Proc. Nat. Acad. Sci. USAVol. 72, No. 10, pp. 4185-4188, October 1975Zoology

Chemoreceptors and feeding in calanoid copepods (Arthropoda:Crustacea)

(sensillum/zooplankton/selective feeding/oil-spill dispersal)

MARC M. FRIEDMAN* AND J. RUDI STRICKLERDepartment of Earth & Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218

Communicated by Hans P. Eugster, August 6, 1975

ABSTRACT Ultrastructural studies of the mouthparts ofthe calanoid copepod Diaptomus pallidus have revealed thepresence of numerous chemoreceptors, and the apparent ab-sence of mechanoreceptors. The setae contain no muscles,and the setules are noncellular extensions of their chitinwall. This allows a new insight into the selective feeding ofzooplankters.

On February 4, 1970, the tanker Arrow was wrecked inChedabucto Bay, Nova Scotia, and a quantity of bunker Coil was released into the sea. Conover (1) studied the effectsof the oil on the dominant calanoid copepod, Temora long-icornis. He found that the copepod: (1) actively ingested oildroplets which were within its normal food size range, andmay actually have selected the oil droplets over natural par-ticulate food, and (2) was apparently unharmed by the oil.Conover calculated that a significant portion of the spill wasremoved from the water column via sedimentation in theanimals' fecal pellets, leading him to suggest that grazersmight be ". . the single most important natural agent lead-ing to eventual dispersal and degradation of oil spills ...

SELECTIVE FEEDINGTo us, the critical question here is why do the copepods eatthe oil particles? Certainly, they cannot be used to them as anatural food. Perhaps the most obvious answer is that cope-pods are nonselective feeders and will ingest any particleswithin their normal food size range. There is, however, alarge body of evidence which suggests that these animals areindeed selective feeders (cf. 2 and 3), and Marshall (3) hasconcluded: "It is clear then that copepods can and do selectparticular foods but they do not do so all the time, and theirpreferences may change." The ingestion of the oil dropletsmight have been facilitated by some physical propertywhich makes them sticky and therefore more easily trapped,but then they would have been correspondingly harder tohandle and "swallow." Visual cues can be eliminated be-cause selective feeding has been demonstrated in the dark(4), and because most calanoids feed near the water's surfaceat night. We have, however, from our own experiments andfrom the literature (e.g., 2, 3, 5-7), evidence to support thesuggestion that chemical information is used by copepods tomake feeding decisions. We have observed individuals ofDiaptomus minutus and D. pallidus grasp and then rejectdetritus which was carried to the mouth by the feeding cur-rent. Before rejection, the detritus was held in the mouth-parts and handled, as if it were being tasted. Conover (6)found that individuals of Calanus, offered their own fecalpellets, tore open the surrounding membranes as if to tastethe pellets before rejecting them. Marshall and Orr (8) and

Conover (6) have reported that Calanus becomes accus-tomed to a particular food and will feed on it in preferenceto other, even larger, food. Additionally, the animals' filter-ing rate is drastically decreased in senescent algal food cul-tures (9). There are also field reports of copepods feedingpreferentially or exclusively on one alga when severalspecies are available (cf. 2 and 3).

There is another line of evidence which suggests that in-formation about particle size or shape can be used to choosefood items. For example, experiments by Richman and Rog-ers (4) and Wilson (10) have indicated that copepods preferlarger particles to smaller ones, prompting Wilson to proposea model for mechanical size-selectivity.

SENSORY RECEPTORSThe unifying theme here is that it is impossible to accountfor all of the observed feeding behavior without assumingsome kind of sensory input. C. M. Boyd (personal communi-cation) has offered a simple model which can partially ex-plain the apparent preference for larger particles: the filter-ing mesh is pictured as a leaky sieve, and the larger a parti-cle is, the greater its chance of being retained once encoun-tered, resulting in an apparent selection for the larger parti-cles. However, any selective feeding mechanism which re-quires an adjustment of the ongoing feeding process also re-quires an input of information about some property of thefood.We and other workers have found sensory receptors in the

head region (11, 12) and on the body surface (13, 14) of co-pepods. There are, however, few reports of electron micro-scopic studies of the mouthparts which do the actual foodhandling. Ong (15) reported finding receptors inside themouth of the brackish water calanoid Gladioferens, in themandibles and labrum. These receptors could not be used togather information usable during handling of food, before itenters the mouth. The purpose of our research was to deter-mine (1) what kind of receptors are located on or in themouthparts of a filter-feeding copepod, and (2) how theymight be employed for selective feeding. We chose the freshwater Diaptomus pallidus, about 1 mm long, which is easilyobtained and kept in our laboratory. The animal is an obli-gate filter-feeder and any mouthpart receptors can be ex-pected to function during filter-feeding. Possible complicat-ing sensory factors which might be introduced by a raptorialfeeding mode are thus precluded.

Animals to be examined by transmission electron micros-copy were fixed in Karnovsky's fixative, posf-fixed in 2% os-mium tetroxide, and embedded in Maraglas. Specimens forscanning microscopy were processed as above through os-mium post-fixation, dehydrated in an ethanol series and ace-tone, and critical-point dried. They were then coated withgold-palladium and kept vacuum dessicated.

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* Present address: Department of Anatomy, School of Medicine,The Johns Hopkins University, Baltimore, Md. 21205.

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4186 Zoology: Friedman and Strickler

FIG. 1. Diaptomus pallidus, Anterior ventral view; X 180.A1,A2: antennae 1 and 2. M: mouth. M1,M2: maxillae 1 and 2. MP:mandibular palps. MX: maxilliped.

We have examined all of the head and mouth appendagesof Diaptomus, shown in Fig. 1. Scanning microscopy failedto reveal any external structures for which we might proposea receptor function, except on the first and perhaps secondantennae (cf. 12). However, transmission microscopy has re-vealed receptors in all of the appendages except the labial

palps (lower lips). We have examined in particular detail thereceptors in the mandibles (inside the mouth), mandibularpalps, and in the first and second maxillae-appendageswhich handle the food. The receptors belong to one of twodistinct general morphologies, which we shall designate hereas types I and II, for convenience.Type I receptors (cf. 15, Figs. 2 and 9) are found mainly

in the mandibles, and each is enclosed distally, beyond theciliary region, in a cuticular sheath. According to Ong (15),in Gladioferens this cuticular sheath ends in a pore at the tipof the mandible. Each receptor contains one or two ciliarydendrites and sometimes also a smaller dendrite containing afew neurotubules. No evidence of typical basal body struc-ture has been found in the smaller dendrites. Ong reportedthat some of the neurons innervating similar mandibular re-ceptors in Gladioferens were nonciliary and called themchemoreceptors, while the ciliary neurons, he hypothesized,were mechanoreceptors. It is now known that both chemo-and mechanoreceptors possess ciliary neurons (16, 17), andwe have determined that the neurons reported by Ong to benon-ciliary in Gladioferens are ciliary in Diaptomus (it israther easy to miss the basal bodies). There is currently nobasis to assign a mechanoreceptor function to any of thesereceptors, as they do not possess the microtubule bundlewhich is characteristic of known mechanoreceptors (17, 18).On the contrary, these mandibular receptors resemble con-tact chemoreceptors described by Slifer (16) as peg-in-pitsensilla and by Kaissling (19) as sensilla ampullacea. Electro-physiological studies have shown that similar receptors inthe horseshoe ctab (Limulus) are chemoreceptors (20).Type II sensilla are found in the first and second maxillae

and the mandibular palps. More precisely, the setae of theseappendages are the sensilla (Figs 1 and 2). These setal sensil-

':1 JVN , .1M.iR . ' _

FIG. 2. (A) terminal portion of a mandibular palp, showing setae; X4000. (B) enlargement of one of the setae (from another thin section)showing two basal bodies (arrows) and an accompanying dendrite (d); X14,600. (C) setal wall (SW), showing pores (arrows). S: interior ofseta; X120,000.

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Proc. Nat. Acad. Sci. USA 72 (1975) 4187

la are characterized by a pore system in their distal regions,permeating the setal wall, which is about 0.25 Am thick (Fig.2G). The numerous pores are about 100 A in diameter, butdue to positioning difficulties we have not yet determinedtheir exact distribution. The setal wall in this region appearsdistinctly different from the normal chitin in the basal por-tion of the seta. Each sensillum contains 1 to 5 dendrites, 1or 2 of which are ciliary and form 9 + 0 basal bodies, withthe accompanying dendrites forming only an unstructuredarray of doublet microtubules. The presence of several neu-rons which can apparently communicate with the outside ofthe seta through a pore system (16, 19), and the absence of a"tubular bundle" (17, 18) clearly indicate that these setalsensilla are chemoreceptors. They are structurally similar tothe hair chemoreceptors found in other arthropods (21-24).

DISCUSSIONWhat kinds of chemical information might the mouthpartchemoreceptors respond to, and how could they then affectfeeding behavior? We have evaluated the available experi-mental and field data and consider the following hypotheti-cal functions worthy of investigation:

(1) Attempts to culture copepods have shown that for thelong term maintenance of reproducing populations, multi-algal diets and/or bacteria, vitamins, or other "critical sub-stances" must be supplied (25, 26). The precise limiting sub-stances for copepods are as yet unidentified, but chemore-ceptors of crustaceans and insects are known to respond tosuch substances as amines, amino acids, sugars and salts(27-30). Lee et al. (31), have found that copepods in deepwater and high latitudes depend on energy from stored waxesters for reproduction and overwintering. These wax estersare synthesized from algal fatty acids which are available inthe diet during the seasonal algal bloom and which would belogical targets of selective feeding. Additionally, if copepodsemploy allelochemics (32) such as pheromones during mat-ing (33), then they or their precursors could be detected inthe appropriate algae. Furthermore, the role of chemorecep-tors as regulators of the feeding process has already beendemonstrated in some insects (29, 34) and crustacenas (35,36). Therefore, the location of the copepod chemoreceptorsin the feeding chamber is ideal in that it would allow the an-imals to monitor both the incoming food and water.

(2) If copepods can discriminate between algal types bysmell, then chemoreceptors tuned to these smells would beeffective guides for the separation of feeding niches.Lowndes' (5) observation that Eudiaptomus gracilis, al-though found in the plankton, was nevertheless feeding ex-clusively on a species of benthic desmid, could be explainedby such receptor tuning.

(3) The use of mouthpart chemoreceptors to recognizeand refrain from eating con-specific larvae might be advan-tageous, particularly in species which produce few eggs andretain them in egg sacs. Recognition of the larvae could leadto the inhibition of the feeding mechanism and the subse-quent release of the larvae unharmed. We note that moststudies in which cannibalism has been reported have usedadults which were starved to varying degrees (e.g., 37).

(4) It is even conceivable that chemoreceptors could beused to differentiate between large and small algae, ef-fecting "size-selective" feeding. Each collision between analga and a setal receptor would constitute a stimulus, andwould trigger a receptor response. A larger alga would con-tact a larger area of the seta and thereby a larger number ofpores and receptor sites, and so the response could be pro-

portional to the size of the alga. A higher response frequencymight then indicate to the animal a larger alga. It is notknown, however, whether the copepod nervous system is ca-pable of processing this information. At any rate, the sameresults might be obtained more simply by using, for exam-ple, gut stretch receptors. In either case the animals wouldneed to have some means of adjusting the feeding mecha-nism in response to the information received. The rate offeeding could also be adjusted, in response to changes infood concentration (cf. 9), and feeding could be shut downentirely during high concentrations of algal toxins.

FURTHER CONSIDERATIONSThe experimental results of Bainbridge (38) suggest thatzooplankters might locate food patches via chemotaxis.Mechanisms for distance orientation were analyzed bySchone (39), and the copepod receptors as a group could beused for "chemotropotaxis" (Sch6ne's Table I, p. 16). A gra-dient would be detected by measuring the odorant concen-tration at one point, moving a known distance in a knowndirection, then remeasuring and comparing. By "remem-bering" the direction of movement through the use of pro-prioceptors (40), the animal could calculate the gradient.However, the slow gliding motion of herbivorous calanoidsmoves the animals at most about 20 cm between jumps, adistance too short to measure the diffuse gradients in aquaticfeeding environments. Also, infrared movies have shownthat the jumps are random, and the direction of any particu-lar jump is not predictable. To verify our conclusion weplaced zooplankters in the base of a "Y" apparatus whichwas sealed off from the arms of the "Y" by a removable par-tition. Into one arm we perfused filtered pond water, andinto the other arm an algal suspension. The partition wasthen removed and the animals moved up the base and intoone or the other of the arms, displaying negative rheotaxis.We have repeatedly been unable to detect any bias towardthe arm with the algae. The animals will, however, respondto a temperature gradient of 50, showing that the "Y" canwork. Thus we must tentatively conclude that chemotropo-taxis is not normally used to locate algal patches.We also investigated the possibility that the setules (cf. 41.

Fig. 23) might be mechanoreceptors by which a mechanicalsize selectivity could be carried out. Examination of the set-ules revealed that they are non-cellular extensions of thesetal chitin wall and contain what appear to be groups of fi-bers which may serve to keep the setules properly posi-tioned. The absence of mechanoreceptors and muscles in thesetae and setules of Diaptomus precludes the possibility ofany form of mechanical size selection which requires themeasurement of the linear dimensions of food particles, orfine-scale setal adjustments.

In summary, it is most likely that herbivorous zooplanktonselect their food on the basis of olfaction rather than size.Rejection of bad-tasting particles has been observed as an ac-tive process; the question remains if the perception of pre-ferred food induces positive selective behavior. In the case ofthe oil droplets discussed earlier, they were obviously not re-jected as toxic or unnatural food particles.

M.M.F. thanks Dr. F. L. Schuster, Mrs. B. Hershenov, and Dr. D.D. Hurst of Brooklyn College for technical training and for the useof the electron microscopes, and the Coates & Welter InstrumentCo. for the loan of a scanning microscope. We thank Prof. D. R.Idler, Director, Marine Sciences Research Laboratory, MemorialUniversity of Newfoundland, for the use of facilities under his di-rection. J. Gerritsen, R. Sommers, and T. Stenovec conducted the

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"Y" tests under the guidance of J.R.S. A Baltimore Gas & ElectricCo. fellowship and a Sigma Xi grant helped support M.M.F. in thisresearch. J.R.S. gratefully acknowledges the support of the donorsof the Petroleum Research Fund, administered by the AmericanChemical Society (Grant 2740-Gl). This is contribution no. 217 ofthe Marine Sciences Research Laboratory.

1. Conover, R. J. (1971) J. Fish. Res. Board Can. 28, 1327-1330.2. Hutchinson, G. E. (1967) A Treatise on Limnology (John

Wiley & Sons, New York), Vol. II, 1115 pp.3. Marshall, S. M. (1973) in Advances in Marine Biology, eds.

Russell, F. & Yonge, M. (Academic Press, London), Vol. 11,pp. 57-120.

4. Richman, S. & Rogers, J. N. (1969) Limnol. Oceanogr. 14,701-709.

5. Lowndes, A. G. (1935) Proc. Zool. Soc. London 3,687-715.6. Conover, R. J. (1966) in Some Contemporary Studies in Ma-

rine Science, ed. Barnes, H. (Allen & Unwin, London), pp.187-194.

7. Poulet, S. A. & Chanut, J. P. (1975) J. Fish. Res. Board Can.32,706-713.

8. Marshall, S. M. & Orr, A. P. (1955) The Biology of a MarineCopepod (Oliver & Boyd, Edinburgh), 188 pp.

9. Mullin, M. M. (1963) Limnol. Oceanogr. 8,239-250.10. Wilson, D. S. (1973) Ecology 54,909-914.11. Elofson, R. (1971) Acta Zool. 52,299-315.12. Strickler, J. R. & Bal, A. K. (1973) Proc. Nat. Acad. Sci. USA

70,2656-2659.13. Fleminger, A. (1973) Fish. Bull. 71, 965-1010.14. Strickler, J. R. (1974) Verh. Int. Ver. Theor. Angew. Limnol.,

in press.15. Ong, J. E. (1969) Z. Zellforsch. Mikrosk. Anat. 97, 178-195.16. Slifer, E. (1970) Annu. Rev. Entomol. 15, 121-142.17. Schmidt, K. (1972) Verh. Dtsch. Ges. Zool. 66, 15-25.18. McIver, S. B. (1975) Annu. Rev. Entomol. 20, 381-397.

19. Kaissling, K. (1971) in Handbook of Sensory Physiology, ed.Beidler, L. M. (Springer-Verlag, Berlin), Vol. IV, part I, pp.351-431.

20. Hayes, W. F. (1971) J. Morphol. 133,205-240.21. Ernst, K. (1969) Z. Zellforsch. Mikrosk. Anat. 94,72-102.22. Altner, H. & Thies, G. (1972) Z. Zellforsch. Mikrosk. Anat.

129,196-216.23. Mustaparta, H. (1973) Z. Zellforsch. Mikrosk. Anat. 144,

559-571.24. Steinbrecht, R. A. (1973) Z. Zellforsch. Mikrosk. Anat. 139,

533-65.25. Provasoli, L., Shiraishi, K. & Lance, J. (1959) Ann. N.Y. Acad.

Sci. 77,250-261.26. Paffenh6fer, G.-A. (1970) Helgol. Wiss. Meeresunters. 20,

346-359.27. Laverack, M. S. (1963) Comp. Biochem. Physiol. 8, 141-151.28. Case, J. (1964) Biol. Bull. (Woods Hole, Mass.) 127,428-446.29. Dethier, V. G. (1970) in Chemical Ecology, eds. Sondheimer,

E. & Simeone, J. (Academic Press, New York), pp. 83-102.30. Ache, B. W. (1972) Comp. Biochem. Physiol. A42, 807-811.31. Lee, R. F., Nevenzel, J. C. & Paffenh6fer, G.-A. (1972) Natur-

wissenschaften 59, 406-411.32. Whittaker, R. H. & Feeny, P. P. (1971) Science 171,757-770.33. Katona, S. K. (1973) Limnol. Oceanogr. 18,574-583.34. Galun, R. & Margalit, J. (1969) Nature 222,583-584.35. Case, J., Gwilliam, G. F. & Hanson, F. (1960) Biol. Bull.

(Woods Hole, Mass.) 119,308 (abstract).36. Levandowsky, M. & Hodgson, E. S. (1965) Comp. Biochem.

Physiol. 16,159-161.37. Gaudy, R. (1974) Mar. Biol. 25, 125-141.38. Bainbridge, R. (1953) J. Mar. Biol. Assoc. UK 32,385-447.39. Sch6ne, H. (1972) Fortschr. Zool. 21, 1-19.40. Strickler, J. R. (1975) Proc. First Internat. Symp. on Swim-

ming and Flying in Nature, Caltech., in press.41. Gharagozlou-van Ginneken, I. D. & Bouligand, Y. (1973) Tis-

sue & Cell 5, 413-439.

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