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J. exp. Biol. 140, 381-391 (1988) Printed in Great Britain © The Company of Biologists Limited 1988
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THE FUNCTIONS OF EYE AND BODY MOVEMENTS IN LABIDOCERA AND OTHER COPEPODS BY MICHAEL F. LAND School of Biological Sciences, University of Sussex, Brighton BN1 9QG, UK Accepted 28 April 1988 Summary Male Labidocera have three eyes derived from the nauplius eye. The dorsal pair have large lenses and are fused to give a single mobile eyecup containing a transverse row of 10 rhabdoms. This rhabdom row scans across 35° of the dorsal visual field at rates of up to 3 Hz, and it is suggested that the scanning movements are concerned with the detection of conspecifics. In addition there are movements which enable the eyecups to track a source of light, and these are mediated by three other receptors in each eyecup. The tracking movements are closely linked to tail movements which tend to keep the animal's back directed towards the light. The total effect is to stabilize both body and eye direction against involuntary disturbance. Other pontellid copepods show similar visually controlled tail movements, but without the eye movements. Introduction
In most copepods the small tripartite nauplius eye is immobile, and its functions are not well understood. It is generally agreed that the eye plays a role in orientation by kinesis (changes of speed) or taxis (guided direction changes) (Bainbridge, 1961). In a few copepod groups, however, the eyes are larger, better developed and often mobile. The best known example is probably Copilia (Sapphirinidae) whose scanning eye movements have been described by Exner (1891) and more recently by Downing (1972). Equally interesting, however, are the eyes of Pontellidae. In these large oceanic copepods the components of the nauplius eye have separated into three distinct eyes, two dorsal and one ventral (Vaissiere, 1961; Land, 1984). They are sexually dimorphic, with either the ventral eye {Pontella, Anomalocera) or the dorsal eyes {Labidocera) enlarged in the male, so there is a strong presumption that the eyes are involved in locating potential mates. What makes this idea particularly intriguing is the small numbers of receptors involved; the ventral eyes of male Pontella have only six receptors, and the dorsal eyes in male Labidocera only eight receptors each. The dorsal eyes of male Labidocera are unique in several ways. They have spherical lenses like those of fish; the two retinae are joined to form a single eyecup, with five of the eight receptors in each cup arranged as a transverse line Key words: copepods, eye movements, vision, scanning
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Fig. 1. (A) Head of a male Labidocera acutifrons seen from the side, showing the eyecup in its extreme anterior and posterior positions. Note the muscle behind the cup and elastic ligament in front. (B) Projection of the line of rhabdoms in the eyecup onto a sphere around the animal, seen from above. The two lines show the rhabdoms at the extremes of a scan. Because the lenses invert the image, the outermost rhabdoms in the image of the line are numbers 4 and 5 (see Fig. 2) and the central pair are the joined images of rhabdoms 2 and 1. Partly from Land (1984).
rather than a two-dimensional surface; and finally the eyecups can be moved in the anteroposterior direction by muscles at right angles to the line of receptors (Fig. 1). These scanning movements were first described by Parker (1891), and consist of bouts of back-and-forth movements with an amplitude of about 35°. As the projection of the retinal line is also about 40°, the eyes sweep out a roughly square region of space above the head with each scan (Land, 1984). These scanning eye movements are spontaneous in the sense that they have no obvious trigger and seem to be unaffected by the presence or absence of visible structures in the surroundings. In this paper I describe an additional kind of eye movement which differs from scanning in that it is driven directly by the movement of a light source around the animal. As well as causing eye movements, moving the light also makes the tail move up or down, resulting in pitching movements of the animal. In the Discussion it is argued that both eye and tail movements are parts of a system for ensuring that the retinae point accurately upwards, presumably so that the scanning system can function effectively in spite of wave action. Materials and methods
The observations described here were made during Cruise 168 of the RRS Discovery, at stations in the North Atlantic in the vicinity of 20°N20°W. Animals were collected from the surface waters with either a neuston or plankton net. Labidocera males are immediately recognizable by the large dorsal lenses. The species was not established. The animals swam vigorously in a dish, and frequently
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became caught with their antennae in the meniscus at the edge. With the animals trapped in this way the eye movements were filmed through a dissecting microscope using a portable video camera (Sanyo VC500) and recorder (VTC7100) which had a single-frame playback facility. The films were analysed frame by frame, the position of the retina relative to the lens centre being measured directly from the monitor screen. To induce movements of the eyes and tail a light guide was rotated around the animal in various planes while the animal was filmed, or more generally just watched. For anatomical work animals were fixed for routine light microscopy, sectioned in wax at 4/^m and stained with haematoxylin and eosin. Rhabdoms take up eosin strongly and are thus easy to identify. Results Anatomy The remarkable anatomy of the eye of male Labidocera was first described by Parker (1891). At that time he identified the genus as Pontella. He gives drawings of a complete series of sections of the eyecup, and it is from these that the reconstruction in Fig. 2 was made. My own sections confirm Parker's work in detail. His description identifies the five slab-like rhabdoms that make up a line that lies in the focal plane of the lens (Land, 1984). However, what is interesting in the present context is Parker's description of three other rhabdoms, two (7 and 8) lying anterior to the line, and one (6) behind it. All three are much closer to the lens than the line, and are thus relatively out of focus. I shall argue later that the line (rhabdoms 1-5) and the three other rhabdoms (6-8) are functionally distinct systems. Parker's numbering system is used here. The smaller eye of the female contains the same elements as the male, and it is interesting to see how the two are related. Fig. 2 shows the disposition of the rhabdoms within the female eye, which has not been described before. Seen from above or the side it appears to be made up of three lobes and, going from anterior backwards, these are found to contain three, three and two rhabdoms, respectively. In the central lobe there is a hint of the linear arrangement of the male, but with three rather than five rhabdoms. What seems to have happened in the male is that the central lobe has 'borrowed' the nearest rhabdom from each of the adjacent lobes, and added this to the line. Thus rhabdoms numbered 2 and 4 of the female eye are probably homologous with the same-numbered rhabdoms in the line of the male. This then leaves only rhabdom 6 in the posterior cup, and 7 and 8 in the anterior cup. The female eyes are not linked to each other in any way, whereas the male eyecups are joined physically. Scanning Parker (1891) also gave an excellent description of the eye movements of Labidocera. 'The two retinas may be rotated on their lenses through an angle of about
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forty-five degrees. The plane of rotation corresponds to the sagittal plane of the body, and the rotation is accomplished by two pairs of muscles, one for each retina By contraction of the posterior muscle, the retina may be drawn upward and backward over the surface of the lens, till its axis, instead of pointing dorsally, is directed forward and upward at an angle of about forty-five degrees with its original position. The retina is not usually held for any great length of time in this position, but is soon returned by the contraction of the anterior muscle to its normal place. The backward motion of the retina is accomplished with such rapidity that the animal has the appearance of winking. The forward motion is rather slower.' My films of male Labidocera eye movements confirm Parker's description in most respects. The movements occur in bouts lasting from a few seconds to a minute. Often there were many minutes between bouts, and at least in the dish the animals spent only a small fraction of their time scanning. Fig. 3 is a record of a 10-s bout of scanning. It shows that the eye's 'rest' position is with the retina
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Fig. 2. Horizontal and saggittal sections through male (top) and female eyecups, showing the locations of the rhabdoms. A—P is the anteroposterior body axis, D and V are dorsal and ventral, ON is the optic nerve. The explanation of the numbering is in the text. The male lens is about 150jum in diameter, and the female lens 70fim.
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Fig. 3. Spontaneous scanning movements of the retina measured from a video film over a 10-s period. Upper and lower records are continuous. Note fast forward and slower rearward movements of the eyecup.
forwards, and that each scan involves a backward movement of the retina, followed after a pause of about one-third of a second by a return to the forward position. In contrast to Parker's remark, the backward motion was the slower in these films. The calculated angular velocities of the eye axis were 219 ± 90degreess" 1 (S.D.) for the backward movements of the retina, and 455 ± 161 degrees s" 1 for the forward movements. These values are based on the 11 movements analysed in Fig. 3, and the difference in the means is highly significant ( P < 0-0001); all animals studied (about 10) showed qualitatively similar speed differences between the backward and forward movements. It is probable that the forward movements are an elastic recoil; the only striated muscles I could find were the ones attached to the rear of the eyecup, the much thinner band at the front having no detectable banding. In the scanning bout shown in Fig. 3 the excursion of the eye's axis is only around 20°. Larger scanning movements were seen, but the maximum seemed to be in the range 35-40°, rather than 45°. The frequency of scans varied between bouts, up to a maximum of about 2-7 Hz, although l H z was a more common rate. The variation was nearly all due to differences in the time that the retina stayed in the forward position; the times spent in the rear position were much more constant (0-3 ± 0-12s compared with 0-35 ±0-34s for 34 scans). It thus seems that the 'unit scan' involves a pulse of activity in the posterior muscle lasting about 0-3 s, there being rather variable intervals between these pulses. Moving a light around the eye does not seem to interfere with the rhythm of scanning. Scanning movements are not accompanied by movements of the tail, as are the orienting movements discussed below. Orienting movements of the eyes and tail in Labidocera Moving a light source around an animal from anterior to dorsal to posterior has the effect of pulling the eye with it. The eye moves in such a way that its axis always
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points towards the light over the full range of possible motion: that is over a 20-25° excursion on either side of the vertical. If the light moves beyond this range, then the eyes remain stuck in the direction closest to the light. If the light source comes from the front or rear the eyes move to centre the light well before it has crossed the eyes' axis, indicating that the narrow lateral line of in-focus receptors is not likely to be involved. Turning the light on and off also produces an eye movement towards the light and then back to a rest position. In other words, the eyes behave exactly as position-sensitive tracking devices, acquiring and tracking the light source over a 45° range. Interestingly the eyes of females, which are mobile but lack the deep eyecups of the males, also showed these tracking movements. It was not possible to measure the time course of the tracking responses without using more elaborate equipment, but it was certainly rapid, the eyes moving as fast as the light source could be moved by hand. On the few occasions where scanning movements (in males) occurred during imposed tracking it seemed that the scanning movements were superimposed on the orienting movements, but it was hard to see exactly what was going on, and no films were made. The most striking feature of the responses to light-source movement was that they involved the tail as well as the eyes. Movements of the light from front to rear cause the retina to move forwards, and at the same time cause the tail to move upwards so that it comes to lie horizontally or even directed slightly upwards (Fig. 4A). Conversely, movement of the light forwards causes the retina to move back and the tail to move down, so that its tip bends through almost a right angle. Intermediate positions of the light result in tail positions between these extremes. These movements have the character of very tightly coupled reflexes. When the light is moved the tail moves as though joined to the light by a piece of string, and the tail never moves in the 'wrong' direction. The consequences of the tail movements were evident in a few individuals that were trapped on their sides. A downward movement of the tail deflects the swimming current downwards, forcing the tail up and resulting in forward pitching. Upward tail movements result in backward pitching. The overall effect of these movements will be to cause the body to rotate in the same direction as the source of light, and if light normally comes from above they will tend to keep the animal's back directed upwards. Their possible role in vision is explored in the Discussion. Orientation movements in other pontellids Working out the role of body movements in Labidocera is complicated by the fact that the eyes also move. However, there are other pontellids with immobile eyes, and these too show visually induced tail movements. The following observations show that other pontellids can use their tails to steer towards or away from a light source. (1) About 10 male Pontella atlantica were all found to be photonegative, accumulating at the side of the dish furthest from the light. When trapped in the meniscus at the edge it was found that the tail could be made to move up or down by moving a light behind the animal. When the light was below the body axis the
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-oFig. 4. (A) Proposed function for the tracking movements seen in male Labidocera. (1) Animal disturbed by a wave. (2) Retina moves backwards to centre the light, and tail moves down. (3) This causes the animal to rotate (pitch) tail upwards. (4) Retina moves forwards to recentre the light, and tail moves upwards. (5) Equilibrium restored. (B) Tail movements in Pontella, which has fixed eyes. Left: dorsoventral movement of a light behind the animal causes tail movements tending to turn the animal away from the light. Right: similar lateral tail movements caused by movements of the light from one side to the other.
tail moved up, and it moved down when the light was moved above the axis. Similarly, the tail moved to the left when the light was to the right, and vice versa (Fig. 4B). The responses were very sensitive; a movement of the light through few degrees behind the animal caused much larger movements of the tail. Since a leftward movement of the tail directs the swimming current to the left, it must ultimately drive the animal's rear to the right, in the direction of the light. Thus all these movements can be thought of as ensuring that the animal's front end continues to point away from the light. The animals' original photonegativity was evidently caused by a negative phototaxis, driven from the eyes and executed by the tail. (2) A few individuals were found that behaved in quite the opposite way to that just described. Light from the left caused tail movement to the left, and vice versa. These individuals headed towards the light, not away from it. Thus the nature of the coupling between eye and tail appears not to be fixed, and it is reasonable to
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assume that individuals can change from negatively to positively phototactic by changing the polarity of this coupling. (3) Changes in illumination conditions also affected the rate of beating of the maxillipeds and swimming legs, and hence the swimming speed. There is thus a 'kinetic' component to orientation in addition to the 'taxic' mechanism demonstrated above. Observations on photonegative Pontellopsis regalis (male) showed that an increase in the intensity of illumination resulted in faster maxilliped beating, whether the tail was in the up or the down position. In other words, the stimulus for steering is not the same as the stimulus for altering speed. The former is related to differences in illumination of different receptors, whereas the latter could be driven by something more like the sum of their responses. These observations indicate that the fixed dorsal eyes of Pontella are used to steer the animal in relation to the light, which in the sea would presumably be either the sun in calm conditions, or the peak of the downwelling light distribution. The ventral eye is not likely to be involved because it has a narrow, anteriorly directed field of view. In Labidocera the dorsal eyes have this steering function too, but there are two differences. First, the eyes are part of a double feedback loop involving both eye and body movements and, second, the Labidocera steering system tends to keep the animal's back pointing towards the light, whereas in Pontella the animal seems to adopt directions directly towards or directly away from it: up or down in the real world. Discussion Stabilization of the eyes and body in Labidocera Vision in Labidocera involves three kinds of movements: (i) spontaneous scanning movements of the eyes, (ii) eye movements concerned with keeping the eye centred on a light source, and (iii) tail movements linked to the second kind of eye movement that keep the animal's back directed towards the light. It is proposed here that movements ii and iii are a double arrangement for ensuring that the eyes point towards the sky, so that the scanning movements (i) can be brought to bear, presumably in a search for conspecifics. The best way to understand the orienting movements (ii and iii) is to consider what happens to a horizontal animal that has a backward pitching movement imposed on it by a wave (Fig. 4A). Assuming the major source of light is vertically above (this doesn't have to be the sun, it could be the sky seen through Snell's window), the pitching movement will be compensated initially by an eye movement that brings the eye axes back to the vertical, in this case a backward movement of the eyecup. This is accompanied by a downward tail movement which directs the swimming current downwards, ultimately pushing the tail up and rotating the animal back to the horizontal. As this counter rotation occurs the eye returns to its original vertical position, and the tail straightens up. All the elements of this behaviour have been observed, the only thing missing being a demonstration that this is what actually happens in the sea. If it does,
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however, we can ask why Labidocera should have this double mechanism for stabilizing its vision. The most likely answer is that the stable upward-looking field of gaze, which the eye and tail movements ensure, is required for the effective use of the scanning mechanism. Although we do not know what the scanning itself is for, the linear retina suggests that the eyes may be maximally stimulated by the elongated body outlines of conspecifics, and obviously these would be easier to detect against the bright sky than in any other direction. Again, it is a pity that the small size of the animal makes it difficult to see what is happening under natural conditions. The system of eye and body movements described here is almost identical to the arrangement in euphausiid shrimps (Land, 1980). In Nematoscelis atlantica, which has a double eye, the eye rotates so that the upper part seeks the light, and at the same time the tail moves so as to keep the animal's back directed towards the light. This mechanism in euphausiids appears to have three functions: it ensures that the higher resolution part of the eye is directed upwards and also that the photophore on the eye, which is used to camouflage the eye from below, points exactly downwards. The third function is to ensure that the body makes a constant angle with the light, and hence the vertical. In euphausiids this angle can be altered, allowing the light direction to be used to steer upward or downward courses. Probably only the first function is important in Labidocera which has no photophores and does not migrate. A double eye and body movement orienting system is also present in the cladoceran Daphnia, and was first described by Woltereck (1913). This is summarized by Lochhead (1961). 'Ocular muscles rotate the eye in response to changes in the intensity or position of the light.... Presumably in response to proprioceptors in the ocular muscles, the antennae correct any displacements of the animal's orientation so that the eye can return toward its normal position. This complex response keeps the dorsal surface of the animal directed towards the light, usually slanted with the head end up.' The functions of this behaviour in Daphnia are still obscure, but it is clearly the same kind of mechanism as in Labidocera. The question of the involvement of proprioceptors is an open one. However, in euphausiids the evidence suggests that the tail receives an efference copy of the command to the eye muscles, rather than a proprioceptive signal. Functions of the retina during orientation The transverse line of 10 receptors that lies across the bottom of the fused eyecups in male Labidocera is not in the best position to detect whether the light source is forward of the axis of the eye or behind it. The line may, in fact, have no function in the orienting response at all. As we have noted earlier, each eyecup contains three other rhabdoms in a more dorsal position up close to the lens, two anterior to the main 'line' retina and one posterior. If the function of these rhabdoms is simply to monitor general light direction the fact that they are out of focus will be no disadvantage. It thus seems highly probable that the two classes of
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eye movement are associated with different receptors, with the scanning mechanism using the line of focused receptors (1-5; Fig. 2), and the orienting response the other three (6-8). Functions of the retina during scanning With regard to the scanning itself, this study has not advanced our understanding of its function very much. The sexual dimorphism in the eyes strongly suggests that the scanning is concerned with the seeking out of sexual partners, and this is made more plausible by the fact that the animals are strongly coloured, both sexes being dark blue. This will make the animals visible from below. The movements, as recorded in Fig. 3, appear almost to be 'jumps', from one position to another, and that does not seem to be a particularly good way to look for targets, since it only effectively samples two positions. In proper scanning the retina would sample all positions in between, as it moves through the 20° intervening space. In the only other known example of a scanning line of receptors, the heteropod sea-snail Oxygyrus (Land, 1982), this clearly is what happens, as the retina moves slowly in one direction, and then quickly resets in the other. This may be happening here. The rearward motion of the retinae is relatively slow (219 degrees s" 1 ), half that of the forward motion. The angular velocities in Oxygyrus are both slower (80 and 250degreess" 1 , for the slow and fast phases, respectively). However, when one takes into account the different angular widths of the individual rhabdoms (1-1° in Oxygyrus and 3-5° in Labidocera), there is very little difference in the time that the receptors in the two eyes spend sampling the same spatial point during the slow phase of the movement. This is 14ms in Oxygyrus and 16 ms in Labidocera. For insect eyes 16 ms would certainly give enough time for the generation of a usable response, although not by a large margin (Howard et al. 1984). These comparative considerations strongly suggest that the important phase of scanning in Labidocera is the slower rearward movement of the retina, when the eye muscle is contracting actively. Steering in other copepods The observations on Pontella, whose dorsal eyes are fixed, show that steering and eye movements are not necessarily connected. Tail movements that turn the animal in both the pitch and yaw planes are driven from these eyes, and result in either positive or negative phototaxis. It seems likely that the situation in Pontella is similar to that in copepods with the typical undivided nauplius eye. Most of these eyes have no lenses, but the three partially shaded eyecups often have a reflector behind them, and they each have 6-10 receptors (Vaissiere, 1961). This 'wide pinhole' design cannot give a sharp image, but it would be perfectly adequate for determining the direction of the major source of light, and thus could mediate steering responses like those shown by Pontella. There have been many observations demonstrating that light affects the behaviour of copepods, and that it plays a major role in vertical migrations (Bainbridge, 1961), although I know of no work that indicates decisively how these responses are mediated. On the basis of
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the present studies I would expect that phototaxis in copepods is brought about by tail movements controlled by the differential stimulation of the small number of receptor cells of the nauplius eye. I am very grateful to Frances Burton for technical help, especially with the histology. My thanks are due to the organizer of the Discovery cruise, Dr Peter Herring of the Institute of Oceanographic Sciences, Wormley, UK. Also to the NERC, who funded the cruise and provided a travel grant, and to the SERC who provided laboratory support. Peter Herring and Howard Roe of the IOS and Justin Marshall of Sussex University were kind enough to read and comment on the manuscript.
References R. (1961). Migrations. In The Physiology of Crustacea, vol. II (ed. T. Waterman), pp. 431-463. New York: Academic Press. DOWNING, A. C. (1972). Optical scanning in the lateral eyes of the copepod Copilia. Perception 1, 247-261. EXNER, S. (1891). Die Physiologie der facettirten Augen von Krebsen und Insecten. Leipzig, Wien: Deuticke. HOWARD, J., DUBS, A. & PAYNE, R. (1984). The dynamics of phototransduction in insects. A comparative study. /. comp. Physiol. A 154, 707-718. LAND, M. F. (1980). Eye movements and the mechanism of vertical steering in euphausiid Crustacea. J. comp. Physiol. 137, 255-265. LAND, M. F. (1982). Scanning eye movements in a heteropod mollusc. J. exp. Biol. 96, 427-430. LAND, M. F. (1984). Crustacea. In Photoreception and Vision in Invertebrates (ed. M. A. Ali), pp. 401-438. New York: Plenum Press. LOCHHEAD, J. H. (1961). Locomotion. In The Physiology of Crustacea, vol. II (ed. T. Waterman), pp. 313-364. New York: Academic Press. PARKER, G. H. (1891). The compound eyes in crustaceans. Bull. Mus. comp. Zool. Harvard 21, 44-140. VAISSIERE, R. (1961). Morphologie et histologie comparees des yeux des crustace's copepodes. Archs. Zool. exp. gen. 100, 1-125. WOLTERECK, R. (1913). Uber Funktion, Herkunft und Entstehungsursachen der sogen "Schwebe-Fortsatze" pelagischer Cladocera. Zoologica Stuttgart 26, 475-550. BAINBRIDGE,
