Journal of Chemical Ecology, Vol. 7, No. 6, 1981

SELECTIVE IMPROVEMENT IN RESPONSES TO PREY ODORS BY THE LOBSTER, Homarus americanus, FOLLOWING FEEDING EXPERIENCE

C H A R L E S D. D E R B Y and J E L L E A T E M A Boston University Marine Program Marine Biological Laboratory Woods Hole, Massachusetts 02543

(Received January 19, 1981; revised February 19, 1981) Abstract--Lobsters can detect odors from two natural prey species, the horse musselModiolus modiolus and the blue musselMytilus edulis. When lobsters fed exclusivelyon one of these two prey speciesfor one month, their behavioral response threshold for the prey odor from this species was lowered relative to the threshold for odor from the nonexperienced prey. Key Words--Lobster, Homarus americanus, chemoreception, feeding

behavior, behavioral plasticity. INTRODUCTION

Clawed and spiny lobsters can locate distant prey by means of their sensitive chemoreceptors (McLeese, 1970; Mackie, 1973; Shepheard, 1974; Fuzessery et al., 1978; Reeder and Ache, 1980). Odors from live prey are sufficient to initiate searching behavior by lobsters (Hirtle and Mann, 1978). The American lobster is a predator that can consume a variety of live prey species (Squires, 1970; Weiss, 1970; Miller et al., 1971; Ennis, 1973). Individual lobsters, however, can be selective in their food preferences, as demonstrated by gut analyses (Squires, 1970; Weiss, 1970; Miller et al., 1971; Ennis, 1973; Leavitt et al., 1979) and behavioral observations (Evans and Mann, 1977; personal observations). For many predators, repeated experience with individuals of a prey species can lead to a preference for that species (Krebs, 1978). Anecdotal information suggests that this may be true for lobsters (Wilson, 1949; Lindberg, 1955). Many features could be learned to form such prey 1073 0098-0331/81/1100-1073503.00/0 9 1981 Plenum Publishing Corporation

1074

DERBY AND ATEMA

preferences (Dawkins, 1971; Krebs, 1973; Curio, 1976). For predators with acute chemical senses, such as lobsters, it may be possible that feeding experience with a single prey species will result in an increased responsiveness to chemicals emanating from that species. Vertebrates capable of such behavioral plasticity of their olfactory responses include fish (yellowfin tuna Thunnus albacares) (Atema et al., 1980) and snakes (garter snake Thamnophis sirtalis) (Fuchs and Burghardt, 1971; Arnold, 1978). Invertebrates with this capacity include symbionts, herbivores, and predators such as insects (parasitic wasp Nemeritis canescens) (Thorpe and Jones, 1937), terrestrial snails (Achatinafulica) (Croll and Chase, 1977, 1980), and marine species [oyster drill Urosalpinx cinerea (Wood, 1968), polynoid polychaeteArctonoe pulchra (Dimock and Davenport, 1971), starfish Asterias rubens (Castilla, 1972), and pea crab Pinnotheres maculatus (Derby and Atema, 1980)]. We present here data demonstrating that after feeding on a species of prey, lobsters show a selective increase in their responsiveness to odors from this prey species. METHODS AND MATERIALS

Ten American lobsters of 6-7 cm carapace length were used in this study. Their behavior was observed in two troughs, each 2.50 m long X 0.30 m wide X 0.30 m high. All observations were recorded under red illumination. Sea water from a head tank entered the head of each trough at a rate of 1 liter/min. A 20-ml stimulus was introduced through a funnel that led directly into the sea water inflow and was washed through with 20 ml of filtered sea water in order to remove any residual stimulus from the funnel. A clear Plexiglas shelter was located in the trough 1 m from the funnel tip. Lobsters normally rested in this shelter when not active. A spectrophotometric dye study revealed that an introduced stimulus was diluted 1000-fold when it reached the shelter, 2.0-2.5 min after introduction. Prey odor was prepared by placing five live undamaged mussels, each of dpproximately l0 g soft tissue wet weight, in I liter of filtered sea water for l0 hr. This water was filtered, diluted in logarithmic steps, and stored in 20-ml aliquots. Prey odors were always used within 36 hr of preparation and were refrigerated when not being used. Stimuli tested included odors of the horse mussel Modiolus modiolus and the blue mussel Mytilus edulis (two natural prey species for lobsters), as well as a filtered sea water control stimulus. The effect of feeding experience on the responses of lobsters to prey odors was examined with ten lobsters which for several weeks before the start of the experiment had been maintained on a diet of sea star (Asteriasforbesi) and cod (Gadus callarias). Response thresholds of each lobster to the two mussel prey odors were determined according to the following method. Each lobster

IMPROVED RESPONSESBY LOBSTER TO PREY ODORS

1075

was allowed to acclimate in the trough for 1-2 days before beginning a series of stimulus introductions. At the time of each stimulus introduction, a lobster was observed for 20 rain, 10 rain before and 10 rain after introduction. Responses were analyzed by observing specific movements of the sensory and feeding appendages (chelae, walking legs, maxillipeds, antennules, and antennae), as well as such general movements as walking and shifting. Table ! lists these units. The number of occurrences of each behavioral unit for the pre- and postintroduction periods was noted as was the total number of occurrences of all behavioral units for the pre- and postintroduction periods. A response was considered significant when a chi-square test demonstrated a significant ( P < 0.05) increase in the total number of occurrences of all behavioral units following stimulus introduction. The introductions of filtered sea water and dilutions of each of the two types of prey odors were interspersed, with at least 30 rain separating each trial. The response threshold, defined as the lowest dilution to which a lobster responded, was determined for each type of odor. If the threshold of any individual was uncertain upon completion of the first dilution series, dilutions near the threshold were again tested. Upon completion of this series of tests for all ten lobsters, five lobsters were put in each of two 675-liter aquaria (1.25 m long X 0.9 m wide X 0.6 m high). Each aquarium, provided with a surplus of shelters, contained only one species of mussel, either Mytilus edulis or Modiolus modiolus, at a density of ten mussels per lobster which was maintained throughout the experiment. These two natural prey species of TABLE | . DESCRIPTION OF LOBSTER BEHAVIORAL UNITS

Behavioral unit Chela raise Dactyl wave Dactyl rake Antennule burst Antennule point Antennule wipe Antenna wave Antenna point Antenna wipe Maxilliped wave Maxilliped wipe Maxilliped rake Walk toward funnel Walk away from funnel Shift

Description Lift claws high Move legs through water without touching substrate Move legs with tips digging into substrate Sudden dramatic increase in flicking rate Point antennules, often in direction of stimulus Groom antennules, usually with 3rd maxillipeds Sweep antenna through water Place antenna directly in front of body Groom antenna, usually with 3rd maxillipeds Slow back-and-forth movement of 3rd maxillipeds without touching one another Rub 3rd maxillipeds against each other Touch substrate with 3rd maxillipeds Move toward site of stimulus introduction Move away from introduction site Change body position without walking

1076

DERBY AND ATEMA

lobsters were c h o s e n since they are p h y l o g e n e t i c a l l y r e l a t e d a n d are similar in t e r m s o f size, shape, ease of c a p t u r e by lobsters, a n d p r o b a b l y also f o o d value. A f t e r 28-30 days in these aquaria, thresholds for the two prey o d o r s were again d e t e r m i n e d as p r e v i o u s l y described. A n y changes in t h r e s h o l d s for the two p r e y o d o r s following feeding experience were a n a l y z e d with a W i l c o x o n m a t c h e d - p a i r s s i g n e d - r a n k s test; since other species that had been tested for similar effects are k n o w n to show an increased responsiveness to experienced o d o r s (see I n t r o d u c t i o n ) , the e x p e c t a t i o n of such an effect in this e x p e r i m e n t p e r m i t t e d usage o f a one-tailed test.

RESULTS F o u r weeks of feeding experience with a single prey species resulted in b o t h increased sensitivity to o d o r s of t h a t species ( m e a n a b s o l u t e t h r e s h o l d change = 0.5 log units) a n d decreased sensitivity to o d o r s f r o m n o n e x p e r i enced prey ( m e a n a b s o l u t e t h r e s h o l d change = - 1 . 4 log units). T h e effect is m o r e r e a d i l y d e m o n s t r a b l e by c o m p a r i n g the relative c h a n g e in response to e x p e r i e n c e d o d o r s versus that to n o n e x p e r i e n c e d o d o r s for each i n d i v i d u a l ( T a b l e 2). This analysis shows that eight lobsters b e c a m e m o r e sensitive to the e x p e r i e n c e d o d o r , one showed no change, a n d one became less sensitive. TABLE 2. CHANGESIN THRESHOLD RESPONSESTO PREY ODORS FOLLOWING FEEDING EXPERIENCEa Change in threshold (log units) ~ Lobster

Prey species

Modiolus odor

Mytilus odor

Relative change in threshold (log units)c

I 2 3 4 5 6 7 8 9 10

Modiolus Modiolus Modiolus Modiolus Modiolus Mytilus Mytilus Mytilus Mytilus Mytilus

2.0 6.2 - 1.8 1.2

-5.0 1.2 -3.8 -0.8

7.0 5.0 2.0 2.0

0.0

- 1.0

t .0

- 1.9 -6.0 -0.9 -0.9 5.2

I. 1 -4.0 -0.6 -0.9 2.2

3.0 2.0 0.3 0.0 -3.0

~Wilcoxon matched-pairs signed-ranks test (one-tailed) demonstrates an increase (P < 0.05) in responsiveness to the experienced odor relative to the nonexperienced odor. bRepresents the change of response threshold to each prey odor after 30 days of feeding on only the prey species listed; a positive value signifies an increase in responsiveness. ' Represents the difference between "change in threshold" for the experienced prey odor and the nonexperieneed prey odor; a positive value signifies that the behavioral response to the experienced prey odor became more sensitive relative to the response to the nonexperienced odor.

IMPROVED RESPONSES BY LOBSTER TO PREY ODORS

1077

DISCUSSION

These results demonstrate that feeding experience results in a significant improvement in the response of lobsters to odors from experienced prey, i.e., exposure of lobsters to a single species of mussel improves the responsiveness of the lobsters to odors of that species relative to the responsiveness of the lobsters to odors from another mussel species. For some individuals, these changes are very large, e.g., after lobster 1 (Table 2) fed on Modiolus for one month, its threshold to Modiolus odor improved by 100-fold while its threshold to Mytilus odor became 100,000-fold less sensitive. In only one animal (lobster 10) was the change in the direction of the nonexperienced prey. These effects are striking in light of the fact that these two complex mixtures are probably quite similar (however, their exact compositions remain unknown). These results contradict the opinion of Mackie and Shelton (1972), who concluded that responsiveness of lobsters (Homarus gammarus) to tissue extracts of squid (Loligo vulgaris) does not change as a result of exclusive feeding on a squid diet. This discrepancy may be due to the fact that their experiments were not specifically designed to examine this hypothesis or possibly that feeding experience with only certain types of foods may result in chemosensory threshold changes. Although several symbionts, predators, and herbivores that possess sensitive chemical senses can show improved responsiveness to odors from their hosts, prey, or plant foods following experience with them (see Introduction), the basis for such changes has rarely been examined. Croll and Chase (1977, 1980) showed that such plasticity of olfactory orientation in the land snail Achatinafulica occurred following exposure to food odors only after ingestion, demonstrating that this behavioral change was not due to sensitization but rather to an associative learning phenomenon. The neuronal basis of associative aversion learning involving changes in chemoreceptive behavior of a number of invertebrate species is presently being studied (Carew et al., 1980; Chang and Gelperin, 1980; Mpitsos et al., 1980). Future studies such as these may shed light on the basis of chemoreceptive response plasticity. Animals which have demonstrated an increased responsiveness to experienced prey may have formed search images. However, there are many features of a recently experienced prey species that can be learned by a predator which result in the formation of a species-specific preference. These include learning to search in a particular area or type of habitat, learning to capture and handle prey more efficiently, developing a change in acceptability of prey, or learning to detect the prey. Only the last case, where detection of a prey species improves as a result of encounters with that species, can truly be called search image formation (Tinbergen, 1960; Dawkins, 1971; Krebs, 1973; Curio, 1976; Pietrewicz and Kamil, 1979; Atema and Derby, 1981). Search

1078

DERBY AND ATEMA

images have most frequently been attributed to visual predators, and the best behavioral experimental evidence for the existence of search images involves birds feeding on cryptic prey (Pietrewicz and Kamil, 1979); however, it is possible that search images may operate through sensory modalities other than vision, e.g., the chemical senses (Atema, 1977, 1980; A t e m a and Derby, 1981). The distinction between changes in prey acceptability and changes in prey detection capabilities of a predator is often difficult to differentiate. This distinction, however, is crucial to the definition of search images (Dawkins, 1971; Krebs, 1973). Behavioral threshold determinations are generally good methods for demonstrating a change in detection ability (Blough, 196 I), and procedures similar to those used here have been used by others to determine the ability of lobsters to detect various chemical stimuli (McLeese, 1970; Mackie, 1973; Hirtle and Mann, 1978). If the experiments described here do actually measure detection thresholds, then they provide behavioral support for the existence of chemical search images. The existence of a relationship between feeding experience and formation of prey preferences by lobsters is suggested by anecdotal information of Wilson (1949) and Lindberg (1955). Wilson described "an apparent example of learning in Panulirus vulgaris": several lobsters that had never eaten hermit crabs (Eupagurus bernhardus) finally did so when no other food was available; subsequently, hermit crabs became one of the most preferred prey of these lobsters, even when other prey species were provided. Lindberg stated that in trapping Panulirus interruptus, "the effectiveness of a bait is sometimes dependent on the locality being fished. Crushed mussels are a good bait in areas where extensive mussel beds are normally present, but not elsewhere." The effect of feeding experience on chemoreceptive behavior of lobsters described by us could be one cause of such selective predation by lobsters, especially in areas where the same prey species is available over an extended period of time. This chemoreceptive plasticity, by enabling lobsters to more readily initiate searching in response to chemical signals released by nearby potential prey, could lead to more efficient location and subsequent ingestion of their prey. Acknowledgments--We wish to thank Barry Ache, Bruce Bryant, Ronald Chase, Jennifer G. Smith Derby, Alan Kamil, and Thomas Trott for critically reviewingthe manuscript.

REFERENCES ARNOLD, S.J. 1978. Some effectsof early experienceon feeding responses in the common garter

snake, Thamnophis sirtalia. Anirn. Behav. 26:455-462. AXEMA,J. 1977. Separation of smell and taste in fish and crustacea, pp. 165-174,in J. LeMagnen and P. MacLeod (eds.). Olfaction and Taste VI. Information Retrieval, Ltd., London. ATEMA,J. 1980. Chemicalsenses, chemicalsignals, and feedingbehavior in fishes, pp. 57-101, in

IMPROVED RESPONSESBY LOBSTER TO PREY ODORS

1079

J.E. Bardach, J.J. Magnuson, R.C. May, and J.M. Reinhart (eds.). Fish Behavior and Its Use in the Capture and Culture of Fishes. International Center for Living Aquatic Resources Management, Manila. ATEMA,J., and DERBY,C. 1981. Ethological evidence for search images in predatory behavior, pp. 395-400, in E. Graystyan and P. Molnar (eds.). Advanced Physiological Sciences, Vol. 16, Sensory Functions. Pergamon Press, New York. ATEMA,J., HOLLAND,K., and I KEHARA,W. 1980. Olfactory responses of yellowfin tuna~ Thunnus albacares) to prey odors: Chemical search image. J. Chem. EcoL 6:457-465. BLOUGH, D.S. 1961. Experiments in animal psychophysics. Sci. A m . 205:113-122. CAREW, T.J., WALTERS,E.T., and KANDEL,E.R. 1980. Defensive classical conditioning in Aplysia: Functionally distinct motor systems show different neurophysiological correlates. Soc. Neurosei. Abstr. 6:590. CASTILLA,J.C. 1972. Responses of Asterias rubens to bivalve prey in a Y-maze. Mar. Biol. 12:222-228. CHANG, J.J., and GELPERIN,A. 1980. Rapid taste-aversion learning by an isolated molluscan central nervous system. Proc. Natl. Acad. Sci. U.S.A. 77:6204-6206. CROLL,R.P., and CHASE,R. 1977. A long-term memory for food odors in the land snail, Achatina fulica. Behav. Biol. 19:261-268. CROLL, R.P., and CHASE,R. 1980. Plasticity of olfactory orientation to foods in the land snail A chatinafulica. J. Comp. Physiol. 136A: 267-277. CURIO, E. 1976. The Ethology of Predation. Springer-Verlag, Berlin. 250 pp. DAWKINS, M. 1971. Perceptual changes in chicks: another look at the "search image" concept. Anim. Behav. 19:566-574. DERBY, C.D., and ATEMA, J. 1980. Induced host odor attraction in the pea crab Pinnotheres maculatus. Biol. Bull. 158:26-33. DIMOCK, R.V., JR., and DAVENPORT,D. 1971. Behavioral specificity and the induction of host recognition in a symbiotic polychaete. Biol. Bull. 141:472-484. ENNIS, G.P. 1973. Food, feeding, and condition of lobsters, Homarus amerieanus, throughout the seasonal cycle in Bonavista Bay, Newfoundland. J. Fish. Res. Board Can. 30:1905-1909. EVANS, P.D., and MANN, K.H. 1977. Selection of prey by American lobster (Homarus americanus) when offered a choice between sea urchins and crabs. J. Fish. Res. Board Can. 34: 2203-2207. Ft;CHS, J.L., and BURGHARDT,G.M. 1971. Effects of early feeding experience on the responses of garter snakes to food chemicals. Learn Motiv. 2:271-279. FUZESSERY,Z.M., CARR,W.E.S., and ACHE, B.W. 1978. Antennular chemosensitivityin the spiny lobster, Panulirus argus: Studies of taurine sensitive receptors. Biol. Bull. 154:226-240. HIRTLE, R.W, M., and MANN, K.H. 1978. Distance chemoreception and vision in the selection of prey by American lobster ( Homarus americanus ). J. Fish. Res. Board Can. 35: 1006-1008. KREBS, J.R. 1973. Behavioral aspects of predation, pp. 73-111, in P.P.G. Bateson and P.H. Klopfer (eds.). Perspectives in Ethology. Plenum Press, New York. KREBS, J.R. 1978. Optimal foraging: Decision rules for predators, pp. 23-63, in J.R. Krebs and N.B. Davies (eds.). Behavioural Ecology. An Evolutionary Approach. Sinauer Associates, Inc., Sunderland, Massachusetts. LEAVITT, D.F., BAYER,R.C., GALLAGHER,M.L., and RITTENBURG,J.H. 1979. Dietary intake and nutritional characteristics in wild American lobsters (Homarus americanus). J. Fish. Res. Board Can. 36:965-969. LINDBERG, R.G. 1955. Growth, population dynamics, and field behavior in the spiny lobster Panulirus interruptus (Randall). Univ. Calif. Publ. Zool. 59:157-247. MACKIE,A.M. 1973. The chemical basis of food detection in the lobster Homarus gammarus. Mar. Biol. 21 : 103-108. MACKIE,A.M., and SHELTON,R.G.J. 1972. A whole-animal bioassay for the determination of the food attractams of the iobster Homarus gammarus. Mar. Biol. 14:217-221.

1080

DERBY AND ATEMA

MCLESSE, D.W. 1970. Detection of dissolved substances by the American lobster (Homarus amerieanus) and olfactory attraction between lobsters. J~ Fish Res. Board Can. 27: 1371-1378. MtLLER, R.J., MANY, K. H., and SCARRATT,D.J. 1971. Production potential of a seaweed-lobster community in eastern Canada. J. Fish. Res. Board Can. 28:1733-1738. MPITSOS, G.J., COHAN, C.S., and MCCLELLAN,A.D. 1980. Pavlovian conditioning in Pleurobranchaea" Discriminative responses and electrophysiological correlates. Soc. Neurosci. Abstr. 6:590. PIETREWlCZ, A.T., and KAMIL, A.C. 1979. Search image formation in the blue jay (Cyanocitta cristata). Science 204:1332-1333. RErDrR, P.B., and ACHE, B.W. 1980. Chemotaxis in the Florida spiny lobster, Panulirusargus. Anim. Behav. 28:831-839. SHEPHEARD,P. 1974, Chemoreception in the antennule of the lobster, Hornarusamericanus. Mar. Behav. PhysioL 2:261-273. SQUIRES, H.J. 1970. Lobster (Homarus americanus) fishery and ecology in Port au Port Bay, Newfoundland, 1960-1965. Proc. Natl. Shel(fish Assoc. 60:22-39. THORPE, W.H., and JONES, F.G.W. 1937. Olfactory conditioning in a parasitic insect and its relation to the problem of host selection. Proc. R. Soc. London, Ser. B 124:56-81. TINBERGEN, L. 1960. The natural control of insects in pinewoods, t. Factors influencing the intensity of predation by songbirds. Arch. Neerl. Zool. 13:265-343. WHss, H.M. 1970. The diet and feeding behavior of the lobster, Homarus americanus, in Long Island Sound. PhD thesis, University of Connecticut. 80 pp. WILSON, D.P. 1949. Notes from the Plymouth Aquarium.J. Mar. Biol. Assoc. U.K. 28:345-351. WOOD, L. 1968. Physiological and ecological aspects of prey selection by the marine gastropod Urosalpinx cinerea (Prosobranchia: Muricidae). Malacologia 6:267-320.