Interactions between piscivorous fishes and their prey, ES Hobson

Tags: Hobson, predators, reef fishes, fishes, Gulf of California, species, quiet period, predator, McFarland, schooling fishes, tropical reefs, fish schooling, shore fishes, Freiwassehmbachtungen zur Deutung des Schwarmverhaltens verschiedener Fische, schooling species, Press Hawaii, shelter, California, feeding behavior, predators and prey, Piscivorous Fishes, interactions, Edmund S. Hobson, feeding mechanisms, aquatic organisms, piscivores, Coral reef fishes, underwater light, bright light, Copeia Edmund S. Hobson
Content: Interactions Between Piscivorous Fishes and Their Prey Edmund S. Hobson
Most fishes that prey on other fishes face what might seem a simple enough task: they must run down, or otherwise capture, organisms that are smaller, and weaker, than they are. Generally, their attacks are direct approaches to exposed prey that are large enough to grasp, yet small enough to manipulate. And once ingested the prey are usually swallowed whole. These predators are well built for the job. Fish-eaters, or piscivores, generally have large, but relatively simple mouths, and short uncomplicated digestive tracts. These features have remained relatively unchanged during a long period of evolution that has seen major alterations in the feeding mechanisms of species which forage on other kinds of aquatic organisms (see Gosline 1959, Hobson 1974). Obviously, they attack with proven equipment. Nevertheless, despite the conservative nature of their feeding apparatus, and what might seem a straightforward predatory task, other parts of their anatomies, and also their feeding related behaviors, have diversified greatly to meet severe problems that stem from capturing prey. And the finely tuned interactions between these predators and their prey are further evidence of powerful evolutionary forces. Attacks by fish-eating predators during the evolution of modem species have pressured prey to acquire effective defensive adaptations. But every successful defense has in turn evoked an appropriate offense, so that a delicate balance now exists. The opposing forces that maintain this balance, as expressed in adaptations of both predators and prey, and the patterns of predation that have resulted, are the topics of this chapter. I have developed a synthesis based on published reports that should have widespread
relevance even though most of the material draws from fishes in tropical and warm-temperate seas. These wanner marine habitats have proven most fruitful for study of the topics considered here, because feeding interactions among members of their exceptionally rich and diverse faunas exhibit especially well-defined patterns. This is largely because so many of the interacting species in these places have highly specialized feeding habits. It is one of the axioms of biology that interacting species become more specialized as their numbers increase. Nevertheless, the principles that emerge from this synthesis should apply even where species are fewer and less diverse, and where feeding patterns perhaps are less distinct. Major Offenses of Piscivorous Fishes Most fishes that prey on other fishes use one of four predatory strategies, each strongly reflected in the user's body features. They either (1) run down prey, (2) ambush prey, (3) habituate prey to an illusion that they are nonaggressive, or (4) stalk prey. Predators that run down their prey are the most widespread because their major asset speed - is particularly suited to the most widespread habitat - open water. They tend to be highly streamlined, with cylindrical, heavily muscled bodies and deeply forked tail-fins. The tunas (Scombridae)and the billfishes (Istiophoridae) are oceanic examples, whereas jacks (Carangidae) are prominent representatives in nearshore marine waters (Figure 1). The straightforward attacks that characterize these predators, however, are less effective in the confined spaces of shallow water and near the bottom, especially where prey are just a quick dart from
PREDATOR-PREY SYSTEMS IN fisheries management
a sheltering reef or grass bed. Such circum- in prey they have managed to approach.
stances favor predators that use one of the fol- The last three saategies cited are described
lowing three ways to catch prey.
in pure form. Although the examples given are
Predators that ambush prey generally rest highly distinctive, many piscivores, for ex-
immobile on the bottom and capture organisms ample, certain sea basses, incorporate compon-
that have inadvertently come within range of a ents of two, or even all three, of these strategies.
short, explosive charge. The color and texture Furthermore, some piscivores have adopted
of some, for example, certain scorpionfishes specialized variations of these strategies. The
(Scorpaenidae), are similar to that of the sub- leather bass, Derniatolepis dermum1rpi.s(Figure
strate on which they lie, while others, like cer- 6). for example, commonly gains predatory
tain flatfishes (Pleuronectiformes), may rest advantages by associating with both schools
under a layer of sand (Figure 2).
and individuals of other species. Sometimes
Some ambusherscapture prey without leaving the leather bass approaches prey from behind
their position on the bottom. When prey ap- these other fishes, apparently using them as a
proach closely to the Hawaiian scorpionfishes blind to get within striking range unnoticed;
of the genus S~.orl'uc'tii) 3), for ex- other times the bass follows alongside herbi-
ample, these big predators simply snap open vores and other grazing fishes and captures prey
their cavernous mouths and gill cavities and that are driven from cover as the grazers disturb
suck their victims in (Hobson 1974). A few the substrate. (Montgomery 1975).
species have embellished the ambushing tactic
by acquiring features that actively draw prey to them; thus, certain anglerfishes (Antenna-
Major Defenses of Prey
riidae) have a fin spine modified as a lure that The strategies used by piscivorous predators
attracts potential prey (Figure 3).
are responses to the defenses of their prey.
Predators that habituate prey to an illusion Major defenses include both behavioral and
that they are nonaggressive, including certain anatomical adaptations, and are used by most
basses (Serranidae) and snappers (Lutjanidae), prey species in some combination. Schooling
often live side by side with their ultimate vic- and staying within reach of shelter are important
tims (Figure 4). Most are sluggish, sometimes defensive behaviors, whereas spininess and
large fish, but can explode upon prey with a body armorare important anatomical adaptations.
burst of speed over short distances. These pred- Virtually all small fishes school when they
ators often hover above the bottom in full view, are away from a sheltering reef or other structure
which distinguishes them from the ambushers. during the day, and almost invariably smaller
Generally, their coloration and demeanor render species that spend all their time in open water
them inconspicuous, so they regularly go un- school habitually. It is widely believed that
noticed, if not unseen. Their success as preda- fishes are protected from predators when they
tors depends on prey becoming accustomed to school, although opinions vary on how this
their presence, lulled by their peaceful mien occurs. Some suggest that when fishes school
and ultimately careless.
they reduce their encounters with predators
Predators that stalk prey typically include (Brock and Riffenburg 1960, Olson 1%4),
long, attenuated fishes like barracudas (Sphy- whereas others believe that schools increase
raenidae), pikes (Esocidae), needlefishes (Be- security because predators regard the group as
lonidae), and cornetfishes Wistulariidae). By some inedible, or threatening, object (Crawford
drifting forward slowly, showing overt aggres- and Powers 1953, Springer 1957).
sion only when close upon an unwary victim, Some note that the greater number of eyes
they amve within striking range of their target and other sensory receptors in the group make
despite being in full view. Many stalkers have it significantly more probable that a threatening
exceptional feeding structures that are in cnn- predator will be recognized (Bowen 1931);
trast to the generalized equipment of most other others suggest that schools have resulted be-
fish-eaters. The cometfishes (Figure S), for cause individuals in the center are protected by
example, have an exceptionally long, tubular those of their kind between themselves and the
snout, which, when suddenly expanded, sucks predators (Williams 1964, Hamilton 1971).
Any one of the above possibilities could be a 1916). But so long as the vast majority offish-
factor under appropriate circumstances, but eating predators is frustrated by the defensive
generally I favor the theory that advantage is aspect of schools, their failures will cancel out
gained through a "confusion effect" (as advo- the successes of the relatively few specialized
cated by Allen 1920a, 1920b, Manteifel and forms that seem to have solved the problem.
Radakov 1961, Eibl-Eibesfeldt 1962, Hobson The advantage of schooling as a protective
1965, 1968, Starck and Davis, 1%6, Neil1 and device is unneeded by smaller fishes that live
Cullen 1974).
close to the sea floor, because these find shelter
The confusion effect theory contends, in in or close to reefs and other structures. Some,
essence, that predators confronted by the many like the razorfishes (Labridae, Hernipteronotus
targets in a school commonly fail to concentrate spp.) are specialized to dive into the sediments.
on an individual. Many characteristics ofschools Still others, like certain goatfishes (Mullidae),
are understandable when considered as enhanc- seem secure simply by being close to the sub-
ing a confusion effect. Schooling fishes empha- strate; perhaps their immediate proximity to the
size features that make individuals difficult to sea floor sufficiently inhibits the space-den~and-
distinguish. Thus, all members of a school are ing attacks of predators that might otherwise
about the same size and look much alike. School- threaten them, or perhaps they simply go un-
ing fishes are noted for lacking external differ- noticed by most predators. In any event, as long
ences between the sexes. When threatened, as these various species are close to shelter
schooling fishes typically close ranks, a ma- many of them feed, and carry on other pursuits,
neuver that places additional individuals within as solitary individuals. Some of them, however,
the attacker's field of vision, thus further troub- for example the damselfish Abudefduj troschelli,
ling those that have difficulty with multiple periodically swim up into midwaters, and at
these times generally school with others of their
Schoolers under attack also swim faster and kind (Hobson 1968).
begin weaving in and out among one another, Although small reef fishes face dl1 intense
a maneuver that appears to increase the confu- threat from predators when they move away
sion effect. At this point the silvery hues that from shelter, some nevertheless spend their
characterize so many schoolers come into play, active hours in the midwaters. But even these
as sunlight reflecting from their sides at rapidly are careful to stay within reach of shelter on the
changing angles would seem to present the sea floor. Many such fishes, including various
attacker with a bewildering shower of brilliant damselfishes (Pomacentridae, especially
flashes (Hobson 1968). Similarly, the bars or Chrornis spp.) feed on zooplankton, and though
stripes that characterize the color patterns of they acquire a measure of security by aggregat-
many schoolers tend to blend together as a ing, they nevertheless remain dependent on
shifting maze of lines that conceal individuals shelters that lie far below thern. Significantly,
(Starck 1966). And just as the protective aspect most of these plankton-feeders, or planktivores,
of the school requires its members to present have acquired features that increase their swim-
a uniform appearance, it also requires them to ming speed and thus hasten their descent to
present uniform behavior. Thus, it seems likely cover when threatened (Hobson 1974).
that any individual that swims abnormally, for Compared to close relatives that spend all
example, because of injury, becomes a distinc- their time near the reef, these planktivores tend
tive target (Hobson 1968).
to have more cylindrical bodies and more deeply
Every successful defense in prey will be incised tailfins (Davis and Birdsong 1973) -
answered among predators by an appropriate two tendencies that increase their speed. Fur-
offense. So it should be expected that some thermore, these speed-inducing features are
predators have solved the defensive features of most pronounced in those species that habitu-
schools. The sword of the swordfish (Xiphiidae) ally range farthest into open water (Hobson and
and the saw of the sawfish (Pristidae) may be Chess 1978). That they use this speed is demon-
effective weapons when attacking fish schools strated when certain predators appear -notably
(Williams 1964), as may the long tail of the jacks - and the entire assemblage abruptly
thresher shark (Alopiidae)(Nichols and Murphy closes ranks and dives headlong toward the reef
234 Figure 1. This large jack. Curuiu rgnohrlrs. here patrolling near shallow reefs at Midway Atoll, Hawaii, is built to run down prey with an aggressive, straightfonvard charge. Figure 2. This California halibut, Purulic / i d i n (uL/ortiicu.\, will erupt from its concealed position under h e sand off THE CALIFORNIA coast to ambush small fishes that hare come within range of a short, explosive charge.
Figure 3. With a sudden expansion of its cavernous mouth and gill cavities, this scotpionfish, Scorpoenopir sp., sucks in prey without leaving i h perch on this Hawaiian reef. The small. white appendage just inside its lower lip, which often is moving and highly visible while the fish rests motionless and otherwise virtually u n m n , may lure prey to within range of capture. Figure 4. Even though these large groupers, iM\.~rt.roper~ruo\o('cu. hover in lull v i e h above thls reef in the Gulf of California. they nevertheless feed on the small fishes about rhern. Perhaps their sluggish countenance habituates prey to an illusion that they arc nonagpressive, and ulrimately the prey become fatally careless.
Figure 5. The long. attenuated body of this cornetfish. I /i/u/cirw~ o m n i e n ~ gliding above a reef in the Gulf of California, accentuates Ieatures that characterize stalking predator\ Figure 6 . A leather bass, D c r r m ~ m / ~ ,d/~~~/ tr t m i r o / va/ ir~n~. da whool of the chaetodontid Hrfiroc/ I N rii,qriro.\iri.\ in the Gulf of California. Thi\ chaetodontid i s not prey o f l ) . &rtmiiok/ii<, hui apparently the s n a n i d enhances its chances of approaching unnoticed t o H ithin striking range of prey hy nimgling with thew and other lishes.
-,> .1,
below. Because there is a direct relation be- led to more cylindrical bodies and deeply incised
tween swimming syxcd and body Icngth. it is caudal fins in many other planktivorous damsel-
unsurprising that within ii group of closely fishes, the evolution of these species seems to
relatcd planktivores the xrnaller species remain have taken the opposite course. Where a cylin-
closest t o cover: thus. the smallest o f the Ha- drical body (and deeply incised tail fin) pro-
waiian planktivorous damselfishes. C'hwmis motes eluding predators, the deep body (with
\xoidet?if/fi, rarely feeds more than 0 . 5 i n above long fin spines) would seem to promote dis-
the reefs. whereas Iugerrelati\es. like C . o\.rr/i.c, couraging predators - two opposing routes to
typically feed 2 to 5 m above (Hobson 1972, the same end: to reduce the threat of attack
1974, Munr and McFarland 1973).
(Hobson and Chess 1978).
Ultimately, even the most elusive prey will
likely find itself between the jaws of a predator. But even from this position the attacker can be
Patterns of Predation
deterred. The strong fin-spines that characterize Interactions between predator and pl-ey are
many fishes certainly complicate the predator's shaped by the way these animals perceive their
task. A predator that has successfully taken a surroundings. Their orienting senses, therefore,
relatively large spiny-finned fish into its mouth are presumably major elements in defining
now must position the prey to enter its narrow predation patterns. Clearly, most piscivorous
digestive tract head-first. An attempt to swal- fishes depend mainly on vision to capture prey.
low such prey tail-first would likely result in Despite notable exceptions, especially among
the spines locking erect and becoming lodged species that frequent dark or turbid waters,
in the pharynx or esophagus. I have seen many the predominance of vision on a broad scale is
predators fatally choked this way. Also, during unquestionable. Of course. the range of vision
the split second that the predator must relax its underwater is sharply limited. Light diminishes
grasp for proper positioning, I have seen preda- rapidly as it passes through even the clearest
tors lose the prey fish they had held firmly natural water because it is scattered by sus-
between their jaws.
pended particles and is absorbed by the water
Spininess, like most successful defenses, is itself; consequently underwater objects no
developed to extremes in some species, but mattFr how large, are invisible beyond about
these extremes generally limit other abilities. 70 m (Brock and Riffenburg 1960). So preda-
Thus, certain rockfishes, Sebusres spp., have tors must use some other sense to detect prey
large bony heads that carry many fixed spines, at greater distances.
and surely present predators with a troublesome Thus, certain sharks track injured or distres-
mouthful. But the added weight of these struc- sed fishes over long distances by orienting on
tures ties those that bear them to a largely seden- odors characteristically emitted by these prey
tary life on the sea floor. Significantly, more (as long as the odor trails out in a current, or
motile rockfishes, like the olive rockfish. Se- behind moving prey). But even sharks drawn hnstrs srrrunoirlc~s. have smaller heads with in by odors generally switch to visual cues
fewer and less pronounced spines.
when close to their target (Hobson 1963). So
A n effective combination of strong fin spines with Visual ranges in the aquatic realm limited,
and exceptionally deep bodies probably protects the major interactions between predators and
butterflyfishes (Chaetodontidae) and angel- their prey are at close range - a set of circum-
fishes (Pomacanthidae) from most of the preda- stances that undoubtedly has contributed to
tors that threaten their neighbors on coral reefs most large predators being nearsighted (Walls
(Hobson and Chave 1972). Furthermore, some 1942). And with visual orientation so important,
reef fishes that habitually swim into the exposed midwaters and feed on plankton, for example, damselfishes of the genera Da.~c~ylI~ani.d\ A n h1~~~lyr) longer fin spines and
feeding interactions are strongly influenced by the changes in underwater light that characterize the different times of the day-night cycle. Because most piscivorous fishes orient visu-
much deeper bodies than d o their relatives that ally when they feed, one might expect attacks remain close to sheltering reefs. Thus in re- to increase with the brightness of underwater sponse to the same predatory threat that has light. By this reasoning, however, attacks
_ 7 3.8
should be most frequent during midday, when pany schooling prey for hours without making
in fact activity among predators is at a low ebb. an aggressive move, then suddenly attack (Hiatt
Probably this mild paradox reflects a long, and Brock 1948, Hobson 1%8), presumably
successful evolution that has given prey de- having sensed a vulnerable individual. But
fenses that effectively counter the offensive these are relatively infrequent successes; preda-
strengths of their predators. If predators can see tors of this type remain better suited to hunt
better in bright light, so can their prey, and the under other conditions.
lack of activity among predators during midday
Adaptations that place predators within strik-
suggests that during this period defense has an ing range when smaller fishes become momen-
edge over offense.
tarily vulnerable during the day characterize
Certain prey defenses, at least, are best suited the ambushers, those that habituate prey to their
to bright light. Features of the school that would apparent nonaggressiveness, and the stalkers.
enhance a confusion effect should be most Included here are most of the species that cap-
effective under bright light. These features ture smaller fishes through the day on tropical
include flashing silver sides and color patterns marine reefs (Hobson 1974, 1975). Their closely
consisting of bars or stripes. Even the confusion related feeding strategies express a common
effect itself, which entails presenting the at- goal: to catch prey unaware in a vulnerable
tacker a complex visual image, may increase position. That their most distinctive features
or decrease with light. Whether or not this is seem designed to exploit relatively infrequent
so, schooling fishes appear relatively safe from lapses in the defenses of their prey testifies
predators during most of the day (Hobson 1968). strongly for the considerable strengths of these
No matter how effective the daytime, or defenses.
diurnal, defenses may be, however, there are If the major defenses of smaller fishes are
bound to be lapses during which the prey are most effective in daylight, what becomes of
briefly vulnerable. For example, small fishes these fishes at night? It appears that after dark
that ordinarily stay within reach of shelter will they are comparatively free of the intense threats
sometimes stray too far into the open, and from predators that so strongly influence their
others normally secure in schools will briefly every move during the day. Predators that threat-
separate from their group. Still others, usually en smaller fishes on tropical marine reefs by
alert to developing dangers, will be momentar- day are largely inactive, or shift to some other
ily distracted. The most successful diurnal prey, at night (Hobson 1968, 1973, 1974). This
piscivores are those best able to exploit such observation is true despite some fish-eaters that
defensive mistakes.
are specially equipped to capture their prey
But prey are unlikely to make errors of this at night. One such predator is the big-eye jack,
sort in the presence of jacks and other aggres- Curcinx marginatus, a large fish whose name
sive free-swimming predators. Prey recognize describes the distinctive feature that permits it
signs that mark hunting predators, and take to see better in the dark; it hunts smaller fishes
appropriate action when aware that one is about. at night in the eastern Pacific ocean (Hobson
Those that find safety in the reef move toward 1968). But compared to the number of pis-
these shelters, and those that are secure in civores abroad in daylight, few are active at
schools tighten their ranks. Above all, the prey night.
are alerted, so that few suitable targets are avail- Some of the smaller night-active, or noctur-
able to those predators that characteristically nal, predators that feed chiefly on crustaceans
run down their prey with a highly overt, straight- and other free-swimming invertebrates - in-
forward charge.
cluding soldierfishes (Mjripristis spp.), glass-
Of course, such predators enjoy occasional eyes (Priucunthus spp.) and cardinalfishes
midday successes. Sometimes groups of jacks (Apogotz spp.) - take small fishes, especially
charge into schools of small fish, seemingly larval forms, but only as a relatively minor
in a coordinated effort to scatter the school and part of their diets (Hiatt and Strasburg 1960,
thus isolate individuals (Eibl-Eibesfeldt 1962, Randall 1967, Vivien 1973, Hobson 1974).
Starck and Davis 1966). Using a different tactic, Perhaps smaller fishes fail to generate the stim-
jacks and skipjacks (Scombridae) may accom- uli that orient nocturnal predators, or perhaps
23 9
they simply are too elusive to be caught consis- a drop in water transparency, as when turbid
tently by most predators that feed in the dark. water inundates an area (Hobson 1972, Steven-
The smaller fishes themselves offer evidence son 1972). Presumably this descent by the
that at night they enjoy a sharply diminished planktivores is in response to a measurably
threat from predators. The two major defensive greater threat from predators that accompanies
behaviors that protect them by day - schooiing even slightly decreased visibility. As light con-
and staying close to shelter -are greatly relaxed tinues to fade with the advancing afternoon, the
at night. Breder (1959) stated, "The dispersion planktivores descend progressively closer to the
of schools in darkness has been so often reported bottom - a descent that probably provides a
that i t is to be expected unless otherwise shown." rough index of increasing danger.
Although some fishes maintain their schools As day`s end approaches, the fishes that had
during the night, especially certain open-water been active above many tropical reefs show a
species, generally the assemblages are much clear relation between their size and the time
looser, and individuals are much farther apart. they go under cover. The smaller individuals,
Proximate shelter certainly becomes less which are most vulnerable to predators, seek
critical at nightfall, as many species that stay shelter first, and by sunset many of them already
close to reefs in daylight range into open regions are out of sight (Hobson 1972). Many of the
after dark. Thus, the open sandy expanses larger diurnal fishes, however, remain above
adjacent to reefs in the Gulf of California are the bottom for some time into twilight. Then,
largely without visible signs of life during the about 1S minutes after sunset, these too move
day, but are transformed into centers of noctur- from exposed positions, and the reef experiences
nal activity as fishes flood from the reefs into what has become known as the "quiet period"
these expanses at nightfall (Hobson 1965, (Hobson 1972, Munz and McFarland 1973,
1968). Evidence of a diminished threat at night Major 1977).
also exists among the body features of the It is during this "quiet period," a short span
smaller fishes. For example, the widespread of about 20 Minutes during both morning and
tendencies among planktivomus reef fishes evening twilight, that smaller fishes appear to
toward more cylindrical bodies and deeply be most vulnerable. The evening quiet period
incised tails - features that permit a speedier on tropical reefs is that time shortly after sunset
retreat to cover when threatened - occur only- when the day-active fishes have retired to shel-
in species that feed in the midwaters by day. ter, but the night-active fishes have not yet
Features of this sort are lacking among noctur- emerged, and the morning quiet period is the
nal counterparts, including certain soldierfishes, comparable time shortly before sunrise when
and cardinalfishes (Hobson 1974, 1975).
the sequence is reversed. The term quiet period,
So far I have described a system that would then, describes a general absence of fishes in
seem to favor the prey; they are well protected exposed - and therefore vulnerable - loca-
by effective defenses during most of the day tions. Although descriptive of most tropical
and face only a relatively minor threat from reefs, the term clearly is a misnomer in reference
predators at night. But conditions change drasti- to others. Where schooling fishes abound, this
cally during the morning and evening transi- often is a time when large predators attack most
tions between day and night. Defenses effective intensely. The major attackers under these con-
in bright daylight falter as light diminishes, so ditions are those aggressive piscivores like
that the transition periods. though relatively jacks whose feeding seems most inhibited dur-
brief, provide many piscivorcws fishes their ing midday. Unlike so many other smaller
major feeding successes.
fishes, the schoolers generally are unsuited to
Actually the balance hegins to shift from prey shelter in the reefs and so remain exposed to
to predator long before twilight. Even during u hat obviously are precarious circumstances.
midday, in fact. small fishes that are feeding on `Thus. the waters around hemng (Hcirmgulo)
plankton in the midwaters descend closer to schools in the Gulf of California (Hobson 1965,
sheltering reefs whenever visibility ib reduced. 1968), and silverside (\) schools in
This descent occurs either with a drop in light, Hawaii (Major 1977), often are whipped into
as when clouds pass before the sun, or through a frenzy of predatory activity at a time whe,n
conditions appear tranquil elsewhere. The end of the quiet period, about 35 minutes after sunset, is marked by a surge of nocturnal fishes, notably soldierfishes, that abruptly emerge from their daytime shelters, and swim straight away into the midwaters. The intense predation on schooling fishes has subsided, and the schools are dispersing, or migrating to nighttime feeding grounds. It is almost dark - onlj a trace of fading sunlight remains on the water's surface overhead. The large piscivores have withdrawn, and apparently the severe danger that prevailed moments earlier has passed (Hobson 1968, 1972). Clearly twilight is a critical period for many fishes. It is so critical, in fact, that certain visual features of many species seem specially designed for better vision during this brief period even though it is only a small segment of the 24-hour day. Important studies by Munz and McFarland (1973) have shown that twilight is slightly bluer than the light of day or night. Significantly, they have also shown that some of the light-receiving elements in the eyes of many coral-reef fishes appear more sensitive to this unique quality of twilight (which is apart from the light's brightness) than to the quality of light at other times. This is a striking find, because the study included both diurnal and nocturnal fishes. Munz and McFarland hypothesized that this added sensitivity to the quality of twilight, which presumably lets them see slightly better at this time than they could otherwise, is an adaptation to the severe evolutionary pressures that have long been exerted on predator-prey interactions during this critical period. But sensitivity to twilight blue is a feature of predators as well as prey. We still must ask why predators become relatively more effective during twilight. The visual equipment of piscivorous fishes may be better suited after all to the half-light of dusk than is the visual equipment of their prey. Fishes on tropical reefs, including prey of the piscivores, tend to be most active either by day, or by night (Hobson 1965, 1968, 1972, 1974, Starck and Davis 1966, Vivien 1973), and adaptations to these activities quite likely limit their abilities during the transition periods. The typical diurnal fishes, including various wrasses, butterflyfishes, and damselfishes, feed on small organisms, a task requiring exception-
ally sharp vision. On the other hand, most of the nocturnal fishes, including cardinalfishes, squirrelfishes, and soldierfishes. feed on comparatively large prey, but in relative darkness, a task requiring sensitivity to dim light (Hobson 1974). The two groups, then, have different visual needs; in fact, they accentuate aspects of vision that tend to be mutually exclusive; visual structures that stress perception of detail are poorly suited to dim light, whereas visual structures that stress sensitivity to dim light are poorly suited to perceiving detail (Walls 1942). Recognizing these relationships, Munz and McFarland (1973) examined the eyes from a limited number of coral reef fishes and found that visual structures in the diurnal species differed from those in the nocturnal species precisely as might have been predicted based on their contrasting feeding circumstances. There were only two piscivores in their sample - the jack, Curutipidrs ujur. and the grouper, Epitwplic1ii.s mrrru - but significantly these constituted a third group which have eyes with visual structures intermediate between those in the diurnal and nocturnal groups, and which Munz and McFarland assumed to be twilight feeders. Perhaps this development should have been expected. These piscivores take relatively large prey, so are free of the need to perceive great detail; and they do not feed at night. so they are free of the need for extreme visual sensitivity. Consequently, during the transitions between day and night these predators should have a marked visual edge over at least many of their prey. They can see well enough in light that is too dim for those diurnal prey that have sacrificed visual sensitivity for visual acuity, and they have better attention to detail than is possible for those nocturnal prey that have sacrificed visual acuity for visual sensitivity. Apart from any sensory advantage that piscivores may have during twilight, however, they unquestionably gain an edge by choosing the time and place of the attack. The prey can only respond. During most of twilight the sky is light, but little of the light penetrates the water. As a result, predators positioned near the bottom find prey in the water above them readily visible against a light background, but prey in the midwaters find predators below them hidden in the gloom (Hobson 1966, 1968, Munz and McFarland 1973). Obviously, under
24 I
these circumstances small fishes find life in the midwaters especially risky, and their reason for vacating this region is obvious. Schools, however, must remain in this highly vulnerable position through twilight even though many of the features that protect them in bright light have lost effectiveness. The colorations of their members, for example, are no longer helpful in masking individuals. Predatory fishes typically attack from below at this time, and each schooler is silhouetted against the bright surface -. a distinct target (Hobson 1966). Summary Most fishes that prey on other fishes use one of four predatory tactics each of which is strongly reflected in its body features. They either ( I ) run down their prey with an overpowering charge (examples: jacks and billfishes), ( 2 ) ambush their prey (examples: scorpionfishes and flatfishes), (3) habituate prey to their apparent nonaggressiveness (examples: basses, snappers), or (4)stalk their prey (examples: barracudas and pikes). These tactics relate closely to the defenses of their prey. Some prey defenses are behavioral, such as schooling and staying within reach of shelter, whereas others involve some adaptive body feature; for example, the development of spines, or concealing colorations. Generally, these defenses are used in combination. Schooling fishes, for instance, often have color patterns and other body features that make individuals in the assemblage less conspicuous and therefore poorer targets for predators. And small reef fishes that characteristically swim a distance from shelter often have body features that permit them to swim faster and so speed their retreat to cover when threatened. Predator-prey interactions are strongly influenced by the varying characteristics of underwater light at different times of the day-night cycle because most of the fishes involved orient mainly by vision. Prey defenses are most effective during midday, and the smaller fishes are relatively secure then. The most successful predators at this time are those at the scene when prey fishes make momentary errors. Those best able to succeed are among the ambushers, stalkers, and those that have habituated prey to their apparent nonaggressiveness.
Predators that characteristically run down their prey with a highly overt, straightforward charge find suitable targets rare during midday because prey are unlikely to make a defensive error in their obvious presence. Most large piscivores find light at night insufficient for hunting, thus greatly reducing their threat to small fishes then. Many prey defenses, includinG Schooling and remaining within reach of shelter, are greatly relaxed at this time. Piscivores are most active during the bansi- tion between day and night, with their attacks peaking during twilight. Their major prey are primarily diurnal or nocturnal species ill-equipped for the rapidly changing conditions that prevail during the transition periods. At these vulnerable times most of the smaller reef fishes abandon the exposed midwaters, but schooling species, which are unsuited to shelter in the reefs, cannot do so, and are attacked severely. The major twilight attackers are those large, aggressive predators that are most inhibited in their feeding during midday. Literature Cited Allen, W . E. 1920a. Behavior of loon and sardines. Ecology 1(4):309-310. 1920b. Behavior of feeding mackerel. Ecology l(4):3IO. Bowen, E. S . 193 I . The role of the sense organs in aggregations of Ameriurus melus. Ecol. Monogr. I ( I ) : 1-35. Breder, C . M . , Jr. 1959. Studies on social groupings in fishes. Bull. Amer. Mus. Nat. Hist. 117(6):397-481. Brock, V. E. and R. H . Riffenburg. 1960. Fish schooling: a possible factor in reducing predation J . Const. Int. Explor. Mer 25(3):307-3 17. Crawford, R. W . andC. F. Powers. 1953. Schoolingof the orange filefish, Aluteru schoepfi. in New York Bight. Copeia 1953 (2):115-1 16. Davis, W. P. and R. S . Birdsong. 1973. Coral reef fishes which forage in the water column. Helgolander wiss. Meersunters 24:292-306. Eibl-Eibesfeldt, I . 1%2. Freiwassehmbachtungen zur Deutung des Schwarmverhaltens verschiedener Fische. 2. Tierpsych. 19(2):165-182. Gosline, W. A. 1959. Modeof life, functional morphology, and the classification of modern teleostean fishes. Syst. Zool. 8:160-164. Hamilton, W. D . 197t. Geometry for the selfish herd. J . Theor. Biol. 3 I :295-3I I . Hiatt, R . W. and V . E. Brock. 1948. On the herding ofprey and the schooling of the black skipjack Eurhynnus yaifo Kishinouye. Pac. Sci. 2(4):297-298. Hiatt, R . W. and D. W. Strasburg. 1960. ecological relationships of the fish fauna on coral reefs of the Marshall Islands. Ecol. Monogr. 30:65-127. Hobson, E. S . 1963. Feeding behavior in three species of sharks. Pac. Sci. 17(2):171-194.
_ _ _ 1%5. Diurnal-nocturnal activity of some inshore Randall, J. E. 1967. Food habits of reef fishes of the West
fishes in the Gulf of California. Copeia 1%5(3):291-302. Indies. Stud. Trop. Oceanogr. (Miami) 5:655-847. ___1966. Visual orientation and feeding in seals and Springer, S. 1957. Some observations on the behavior of
sea lions. Nature 210(5033):326-327.
schools of fishes in the Gulf of Mexico and adjacent
_ _ _ 1968. Predatory behavior of some shore fishes waters. Ecology 38(1):166-171.
in the Gulf of California. U.S. Fish Wildl. Serv. Res. Starck, W. A,, 11. 1966. Marvels of a coral realm. Nat.
Rep. 73, 92pp.
Geogr. Mag. 130(5):710-738.
-~ 1972. Activity of Hawaiian reef fishes during the Starck, W . A., I1 and W. P. Davis. 1966. Night habits of
evening and morning transitions between daylight and fishes of Alligator Reef, Florida. Ichthyologica, Aquar.
darkness. Fish. Bull., US. 70(3):715-740.
J. 38(4):313-356.
1973. Diel feeding migrations in tropical reef Stevenson, R. A , , Jr. 1972. Regulation of feeding behavior
fishes. Helgolander wiss. Meeresunters. 24:361-370. of the bicolor damselfish (Eupomacentrus punifus Poey)
1974. Feeding relationships of teleostean fishes
on coral reefs in KOM, Hawaii. Fish. Bull., U.S. 72(4):
by Environmental Factors. In H. E. Winn and B. L. Olla (editors), Behavior of marine animals Vol. 2:Vertebrates.
9 15-103I .
pp. 278-302. Plenum Press, N.Y.
_ _ 1~ 975. F~ eeding patterns among tropical reef fishes. Vivien, M. C. 1973. Contribution a la conaissance de
Amer. Sci. 63(4):382-392.
I'ethologie alimentak de l'ichthyofaune du platier in-
Hobson, E. S. and E. H. Chave. 1972. Hawaiian reef animals. Univ. Press Hawaii, Hono.
terne des recifs coralliensde Tulear (Madagascar)Tethys SUPPI. 5:221-308.
Hobson, E. S. and J. R. Chess. 1978. Trophic relationships Walls, G. L. 1942. The vertebrate eye and its adaptive radi-
among fishes and plankton in the lagoon at Enewetak ation. Reprinted 1%3, Hafner Publ., New Yo& 785 pp.
atoll, Marshall Islands. Fish. Bull., U.S. 76(1):133-153. Williams, G. C. 1964.Measurement of consociationamong Major, P. F. 1977. Predator-prey interactions in schooling Fishes. Mich. St. Univ. Mus. Biol. Ser. 2(7):349-384. fishes during periods of twilight: a study of the silverside
It ti
Prunesus insultrum in Hawaii. Fish. Bull., U.S. 75(2):
4 15-426.
Manteifel, B. P. and D. V. Radakov. 1%1. The adaptive
significance of schooling behavior in fishes. Russ. Rev.
Biol. 50(3):338-345.
Montgomery, W. L. 1975. Interspecific associationsof sea
basses (Serranidae) in the Gulf of California. Copeia Edmund S. Hobson is a fisheries research
biologist with the Southwest Fisheries Center,
M u m , F. W. and W. N. McFarland. 1973. The significance of spectral position in the hcdopsins of tropical marine fishes. Vis. Res. 13(10):1829-1874. Neill, S. W. SI. J. and J. M. Cullen. 1974. Experiments on whether schooling by their prey affects the hunting be-
Tiburon Laboratory, National Marine Fisheries Service, Tiburon, California, and a research associate with the Scripps Institution of Oceanography. He holds a doctorate from the Univers-
havior of cephalopods and fish predators. J. Zool. Lond. ity of California, Los Angeles. His research dur-
172549-569. Nichols, J. T. and R. C. Murphy. 1916. Long Island fauna IV, the sharks. Brooklyn Mus. Sci. Bull. 3( l):l-34. Olson, F. C. W. 1964. The survival value of fish schooling. J. Const. Int. Explor. Mer 29:115-1 16.
ing the past twenty years has centered on predator-prey relationships among fishes. He is the author of over thirty-five scientific and popular articles, most of them on predation in fishes.
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