Egg-care behaviour of Aequidens paraguayensis (Pisces, Cichlidae) in relation to predation pressure and spawning substrate

HBLBNE M. C. DUPUIS AND MILES H. A. KEENLEYSIDE Department of Zoology, University of Western Ontario, London, Ont., Canada N6A 5B7

Received September 29, 198 1

DUPUIS, H. M. C., and M. H. A. KEENLEYSIDE. 1982. Egg-care behaviour of Aequidensparaguayensis (Pisces, Cichlidae) in relation to predation pressure and spawning substrate. Can. J. Zool. 60: 1794- 1799.

Aequidens paraguayensis is a cichlid fish with biparental care that spawns on a leaf taken from substrate litter and, after the eggs have hatched, broods the progeny orally. We tested the hypothesis that increasing predator density causes proportional changes in the frequency or duration of parental care activities, including moving the spawn. We predicted that at high predator densities, the parents would pick up the eggs and begin mouthbrooding earlier than usual to prevent egg loss.

Increasing predator density led to greater parental sex-role differentiation, the males defending the spawning site while the females tended the eggs. Leaf tugging was performed by the females only and did not increase significantly with predator density. Pairs that spawned on a fixed substrate did not exhibit this sex-role differentiation.

Predator density and type of spawning substrate did not affect brood survival or time between spawning and beginning of mouthbrooding. Hatching time is temperature dependent and parents apparently must wait for the egg membranes to break before picking up the hatchlings.

DUPUIS, H. M. C., et M. H. A. KEENLEYSIDE. 1982. Egg-care behaviour of Aequidens paraguayensis (Pisces, Cichlidae) in relation to predation pressure and spawning substrate. Can. J. Zool. 60: 1794- 1799.

Chez Aequidens paraguayensis, poisson cichlide, les deux parents prennent part au soin des petits, les oeufs sont pondus sur une feuille prelevee a m2me le substrat du fond et, aprks l'eclosion, sont gardes dans la bouche du parent. Cette recherche a ete entreprise dans le but de verifier l'hypothkse selon laquelle l'augmentation de la densite de predation risque de causer des changements proportionnels dans la frequence et la duree des soins parentaux, y compris celui de demenager la ponte. Nous avons predit qu'a des densites de predation eleves, les parents demenageraient les oeufs et entreprendraient l'incubation dans la bouche plus t6t qu'a l'ordinaire afin d'emp2cher la perte d'oeufs.

L'augmentation de la predation augmente la differenciation des r6les parentaux entre mile et femelle: le mile defend le territoire de ponte alors que la femelle s'occupe des oeufs. Le deplacement de la feuille est l'apanage exclusif de la femelle et cette activite n'augmente pas significativement en fonction de la densite des predateurs. Les couples qui pondent sur un substrat fixe ne manifestent pas cette differenciation des r6les.

La densite des prkdateurs et le type de substrat de ponte n'affectent ni la survie des portees ni la duree du temps ecoule entre la ponte et la garde des petits dans la bouche. Le temps necessaire a l'eclosion est fonction de la temperature et les parents doivent semble-t-il attendre la rupture des membranes de l'oeuf avant de pouvoir transporter les petits dans leur bouche.

[Traduit par le journal]

Introduction Three distinct parental-care strategies are found

among cichlid fishes. The first, considered to be the most primitive (Breder 1934; Myers 1939; Lowe (McConnell) 1959; Wickler 1966; Iles and Holden 1969; Oppenheimer 1970; Fryer and Iles 1972), is substrate brooding, in which the eggs are aerated and guarded by both parents. The wrigglers (nonfree- swimming hatchlings) and, later, the fry (free-swim- ming young fish) are defended by the parents, usually with some division of labour (Barlow 1974; Smith- Grayton and Keenleyside 1978; Keenleyside 1979). Most cichlids of Central and South America and riverine African species are substrate brooders. The second strategy, mouthbrooding, is probably derived from sub- strate brooding (Myers 1939; Lowe-McConnell 1969; Oppenheimer 1970; Fryer and Iles 1972; Hanon 1975).

Typically, the female releases a few eggs at a time into the male's nest and immediately picks them up in her mouth where they are fertilized (Hanon 1975). The female then leaves the male's territory while carrying the eggs. The offspring are released after they become free swimming, although they continue to use the female's mouth as a temporary shelter for several weeks. Female mouthbrooding is used by most lacus- trine African cichlids and by a few American species (Sprenger 1971; T. R. L. Carlisle, personal com- munication). In a very few African species the male alone or both parents brood the young orally (Fryer and Iles 1972). In the third strategy, the eggs are guarded on the substrate by both parents until they hatch. The young are then brooded orally by both parents until they are free swimming. The fry use the parents' mouths as temporary refuges for several weeks.

0008-430 1 182 108 1794-06$0 1 .00/0 0 1982 National Research Council of Canada/Conseil national de recherches du Canada

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DUPUIS AND KEENLEYSIDE 1795

This form of parental care is known only in a few New World species (Reid and Atz 1958; Timms and Keenleyside 1975).

Some species using substrate brooding or combined substrate- and mouth-brooding increase clutch mobility by spawning on a moveable substrate such as a piece of wood or leaf litter (Barlow 1974; Timms and Keenley- side 1975; Cichocki 1976; Keenleyside and Bietz 198 1). Such spawns are moved by one parent grasping the edge of the substrate with the mouth and swimming back- wards. A moveable spawning site may be useful to breeding fish that are subjected to water level fluctua- tions during the parental cycle (Barlow 1974; Lowe- McConnell 1975; Timms and Keenleyside 1975; Ci- chocki 1976, 1977). Alternatively, the use of a move- able site may have been favoured as an antipredator tactic; cichlid pairs guarding eggs on leaf litter in Panama and Surinam, South America, move their spawn into areas of greater cover when alarmed (Barlow 1974; Keenleyside and Bietz 1981). Full-time mouth- brooding should be even more effective in dealing with such environmental pressures (Keenleyside 1979).

Aequidensparaguayensis is a South American cichlid that follows the third strategy described above. Adult pairs spawn on a moveable substrate and later both parents brood the hatched embryos orally (Timms and Keenleyside 1975). Keenleyside and Prince (1 976) found that egg-tending A. paraguayensis move the spawn into areas of relatively dense plant cover in response to a model predator. In this study, we tested the hypothesis that egg-tending pairs respond to the pres- ence of egg predators by changing the frequency or duration of parental care behaviour, including moving the spawn (leaf tugging). We predicted that at relatively high predator densities these changes would be ineffec- tive and ,that parents would pick up the eggs earlier than usual to avoid losing them. We tested this hypothesis by subjecting egg-tending pairs to four densities of con- specific juveniles, which will quickly devour unattended eggs. We also predicted that if moving the spawn to a sheltered area is an antipredator tactic, then pairs having spawned on a fixed, unmoveable substrate would suffer greater brood predation than pairs that spawned on leaves.

Methods Holding conditions

The animals used in this study were descended from pond-reared fish obtained in 1974 from Socolof Farms, FL. Adults (7 .O- 13.0 cm total length) of unknown breeding history were held in four 100-L aquaria containing bricks, stones, pieces of slate, and natural and polyethylene leaves as potential spawning substrates. They were fed daily with dried flake food, trout pellets, frozen adult Artemia salina, or live Daphnia. When spawning occurred, the pair was isolated,

without their eggs, in a 90 X 60 x 45 cm experimental aquarium to begin another breeding cycle. Each of the 10 aquaria used contained under-gravel filters, 3-5 cm of gravel, and one hollow brick, one stone, and six plastic plants positioned in a standard pattern so that cover was available in the back of the aquarium only. Two loose polyethylene leaves (8.0 x 10.0 cm, 1.5 g) were also placed on the gravel. Six adult guppies (Poecilia reticulata) were introduced into each aquarium as "dither" fish (Barlow 1968). Illumination was provided by a 30 W fluorescent tube 8 cm above each aquarium, from 0730 to 2130. Ceiling lights were on irregularly during the photoperiod. Water temperatures were maintained at 30 + 0.5"C. Twenty-six pairs were used for this study.

Leaf-spawning experiment The experimental pair always spawned on one of the

polyethylene leaves. The exact time of the beginning of spawning (deposition of the first eggs) was noted. When spawning had ceased (about 1 h later), we photographed the clutch for subsequent egg counts by temporarily placing the leaf at a 45" angle against the front of the aquarium.

Twenty-four hours after spawning began, we introduced 0, 4, 12, or 20 conspecific juveniles (standard length 2 SE, 3.80 + 0.05 cm) into the experimental aquarium. Five minutes later, we recorded several parental-care activities (see below) simultaneously for both sexes for 10 rnin using an 8-channel Esterline-Angus event recorder. Similar recordings were taken 26 and 28 h after the beginning of spawning. We adopted this schedule to ensure that recordings occurred at regular intervals during daylight hours. The juveniles were introduced into the aquarium 24 h after spawning to avoid overnight egg pre- dation.

The parental-care activities (see Tirnms and Keenleyside 1975) recorded were the number of bouts and the total time spent fanning the eggs, mouthing the eggs, leaf tugging, and defending the clutch. The latter included all aggressive acts toward intruders and the mate (except the "greeting" behaviour (Baerends and Baerends-van Roon 1950) performed when one parent relieves the other from egg-care duties).

We noted the time at which the parent(s) began picking up the embryos to calculate the length of the substrate phase (from the beginning of spawning to picking up the first embryos). Forty-eight hours after spawning began, we removed the parents, gently flushed out their mouths, and counted the surviving wrigglers to determine brood survival. We obtained 10 replicates for each treatment (0, 4, 12, or 20 juveniles per aquarium) and conducted the experiments sequentially, in order of increasing predator density. We subjected most pairs to two or more treatments.

We examined the parental behaviour data by multivariate analysis of variance. Data obtained 28 h after spawning were excluded from this analysis, because some pairs had begun picking up the embyos from the leaf by that time. Therefore, we combined the data collected 24 and 26 h after spawning and transformed each variable according to Taylor's power law (Taylor 1961) to normalize the distributions and eliminate heteroscedasticity. We also performed an analysis of variance to determine the effects of the number of juveniles present on the length of the substrate phase and on brood survival.

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1796 CAN. J. ZOOL. VOL. 60. 1982

Fixed-substrate experiment Ten pairs spawned under conditions identical to those of the

first experiment, except that instead of two plastic leaves, we placed a slate rectangle (9.0 x 5.0 X 0.7 cm, 90 g) at the centre of the aquarium so that the distance to the nearest cover was 15 cm. All pairs spawned on the slate in this experiment. We followed the same procedure as for the first experiment except that we tested only one predator density (20 conspecific juveniles per aquarium).

We compared the results by analysis of variance with those from the two extreme treatments of the first experiment (0 and 20 juveniles). We applied Newman-Keul's multiple range tests on the variables which differed significantly across the three treatment groups.

Results The two-way multivariate ANOVA (leaf-spawning

experiment) revealed a significant interaction between the two factors, predator density and sex of the parent (p < 0.001, Wilk's test), suggesting that the sexes were reacting differently to predator density. We therefore reanalyzed the behavioural data separately for each sex.

Although we recorded both the number of bouts and the total time spent leaf tugging and defending the spawn, we present only the number of bouts, since bout duration of these two activities was clearly influenced by the artificial confines of the aquarium. On the other hand, bout frequencies of fanning and mouthing the eggs are not presented, because they clearly reflected the number of interruptions by the juveniles, rather than the degree of attention paid to the clutch by the parents.

Sex-role differentiation in defending the spawn (Fig. 1) occurred at the higher predator densities in leaf- spawning pairs (p < 0.001). Males exhibited a marked increase in the number of defending bouts with increas- ing predator density ( p < 0.00 1). Females responded to the presence of the juveniles (p < 0.00 l) , but not to their abundance. The males from slate-spawning pairs (Fig. 1) were no more aggressive towards 20 juveniles than leaf-spawning males had been (p > 0.05). The females, however, performed more defending bouts than females guarding clutches on leaves, so that the male-female difference in frequency of defending bouts was not present in slate-spawning pairs (p > 0.25).

Times spent fanning (Table 1) and mouthing (Table 2) remained similar across the four predator densities within each sex (fanning, p > 0.5; mouthing, p > 0.25). Leaf-spawning females performed these egg-care activi- ties more often than did males (fanning, p < 0.05; mouthing, p < 0.001), particularly at the higher predator densities. Slate spawners showed no sex-role differentiation in these activities (p > 0.25 for both behaviours). Slate-spawning females spent somewhat less and males somewhat more time fanning and mouthing the eggs than did the leaf-spawning adults dealing with 20 juveniles.

1'2 I

2 0

NUMBER OF JUVENILES

FIG. 1. Relationship between number of juveniles present, type of spawning substrate, and number of defending bouts performed by each parent (i 5 SE, n = 10 for each treatment). 0, leaf-spawning females; H, leaf-spawning males; A , slate- spawning females; A, slate-spawning males.

The number of tugging bouts (Table 3) performed by leaf-spawning pairs did not change with increasing numbers of juveniles (p > 0. l) , but sex-role differentia- tion was marked (p < 0.00 l) . Slate spawners often tried to move the spawn when disturbed by us shortly after spawning, but never did so during the recording ses- sions.

Pairs subjected to potential egg predators did not begin oral brooding sooner than undisturbed pairs (Table 4; p > 0. l) , regardless of the type of spawning substrate (p > 0.5). Apparently, sex-role differentiation in parental-care behaviours and increased defense of the spawn were effective, since brood survival remained similar across all treatments (Table 4, p > 0.25). Slate spawners did not suffer higher brood loss than leaf spawners as we had predicted (Table 4, p > 0.5). In other words, a moveable spawning substrate con- ferred no advantage on pairs dealing with egg predators.

Discussion We never witnessed egg predation during the experi-

ments. However, brood survival was always less than 100%, possibly because of predation outside the obser- vation periods, unfertilized or fungused eggs, or swal- lowing of some wrigglers by mouthbrooding parents. The juveniles were never seen mobbing the egg-tending parents, but tried reaching the spawn individually while one parent was chasing other intruders away. This behaviour persisted until the parent(s) had picked up all the hatching embryos from the substrate, about 28 h after spawning (Table 4).

The sex differences in parental-care behaviour ob- served in our first experiment agree with earlier studies on A. paraguayensis (Timms and Keenleyside 1975;

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DUPUIS AND KEENLEYSIDE

TABLE 1. Relationship between type of spawning substrate, number of juveniles present, and time (seconds per 10 min) spent by each parent fanning the eggs

( i 2 1 SE, n = 10 for each treatment)

Spawning substrate Number of juveniles Females Males

Leaf Leaf Leaf Leaf

Slate

TABLE 2. Relationship between type of spawning substrate, number of juveniles present, and time (seconds per 10 min) spent by each parent mouthing the eggs

(i + 1 SE, n = 10 for each treatment)

Spawning substrate Number of juveniles Females Males

Leaf Leaf Leaf Leaf

Slate

TABLE 3. Relationship between type of spawning substrate, number of juveniles present, and number of tugging bouts per 10 min performed by each parent

( i 2 1 SE, n = 10 for each treatment)

Spawning substrate Number of juveniles Females Males

Leaf Leaf Leaf Leaf

Slate

TABLE 4. Relationship between type of spawning substrate, number of juveniles present, length of substrate phase, and brood survival ( i 2 1 SE, n = 10 for

each treatment)

Length of the substrate phase Brood survival

Spawning substrate Number of juveniles (min)

Leaf Leaf Leaf Leaf

Slate

Keenleyside and Prince 1976). Keenleyside and Prince the females taking over more of the egg-tending noted that the males performed more aggressive acts activities and the males concentrating on the defense towards a predator model than did the females. We of the spawn as the number of juveniles increased. observed this difference at the higher predator densities, This greater division of labour seems more efficient

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1798 CAN. J. ZOOL. VOL. 60, 1982

than having all parental-care duties shared evenly by both parents, since adult males are generally larger than adult females and therefore presumably more effective in keeping small fish away from the eggs.

The lack of parental sex-role differentiation in slate spawners may be related to the impossibility of moving the spawn into a more sheltered area when disturbed. Perhaps this added stress led the males to spend more time at the spawning site and thus share parental duties with the females more equally than did leaf-spawning males.

In both experiments, the behavioural adaptations by the breeding pairs minimized egg predation; brood survival was similar across all five treatment groups. The parents did not shorten the substrate phase as we had expected them to do. We later concluded that the substrate phase probably cannot be shortened by the parents; 10 batches of eggs developing in well-aerated 30°C water without parental assistance required 27 h 55 min (SE, 3.26 min) to reach the hatching stage (the instant the shell first ruptures). This is significantly shorter than the 28 h 33 min (Table 4) between the beginning of spawning and the beginning of oral brooding (i.e., the substrate phase) for the control leaf spawners (p < 0.05, F-test, n = 10). This means that the eggs hatch several minutes before the parents begin picking up the embryos. Nonexperimental pairs continuously nipped at their eggs for at least 20 min following the first shell rupture before any embryos were sucked out of their shells. Without parental assistance, however, the embryos required several additional hours to free themselves from their broken shells. It appears, therefore, that repeated parental mouthing of the eggs does not speed up the events leading to hatching, but once hatching has occurred, mouthing does help to widen the shell rupture made by the embryo's tail. The adults appear to suck the embryos out of their broken shells as soon as possible.

At hatching, the embryos are relatively undeveloped, compared with those of substrate spawners. Balon (1977) believes that this is possible only because the hatchlings are then protected orally. Aequidens para- guayensis may have evolved relatively early hatching and subsequent mouthbrooding to minimize the length of the substrate phase. Perhaps selection has reduced the substrate phase as much as possible, short of doing away with it altogether, so that a breeding pair can reduce it no further, even when its brood is threatened.

Our results do not support the hypothesis that a moveable spawning substrate is an antipredator device. However, other workers (Barlow 1974; Timms and Keenleyside 1975; Keenleyside and Prince 1976) have provided evidence for such a function. Noting the position of the leaf in the aquarium, .rather than the frequency with which it was tugged, would perhaps

have been more revealing, since a spawn located in a sheltered area need not be moved elsewhere for greater protection. Another explanation is that a mobile spawn- ing platform may be more useful in dealing with sudden water level changes than in reducing egg predation. Floods commonly occur in South American drainage systems (Lowe-McConnell 1975), and breed- ing pairs could avoid areas of strong current by tugging the spawn into quiet backwaters (Keenleyside and Bietz 1981).

Full-time mouthbrooding would seem to be a more efficient solution to environmental pressures such as egg predation and sudden water level changes than using a moveable spawning site. It is surprising that this form of parental care is so rare in New World cichlids. Only Geophagus hondae (Sprenger 197 1 ) and Geophagus crassilabris (T. R. Carlisle, personal communication) are known to be maternal mouthbrooders. The combined substrate- and mouth-brooding parental strategy, as followed by A. paraguayensis, is also rare among New World cichlids (Cichocki 1976). This may represent an intermediate step in the evolution of true mouthbrooding (Keenleyside 1979) or it may have evolved in response to greater predation pressure on the wrigglers and fry than on the eggs. Clearly, the best method of addressing this question is to gather information about the behaviour of this species, and the conditions that it deals with, in its natural habitat.

Acknowledgements We wish to thank R. H. Green and B. F. Bietz for

statistical advice; A. C. J. Vincent for typing the manuscript and drawing the figure; and S. C. Courtenay, G. P. Goff, P. T. Handford, M. I. Kavaliers, D. B. Noltie, R. J. J. Roy, D. M. Scott, and anonymous reviewers for helpful comments on the manuscript. This research was supported by an NSERC grant to M . H. A. K. and Ontario Graduate Scholarships to H.M.C.D.

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