Defenses
 
column spacer Defenses
 
 

Fish defenses

hot buttons for defensive "tasks" for fishes hot button for avoid detection part of the section on fish defenses in the Biology of Caribbean Coral Reefs website hot button for take evasive action part of the section on fish defenses in the Biology of Caribbean Coral Reefs website hot button for prevent capture part of the section on fish defenses in the Biology of Caribbean Coral Reefs website hot button for prevent being eaten if captured part of the section on fish defenses in the Biology of Caribbean Coral Reefs website

Defenses of fishes can be categorised into an heirarchical "cascade" beginning with:

1) avoid detection
2) take evasive action if spotted
3) prevent capture
4) prevent being eaten if captured
5) escape

Of these, the last is obviously the most important, but success in any of the others will aid in achieving it. Note that costs and risks to the prey escalate from first to fourth. The fifth and ultimate category, that of escape, can come at any time and won't be considered further here. The first task of any potential prey fish is to avoid detection. Helfman et al. 1997 The diversity of fishes. Blackwell Sci Publ.

 
 

Avoid detection

  There are several ways for a potential prey fish to avoid detection, but some of the main ones are to confuse the predator, considered in this section, and to HIDE AWAY or to employ CAMOUFLAGE, considered in other sections.
 
 

Avoid detection: confuse the predator

 
  There are 3 main ways used by an intended prey fish to confuse a predator. These are shoals & schools, considered here, and MISDIRECTION and SOUNDS considered in another section.
 
 

Avoid detection: confuse the predator: shoals & schools

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of mixed Caribbeanreef fishes "Oh, what a beautiful school of fishes. Believe it or not, you're safer in the school than out on your own. I know it seems counter-intuitive, but in the open ocean schooling lowers the probability of a predator finding its prey. First, it must find a school, and that requires much time and effort, then it must find, and catch, its prey." - Turneffe Island, Belize 2002. Video courtesy Andy Stockbridge, Belize.
 
 

photograph of a school of
While it is true that fishes in a school have a lower probability of being detected than if they were solitary, this only applies to oceanic species that occupy three-dimensional space. In the close confines of a reef, where schools are not so difficult to locate, inducing confusion in a predator seems to be the paramount function of schooling.

NOTE a school is a disciplined aggregation, with its members swimming in the same direction and at the same speed, while a shoal is an undisciplined aggregatio

 

A school of bluestriped grunts Haemulon sciurus 0.15X. In
reef-based shoals and schools the participants benefit from
confusing kaleidoscopic changes of colour, shape, and pattern

 

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a school of schoolmasters taken from a video

"Quite a pretty shoal of schoolmasters...Oh, that's a sight! You might wonder how fish get formed into schools...well, it looks like Bermuda chums act like border collies...they zip around and keep everyone tightly bunched." - Cayman Brac 2002

NOTE of course, that this comment is meant to be just "tongue-in-cheek"

 
 

photograph of a shoal of schoolmasters
Whether oceanic or near the reef, the collective vigilance of a shoal or school photograph of a school of shinersincreases the chance of an approaching predator being detected.

Shoal of schoolmasters
Lutjanus apodus 0.1X

 

A school of shiners is
vigilant and responsive,
and its movement
creates a bewildering
visual display 0.1X

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs photograph of a school of mackerel scad taken from a video

"Being in a school like these mackerel scad is protective in that the pedators are confused by the shifting colours and patterns. It's hard for them to pick out a target." - Turneffe Island, Belize. Video courtesy Andy Stockbridge, Belize.

NOTE Decapaterus macarellus

 
 

photograph of a school of unidentified oceanic fishes
Studies have shown that predators catch fewer prey from large than from small schools. Confusion arises as the predator switches attention from one to another of the multiple edible objects passing through its field of view.

 

 

 

 

 

A school of unidentified oceanic fishes. A
predator must first find the school, then attempt
to identify and isolate one of its members

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a shoal of yellowtail snappers taken from a video

"Okay, here we are, voracious predators, sneaking up on these yellowtail snappers. They're in a shoal, making it easy for us to grab an individual...it is!? But now they're schooling, making it even more difficult. Hmmm!...seems to me its going to be hard in either case!" - Turneffe Island, Belize. Video courtesy Andy Stockbridge, Belize.

NOTE Ocyurus chrysurus

 
 

school of blue tang being eyed by a predatory barracuda
A barracuda has come across a school of blue tangs and is looking for an individual to attack. The tangs are watchful and attend carefully to the actions of the predator. No-one said it was going to be easy. The barracuda is attentive to all aspects of the school, an outlier, a weaker individual, or any sign of vulnerability.

In this standoff scenario the predator may go away hungry.

 
 
Because of the usual homogeneity of a school, a predator's attention will be caught by any unusual sign. photograph of a school of silversides  
photograph of a Nassau grouper

In this scenario, it is not difficult to imagine which
of these schooling silversides has caught the attention
of the Nassau grouper. In any school, it is the
oddly shaped, oddly coloured, and oddly behaving
individual that is likely to be noticed

 
  A predator's success rises when the effect of confusion created by a mass of similar-appearing fishes is alleviated by another fish joining the group.
 
photograph of shoal of blue tangs with an iniital-phase queen parrotfish photograph of a shoal of bluestriped grunts with an ocen surgeonfish
This shoal of blue tangs Acanthurus coeruleus is accompanied by a visually distinctive initial-phase queen parrotfish Scarus vetula (both photos 0.15X) A contrastingly coloured surgeonfish Acanthurus sp. in a shoal of bluestriped grunts Haemulon sciurus might as well have a sign saying "here I am"
  drawing of school of blue tangs with a visually obvious member
In this scenario the predator has a well-defined target and may be successful in its attack.
 

barracuda eyes an heterospecific shoal with one member standing out visually, even though darker
Mixed-species aggregations of fishes are called heterospecific shoals or heterospecific schools. Here, an unidentified dark fish has joined a shoal of grunts Haemulon spp. Here, it would be no surprise if the barracuda were to attack the dark fish, even though it is surrounded by brighter images, because of its visual obviousness.

NOTE lit. different species

 
  list of "do's" for a heterospecific fish in a school that may aid its survivallist of "don'ts for a heterospecific fish in a school that may lead to decreased survival
What is the best survival strategy for a fish that is heterospecific to a school? The following may help:
 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a shoal of grunts taken from a video "Hmmm...nice shoal of grunts, and a nice example of how confusing a shoal of fishes can be to a predator. The grunts mill around, providing a confusing view to the predator, that is, us, as we get closer. Now we're quite close and prey fishes start to stream away...which one do we select to attack?" - Little Cayman Island 2002
 
 

photograph of a mixed school of fishes following a large, unidentified fish, possibly a sharkThis photograph is interesting, as it shows a mixed school of fishes swimming behind a larger fish. The smaller fishes appear to be following the larger one, but what actually is going on? There are a number of possibilities relating to different manifestations of behaviour. Think of what these might be, then CLICK HERE to see a list of possibilities.

 

 

A mixed school of fishes, possibly including
some jacks, follows a large, predatory fish,
perhaps a shark (approx. 1/50th life size)

 
 

photograph of a school of silversides
Herring and anchovies are collectively known as silversides. The silvery colour derives from reflection of light from inclusions within the skin cells known as iridophores. Silversides feed at night on plankton and rest in large schools during the day, often in caves or under shelter. Their reflectivity and how it changes when they swim in a school add another dimension to confusing a predator.

NOTE lit. "rainbow carry"; in other words, bearers of colour. These types of structural colours and other agents of colour are dealt with in more detail in: COLORATION IN REEF ANIMALS: HOW COLOURS ARE CREATED

 

 

School of silversides 0.05X

 
  What sensory information is used by a predatory fish to locate and identify a potential prey fish in a school?
 
illustration of sensory devices in a predatory fish, such as a Nassau grouper
  Let's see what the schoolmasters have to say on the subject. Ideas from Pitcher et al. 1976 Science 194: 963.
 
cartoon 1 in a series showing two schoolmasters discussing the mechanism of schooling cartoon 2 in a series showing two schoolmasters discussing the mechanism of schooling
cartoon 3 in a series showing two schoolmasters discussing the mechanism of schooling cartoon 4 in a series showing two schoolmasters discussing the mechanism of schooling
cartoon 5 in a series showing two schoolmasters discussing the mechanism of schooling cartoon 6 in a series showing two schoolmasters discussing the mechanism of schooling
cartoon 7 in a series showing two schoolmasters discussing the mechanism of schooling cartoon 8 in a series showing two schoolmasters discussing the mechanism of schooling
 
 

photograph of different life stages of bluehead wrasses
Alarm chemicals released from molested school-members may elicit escape behaviour or erratic swimming in the other school-members.

 

 

 

 

Erratic and fast-moving mixed group
of juvenile- and terminal-stage bluehead
wrasses Thalassoma bifasciatum 1X

 

 


photograph of juvenile clown wrasses Halichoeres maculipinnaAn attack by a predator on a school may not actually be an attack on a single, intended prey; rather, the attack may function to separate individual prey from the school, which can then be individually chased. Such strategy produces several-fold greater success than attacks on the main school.

 

 

 

Small shoals of mostly juvenile clown
wrasses Halichoeres maculipinna 0.15X
 
 

Although the major function of schooling is thought to relate to minimising predation, other functions may be served. Think about what these may be, then check the following list of past and current ideas:

1. Increases foraging success: YES: increased vigilance in the school leads to more feeding time being available.

2. Synchronises breeding: YES.

3. Increases hydrodynamic efficiency: YES: a fish gains large energy savings by swimming with others. Both speed and duration of swimming are increased. Weihs 1975, p. 703 In, Swimming and flying in nature. Vol. 2, Plenum Press.

4. Creates greater oxygenation of the water: NO: as the school moves along it actually leaves behind a plume of oxygen-depleted water.

5. Maintains the same individuals together: NO: surprisingly, most schooling fishes have little fidelity to a particular school and individuals may switch schools several times a day.

6. Mucus produced by leading fishes permits following fishes to swim through the water more easily: NO. There is no evidence for this.

7. Schooling disguises the smell of individual fishes: YES: this is true, but overall the school produces an enormous chemical plume of urine, feces, mucus, and pheromones, one that a predator can easily follow.

NOTE chemicals released from one individual of a species that affect the behaviour of other individuals of the same species at a distance. Pheromones are most often associated with sexual behaviour

 
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