Experiments on the Influence of Light on the Behaviour of Polyzoan Larvae

The majority of marine polyzoans have a larva which hatches at an advanced stage of development and settles soon after liberation. Throughout its short pelagic life the behaviour of the larva is strongly influenced by light. In fact, the hatching process itself appears to occur in response to illumination in Bugula simplex (Grave, 1930), B. neritina (Lynch, 1947) and B. turrita (Lynch, 1949). (The nomenclature of the various species of Bugula has been corrected where necessary, and follows the account of Ryland (1960).) In all other species investigated during the present work also, light was found to be the stimulus required to initiate hatching. It should be noted, however, that all the present, and previously published, observations refer to species found in shallow water or on the shore; there are no data referring to deep-water species.

Two related but distinct types of behaviour will be discussed. First, the responses of larvae to directional illumination during the free-swimming phase and, secondly, the effect of illumination on the settlement behaviour. The experiments appropriate to each will be described and discussed separately.

It is well known that the behaviour of many marine larvae and other planktonic organisms is strongly influenced by light. These may display markedly photopositive or photonegative reactions, and Spooner (1933) has shown that the response is to the direction rather than the intensity of illumination. It also seems probable that the responses of polyzoan larvae to light are accompanied by responses to gravity. Thus, when first liberated, many larvae, such as those of Celleporella hyalina and Bowerbankia imbricata, accumulate near the surface of the water on the illuminated side of the container. Grave (1930) and Lynch (1947) make similar observations with reference to other species.

Previous work on light as a factor influencing larval behaviour has sometimes suffered from certain defects. First, the conditions of illumination have been badly defined, making it difficult for other workers to repeat them with accuracy. A second criticism is that the light intensity employed was usually very low and has borne little relation to that existing normally in the surface waters of the sea. In summer, at latitude 50°N. with the sun at zenith and shining, for example, the illumination at the surface might amount to 120 kilolux or more, of which some 4−25 % would be reflected according to the state of the surface. At a depth of 10 m. in average coastal water the illumination would be in the region of 6800 lux, and about 2600 lux in turbid water (combined from Sverdrup, Johnson & Fleming, 1942; and Harvey, 1955).

Thirdly, no attempt has been made to describe the behaviour in quantitative terms. A method was required, therefore, which did not suffer from these three defects, and would permit the investigation of the influence of directional illumination unaffected by any possible effects due to gravity.

The apparatus which was designed for these experiments, somewhat modified in the light of later experience, is shown in Fig. 1. The light source employed was a theatre spot-lamp fitted with a 500 W. tungsten filament bulb. The lamp had a silvered reflector and an adjustable front lens which permitted focusing of the light beam. The lamp provided a virtually parallel beam of considerable intensity. In all the experiments described below, the illumination employed was 5000 lux, which is considerably higher than has generally been used for studying the reactions of marine organisms to light. Since most polyzoans breed in the latter part of the summer, this intensity was considered a suitable one (see above), and it also showed up the larvae well.

The intensity could be reduced by inserting neutral density filters. (A rheostat is unsuitable for this purpose as it would alter the colour temperature.) The spectral composition of the light could also be altered by the use of colour filters, notably the Ilford 810 which can be used with a tungsten source to provide light approximating closely in quality to mean noon daylight. A mask in front of the lamphouse reduced the size of the beam, while another was placed in front of the apparatus to exclude stray light. A long distance between the lamp and the troughs is an important feature essential for a well-collimated beam.

The experimental vessels in which the larvae were placed were rectangular Perspex troughs, 20 × 2 × 2 cm., whose long axes were orientated in the plane of the light beam. The first set consisted of six parallel troughs, but it was later found desirable to have two trough units which could be used independently. Each unit consisted of a pair of troughs either one of which acted as control for the other. (While the experiments described in this paper were conducted in rectangular troughs, the apparatus now in use incorporates the paired U-shaped trough units shown in Fig. 1.)

The troughs stood over a black card divided into ten 2 cm. divisions by thin white lines. These were numbered I-X, but in some of the earliest experiments a background divided into eight divisions was used. When the light was projected along the troughs, the larvae stood out clearly against the black background, and it was possible to record the number of larvae above each of the divisions marked on the card. The larvae were most difficult to count against the trough end-walls, partly because numbers were usually highest at one or other of the ends, and partly because of the presence of corners which seemed to attract the larvae. The first difficulty was not serious if only about twenty larvae were used per trough. The use of troughs of U-shaped section eliminates the corners, and these also have the added advantage of a larger and more intimate contact with the water in the constant temperature bath.

The set of six troughs, or the trough units, stood on a glass base-plate supported in an aquarium tank filled to just above trough water level to maintain a constant temperature. The water-bath incorporated a thermometer at trough level and an electric stirrer (of heater-stirrer in experiments with Bugula neritina). Fresh cold water could be added if required, and the requisite level was maintained by a siphoning device. Ordinary moulded glass tanks have been used for the water bath, but the framed type with plane plate-glass walls, now being used, is to be preferred.

The trough units were covered by a sheet of glass. This helped to prevent their being moved by agitation of the water in the tank, kept the trough water free from dust, and reduced evaporation.

The polyzoan colonies were kept in running sea water in total darkness. When larvae were required for experiments, the colonies were illuminated by tungsten or daylight. Wherever possible, as with the tufted colonies of species of Bugula, the zoaria were placed in the troughs so that the larvae were liberated in situ. With incrusting species it was necessary to keep large quantities of polyzoan-covered alga in tanks, and pipette the larvae gently into the troughs. Pipetting, even when carried out gently with a wide-mouthed pipette, sometimes influenced the behaviour of the larvae. In B. neritina and Escharoides coccineus some of the larvae reacted by immediately metamorphosing on the surface film, while in other species a few larvae changed the sign of their reaction to light and became prematurely photonegative. When this happened the experiment had to be restarted with fresh larvae.

The water in the troughs was filtered to remove planktonic organisms, and prior to the experiment was stored in a container placed, with the troughs, in the tank containing the polyzoan colonies. In this way it was possible to avoid subjecting the larvae to abrupt changes of temperature. The apparatus was left for about | hr. before the first reading was taken. Then and at regular intervals the positions of the larvae were recorded.

The behaviour of the larvae investigated in the troughs conformed to one of four patterns (Tables 1−9; Fig. 2) referred to as types I, II, III and IV. In type I (Fig. 2 a) the larvae were at first photopositive, almost the whole population swimming against the end-wall nearer the light source. After the lapse of an hour or two the population as a whole tended to become photonegative and eventually became almost completely so. Since at any moment larvae could be seen swimming along the troughs in both directions, there was no definite and clear-cut reversal of behaviour, but rather a trend shown by the population. Some species which behave in this way are Celleporella hyalina, Bugula fulva, B. plumosa, Scrupocellaria scruposa and, also, the serpulid worm Spirorbis borealis (Tables 1−5). A similar reversal has been noted in the polyzoans Bugula simplex and B. turrita (Grave, 1930 ; Lynch, 1949) and Scrupocellaria reptans (Lutaud, 1953).

Larvae showing type II behaviour tended to remain photopositive, although a proportion eventually swam away from light. This type of behaviour was most clearly shown by Alcyonidium hirsutum (Table 6, Fig. 2,b). Type III behaviour, in which the larvae remained photopositive throughout the experiment, was clearly shown by Flustrellidra hispida (Table 7, Fig. 2c).

Bugula neritina larvae were found to belong to type II (Table 8). Many of the larvae metamorphosed, the highest settlement occurring at the end of the trough toward the source of illumination, but there was a definite, though partial, swing to photonegative behaviour. Both McDougall (1943) and Lynch (1947) found that the larvae remained photopositive throughout their free-swimming phase, but Mawatari (1951) recorded a reversal of response. In other experiments Lynch considered that the larvae became ‘more or less indifferent to light’. Table 8 shows, on the contrary, that the larvae were showing either a positive or a negative response : if the larvae were indifferent the distribution in the troughs would follow a random pattern.

Lynch (1947) considered that in his experiments the failure of B. neritina larvae to become photonegative was due to the low intensity of illumination employed, although, in the same paper, he failed to induce any change by increasing the intensity. Both he and McDougall (1943) thought that in their natural environment the larvae must become photonegative. This seems probable ; but McDougall’s demonstration that the larvae preferred to settle in a region of low light intensity does not necessarily imply a negative phototaxis. Further work seems desirable to try to relate the behaviour of these and other type II larvae in the laboratory to their behaviour in the sea.

Nitsche (1870) found that B.flabellata larvae metamorphosed on the illuminated side of glass containers, but preliminary observations suggest that they may sometimes become photonegative.

Whatever their subsequent behaviour, immediately after liberation the larvae of the great majority of polyzoans are positive to light. In addition to the species already mentioned this has been found true in Cryptosula pallasiana. Microporella ciliata, Fenestrulina malusi, Escharoides coccineus and Bowerbankia imbricata, while Mawatari (1952) made the same observation for Watersipora cucullata. Alcyonidium polyoum seems exceptional in that the behaviour of the larvae appeared uninfluenced by light (Table 9, Fig. 2 d)—a result which confirms the earlier observation of Zschiesche (1909). This has been called type IV.

The experiments were generally performed at a temperature about equal to that of the sea surface at that time. It is possible that larval responses to light may be influenced by temperature. In Bugula neritina a decrease in temperature prolongs the natant stage (Lynch, 1947). Gravity responses of polyzoan larvae are also affected by temperature (Lynch, 1947). This subject is being investigated at the present time.

It seems probable that the majority of polyzoan larvae show type I behaviour, being photopositive when liberated, but later swimming away from light. It was clearly of importance to ascertain the cause of this change in behaviour: was it purely a question of time, or the result of a certain amount of light energy being received by the larvae? The observation of Lynch (1947) that B. neritina larvae ‘swim away from regions receiving the direct rays of the sun after a certain amount of exposure to direct light’ appears to imply the latter. To decide the question, two paired trough units were used. Approximately equal numbers of freshly liberated, photopositive larvae of Celleporella hyalina were pipetted into each. One unit was placed in the light beam while the other was kept in the dark. When the majority of larvae in the illuminated troughs had become photonegative, the second unit was placed in the light beam beside the first. The results are given in Table 10, and show that, in this species at least, the reversal of behaviour is dependent on the passage of time and not on illumination.

These experiments raise a second problem. To what extent is the reversal of the reaction to light related to the normal settlement of the larvae? Previous experiments had already shown that illumination affected settlement of C. hyalina larvae on algal substrates (Ryland, 1959). In another experiment with C. hyalina, similar disks cut from Fucus serratus fronds were placed at intervals along the length of troughs containing photopositive larvae. The results (Table 11) show that the greatest numbers settled furthest from the light source ; it is therefore quite clear that the majority of larvae had developed a photonegative reaction before they settled.

This problem deserves further attention, for it seems desirable to try to relate this pattern of settlement to the amount of exploratory behaviour shown by the larvae. Observation suggests that larvae whose reaction has become photonegative ‘explore’ more than those still positive to light. That is to say, larvae with a negative reaction do not swim against the end wall so persistently as those which are photopositive. This was also implied by Lynch (1947) who said that the photonegative response ‘is never so intense as the positive one’. The results of the experiment just discussed indicate that the pattern of settlement may well be related to the amount of ‘exploration’.

The influence of surface angle on the settlement of marine larvae has received considerable attention, notably in relation to the cultivation of oysters. The settlement of organisms has been recorded on panels of various kinds and in different orientations in the sea. The results of earlier workers which related to shell-fish culture were reviewed by Cole & Knight-Jones (1949), who also discussed their own observations. The results of Pomerat & Reiner (1942) and of Maturo (1959) are of special interest since Polyzoa figured largely in their studies. The practice, now conventional, of indicating the orientation of a surface is as follows:

It has been recorded by Pomerat & Reiner (1942) and Maturo (1959) that abundant settlement of the polyzoans Acanthodesia tenuis, Bugula neritina, Electra hastingsae and Schizoporella unicornis only occurred on the 0° and 45° surfaces. Knight-Jones (1951) found that ‘Membranipora lacroixi’^* settled more abundantly on the lower surfaces of horizontally placed slates. Pomerat & Reiner found that the settlement of bivalves was markedly heavier on the 0° surface, and this has been confirmed by Cole & Knight-Jones (1949) for Ostrea edulis—although both they and Korringa (1941) occasionally obtained anomalous results in which spatfall was higher on the upper surfaces. Many other animals, but not all, show a preference for lower surfaces (Knight-Jones, 1951).

In the present studies panels fixed on various types of collector were immersed in the Menai Straits with the principal objective of determining the settlement seasons of the commoner Polyzoa. In view of the preference for lower horizontal surfaces reported in the literature, some of the panels were placed vertically and some horizontally (Fig. 3). In fixing the panels a space of 1−2 mm. was left between the panels and the base-plate by inserting washers around the attaching bolts : thus every panel had an outside and an inside surface available for settlement. At locality A, during 1957 and 1958, panels were attached on poles mounted below a raft moored in the Menai Straits. In 1958 a second set was mounted on the swivel device described by Crisp & Stubbings (1957) and located between the floating pontoons of Menai Bridge pier (locality B). In both cases, therefore, the orientation of the panels would follow the direction of the current, while they would also remain at a constant (2−4 ft.) depth below the surface. The settlement recorded on each of the various surfaces is shown in Table 12.

The conditions at A, under the raft, differed in one important respect from those obtaining under the pier (B) : the greater amount of illumination allowed the formation of a dense carpet of algae (Ectocarpus and Pilayella) on the panels during the summer months. The maximum settlement of some polyzoans, notably Celleporella hyalina, coincided with the period of greatest development of this algal cover. It is not surprising that the presence of this weed inhibited the settlement of polyzoans. Thus at A in 1957 the upper set of vertical panels, 1−2 ft. below the water surface, supported a dense algal growth; consequently the majority of polyzoans settled on the inside surfaces of the panels. On the lower set, 3−4 ft. below the surface, algal growth was sparse, and the settlement of Celleporella was about equal on the inside and outside surfaces (Table 12).

The horizontal surfaces showed even more striking differences. The general picture was one of high settlement on surfaces which face downward, and of low settlement on those facing upward.

No algae developed on the panels at B, although, as at A, sediment collected on upward-facing (180°) surfaces.

The data from both localities differ from those previously published in that vertical (90°) surfaces were as favourable, or even more so for some species, than the 0°surfaces. It is clear, however, that those surfaces on which algae grow densely and/or collect sediment do not receive a high settlement of polyzoans, and Maturo (1959) considered silting of the 135° and 180° surfaces to be the major factor preventing their establishment. But this is not necessarily the only explanation of the results ; there is another, although its effect may have been masked by the presence of algae plus sediment. The settlement pattern may in fact reflect the general swimming and searching behaviour of the larvae, and some previous workers (e.g. Cole & Knight-Jones, 1949) have explained their results in this way. Since this could only be decided by direct observation in the absence of sediment, further investigations were pursued in the laboratory.

Experiments have been conducted with Celleporella hyalina larvae, using filmed opaque (Tufnol) or transparent (Perspex) panels placed vertically, obliquely and horizontally in aquarium tanks. Each of the panels was suspended from supports above the tank by threads attached to the corners. Three panels were used simultaneously so that, in each tank, six surfaces were offered, orientated at 0°, 45 °, 90° (two), 135° and 180°. The result given for 90° is, therefore, the mean of the settlement recorded on the two vertical surfaces.

The results of these experiments are shown in Table 13. Since the actual number of larvae that settled in any experiment varied so much, the results are expressed as percentages before being converted to means. A mean based on numbers of larvae set could be somewhat misleading, since the results would be unduly weighted by those experiments in which settlement was very high. The percentage of the total number of larvae set on each of the surfaces is, however, also given in Table 13, but it was not used in the construction of Fig. 4.

Lighting conditions were varied. In some tanks diffuse light from a 100 W. bulb could fall on the panels only from above. In others the light from a similar bulb was reflected by a mirror, through a diffusing screen, from below the panels, the tank being otherwise surrounded by a blacked-out box. In other experiments the tank was covered so that the experiment was conducted in the dark.

The results obtained using top light and opaque panels will be considered first (Fig. 4b). It is at once apparent that the trend of these results is very different from those discussed in the previous section. Here the ‘normal’ results have been obtained, with the highest settlement on the 0° surface ; lower, but still high, on the 45° surface; and low on the others. It is not clear why the field results should have been so different. It is possible that the turbidity of the water in the Menai Straits causes a scattering of the light so that, even quite near the surface, the illumination is not markedly directional; this might be especially true in the dim light below Menai Bridge pier.

When the lighting of the tanks was reversed and the panels illuminated from below, the settlement pattern abruptly changed (Fig. 4 e): the majority of larvae settled on the 180° and 135° surfaces—that is, on the shaded side of the panels.

In total darkness it can be seen that settlement on all five surfaces was approximately equal (Fig. 4 d).

The results using top light and transparent panels were quite different. Settlement was highest on the horizontal panel, much lower on the oblique panel, and lowest on the vertical panel ; but in each case it was more or less divided between the two surfaces (Fig. 4c). These results might be expected if the larvae tended to swim up and down rather than in a random manner—and this seems possible in view of the responses discussed earlier (p. 786). It is perhaps a little surprising that settlement on the two surfaces of the horizontal panel was about equal, for if the larvae were at first positive to light and then swam away from it, a heavier settlement might have been expected on the upper surface.

The results obtained when transparent panels were illuminated from below appear to be somewhat anomalous, the high settlement on the 135° surface being difficult to explain (Fig. 4 f). This result, based on two experiments only, may have been due to uneven illumination, or perhaps to some unknown factor. In spite of this anomaly, however, the general picture to emerge from these experiments is clear. The pattern of settlement depends on the direction of illumination, and on opaque panels the larvae settle on the side facing away from the light.

Hyman (1959), reviewing previous work, drew the tentative conclusion that: ‘Avoidance of light probably accounts for the selection of the undersurface of horizontal or nearly horizontal plates or other objects and the avoidance of surfaces much tilted from the horizontal.’ The results just discussed show that, for C. hyalina at least, this can be regarded as certainly true. It seems quite likely that experiments with other marine larvae would show that the preference for 0° and, to a lesser extent, 45° opaque surfaces recorded by previous workers was also due to the selection of a shaded surface.

The behaviour shown by larvae in the troughs as a response to directional illumination appears to be a photoklinotaxis (Fraenkel & Gunn, 1940). The larvae swim along the light beam, either toward or away from the source. Their path follows a spiral course, for their ciliary action causes the larvae to rotate as they swim. Many of the larvae are provided with orange-red pigment spots; for example, those of Bugula plumosa have four and those of B. flabellata and B. fulva ten. In the literature dealing with the Polyzoa these have often been called eye-spots, but there is no evidence that they are photoreceptors (Hyman, 1959). Species like Alcyonidium hirsutum show equally distinct orientation to light yet lack pigment spots. The mechanism of photoreception remains unknown.

McDougall (1943) found that Bugula neritina larvae settled preferentially in the darkest of a series of variously illuminated chambers. He, and also Hyman (1959), regarded this preference for a low light intensity as evidence that the larvae become negatively phototactic (see p. 790). In fact, some sort of high photokinesis might better explain McDougall’s finding of heavy settlement correlated with low light intensity. An experimental approach with this in mind might resolve the apparent paradox of the settlement behaviour of this species. It certainly seems important to show by suitable experiments whether or not polyzoan larvae can react to the intensity as well as to the direction of illumination.

The pattern of settlement on the variously orientated panels, which showed that the larvae metamorphosed on the shaded side, could also be the result of a high photokinesis causing the larvae to accumulate in the dark area. The present experiments have not resolved this point because Celleporella hyalina had already been shown to develop negative phototaxis; it would be of interest to repeat the experiments with a species which apparently displays positive phototaxis throughout its life.

I would like to express my gratitude to Dr D. J. Crisp for his interest in all stages of this work and for his helpful criticism of the manuscript, and to Prof. E. W. Knight-Jones for facilities to work with Bugula neritina in his department at the University College of Swansea, and for his great assistance in obtaining supplies of this polyzoan. I am indebted to the Development Commission for a research grant.