronald L. Peters
Hopkins Marine Station
Stanford University
Pacific Grove, California
Biology 175h
March 30, 1964
In Fretter and Graham (1962, p.14), the following description is
given for part of the sensory apparatus in the snail Littorina littorea:
"Toward its posterior end the head carries a pair of laterally placed tent-
acles.... At the base of each is a cushion-like bulge.... This is the eye
stalk, and the dark spot on it is the eye. The tentacle, which is tactile
and olfactory, is thus the seat of three major senses." The snail Littorina
planaxis (Philippi, 1847), common along the California coast, has tentacles
very similar to those described for L. littorea. Studies have revealed that
the eye definitely is a light receptor and causes the animal to respond pre-
dictably to various light stimuli (Eckert, 1964). However, the portion of the
tentacle distal to the eye has not undergone extensive investigation, and
tactile and olfactory capabilities of this part of the organ are undetermined.
In April and May, 1964, studies were carried out at the Hopkins Marine Sta-
tion of Stanford University, Pacific Grove, California, to determine the be-
havior and function of that part of the tentacle extending beyond the eye in
L. planaxis.
The two cephalic tentacles are situated at the sides and slightly back
of the large blunt snout. The organs are contractile, and when contracted
they fit snugly at the sides of the mouth. Upon extension, they appear as
delicate finger-like structures which exhibit movement patterns that vary de-
pending on the substrate or environmental condition the animal has encountered.
They are innervated from the cerebral ganglion.
The tentacles are used by the animal as a main guide to its movements
in the rocky areas which it so abundantly inhabits. Moving primarily during
the lower temperatures at night and in the film of moisture provided by high
tide and surf, the tentacles generally remain on the stony substrate and
slowly move from side to side. As the moisture decreases, or as obstacles
are encountered, the organs begin an up and down pattern of movement, with
the snail touching the substrate and immediately lifting the tentacle usually
no more than one to one and a half milimeters. When the snail reaches an
obstacle in its path, it undertakes a tactile survey of the impediment by
extending the organs to their full tapering length and moving them about.
When the animals are submerged, tentacular movement is usually restricted
to a continuous motion from side to side, in contact with the substrate. In
any circumstance, movements of the two tentacles may either by highly coord-
inated, as in horizontal swaying motions, or one tentacle may move completely
independently of the other.
In order to determine more precisely the functions of the cephalic tent-
acles, it proved desirable to extirpate the organs in a group of L. planaxis
and to compare their responses with those in a group of normal snails under
various conditions. It was necessary to anesthetize the animals prior to re-
moving the tentacles. An aqueous solution of magnesium chloride isotonic with
seawater proved to be superior to 1% propylene phenoxetol (Owen and Steedman,
1958), 1% chloral hydrate (Sivik, 1953), and 10 parts/million Sevin (Carriker
and Blake, 1959) for the purposes of this investigation. Having adequately
relaxed the snails, it was easy to pull the head a good distance from the shell
and to snip off the entire tentacle distal to the eye with a pair of iridectomy
scissors. The snails were then placed in normal seawater for recovery. In
all instances, the operated animals exhibited activity similar to that of
the normal ones. The wound appeared healed after two or three days, and op-
erated animals placed in the field resumed normal activity and would occasion-
ally be noted travelling three to five feet during a very moist night.
The following experiments and observations were carried out to compare
the responses of normal snails with those of snails in which the tentacles
had been removed.
General movement and righting
In the laboratory, the L. planaxis without cephalic tentacles did not
exhibit striking locomotive inabilities. Glass dishes were used for all of
the tests, and in practically every instance the extirpated animals travelled
across the smooth surface at approximately the same speeds as the normal ones.
When encountering obstacles, however, a definite difference in reaction was
noted. A normal snail, with its tentacles exploring the substrate immediate
in its path, would reach an object, touch it with the tentacles, and stop
before bumping into it with the shell. On the other hand, a tentacleless
snail would encounter the obstacle, bump into it with the shell, and continue
for a time as if trying to push the object over. If the impediment happen-
ed to be another snail, the animal without tentacles could climb onto the
shell, although observably slower than a normal animal.
To see if a lack of the tentacles produced an impairment to the ability
for righting, about 180 animals with tentacles and a like number without
tentacles were placed on their backs in glass bowls containing fresh sea-
water. They were then timed from entry into the water until righted at one
minute intervals. The results, shown on graph number I, page 7, indicate
that of the animals which did complete the maneuver, the normal L. planaxis
were slightly quicker. Further corroborating evidence that the snails are
tactically dependent upon the organs for righting is that a fewer number of
tentacleless animals than normal ones even completed the maneuver after trying.
Response to waterborne extracts of Acanthina spirata
When the predacious inter-tidal snail A. spirata is introduced into
a dish containing normal L. planaxis, a definite evacuation from the area of
the larger snail will be detected within minutes. This response to the pred-
ator is induced through the effects of a waterborne chemical stimulus that
issues from the A. spirata, and is believed to be produced in association with
its mucus (Tittle, 1964). To determine if the point of reception for this
stimulus is the cephalic tentacle, or if its removal in any way affects the
response, animals with and without tentacles were tested in the following man-
ner.
From approximately the same area in the field, 100 1. planaxis were col-
lected and the total population was anesthetized in isotonic magnesium chlor-
ide. After an hour, the tentacles were removed from one half of the animals.
After the operations, all of the snails were placed in fresh seawater for re-
covery. Three to four hours later, the Littorina appeared totally recuperated,
and were then subjected to the tests. Two finger bowls were placed side by
side, each containing 100 ml of seawater. In one bowl, five normal animals
were placed, and five animals without tentacles in the other one. Both groups
were timed to determine the tendency to leave normal seawater. After 20 min-
utes, the snails were placed back into the center of the bowl, and 20 ml of
seawater were added from a jar which had contained 30 A. spirata in 180
ml for two days. Evacuation from both bowls was again timed and an ac-
celerated departure from the water in both bowls was observed. To deter-
mine whether or not the animals were merely leaving the extract contain-
ing water because of having been replaced into water after an initial de-
parture, several tests were run with snails placed directly into the A.
spirata water. Differences in response in the two instances were neglig-
ible. Identical runs were performed for the entire test population, using
fresh seawater and A. spirata extract from the same jar each time.
Results of all trials are summarized on graph number II, page 7, and
show that the escape response in normal L. planaxis and in the animals
lacking tentacles was almost identical; if anything the response in the
latter group was faster. In trying to account for the quicker response
on the part of the extirpated animals, the same population was placed in
an aquarium for two weeks, after which they were run through identical tests.
Graph number III, page 7, demonstrates the results of the second test, and
shows that the tentacleless beasts were slightly slower this time. Perhaps
initially the extirpated animals were either in a more excited state after the
operation, or the wound was being irritated by the seawater and chemical.
In any case, it is evident that the cephalic tentacles are not critically
important as chemo-receptors in the detection of A. spirata at a distance
under water.
Response to waterborne extracts of female Littorina planaxis
If water that has contained a group of female L. planaxis is added to
water containing normal males of the same species, within a short period
definite clustering and increased activity can be noted (Rohe, 1964). To
determine if the cephalic tentacles were pertinent in detecting this water-
borne mulluscan aphrodisiac, tests similar to the Al spirata experiment were
set up, using the same general procedure as described. In one bowl, con-
taining normal seawater, were placed five normal males; in a second bowl,
also containing seawater, five males lacking tentacles were placed. At one
minute intervals, clustering tendencies were timed by recording the number
of snails in contact with other animals, either side by side or one on top
of another. Following this, 20 ml of seawater taken from a jar containing
approximately 200 ml of water in which 25 female snails had been kept for
four hors, was added to each bowl. Again the number of clustered animals
versus time was noted. Tests were run on 50 normal and 50 operated individ-
uals.
Results of all tests are summarized in graph number IV, page 7. All
snails, both experimentals and controls, showed some initial tendency to
pair and thus form clusters. However, this tendency is short lived except
in the normal males exposed to female extract. Perhaps the reason the re-
sponse is not sustained in extirpated males is that once the animal has climbed
onto the back of another snail, he lacks the probing equipment necessary to
determine the sex of his partner or to assume the correct position. The re-
sults again suggest that the tentacles do not play a role in chemo-reception,
though they appear necessary for definite sex recognition on contact.
Response to mucus trails
It has been observed (Miyamoto, 1964) that L. planaxis tend to follow
mucus trails across the rocks. A series of experiments was therefore set
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up to determine the role of the tentacles in such behavior. Because the
tentacles exploratively preceed the animal, it appears that these organs
would be apt for detection of trails.
By cutting off the foot of an L. planaxis, and dabbing the structure
on a glass plate, an artifical mucus path can be applied. Using plates
6 by 6 inches square bearing such mucus trails from female feet, two normal
L. plaaaxis and two males lacking tentacles were placed respectively in the
centers of the two plates. The plates were then taken into a dark room and
sprayed lightly and equally with seawater. After 15 minutes, the snails were
removed and the glass plates were immersed in a dilute suspension of India
ink in seawater to mark the paths of the animals during their movements in the
dark, a technique designed by Eckert (1964). Experiments were carried out
using each sex as a source of mucus and each sex as a test animal. Two spe-
cific examples of typical results are depicted below.
Mucus Trail Experiments
No Tentacles
Normal

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Craphs L, II, and II on page 9 summarise the results of all tests per-
formed. They clearly show that the animals do employ their tentacles in fol-
40


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lowing L. planaxis mucus trails, and that they follow trails regardless
of the sex of the author of the trail. This result suggested that perhaps
the tentacles are sensitive to any mucus or material that noticeably changes
the surface texture of the substrate. To follow this idea, the mucus from
several other inter-tidal molluscs was employed, using the same method as
described. The results for this set of investigations is shown in graph
IV, page 9. In most cases it seems that the mucus from species living in
close proximity to the L. planaxis populations exhibit properties close e-
nough to the Littorina mucus to elicit at least partial following. Art-
ifical trails made with methyl cellulose and granular mucin were tried, but
neither provided positive results.
Field observations
For observations concerning activities of both normal and tentacleless
animals while in regular field conditions, 100 males and 100 females were
taken from a large rock surface. All were anesthetized using MgCl2, the
tentacles were removed from one half of the males and one half of the fe-
males, the animals were marked, and all were placed back of the rock in a
large fenced area from which all other Littorina were removed. Regular daily
observations were recorded pertaining to pairing, clustering and single activ-
ity for 12 days. Graph I on page 11 shows pairing frequencies for all four
combinations of normal males and females and extirpated males and females.
From studies done on mating, the males are believed to locate the females for
copulation. Graph II, page 11, shows the comparative pairing, with any type
female, of normal and extirpated males. Both of these results indicate that
the animals without tentacles, especially males, are less able to locate
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female Littorina for pairing. This result follows the conclusions drawn from
the mucus trail experiments, in that the tentacles are pertinent for following
another mucus pathway and for sex recognition on contact.
Responses after extirpation of a single tentacle
By removing either of the tentacles and placing the animal on a clean
glass plate, the path assumed while in the dark was determined by again using
the carbon bath. Such experiments did not give clear cut results, but merely
hinted at tendencies. Twenty-four snails were tested, 12 with the left tent-
acle removed and 12 with the right taken off. The results were that 8 of the
animals lacking a tentacle on the left side exhibited circus movements to the
right, and 6 snails without a tentacle on the right side moved in circus mo-
tions to the left. Perhaps the nature of the substrate did not lend to more
consistent results, but circus movements are intimated, indicating a depen-
dency on tactile assurance.
Intermittently throughout the study, populations of animals were extir-
pated for various tests, and watched for tentacle regeneration. Using an oc-
ular micrometer on a dissecting microscope, accurate measurements could be ta¬
ken of the tiny regenerated tip that emerged approximately two to three weeks
after removal. In each instance, a regenerated tip would appear out of the
large stump where the original organ was located. The protuberance is flesh
colored, as opposed to the black remainder of the head. By relaxing the ani-
mals to be investigated, a standard contraction was obtained, so that lengths
could be compared on a time basis. These results are graphed on page 11. The
data is based on six stages of regeneration, although not equally spaced. y
making the disputable extrapolation to about twice the length of the graph,
a time for complete regeneration, based on the average of a normal tentacle
(1.3 mm - contracted), is obtained and is about 4.5 months. This result is
far from conclusive because of both the gross extrapolation and the conditions
in which the animals lived, assuming that they did not provide optimum growth
opportunities.
The cephalic tentacles, of Littorina planagis, are not critical to
general movement, but are used, while the snail moves, for tactile
surveillance, and they enable the animal to perform more easily such
maneuvers as righting.
Removal of the tentacles does not impair the ability to detect dif-
2.
fusible substances from the predacious snail A. spirata or from female
L. planaxis.
3. The tentacles appear necessary for sex recognition on contact.
4. The tentacles are employed in following mucus trails on the substrate.
The trails of other L. planaxis are followed more consistently than
are trails laid down by other species of molluscs.
ACKNOLILEDOMENTS
I am happy to acknowledge the advice of Dr. Donald P. Abbott.
BIBLIOGRAPHY
Carriker, Melbourne Romaine and John W. Blake
1959. A method for full relaxation of muricids. Nautilus
73 (1): 16-21.
Eckert, Dieter
1964. Phototaxis and photokinesis in L. planaxis. Unpublished.
Fretter, V. and A. Graham
1962. British prosobranch molluscs. London. Adlard & Son Ltd.
Miyamoto, Allan
1964. Clustering in L. planaxis. Unpublished.
Owen, G. and H. F. Steedman
1958. Preservation of molluscs. Proceedings of the Malaco¬
logical Society of London 33 (3): 101-103.
Rohe, Karin
1964. The role of the osphradium in L. planaxis. Unpublished.
Sivik, Frank P.
1953. A comparison of the effects of various relaxing agents on
slugs. Turtox News 31 (h): 66-68.
Tittle, Kenneth
196. Escape response to A. spirata. Unpublished.
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