Social hierarchy, shelter use, and avoidance of predatory toadfish (Opsanus tau) by the striped blenny (Chasmodes bosquianus)

Anim. Behav ., 1979, 27, 1 11 3-1121

SOCIAL HIERARCHY, SHELTER USE, AND AVOIDANCE OF PREDATORY TOADFISH (OPSANUS TAU) BY THE STRIPED BLENNY

(CHASMODES BOSQUIANUS) BY

ROBERT R . PHILLIPS & STANLEY B . SWEARS

Department of Biology, State University College, Oneonta, New York 13820

Abstract. This paper describes a three-part examination of the predatory relationship between oyster toadfish (Opsanus tau) and striped blennies (Chasmodes bosquianus) . In the first laboratory study, blennies avoided the toadfish when placed in a choice situation . During the second laboratory study, dominant blennies escaped attack significantly more than did subordinates by using a protective shelter . A field study demonstrated that toadfish will attack active but tethered blennies under natural conditions . As predators, toadfish appear to use a mixed strategy . During the day, when they are relatively inactive, they may feed as ambush predators . However, we suggest that at night they move about actively, stalking benthic invertebrates and small fishes . They also feed as scavengers, consuming a variety of foods . The striped blenny (Chasmodes bosquianus Lacepede) is a small, bottom-dwelling fish found in estuarine and coastal waters along parts of the East and Gulf Coasts of the United States . It prefers areas of irregular and cluttered bottom topography . Within such areas, males compete intensely for the breeding shelters, such as empty oyster shells, that they must have for successful breeding . Phillips (1977a) has suggested that these shelters are important for two major reasons : (1) They serve as protected sites for positioning eggs that adhere to a smooth, hard surface . The inside of a shell is probably one of the few surfaces not covered by either silt or encrusting organisms in Chesapeake Bay. (2) The shelters provide protection from eggeating predators and also protect the male parent from larger predators . Stephens et al . (1970) mention that several predatory fishes attack West Coast blennies . It is the shelter as a protective site for adult blennies that we are considering in this study . There are two types of evidence that suggest that the striped blenny is under predatory pressure . First, the fish is basically cryptically coloured . The two morphological regions that bear the bright colours used in displays can be covered. An iridescent, blue ocellus on the dorsal fin can be hidden when the fin is furled . Also, the interior of the mouth is shiny white ; although this area is quite visible in mouth-open displays (Phillips 1971a), it, of course, is covered when the blenny closes his mouth . The cryptic coloration may be an adaptation for escaping detection by daytime predators .

Second, the animal is quite effective in using any substrate irregularity or object in avoiding capture, at least by a human diver. One would assume that these abilities would certainly increase the animal's probability of escaping from other types of attackers . The fish will slip into any available crevice, duck under an overhang, or lie tightly against the side of an object, motionless, becoming hard to detect because of its coloration . Avoidance would be useful against both diurnal and nocturnal predators . Although this set of attributes certainly suggests pressure from predators, no observation of an actual attack has been reported. One of us (RRP) has spent many hours underwater in the habitat of the blenny and has never seen a typical predator close to them . However, the oyster toadfish (Opsanus tau) is quite common in the same area as the blenny. The toadfish were frequently seen, motionless and watchful, either lying on the substrate or peering from their shelters. The blennies and toadfish appeared to behave neutrally toward one another . Blennies often passed close to toadfish with no response on the part of the toadfish . However, since the toadfish is known to be omnivorous and opportunistic, we performed a short pilot experiment where blennies were exposed to toadfish in laboratory pools . Two results of that short study generated the formal study reported in this paper : (1) toadfish did stalk, attack, and eat blennies, and (2) they did so only after dark . This is not to suggest that blennies are a major item in the diet of the toadfish . However, 1113



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ANIMAL BEHAVIOUR, 27, 4

the toadfish is an ever-present predator, able to seize a weakened or careless blenny or perhaps one who is unable to find a suitable shelter . One approach to examining the relationship between these two species was to determine whether the blennies avoided toadfish . If toadfish are dangerous to the smaller fish, we could predict that such a response would have been developed. By presenting blennies with a choice, i .e . an opportunity either to approach or to withdraw from toadfish, it was possible to interpret their response to the predator. This was the format of study 1 . In study 2, we used O . tau as a predator to test the relationship between social dominance, shelter use, and predator avoidance . The link between social dominance and shelter use has already been demonstrated by Phillips (1971b), who found that dominant male striped blennies use shelters more than subordinate males . The dominant fish can drive subordinates from shelters at will . The variable that we tested in study 2 was the importance of the shelter in preventing attack . Study 3 was designed to see whether toadfish would attack blennies made available to them in the field. In this way, we verified, at least to some degree, the results of the laboratory work . Materials and Methods The fishes used in this experiment, the toadfish (Opsanus tau), striped blenny (Chasmodes bosquianus), and hogchoker (Trinectes maculatus, Soleidae), were captured from the waters adjacent to the Chesapeake Biological Laboratory in Solomons, Maryland, and transported to our laboratory at the State University College in Oneonta, New York . We have already mentioned the roles of the toadfish and blenny ; the hogchoker, a flatfish, was used as a control fish in the choice experiment (study 1) . His exact role will be described later . Within a few days after arrival in Oneonta, all the blennies were anaesthetized, measured, and tagged with individually recognizable combinations of beads . The method of tagging has been described by Phillips (1971 a) . The fishes were maintained in fibreglasscovered wooden holding tanks (91 x 118 cm) containing water 15 cm deep . Each species inhabited a different tank . The salt water in all tanks and pools was prepared from a commercial mix, Instant Ocean, and maintained at approximately 15 parts per thousand . The room lights, which were controlled by a timer, came on at

04 .00 hours and went off at 18 .00 hours . The temperature of the water varied between 17 and 26 C . The fish were fed chopped shrimp once a day . Because both blennies and toadfish are shelter-dwelling species, we provided sections of opaque PVC tubing as shelters for them in the stock tanks . Shelters for toadfish were 10 . 2 cm in diameter and those for blennies 4 .0 cm . Both species readily used their shelters . Opaque plastic wading pools, were selected as experimental aquaria for two parts of this study . The pools were 98 cm in diameter, containing 13 cm of water . Approximately 3 cm of sand covered the bottoms . Blinds constructed of black plastic film surrounded the pools . All conditions of maintenance in the experimental situation were similar to those of the stock conditions . The order in which these three studies were performed was : (1) predator recognition, (2) hierarchy and predation, and (3) field verification . Many of the same blennies and toadfish were used in studies 1 and 2 . No blenny or toadfish, however, was used more than once in the same study . We did find it necessary to use the same two hogchokers repeatedly as control fish in study 1 . The toadfish were fed daily throughout the laboratory study . Perhaps we could have made them more voracious by starving them, but we wanted to insure that their interest in pursuing blennies did not stem from extreme hunger . All fish were given at least seven days to acclimate to the laboratory conditions before we used them in the first study . As an indication of the degree of acclimation, the blennies successfully mated and laid eggs in several of the stock tank shelters . Specific methods related to the three experimental studies will be identified below . Predator Recognition In this study the objective was to present the blennies with a choice situation where they could either share an area with a toadfish or, by swimming through a wire grating too small to admit toadfish, share an area with a hogchoker, a control species that does not eat blennies . Figure IA illustrates how this experimental pool was arranged. The pool was divided into halves by a vertical grid formed from welded-steel fencing with rectangular openings 5 . 0 x 2 . 5 cm . For convenience, we have used the abbreviation PS to represent the predator's (toadfish's) side of the



PHILLIPS & SWEARS : FISH PREDATION

pool in this study . The symbol CS signifies the control (hogchoker's) half of the pool . On each side, a toadfish shelter was placed . The shelter on the PS was included for the comfort of the predator . That on the CS was added in an effort to keep both sides as similar as possible . There was one difference in the shelters, however. The entrance to the CS shelter was covered with a transparent plastic film to keep the blennies from entering and being lost from the observer's view . If this precaution were not taken, it could have led to an erroneous conclusion that a blenny had been eaten . We did not expect that blennies would enter the PS shelter, and indeed

Fig . 1 . Diagrams of the arrangement of the experimental pools . A, study 1 . Abbreviations : t, toadfish ; b, blennies ; --, position of wire grid ; c, control fish (hogchoker) . The toadfish shelter in the control half is blocked by a transparent plastic film. B, study 2. Open rectangles represent the larger toadfish shelter and the smaller hlenny shelter.

1 11 5

our later observations showed that blennies did avoid the shelter of toadfish . The hogchoker is a flat fish that is common in the Chesapeake Bay. We selected the hogchoker as a proper control fish for several reasons : (1) they are plentiful in the habitat of the blenny and thus would not constitute a novel stimulus object, which could influence choice of the two halves of the pool ; (2) they are bottom fish and similar to the toadfish in this respect ; (3) they are roughly the same length as a toadfish, although different in shape ; and (4) they can be caught unharmed with a handnet . Another similarity, as it turned out, is that both hogchokers and toadfish are active primarily at night. In the experimental schedule that we designed, a toadfish and a hogchoker were released in the pool on opposite sides of the wire grid on day 1 at noon . Shortly thereafter, three blennies were released adjacent to, the central wire grid . The three blennies had been together in another pool for three days just prior to their release, so that their social hierarchy could be determined . For the next three days, a daytime and a nighttime observation, each lasting 100 min, were taken . These two observation periods began, respectively, at 08 .00 hours and 20 .00 hours. The location of each blenny, i .e . PS or CS, was recorded once every 10 min for a total of 10 counts . At night, this required using a dim, red-filtered flashlight just long enough to find all the blennies . The data from this study were analysed to determine whether blennies display an avoidance or preference for one half of the pool as compared to the other. Secondly, we examined the data to see whether blennies avoid the toadfish side more at night, when the toadfish is active, than during the daytime . Both comparisons were evaluated by means of the Wilcoxon matched-pairs signed ranks test . N was 9 in both cases. Hierarchy and Predation In this study, the relationship between position in the social hierarchy, shelter use, and survival of blennies exposed to toadfish was examined. At noon, three blennies were released in a pool containing one toadfish, a toadfish shelter, and one blenny shelter (see Fig . 1B). The blennies and toadfish remained together for 72 h . A top made of cheesecloth netting covered the pool to keep the blennies from jumping out .



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ANIMAL BEHAVIOUR, 27, 4

In selecting the blennies for each three-fish group, we made sure that there was at least one male per group . Males are more aggressive than females, making it easier to recognize the dominance-subordinance relationships . If we had used any groups of three females, it might have been difficult to recognize the dominant fish without more extensive observations . No other criteria, such as size of individuals, were employed . After the blennies were released, observations were taken at irregular intervals to determine which of the three fish became the shelter user . Also, the shelter users' social interactions with the two shelterless fish were recorded . As we expected from previous work (Phillips 1971b), the individual that dominated the others during social encounters was also the fish that had exclusive use of the shelter. In the social system of the striped blenny, the hierarchies among small groups tend to be linear, and reversals of hierarchical positions are rare . The dominant fish has exclusive or almost exclusive use of the shelters . The animals in the present study displayed the same characteristics of organization . For more complete descriptions of aggressive behavioural patterns and shelter use, see Phillips (1971a, 1971b, 1977a) . The basic type of data that we collected was the death of specific blennies . Since the pilot experiment had indicated that toadfish hunted blennies in darkness and that even dim light would allow blennies to see and avoid toadfish, our primary measure was to check early in the morning after each test day to determine whether any blennies were missing. Additional evidence of predation was the appearance of the blennies' identifying tags in fecal material of the toadfish. Survival information could then be compared with previously determined hierarchical positions and shelter-use information in order to test the appropriate null hypothesis associated with this study . Field Verification The laboratory studies can be criticized on the basis that toadfish ate blennies in the laboratory because they had little choice of food items . For this reason we designed a field study to test the assumption that toadfish attack blennies in the field . Ten blennies were captured from the waters surrounding the pier of the Chesapeake Biological Laboratory at Solomons, Maryland . A fish hook attached to a steel leader was driven

laterally through the epaxial musculature about midway between the head and tail of each blenny . Using SCUBA gear, we tethered each fish to a different pier piling by a length of monofilament fishing line approximately 50 cm long. The line was attached to the base of the piling at substrate level so that the fish could move about the bottom as naturally as possible under tethered conditions . We placed the fish hook in the fleshy area above the vertebral column, avoiding injury to vital organs . This is the location that has been used for tagging striped blennies (Phillips 1971a), producing a similar type of wound . Tagged blennies have survived for at least a year in the field . For this reason, we expected the fish to remain active while tethered . Our observations indicated that this assumption was indeed valid . The blennies were tethered in position just before sundown, when toadfish can be expected to become active (Phillips & Swears, in preparation) . Early the next morning, the blennies were checked . Results Predator Recognition The results of this study are listed in Table I . In order to test the significance of these data, we first added the checks of the three blennies in each pool throughout the three-day experimental period to arrive at a representative value for each side (PS or CS) of the pool . There was a problem, however, stemming from the fact that four blennies leapt out, resulting in a different total number of checks for three of the replicates (1, 2, and 5 of Table I) . We approached this problem in two ways . First, we calculated the percentage of checks on each side for all replicates so that all replicates would have equal weight in the statistical tests . Second, we tested each hypothesis in this section again using the actual number of counts after deleting replicates 1, 2 and 5 . All other replicates had the same number of position counts . The Wilcoxon matched-pairs signed ranks (Siegel 1956) was used to test the significance of the differences between treatments . The level of significance was 0 . 05 . Except where otherwise indicated, all hypotheses in this section were two-tailed . We first tested total values representing the day and night observations . The blennies used the CS of the pool significantly more than they did the PS (percentage : N = 9, T = 0 ; actual counts : N = 6, T = 0) .



PHILLIPS & SWEARS : FISH PREDATION

Our next question was whether the blennies were seen more frequently on one side than the other during the daylight . The same question was posed for night-time checks . The results of the Wilcoxon tests were the same for both day and night : for percentages, N = 9, T = 0 ; for actual counts, N = 6, T = 0 . During both day and night-time, blennies spent significantly more time on the CS . Toadfish have been found to be active primarily at night, while blennies are active during both day and night (Phillips & Swears, in preparation) . One might assume that blennies would be more likely to approach toadfish during the daytime than at night . This could perhaps be part of the explanation for blennies being found near toadfish in their natural habitat during the daytime, as we observed . The data collected during this study permit a convenient comparison of whether they were more active in the toadfish's part of the pool during daylight hours than they were at night . When we tested this one-tailed hypothesis, we found that blennies did use the toadfish's side significantly more during the daytime than at night (percentage : N = 8, T = 3 ; actual counts N = 5, T = 1). However, the difference between mean percentages, 7, is relatively small . A further question with respect to this study is whether the blennies learned over the course of the three-day experimental period to avoid the toadfish . In other words, by stalking or chasing blennies the toadfish might have conditioned the smaller fish to avoid them . Table I. Position Check Data from the Predator-Recognition Study* Percentage of position checks PS Replicates 1J1 2J2 3 4 5J1 6 7 8 9 Averages

CS

day night total

day

night total

11 17 10 8 17 24 11 0 6

5 10 0 8 0 6 0 4 0

9 8 5 8 9 15 6 2 3

89 83 90 92 83 76 89 100 94

95 80 100 92 100 94 100 96 100

91 82 95 92 91 85 94 98 97

12

4

7

88

95

92

*PS, Predator Side ; CS, Control Side . J i : 1 blenny jumped from this pool . J2 : 2 blennies jumped .

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One way of approaching this problem is to compare the percentage of checks in the predator side on night 1 to those on night 3 . It was during night 1 that the blennies first encountered the toadfish in this laboratory study during the larger fish's more active phase . If blennies learned to avoid toadfish, we would predict a decrease in the number of PS checks between the two nights . The difference between nights 1 and 3 did not approach significance (N = 5, T = 5) . Although the number of ties reduced the number of replicates from nine to five, this was still enough to test, i .e . it was possible to reach the 0 . 05 level of significance with N = 5 and T = 0 . The facts that T = 5, well above significance, and that there were four ties, speak strongly against the hypothesis . A tie means that the blennies involved were not seen in the PS on either the first or third night . In addition to the tests of the quantitative data, other observations reveal interesting information about the nature of the relationship between these two species . Four blennies were found dead on the floor in the morning (see Table I) adjacent to the PS of the pools . Obviously, they had leapt out . Also, there were shallow puddles of water on the floor . Striped blennies are not jumpers . One of us (RRP) has kept blennies for a number of years and has never had one jump from the tanks . The puddles of water are also unusual . There was never any water around either the blenny or toadfish stock tanks . This suggests violent motion at the surface near the walls of the pool . Hierarchy and Predation As shown in Table II, 15 of the 30 individual blennies used in study 2 were killed . Of the 15 victims, none were the dominant shelter users . One way to evaluate the effect of dominance on escaping from the predator is to determine whether animals in the two levels of hierarchy have different probabilities of being captured. If position in the hierarchy is not related to the ability to avoid capture, then the probability of capture will be the same for all three fish. On the other hand, if dominance hierarchy and the ability to avoid capture are related, then these probabilities will be different . The hypothesis that we tested is Ho : PD = 0 . 5 Ps versus H1 : PD 0 0 .5 Ps, where P is the probability of being attacked, D represents dominant, and S subordinate blennies . The data used to test the hypothesis are presented in Table III .

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ANIMAL BEHAVIOUR, 27, 4

The chi-square value, calculated from Table III, was 6 . 08 with 1 df. This value is greater than the 0 . 025 level of significance . We conclude that there is strong evidence of a difference in ability to escape predation . Field Verification An examination of the lines attached to the pilings early in the morning revealed seven hooked toadfish . Two hooks were bare, i .e . the blennies were gone . A single female blenny remained on a hook . Interestingly, she had the use of a shelter . Near the piling was a narrowly opened empty oyster shell . This is the preferred shelter of blennies in this region . Her tether was long enough so that she could enter the shelter . Because she was active and in good physical condition, we cut the barb off the hook, withdrew it, and allowed her to swim away . One of us (SBS) actually watched a toadfish attack one of the hooked blennies . Immediately after the tether was tied to a piling in the evening, a toadfish appeared, swimming along the bottom . He darted to the blenny and seized him . As soon as the toadfish discovered that he was hooked, he thrashed about, trying to escape . After a hasty conference, we chose to leave the fish hooked overnight as a test of whether a toadfish, with its heavy teeth and massive jaw musculature, could chew through the steel leader to escape . We found him still on the hook the following morning . Table H. Results of the Social-Hierarchy-and-Predation Study* Days of exposure to toadfish Pools 1 2 3 4 5 6 7

8 9 10 Totals killed

1

2

3

Totals

0 S S S S 2S 2S 0 0 S

0 Ot S 0 0 0 0 0 S S

0 0 0 S 0 0 0 S S 0

0 1 2 2 1 2 2 1 2 2

9

3

3

15

*The hierarchical rank of the blennies killed by the toadfish is indicated . S represents one of the two subordinate fish . t-Dorsal fin of I S individual badly torn .

Discussion The results of this study strongly suggest that the oyster toadfish exerts predatory pressure on the striped blenny. Although recognizable remains of blennies have not been found in studies of toadfish stomach contents, Chrobat (1959) has identified a variety of small fishes, including the naked goby (Gobiosoma bosci, Gobiidae) and small toadfish . The goby shares the habitat of the blenny and, in general, lives in a similar manner . The fact that blennies were not found suggests (1) that blennies are taken infrequently and thus not seen in a relatively small sample, and (2) the defensive mechanisms of these fishes are somewhat effective . However, an ill, injured, or careless blenny might frequently attract the interest of a nearby toadfish . This, we believe, was demonstrated by the toadfish's response to the hooked blennies in study 3 . Although toadfish do not support themselves by catching blennies, they may still exert intense predatory pressure on blennies . We have seen several toadfish wherever we have found blennies . Both are bottom fish, and the toadfish appears to be attracted to the areas of irregular topography that could supply the burrows or breeding shelters that both species require . The results of the field study certainly show that toadfish attack blennies in the wild if the opportunity is present. The direct and vigorous attack observed by SBS did not suggest hesitancy or indecision on the part of the predator . Perhaps the toadfish sensed that the blenny was injured or, since it was early dusk, the time of peak toadfish activity was approaching (Phillips & Swears, in preparation) . During the field study, toadfish were caught on 7 of the 10 hooks . There are two likely explanations for the two bare hooks : (1) toadfish or perhaps other predators seized the blennies either by the posterior or anterior end and tore Table III. Number of Blennies of Each Dominance Level Killed by Toadfish Dominance levels Survival states Captured, nights 1+2+3

Dominant Subordinate

Total

0

15

15

Survived

10

5

15

Total

10

20

30



PHILLIPS & SWEARS : FISH PREDATION

them from the hooks or (2) the blennies managed to free themselves . Of these two possibilities we think that the first explanation is more probable . It is difficult to imagine how the blennies could have gained enough leverage to work the barbed hook free. During study 1, the blennies were counted in the hogchoker's side of the pool, both day and night, significantly more frequently than in the toadfish's side. There are two interpretations of these results : (1) the blennies respond to toadfish as dangerous predators and avoid them when given the choice or (2) blennies are attracted to hogchokers. The first explanation is more reasonable than the second because we have seen blennies actively avoiding toadfish at night under low-intensity red light . We never saw any reactions to hogchokers. The blennies responded neutrally to hogchokers, i .e. they neither approached nor avoided hogchokers . The avoidance of toadfish as shown in this study could be interpreted as behavioural patterns learned in the laboratory, i .e . blennies learned, during night 1, to avoid toadfish because they were chased . However, the fact that there was no significant difference between the first and third nights with respect to the amount of time that blennies spent on the toadfishes' side discredits this explanation . Rather, this behavioural pattern was present in the wildcaught blennies . The blennies did use the predator's side of the pool significantly more frequently during the day than at night. However, the difference was not large enough (day : 12 %, night : 7 %) to imply that blennies are safe from toadfish during the day . Therefore it does not seem that daylight by itself could account for blennies approaching toadfish to within relatively short distances during the daytime in the field, as we have seen them do . The best explanation may involve a combination of several factors . The availability of cover is certainly important . In the field, blennies prefer areas with large amounts of cover to which they can dash at any time . The laboratory pools were bare and open compared to the natural situation . Also, the relative inactivity of toadfish during the day may play a part . In addition, other observers of predatory situations have suggested that some mammalian prey species can often recognize whether predators are hunting and can respond to their degree of proximity appropriately (Estes & Goddard 1967 ; van Lawick-Goodall & van

1 119

Lawick-Goodall 1971) . Perhaps blennies have similar abilities . Probably the most important defensive mechanism that blennies have is their highly developed ability for using cover . Previously, Phillips (1977a) described two types of shelters used by blennies as open and closed shelters . Open shelters are objects that provide visual and/or physical cover for a blenny but do not almost completely surround it as does the closed shelter, e .g . the slightly open valves of a dead oyster. Closed shelters would provide a higher degree of protection against predation and are the only type of shelter used for laying eggs . There are always more adult male blennies than there are suitable closed shelters . During past summers, one of us (RRP) has often collected most of the males from the closed shelters and then found them all re-occupied a week later. Males defend the closed shelters vigorously against one another . Additional evidence concerning the effects of shelters on survival emerged from the field study. The only hooked fish remaining alive by morning was a female who found an oyster shell within her reach that we had not noticed. She was safely inside when the hooks were checked . Blennies are quite agile at using both types of shelter for escape . They can dart to and, turning to fit their laterally compressed bodies through the narrow opening, slide into an oyster shell with no apparent reduction in speed. A blenny will squeeze into any crevice or opening afforded by an open shelter and lie just out of reach . As a diver tries to catch a blenny, the fish will go from one object to the next, using each to its full potential as an obstruction before continuing . We suggest that it is this ability that allows blennies to swim about near toadfish during the daytime in regions containing plentiful shelters, as we have seen them do . The effect of a high position in the social hierarchy with respect to increased protection from toadfish was demonstrated . In the present study and in others (Phillips 1971, 1977a), dominant blennies have displayed the ability to chase other individuals from shelters . In the starkly bare topography of the experimental pool, which contained only one shelter, the dominant, shelter-using fish were never captured . As calculated in the results section, the probability of this occurring as a random event is significantly low. In their natural habitat, the



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ANIMAL BEHAVIOUR, 27, 4

ability to take the safest shelters could be expected to increase the probability of survival and hence increase the fitness of dominant blennies . The feeding techniques of the toadfish can be described as opportunistic . The animal appears to act as both a scavenger and a predator (Gudger 1908) . Hobson (1972) has described a group of small predatory fishes that become active on coral reefs as darkness falls, consuming primarily benthic invertebrates . The toadfish seems to follow a similar pattern . We have found toadfish to be much more active at night than during the day, with the peak of activity occurring shortly after dark (Phillips & Swears in preparation) . Chrobat (1959) reports finding xanthid crabs, amphipods, and shrimp in toadfish stomachs. Blue crabs (Callinectes sapidus) and other crustaceans are listed as preferred food by Gudger (1908) . Small clams were in the relatively few stomachs that we checked . Along with the invertebrates, however, toadfish take fish (Chrobat 1959) . One of those that were identified is Gobiosoma bosci, a bottom-dwelling fish that uses areas containing cover and shelters . In the present laboratory study, we rarely saw the toadfish out of their shelters during the day . They became active just after dark . Under low-intensity red light we have seen toadfish slowly but directly approach blennies, who dashed away from them . The blennies' response is much more vigorous at night than during the day . One of us (RRP) has seen blennies dart from the bottom toward the surface as toadfish continued their dogged pursuit. In the field, this may be a useful emergency technique for a small bottom fish, especially if caught out in the open . In the laboratory, however, it results in a fish dying on the floor . Phillips (1977b) reports that Istiblennius zebra, a Hawaiian tide pool blenny, also skips along the surface when disturbed, but also only at night . During the daytime, however, the toadfish appears to employ another strategy . We, as divers, have seen them either motionless on the bottom or occasionally peering from their shelters . Their coloration helps somewhat in concealment. During the day, they may act as ambush predators, waiting, as Hobson (1974) states in describing reef ambushers, `until a small organism within striking distance makes a defensive mistake' . The fact that blennies do approach them closely as they swim around and do seem to be aware of them may be based, as stated earlier, on the smaller fish's ability to dash to nearby cover . However, one incident

that occurred in the laboratory demonstrated the toadfish's ability to profit from defensive mistakes during daylight hours . A blenny was being transferred from one tank to another by hand net . He leapt out,landing in a pool containing a toadfish, who was in a 'head-out' position from his shelter . The blenny darted toward the only large structure in the pool, the toadfish's shelter. In an action almost too fast to see, the predator shifted his position slightly to intercept the blenny, opened his mouth into which the blenny dashed, and closed it on his unexpected meal . It would be interesting to know what sensory system(s) the toadfish employs to guide him to his nocturnal prey . However, little is known about the relative sensitivities of their receptors . Perhaps future investigations will provide answers to the fascinating problems of nocturnal predation . Acknowledgments Phyllis Phillips aided us in capturing the fishes and in proofreading . Douglas Robinson helped in catching fishes . The personnel of the Chesapeake Biological Laboratory provided housing for us and temporary storage facilities for the fishes . We deeply appreciate the aid and support of these persons . REFERENCES Chrobat, R. J . 1959. The feeding habits of the toadfish (Opsanus tau) based on an analysis of the contents of the stomach and intestine . M.S. thesis, University of Maryland, College Park . Estes, R . D . & Goddard, J. 1967 . Prey selection and hunting behavior of the African wild dog. J. Wildl. Mgmt., 31, 52-70. Gudger, E . W. 1908 . Habits and life history of the toadfish (Opsanus tau) . Bull. U. S . Bur. Fish ., 28, 1073-1109 . Hobson, E . S . 1972. Activity of Hawaiian reef fishes during the evening and morning transitions between daylight and darkness . Fish . Bull., 70, 715-740. Hobson, E . S. 1974. Feeding relationships of teleostean fishes on coral reefs in Kona, Hawaii. Fish. Bull., 72, 915-1031 . van Lawick-Goodall, H. & van Lawick-Goodall, J. 1971 . Innocent Killers. Boston : Houghton Mifflin Co . Phillips, R . R . 1971a . The relationship between social behavior and the use of space in the benthic fish Chasmodes bosquianus Lackpede (Teleostei, Blenniidae) : I. Ethogram. Z. Tierpsychol., 29, 11-27 . Phillips, R. R. 1971b . The relationship between social behavior and the use of space in the benthic fish Chasmodes bosquianus Lac6p6de (Teleostei, Blenniidae) : II. The effect of prior residency on social and shelter behavior. Z. Tierpsychol., 29, 389-408 .

PHILLIPS & SWEARS : FISH PREDATION Phillips, R . R . 1977a . The relationship between social behaviour and the use of space in the benthic fish Chasmodes bosquianus Lac6pede (Teleostei, Blenniidae) : IV. Effect of topography on habitat selection and shelter choice . Behaviour, 60, 1-27, Phillips, R . R . 1977b . Behavioral field study of the Hawaiian Rockskipper, Istiblennius zebra (Teleostei, Blenniidae) : I . Ethogram . Z. Tierpsychol ., 43, 1-22. Phillips, R . R . & Swears, S. B . In preparation. Circadian activity cycles of two Chesapeake Bay fishes, the

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striped blenny (Chasmodes bosquianus) and the oyster toadfish (Opsanus tau) . Siegel, S . 1956 . Nonparametric Statistics for the Behavioral Sciences . New York : McGraw-Hill . Stephens, Jr, J. S ., Johnson, R. K ., Key, G . S . & McCosker, J . E . 1970. The comparative ecology of three sympatric species of California blennies of the genus Hypsoblennius Gill (Teleostomai, Blenniidae) . Ecolog . Monogr ., 40, 213-233 . (Received 13 October 1978 ; revised 23 January 1979 ; MS. number : n2220)