Proc. Nat. Acad. Sci. USA Vol. 72, No. 10, pp. 4185-4188, October 1975
Zoology
Chemoreceptors and feeding in calanoid copepods (Arthropoda: Crustacea) (sensillum/zooplankton/selective feeding/oil-spill dispersal) MARC M. FRIEDMAN* AND J. RUDI STRICKLER Department 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 of the calanoid copepod Diaptomus pallidus have revealed the presence of numerous chemoreceptors, and the apparent absence of mechanoreceptors. The setae contain no muscles, and the setules are noncellular extensions of their chitin wall. This allows a new insight into the selective feeding of zooplankters.
On February 4, 1970, the tanker Arrow was wrecked in Chedabucto Bay, Nova Scotia, and a quantity of bunker C oil was released into the sea. Conover (1) studied the effects of the oil on the dominant calanoid copepod, Temora longicornis. He found that the copepod: (1) actively ingested oil droplets which were within its normal food size range, and may actually have selected the oil droplets over natural particulate food, and (2) was apparently unharmed by the oil. Conover calculated that a significant portion of the spill was removed from the water column via sedimentation in the animals' fecal pellets, leading him to suggest that grazers might be ". . the single most important natural agent leading to eventual dispersal and degradation of oil spills ...
SELECTIVE FEEDING To us, the critical question here is why do the copepods eat the oil particles? Certainly, they cannot be used to them as a natural food. Perhaps the most obvious answer is that copepods are nonselective feeders and will ingest any particles within their normal food size range. There is, however, a large body of evidence which suggests that these animals are indeed selective feeders (cf. 2 and 3), and Marshall (3) has concluded: "It is clear then that copepods can and do select particular foods but they do not do so all the time, and their preferences may change." The ingestion of the oil droplets might have been facilitated by some physical property which makes them sticky and therefore more easily trapped, but then they would have been correspondingly harder to handle and "swallow." Visual cues can be eliminated because selective feeding has been demonstrated in the dark (4), and because most calanoids feed near the water's surface at night. We have, however, from our own experiments and from the literature (e.g., 2, 3, 5-7), evidence to support the suggestion that chemical information is used by copepods to make feeding decisions. We have observed individuals of Diaptomus minutus and D. pallidus grasp and then reject detritus which was carried to the mouth by the feeding current. Before rejection, the detritus was held in the mouthparts and handled, as if it were being tasted. Conover (6) found that individuals of Calanus, offered their own fecal pellets, tore open the surrounding membranes as if to taste the pellets before rejecting them. Marshall and Orr (8) and *
Present address: Department of Anatomy, School of Medicine, The Johns Hopkins University, Baltimore, Md. 21205. 4185
Conover (6) have reported that Calanus becomes accustomed to a particular food and will feed on it in preference to other, even larger, food. Additionally, the animals' filtering rate is drastically decreased in senescent algal food cultures (9). There are also field reports of copepods feeding preferentially or exclusively on one alga when several species are available (cf. 2 and 3). There is another line of evidence which suggests that information about particle size or shape can be used to choose food items. For example, experiments by Richman and Rogers (4) and Wilson (10) have indicated that copepods prefer larger particles to smaller ones, prompting Wilson to propose a model for mechanical size-selectivity.
SENSORY RECEPTORS The unifying theme here is that it is impossible to account for all of the observed feeding behavior without assuming some kind of sensory input. C. M. Boyd (personal communication) has offered a simple model which can partially explain the apparent preference for larger particles: the filtering mesh is pictured as a leaky sieve, and the larger a particle is, the greater its chance of being retained once encountered, resulting in an apparent selection for the larger particles. However, any selective feeding mechanism which requires an adjustment of the ongoing feeding process also requires an input of information about some property of the food. We and other workers have found sensory receptors in the head region (11, 12) and on the body surface (13, 14) of copepods. There are, however, few reports of electron microscopic studies of the mouthparts which do the actual food handling. Ong (15) reported finding receptors inside the mouth of the brackish water calanoid Gladioferens, in the mandibles and labrum. These receptors could not be used to gather information usable during handling of food, before it enters the mouth. The purpose of our research was to determine (1) what kind of receptors are located on or in the mouthparts of a filter-feeding copepod, and (2) how they might be employed for selective feeding. We chose the fresh water Diaptomus pallidus, about 1 mm long, which is easily obtained and kept in our laboratory. The animal is an obligate filter-feeder and any mouthpart receptors can be expected to function during filter-feeding. Possible complicating sensory factors which might be introduced by a raptorial feeding mode are thus precluded. Animals to be examined by transmission electron microscopy were fixed in Karnovsky's fixative, posf-fixed in 2% osmium tetroxide, and embedded in Maraglas. Specimens for scanning microscopy were processed as above through osmium post-fixation, dehydrated in an ethanol series and acetone, and critical-point dried. They were then coated with gold-palladium and kept vacuum dessicated.
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Proc. Nat. Acad. Sci. USA 72 (1975)
palps (lower lips). We have examined in particular detail the receptors in the mandibles (inside the mouth), mandibular palps, and in the first and second maxillae-appendages which handle the food. The receptors belong to one of two distinct general morphologies, which we shall designate here as 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 the ciliary region, in a cuticular sheath. According to Ong (15), in Gladioferens this cuticular sheath ends in a pore at the tip of the mandible. Each receptor contains one or two ciliary dendrites and sometimes also a smaller dendrite containing a few neurotubules. No evidence of typical basal body structure has been found in the smaller dendrites. Ong reported that some of the neurons innervating similar mandibular receptors in Gladioferens were nonciliary and called them chemoreceptors, while the ciliary neurons, he hypothesized, were mechanoreceptors. It is now known that both chemoand mechanoreceptors possess ciliary neurons (16, 17), and we have determined that the neurons reported by Ong to be non-ciliary in Gladioferens are ciliary in Diaptomus (it is rather easy to miss the basal bodies). There is currently no basis to assign a mechanoreceptor function to any of these receptors, as they do not possess the microtubule bundle which is characteristic of known mechanoreceptors (17, 18). On the contrary, these mandibular receptors resemble contact chemoreceptors described by Slifer (16) as peg-in-pit sensilla and by Kaissling (19) as sensilla ampullacea. Electrophysiological studies have shown that similar receptors in the 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 these appendages are the sensilla (Figs 1 and 2). These setal sensil-
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 appendages of Diaptomus, shown in Fig. 1. Scanning microscopy failed to reveal any external structures for which we might propose a receptor function, except on the first and perhaps second antennae (cf. 12). However, transmission microscopy has revealed receptors in all of the appendages except the labial
VN 1M. . iR :1 '
J
,
.
'
_
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 of seta; X120,000.
Zoology:
Friedman and Strickler
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, but due to positioning difficulties we have not yet determined their exact distribution. The setal wall in this region appears distinctly different from the normal chitin in the basal portion of the seta. Each sensillum contains 1 to 5 dendrites, 1 or 2 of which are ciliary and form 9 + 0 basal bodies, with the accompanying dendrites forming only an unstructured array of doublet microtubules. The presence of several neurons which can apparently communicate with the outside of the seta through a pore system (16, 19), and the absence of a "tubular bundle" (17, 18) clearly indicate that these setal sensilla are chemoreceptors. They are structurally similar to the hair chemoreceptors found in other arthropods (21-24). DISCUSSION What kinds of chemical information might the mouthpart chemoreceptors respond to, and how could they then affect feeding behavior? We have evaluated the available experimental and field data and consider the following hypothetical functions worthy of investigation: (1) Attempts to culture copepods have shown that for the long term maintenance of reproducing populations, multialgal diets and/or bacteria, vitamins, or other "critical substances" must be supplied (25, 26). The precise limiting substances for copepods are as yet unidentified, but chemoreceptors of crustaceans and insects are known to respond to such substances as amines, amino acids, sugars and salts (27-30). Lee et al. (31), have found that copepods in deep water and high latitudes depend on energy from stored wax esters for reproduction and overwintering. These wax esters are synthesized from algal fatty acids which are available in the diet during the seasonal algal bloom and which would be logical targets of selective feeding. Additionally, if copepods employ allelochemics (32) such as pheromones during mating (33), then they or their precursors could be detected in the appropriate algae. Furthermore, the role of chemoreceptors as regulators of the feeding process has already been demonstrated in some insects (29, 34) and crustacenas (35, 36). Therefore, the location of the copepod chemoreceptors in the feeding chamber is ideal in that it would allow the animals to monitor both the incoming food and water. (2) If copepods can discriminate between algal types by smell, then chemoreceptors tuned to these smells would be effective guides for the separation of feeding niches. Lowndes' (5) observation that Eudiaptomus gracilis, although found in the plankton, was nevertheless feeding exclusively on a species of benthic desmid, could be explained by such receptor tuning. (3) The use of mouthpart chemoreceptors to recognize and refrain from eating con-specific larvae might be advantageous, particularly in species which produce few eggs and retain them in egg sacs. Recognition of the larvae could lead to the inhibition of the feeding mechanism and the subsequent release of the larvae unharmed. We note that most studies in which cannibalism has been reported have used adults which were starved to varying degrees (e.g., 37). (4) It is even conceivable that chemoreceptors could be used to differentiate between large and small algae, effecting "size-selective" feeding. Each collision between an alga and a setal receptor would constitute a stimulus, and would trigger a receptor response. A larger alga would contact a larger area of the seta and thereby a larger number of pores and receptor sites, and so the response could be pro-
Proc. Nat. Acad. Sci. USA 72 (1975)
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portional to the size of the alga. A higher response frequency might then indicate to the animal a larger alga. It is not known, however, whether the copepod nervous system is capable of processing this information. At any rate, the same results might be obtained more simply by using, for example, gut stretch receptors. In either case the animals would need to have some means of adjusting the feeding mechanism in response to the information received. The rate of feeding could also be adjusted, in response to changes in food concentration (cf. 9), and feeding could be shut down entirely during high concentrations of algal toxins. FURTHER CONSIDERATIONS The experimental results of Bainbridge (38) suggest that zooplankters might locate food patches via chemotaxis. Mechanisms for distance orientation were analyzed by Schone (39), and the copepod receptors as a group could be used for "chemotropotaxis" (Sch6ne's Table I, p. 16). A gradient would be detected by measuring the odorant concentration at one point, moving a known distance in a known direction, then remeasuring and comparing. By "remembering" the direction of movement through the use of proprioceptors (40), the animal could calculate the gradient. However, the slow gliding motion of herbivorous calanoids moves the animals at most about 20 cm between jumps, a distance too short to measure the diffuse gradients in aquatic feeding environments. Also, infrared movies have shown that the jumps are random, and the direction of any particular jump is not predictable. To verify our conclusion we placed zooplankters in the base of a "Y" apparatus which was sealed off from the arms of the "Y" by a removable partition. Into one arm we perfused filtered pond water, and into the other arm an algal suspension. The partition was then removed and the animals moved up the base and into one or the other of the arms, displaying negative rheotaxis. We have repeatedly been unable to detect any bias toward the arm with the algae. The animals will, however, respond to a temperature gradient of 50, showing that the "Y" can work. Thus we must tentatively conclude that chemotropotaxis 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 mechanical size selectivity could be carried out. Examination of the setules revealed that they are non-cellular extensions of the setal chitin wall and contain what appear to be groups of fibers which may serve to keep the setules properly positioned. The absence of mechanoreceptors and muscles in the setae and setules of Diaptomus precludes the possibility of any form of mechanical size selection which requires the measurement of the linear dimensions of food particles, or fine-scale setal adjustments. In summary, it is most likely that herbivorous zooplankton select their food on the basis of olfaction rather than size. Rejection of bad-tasting particles has been observed as an active process; the question remains if the perception of preferred food induces positive selective behavior. In the case of the oil droplets discussed earlier, they were obviously not rejected 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 use of the electron microscopes, and the Coates & Welter Instrument Co. for the loan of a scanning microscope. We thank Prof. D. R. Idler, Director, Marine Sciences Research Laboratory, Memorial University of Newfoundland, for the use of facilities under his direction. 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 & Electric Co. fellowship and a Sigma Xi grant helped support M.M.F. in this research. J.R.S. gratefully acknowledges the support of the donors of the Petroleum Research Fund, administered by the American Chemical Society (Grant 2740-Gl). This is contribution no. 217 of the 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 Marine 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 Marine Copepod (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.
Proc. Nat. Acad. Sci. USA 72 (1975) 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) Naturwissenschaften 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 Swimming and Flying in Nature, Caltech., in press. 41. Gharagozlou-van Ginneken, I. D. & Bouligand, Y. (1973) Tissue & Cell 5, 413-439.
Zoology
Chemoreceptors and feeding in calanoid copepods (Arthropoda: Crustacea) (sensillum/zooplankton/selective feeding/oil-spill dispersal) MARC M. FRIEDMAN* AND J. RUDI STRICKLER Department 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 of the calanoid copepod Diaptomus pallidus have revealed the presence of numerous chemoreceptors, and the apparent absence of mechanoreceptors. The setae contain no muscles, and the setules are noncellular extensions of their chitin wall. This allows a new insight into the selective feeding of zooplankters.
On February 4, 1970, the tanker Arrow was wrecked in Chedabucto Bay, Nova Scotia, and a quantity of bunker C oil was released into the sea. Conover (1) studied the effects of the oil on the dominant calanoid copepod, Temora longicornis. He found that the copepod: (1) actively ingested oil droplets which were within its normal food size range, and may actually have selected the oil droplets over natural particulate food, and (2) was apparently unharmed by the oil. Conover calculated that a significant portion of the spill was removed from the water column via sedimentation in the animals' fecal pellets, leading him to suggest that grazers might be ". . the single most important natural agent leading to eventual dispersal and degradation of oil spills ...
SELECTIVE FEEDING To us, the critical question here is why do the copepods eat the oil particles? Certainly, they cannot be used to them as a natural food. Perhaps the most obvious answer is that copepods are nonselective feeders and will ingest any particles within their normal food size range. There is, however, a large body of evidence which suggests that these animals are indeed selective feeders (cf. 2 and 3), and Marshall (3) has concluded: "It is clear then that copepods can and do select particular foods but they do not do so all the time, and their preferences may change." The ingestion of the oil droplets might have been facilitated by some physical property which makes them sticky and therefore more easily trapped, but then they would have been correspondingly harder to handle and "swallow." Visual cues can be eliminated because selective feeding has been demonstrated in the dark (4), and because most calanoids feed near the water's surface at night. We have, however, from our own experiments and from the literature (e.g., 2, 3, 5-7), evidence to support the suggestion that chemical information is used by copepods to make feeding decisions. We have observed individuals of Diaptomus minutus and D. pallidus grasp and then reject detritus which was carried to the mouth by the feeding current. Before rejection, the detritus was held in the mouthparts and handled, as if it were being tasted. Conover (6) found that individuals of Calanus, offered their own fecal pellets, tore open the surrounding membranes as if to taste the pellets before rejecting them. Marshall and Orr (8) and *
Present address: Department of Anatomy, School of Medicine, The Johns Hopkins University, Baltimore, Md. 21205. 4185
Conover (6) have reported that Calanus becomes accustomed to a particular food and will feed on it in preference to other, even larger, food. Additionally, the animals' filtering rate is drastically decreased in senescent algal food cultures (9). There are also field reports of copepods feeding preferentially or exclusively on one alga when several species are available (cf. 2 and 3). There is another line of evidence which suggests that information about particle size or shape can be used to choose food items. For example, experiments by Richman and Rogers (4) and Wilson (10) have indicated that copepods prefer larger particles to smaller ones, prompting Wilson to propose a model for mechanical size-selectivity.
SENSORY RECEPTORS The unifying theme here is that it is impossible to account for all of the observed feeding behavior without assuming some kind of sensory input. C. M. Boyd (personal communication) has offered a simple model which can partially explain the apparent preference for larger particles: the filtering mesh is pictured as a leaky sieve, and the larger a particle is, the greater its chance of being retained once encountered, resulting in an apparent selection for the larger particles. However, any selective feeding mechanism which requires an adjustment of the ongoing feeding process also requires an input of information about some property of the food. We and other workers have found sensory receptors in the head region (11, 12) and on the body surface (13, 14) of copepods. There are, however, few reports of electron microscopic studies of the mouthparts which do the actual food handling. Ong (15) reported finding receptors inside the mouth of the brackish water calanoid Gladioferens, in the mandibles and labrum. These receptors could not be used to gather information usable during handling of food, before it enters the mouth. The purpose of our research was to determine (1) what kind of receptors are located on or in the mouthparts of a filter-feeding copepod, and (2) how they might be employed for selective feeding. We chose the fresh water Diaptomus pallidus, about 1 mm long, which is easily obtained and kept in our laboratory. The animal is an obligate filter-feeder and any mouthpart receptors can be expected to function during filter-feeding. Possible complicating sensory factors which might be introduced by a raptorial feeding mode are thus precluded. Animals to be examined by transmission electron microscopy were fixed in Karnovsky's fixative, posf-fixed in 2% osmium tetroxide, and embedded in Maraglas. Specimens for scanning microscopy were processed as above through osmium post-fixation, dehydrated in an ethanol series and acetone, and critical-point dried. They were then coated with gold-palladium and kept vacuum dessicated.
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Proc. Nat. Acad. Sci. USA 72 (1975)
palps (lower lips). We have examined in particular detail the receptors in the mandibles (inside the mouth), mandibular palps, and in the first and second maxillae-appendages which handle the food. The receptors belong to one of two distinct general morphologies, which we shall designate here as 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 the ciliary region, in a cuticular sheath. According to Ong (15), in Gladioferens this cuticular sheath ends in a pore at the tip of the mandible. Each receptor contains one or two ciliary dendrites and sometimes also a smaller dendrite containing a few neurotubules. No evidence of typical basal body structure has been found in the smaller dendrites. Ong reported that some of the neurons innervating similar mandibular receptors in Gladioferens were nonciliary and called them chemoreceptors, while the ciliary neurons, he hypothesized, were mechanoreceptors. It is now known that both chemoand mechanoreceptors possess ciliary neurons (16, 17), and we have determined that the neurons reported by Ong to be non-ciliary in Gladioferens are ciliary in Diaptomus (it is rather easy to miss the basal bodies). There is currently no basis to assign a mechanoreceptor function to any of these receptors, as they do not possess the microtubule bundle which is characteristic of known mechanoreceptors (17, 18). On the contrary, these mandibular receptors resemble contact chemoreceptors described by Slifer (16) as peg-in-pit sensilla and by Kaissling (19) as sensilla ampullacea. Electrophysiological studies have shown that similar receptors in the 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 these appendages are the sensilla (Figs 1 and 2). These setal sensil-
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 appendages of Diaptomus, shown in Fig. 1. Scanning microscopy failed to reveal any external structures for which we might propose a receptor function, except on the first and perhaps second antennae (cf. 12). However, transmission microscopy has revealed receptors in all of the appendages except the labial
VN 1M. . iR :1 '
J
,
.
'
_
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 of seta; X120,000.
Zoology:
Friedman and Strickler
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, but due to positioning difficulties we have not yet determined their exact distribution. The setal wall in this region appears distinctly different from the normal chitin in the basal portion of the seta. Each sensillum contains 1 to 5 dendrites, 1 or 2 of which are ciliary and form 9 + 0 basal bodies, with the accompanying dendrites forming only an unstructured array of doublet microtubules. The presence of several neurons which can apparently communicate with the outside of the seta through a pore system (16, 19), and the absence of a "tubular bundle" (17, 18) clearly indicate that these setal sensilla are chemoreceptors. They are structurally similar to the hair chemoreceptors found in other arthropods (21-24). DISCUSSION What kinds of chemical information might the mouthpart chemoreceptors respond to, and how could they then affect feeding behavior? We have evaluated the available experimental and field data and consider the following hypothetical functions worthy of investigation: (1) Attempts to culture copepods have shown that for the long term maintenance of reproducing populations, multialgal diets and/or bacteria, vitamins, or other "critical substances" must be supplied (25, 26). The precise limiting substances for copepods are as yet unidentified, but chemoreceptors of crustaceans and insects are known to respond to such substances as amines, amino acids, sugars and salts (27-30). Lee et al. (31), have found that copepods in deep water and high latitudes depend on energy from stored wax esters for reproduction and overwintering. These wax esters are synthesized from algal fatty acids which are available in the diet during the seasonal algal bloom and which would be logical targets of selective feeding. Additionally, if copepods employ allelochemics (32) such as pheromones during mating (33), then they or their precursors could be detected in the appropriate algae. Furthermore, the role of chemoreceptors as regulators of the feeding process has already been demonstrated in some insects (29, 34) and crustacenas (35, 36). Therefore, the location of the copepod chemoreceptors in the feeding chamber is ideal in that it would allow the animals to monitor both the incoming food and water. (2) If copepods can discriminate between algal types by smell, then chemoreceptors tuned to these smells would be effective guides for the separation of feeding niches. Lowndes' (5) observation that Eudiaptomus gracilis, although found in the plankton, was nevertheless feeding exclusively on a species of benthic desmid, could be explained by such receptor tuning. (3) The use of mouthpart chemoreceptors to recognize and refrain from eating con-specific larvae might be advantageous, particularly in species which produce few eggs and retain them in egg sacs. Recognition of the larvae could lead to the inhibition of the feeding mechanism and the subsequent release of the larvae unharmed. We note that most studies in which cannibalism has been reported have used adults which were starved to varying degrees (e.g., 37). (4) It is even conceivable that chemoreceptors could be used to differentiate between large and small algae, effecting "size-selective" feeding. Each collision between an alga and a setal receptor would constitute a stimulus, and would trigger a receptor response. A larger alga would contact a larger area of the seta and thereby a larger number of pores and receptor sites, and so the response could be pro-
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portional to the size of the alga. A higher response frequency might then indicate to the animal a larger alga. It is not known, however, whether the copepod nervous system is capable of processing this information. At any rate, the same results might be obtained more simply by using, for example, gut stretch receptors. In either case the animals would need to have some means of adjusting the feeding mechanism in response to the information received. The rate of feeding could also be adjusted, in response to changes in food concentration (cf. 9), and feeding could be shut down entirely during high concentrations of algal toxins. FURTHER CONSIDERATIONS The experimental results of Bainbridge (38) suggest that zooplankters might locate food patches via chemotaxis. Mechanisms for distance orientation were analyzed by Schone (39), and the copepod receptors as a group could be used for "chemotropotaxis" (Sch6ne's Table I, p. 16). A gradient would be detected by measuring the odorant concentration at one point, moving a known distance in a known direction, then remeasuring and comparing. By "remembering" the direction of movement through the use of proprioceptors (40), the animal could calculate the gradient. However, the slow gliding motion of herbivorous calanoids moves the animals at most about 20 cm between jumps, a distance too short to measure the diffuse gradients in aquatic feeding environments. Also, infrared movies have shown that the jumps are random, and the direction of any particular jump is not predictable. To verify our conclusion we placed zooplankters in the base of a "Y" apparatus which was sealed off from the arms of the "Y" by a removable partition. Into one arm we perfused filtered pond water, and into the other arm an algal suspension. The partition was then removed and the animals moved up the base and into one or the other of the arms, displaying negative rheotaxis. We have repeatedly been unable to detect any bias toward the arm with the algae. The animals will, however, respond to a temperature gradient of 50, showing that the "Y" can work. Thus we must tentatively conclude that chemotropotaxis 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 mechanical size selectivity could be carried out. Examination of the setules revealed that they are non-cellular extensions of the setal chitin wall and contain what appear to be groups of fibers which may serve to keep the setules properly positioned. The absence of mechanoreceptors and muscles in the setae and setules of Diaptomus precludes the possibility of any form of mechanical size selection which requires the measurement of the linear dimensions of food particles, or fine-scale setal adjustments. In summary, it is most likely that herbivorous zooplankton select their food on the basis of olfaction rather than size. Rejection of bad-tasting particles has been observed as an active process; the question remains if the perception of preferred food induces positive selective behavior. In the case of the oil droplets discussed earlier, they were obviously not rejected 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 use of the electron microscopes, and the Coates & Welter Instrument Co. for the loan of a scanning microscope. We thank Prof. D. R. Idler, Director, Marine Sciences Research Laboratory, Memorial University of Newfoundland, for the use of facilities under his direction. 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 & Electric Co. fellowship and a Sigma Xi grant helped support M.M.F. in this research. J.R.S. gratefully acknowledges the support of the donors of the Petroleum Research Fund, administered by the American Chemical Society (Grant 2740-Gl). This is contribution no. 217 of the 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 Marine 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 Marine Copepod (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
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