Proc. Nat. Acad. Sci. USAVol. 71, No. 3, pp. 966-970, March 1974

Olfactory Basis of Homing Behavior in the Giant Garden Slug, Limax maximus(digitate ganglion/locomotion/orientation/terrestrial pulmonate)

ALAN GELPERIN

Department of Biology, Princeton University, PrincetQn, New Jersey 08540

Communicated by V. G. Dethier, November 14, 1973

ABSTRACT Time lapse photography of slugs living inan experimental enclosure shows that these animals canreturn to a homesite from over 90 cm by a direct route.Slime trail following and vision are not involved in thisbehavior. In the presence of a low velocity wind, homingoccurs upwind. Surgical disconnection of the presumptiveolfactory apparatus (digitate ganglion) from the centralnervous system eliminates homing. Neurophysiologicalrecordings from the receptor surface associated with thedigitate ganglion and the olfactory nerve demonstratethe olfactory function of the digitate ganglion. The olfac-tory acuity and capacity for directed locomotion via olfac-tory cues are also relevant to studies of slug feeding be-havior, ecology, and learning ability.

"Simple" animals often reveal their possession of sophisticatedbehavioral machinery when experimental questions are askedin the proper context. This is nowhere better documented thanin studies of orientation. The sun-compass orientation mecha-nism of bees and ants (1), the ability of noctuid moths tosteer their flight path away from bats using two sense cells(2), and the apparent use of hydrodynamic cues by migratinglobsters (3) are examples of complex neural mechanisms whichbecame apparent when physiological experiments were donein an ethological context. The present experiments on homingin Limax maximus were undertaken to probe the complexityof behavior possible in a preparation amenable to cellularneurophysiological analysis (4).Homing behavior has been documented in a wide variety of

molluscan species. Aristotle described the homing behaviorof limpets and experiments to date are still searching for thesensory basis of this behavior (5, 6). Octopus can return to itsnest after forays covering considerable distances (7). Smallcolonies of the intertidal pulmonate Onchidium nest in rockcrevasses and after a period of feeding away from the nest,all members of a particular colony simultaneously return di-rectly to their nest (8). Capture and release experiments haveshown that the garden snail, Helix pomatia, can return tosites favorable for overwintering with an angular error of lessthan 300 over distances up to 40 meters (9). The sea hareAplysia is diurnally active and returns to a particular locationin its tank of seawater to sleep (10).The literature contains scattered suggestions of homing be-

havior among slugs. The ability of Limax maximus to showhoming behavior has been referred to anecdotally by Taylor(11), Pilsbry (12), and Frdmming (13). Field observations ofthe California banana slug, Ariolimax columbianus, suggestthat animals establish a homesite by excavating a depression

Abbreviation: EOG, electro-olfactogram.

in the soil and forage over an area extending at least 4.5meters from the home (14). Time lapse photography of greyfield slugs, Agriolimax reticulatus, locomoting on an enclosedsoil surface shows that the animals often return to the samehole in the soil from which they emerged earlier in the night(15). The present work documents homing behavior in Limaxmaximus and presents initial physiological investigation ofits sensory basis.

MATERIALS AND METHODS

The behavioral experiments were done on slugs confined to a1.5 by 1.7-meter area of moist filter paper bounded by a 2-inch wide border of crystalline NaCl. An inverted clay flowerpot with four semicircular notches cut in its lip, was centrallylocated and served as the animal's daytime resting site. Thefilter paper was kept moist by inverted water reservoirs. Fooditems such as carrot (Daucus carota), potato (Solanum tubero-sum). dog food (Ken-L-Ration), or mushroom (Boletus edulis)were supplied in a petri dish at one corner of the arena. Fluo-rescent room lights provided illumination and were automati-cally controlled to produce a cycle of 12 hr of light and 12 hrof darkness. In some experiments, a plastic covering was usedto shield the arena from air currents.A 16-mm camera modified for time lapse operation and

equipped with a wide-angle lens was mounted vertically 2.7meters above the experimental arena. A xenon bulb flash unitwith a flash duration of 2 msec was triggered synchronouslywith the camera shutter. The charging capacitor in the flashunit was changed from 300 uf to 40 Mf to produce the leastintense flash which would give a distinct image oln plus-Xnegative film with the lens wide open. A framing rate of 4 perminute was used. Typically the camera was activated from1700 hr to 0900 hr the following day. No behavioral responsecould be detected to the light pulse emitted by the flash unit.More than 1600 hr of activity were filmed and analyzed usingan analytical projector.The 25 slugs used in these experiments were Limax maximus

and included both locally collected animals and individualsreared from eggs in the laboratory. No differences in homingability between these two categories of animals were observed.

Anatomical studies oln the optic tentacles were done usingstandard histological techniques to prepare 8-,Am serial sec-

tions of whole tentacles stained with Mallory Heidenhain'striple stain (16). Neural pathways in the digitate (= tentacu-lar) ganglion were stained using axonal iontophoresis (17) tointroduce Co++ ions into axon.s. The tissue was then treatedwith ammonium sulfide, dehydrated with ethanol, and cleared

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Olfactory Basis of Slug Homing 967

A B' Ce D

FIG. 1. Track of single slug during one night.

in basic methyl benzoate (18). Ganglia so treated were studiedand photographed as whole mounts.

Electrical recordings from the receptor surface of an optictentacle were made with saline-agar filled electrodes of tipdiameter 50-100 /Am connected to a neutralized input capaci-tance dc amplifier. Polyethylene suction electrodes were at-tached to the olfactory nerve after severing its central con-nections. Both types of signals were recorded relative to aground electrode in the saline bath. The signals were dis-played on a multichannel oscilloscope and either photo-graphed directly or recorded using an FM tape recorder.The saline used had the following composition in mM: Na70, K 2.5, Ca 3.4, Mg 0.8, Cl 81, glucose 0.6, Tris 50.

RESULTSThe animals spend the daylight hours in the dark and humidenvironment provided by the homesite. With a latency vary-ing from several minutes to several hours after lights off, theyemerge and move about the arena at speeds ranging between0.069 cm/sec to 0.26 cm/sec. These travels bring them in con-tact with the salt barrier, the food dish, other slugs, and ulti-mately, the homesite. Periods of locomotor activity are inter-slpersed with periods of sleep, sexual activity, or feeding. Thereturn to the homesite is often quite direct and over virginterritory.A representative tracing of the travels of a slug about the

experimental arena is shown in Fig. 1. Three of the trips areshort and in the immediate vicinity of the pot. These shorttrips predominate during the first night in the apparatuswhen the pot and filter paper are clean. Two of the trips aremore extensive and have terminal segments which clearlysuggest a directed locomotion back to the homesite. All of theactivity occurred during the dark period; the animal made itsfinal return home 2.2 hr before light onset. We do not knowwhen the animal actually decided to return home, but thefinal segments of the two long trips demonstrate the animal'sability to return home in a direct I)ath from the outer limitsof the arena over previously untraveled territory.

FIG. 2. Homeward paths of several different slugs. Arrowprovides constant compass direction reference.

Fig. 2 presents 10 return Ipaths selected to represent thevariation in directness of homing observed in this study. Themaximum distance from which homing occurred was 93 cm,the outer limit of the arena. The animals used varied in bodylength from 7.5 to 16.5 cm, and inter-optic tentacle distancevaried from 1.5 to 2.5 cm. Fig. 2 also illustrates that the sameslug can use different paths home on successive nights (PathsC, F, J) and that two slugs living together can use differentpaths home on the same night (Paths A, I).The nonrandom nature of homing was tested mathemati-

cally in the following way. A set of linear path segments withorigins close to the periphery of the arena was selected; onlythose paths which were linear because the animal movedalong the salt barrier were excluded. For the path whose ori-gin was closest to the home, the flower pot subtended 150 ofthe horizon. I assume, for purposes of this test, that the animalselects his direction of travel from a 1800 sector. This assump-tion yields a probability of contacting the home by chance of15°/180° or 8.3%. The sample of 41 linear path segments con-tained 13 (32%) which were homing paths, a clearly non-random distribution.Experiments were then directed to the question of the sen-

sory cue providing direction to the homeward path. The ani-mals are not following slime trails home, although they canfollow slime trails and do so routinely to locate sexual part-ners. The use of visual cues is possible but unlikely. The op-tical system and fine structure of the slug eye suggest poorvisual acuity (19) and light was available for only 2 msecevery 15 sec. The use of vision in homing was tested in twoways. During the dark period, slugs were removed from thehome, placed at the periphery of the arena, and allowed toreturn home in complete darkness, which they did. Two ani-mals in which the optic nerves were successfully sectioned

Proc. Nat. Acad. Sci. USA 71 (1974)

Proc. Nat. Acad. Sci. USA 71 (1971)

FIG. 3. Distribution of homeward path directions relative todirection of prevailing wind. Arrow indicates wind direction.

bilaterally also retained the ability to return home by a directpath if removed from the home and placed at the peripheryof the arena.Two kinds of experiments suggest that olfaction may be the

key stimulus for homing in these exl)eriments. If a very gentlewind is set up across the arena, the animals show a distincttendency to move against the wind. Fig. 3 shows the distri-bution of 42 homeward paths in relation to the wind directionover the app)aratus. Five times as many returns occur fromthe two downwind sectors as from the two upwind sectors(42%o versus 8%7). Application of the x2 one-sample test to thedistribution shown in Fig. 3 indicates a significant deviationfrom the expectation that returns are uniformly distributedaround the home (P < 0.001). A distribution of returns basedon the hypothesis that the two downwind sectors receive be-tween 4 and 8 times as many returns as the two upwind sec-tors is not significantly different from the observed distribu-tion (0.05 < P < 0.1).The digitate ganglion, located at the distal end of the

olfactory nerve in the dorsal or optic tentacles, has beenassigned an olfactory function based on behavioral (20-22)and histological (23-25) observations. To test the idea thatthe digitate ganglion is important in homing behavior, slugswere subjected to bilateral olfactory nerve section. They wereanesthetized with CO2 and an incision made in the lateralbody wall of the head. The olfactory nerves were cut wherethey emerge from the cerebral ganglia. Control slugs receivedthe same operative procedure except the nerves were not cut.Operations were verified by autopsy. The six slugs success-fully operated on, never homed again whereas the controlscontinued to do so. The excursions of the operated animalswere of normal extent and at normal speed. However, eventhough they occasionally passed within 2 cm of home, theoperated slugs did not return to it.

Several behavior patterns exhibited by normal slugs aresuggestive of an olfactory sensitivity. Locomotion of any tyeleis always accompanied by movements of the optic tentacleswhich sweep through an arc of 15-20° on either side of themidline. During homeward locomotion, animals often exhibita characteristic "head-waving" behavior during which theanterior end of the body is lifted above the substrate and thehead moved from side to side with a frequency of approxi-mately 1-2 per sec. Paths toward odorous foods, particularlyfungi, are also often quite direct over distances up to 80 cm.The digitate ganglion is situated within the distal end of

the cylindrical tentacle retractor muscle. The finger-likeprocesses emanating from the ganglion innervate a distinctive

FIG. 4. Digitate ganglion after CoCl2 was iontophoresedtoward the ganglion via axons in the olfactory nerve and aftercobalt was precipitated with ammonium sulfide. Dark areas out

of focus at top are pigment cells in sensory epithelium. Threeneuronal somata in the ganglion are indicated by short arrows.The majority of the fibers in the olfactory nerve terminate in theganglion. scale bar = 500 um.

el)ithelial lpad or sensory zone (26) at the end of the tentacle.Presumptive sensory neurons located in and under the sensoryel)ithelium send lprocesses into the digitate ganglion, as dolarge numbers of 5-7 Azm cells located in the distal extensionsof the ganglion (24). A mantle of neurosecretory cells sur-rounds the ganglion (27). Cobalt backfills of the olfactorynerve reveal six to eight large cells in the body of the ganglionand several fiber tracts ending within it (Fig. 4). This suggeststhat in fact the majority of the axons in the olfactory nerve

are second order processes.To test the olfactory function of the digitate ganglion

physiologically, a preparation of the sensory el)ithelium,digitate ganglion, and olfactory nerve was isolated from theanimal and set up in vitro so that the sensory surface was ex-

l)osed to the air while the ganglion and nerve were immersedin saline. An agar-filled electrode recorded from the sensorysurface (28, 29) and a suction electrode monitored the olfac-tory nerve. Filter paper discs were saturated with odorantsand l)laced in a Swinny adapter ('Millipore Corp.) mountedon a 1-cc syringe. The syringe was mounted so that the tip ofthe adapter was I cm from the receptor surface. Stimuli were

delivered by hand.A puff of air containing an odorant such as amyl acetate

elicits a large compound action l)otential in the olfactorynerve (Fig. 5. A) and a negative electro-olfactogram (EOG)(Fig. 5, A1,). Several small single unit responses are evident in

968 Physiology: Gelperin

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Olfactory Basis of Slug Homing 969

Al preceding and following the compound action potential.The single unit responses typically lasted for the duration ofthe active phase of the EOG. A puff of moist air produces noresponse (Fig. 5, B1, B2). The EOG is not recorded from epi-thelium outside the sensory patch. The size of the compoundaction potential and EOG can be increased by more rapid ap-plication of a given volume of odorant-laden air. Similar re-sponses were obtained to aqueous extracts of carrot and po-tato. These data demonstrate the olfactory function of thedigitate ganglion.

DISCUSSIONThe results presented here demonstrate the homing ability ofLimax maximus. The maximum distance over which Limaxcan forage and still return home is unknown. Estimates ofthis distance require knowledge of the chemical species usedas an olfactory cue and information on the behavioral thresh-old to this chemical (30). The behavioral threshold could bevery low since insect and vertebrate olfactory systems cantrigger a behavioral response to a few molecules of odorant(30). If the initial behavioral response triggered by the odoris movement ul)willd, a detectable gradient is unnecessary andthe odor will be effective in promoting homing even at thefringes of its "active space". Lirnax can show positive anemo-taxis to gentle, l)resumably odor-free winds (31).

Olfactory stimuli are also important in locating food. Slugsnormally eat a variety of plants, including fungi (13). Re-moval of the optic tentacles containing the digitate ganglionreduces the distance at which slugs detect the stinkhorn(Phallus impudicus) from 120 cm to 20 cm (20). A similar re-sult was obtained testing the response of A griolimax reticulatusto potato before and after optic tentacle removal (21). Bothof these exl)erimelits indicated a residual olfactory sensitivityafter optic tentacle removal attributed to the smaller an-terior tentacles. However, this l)henomenon was iiot al)l)arentin my exl)eriments. The olfactory sensitivity and capacityfor directed locomotion via olfactory cues are also relevantto ecological studies of slug distribution in relation to foodplant abundcance (32, 33).The neurophysiological data obtained to date demonstrate

the olfactory function of the digitate ganglion in the slug. Anegative EOG in response to attractive plant odors has beenrecorded from snail tentacles (28), whereas methanol andethanol produced a positive EOG. Based on data from frogolfactory mucosa, Gesteland (34) suggests that substancesproducing a negative EOG excite receptor cells and that sub-stances producing a positive EOG inhibit receptor cells. Itwill be very interesting to determine the odor response spec-trum of single cells and so determine the presence of odorgeneralists or odor specialists (35).

Input from the olfactory nerve enters the metacerebrum(36) and ultimately impinges on pedal neurons that controllocomotion. Slugs move by producing waves of contractionand elongation which move anteriorly over the foot (37). Al-though the production of these pedal waves has been re-ported to be independent of the central nervous system (38,39), we have found that animals with sectioned pedal nervessurvive for weeks with no sign. of pedal wave activity in thedenervated region (Prior and Gelperin, unpublished observa-tion). In the absence of any directive visual or olfactory stim-uli, the circling locomotion which occurs (40) is probably dueto bilateral asymmetries in spontaneous pedal neuron outputs(41).

FIG. 5. Al and B1 are recordings from olfactory nerve; A2 andB2 are recordings from receptor surface. Al and A2 were recordedsimultaneously in response to 0.5 cc of amyl acetate vapor.B1, B2 are the responses to 0.5 cc moist air. Calibration barapplies to A and B and indicates 1 sec., 200 ,uV for Al, B1 and400 ,AV for A2, B2.

The data presented here are very useful in designing experi-ments to probe plasticity of behavior in the slug. Olfaction,taste, and vibration sensitivity (42) are the dominant sensesand learning paradigms must accommodate these facts.Closely related snails have been trained to avoid previouslyattractive plant odors by shocking them in the presence of theodor (43). The aversive taste of quinine was effective in modi-fying the climbing behavior of Helix (44). Snails can alsolearn to keep an optic tentacle retracted to avoid an aversivestimulus (45). The recent demonstration of operant condi-tioning in Octopus (46) has further extended our knowledge ofthe capabilities of molluscan brains.

Note Added in Proof. Recent experiments (47) demonstratethat using olfactory cues, Limax can rapidly learn to avoidnew foodplants if aversive stimulation is paired with ingestionof the new foodplant.

I thank D. Giesker for collecting some of these data. Supportedby N.S.F. Grant GB 20762.

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