Fish as predators and as prey

1. Predator–Prey Arms Race

  • Predation involves hunting → pursuing → attacking → catching → handling, though not always in a strict sequence.

  • Strong selection pressure on prey:

    • Quote: “The prey… have a very strong selective pressure… not to be eaten. Because if an individual gets eaten its fecundity… is zero.”

  • Energetic logic: avoiding detection is best because evasion is costly.

  • Quote: “It’s better… if the fish doesn’t have to swim away at high speed… then it has energy… for growth and developing gametes.”

2. Avoiding Detection

Two major strategies:

  1. Camouflage

  2. Seeing the predator first

2.1 Camouflage Strategies

A. Masquerade

  • Fish evolve to look like non‑fish objects.

  • Examples:

    • Sargassum fish blending with floating weed.

    • Green pipefish resembling vegetation.

  • Quote: “This is masquerading… having evolved to appear like a plant or elements of the environment.”

B. Disruptive Colouration

  • Patterns break up the outline of the fish.

  • Examples:

  • Fish with a large black spot that “makes it harder… to see it’s a fish.”

  • Striped species (e.g., Terapon) where stripes obscure the eye and body shape.

C. Countershading

  • Dark on top, pale underneath.

  • Works because light is strongest from above.

  • Quote: “The end result is you can’t actually see the shark… you can’t actually see it so well.”

D. Reverse Countershading

  • Used by species that swim upside‑down.

  • Examples:

  • Upside‑down catfish: “When it doesn’t want to be seen it flips upside down.”

  • Chromis species that invert when ambushing prey.

E. Background Matching & Pattern Matching

  • Experimental evidence (Karen Cheney’s lab):

  • Predators attack less when background stripe width matches the fish’s body stripes.

  • Quote: “The more its background is similar… the less likely it is to get eaten.”

F. Mirror‑sided Reflective Scales

  • Common in herrings, anchovies, minnows.

  • Silvery scales act like invisibility cloaks by reflecting ambient light.

  • Quote: “What it appears to be is this light coming down… invisibility cloak.”

G. Transparency

  • Common in deep‑sea species and larvae.

  • Example: transparent fish with visible heart.

  • Quote: “Just being see‑through obviously makes it harder to be seen visually.”

3. Early Detection of Predators

A. Using Light Differences

  • Nocturnal fish (e.g., squirrelfish) hide in dark crevices.

  • They can see predators in bright water before being seen.

  • Quote: “They can see out into the bright water better than a shark can see…”

B. Shoaling for Vigilance

  • Many eyes increase detection distance.

  • Quote: “One of these fish will see a predator… before the predator is too close.”

C. Collective Diligence

  • Shoals may switch between shoaling (loose) and schooling (tight, coordinated).

  • Some species inspect predators to assess hunger state.

  • Quote: “They can pick that up… looking at it, smelling it… working out if it’s hungry.”

D. Detecting Non‑visual Predators

  • Cetaceans use echolocation; fish respond behaviourally.

  • Examples:

  • Supercarin drop to the bottom when detecting echolocation.

  • American shad perform a random‑walk escape:

  • Quote: “If it gets louder, they’ll change direction… move away from echolocation.”

4. Avoiding Pursuit

A. Staying Near Shelter

  • Small fish cannot out‑swim large predators.

  • Quote: “A smaller fish can’t swim as fast as a big fish… so they stay closer to shelter.”

B. High‑speed Escape Responses

Two main escape manoeuvres:

1. S‑bend escape (predators)

  • Smooth S‑shaped acceleration.

2. C‑bend escape (prey)

  • Rapid C‑shaped bend with extremely high angular velocity.

  • Quote: “The angular velocity… coming up for 1000 degrees per second.”

  • Predators aim for the midpoint of the prey’s predicted path.

5. Group‑Level Evasion During Attacks

A. Flash Expansion

  • Sudden outward explosion of the school.

B. Fountain / Skittering

  • Rapid upward or erratic movements to confuse predators.

C. Vacuole Formation

  • A hole opens in the centre of the school as fish move outward.

D. Rolling & Silver Flashes

  • Quote: “Scootering is rolling and flashing… all about confusing the predator.”

E. Predator Mobbing

  • Shoals may collectively harass or intimidate predators.

6. Avoiding Capture and Handling

A. Spines and Armour

  • Lionfish: venomous spines deter predators.

  • Quote: “One of the reasons they’re doing so well… no natural predators.”

B. Puffing Up (Pufferfish)

  • Inflate with water; spines protrude; tetrodotoxin present.

  • Quote: “It’ll swallow water… spines appear… contain tetrotoxins.”

C. Mimicry

  • Batesian mimicry: harmless species mimic dangerous ones.

    • Example: striped fish mimicking pufferfish.

  • Predator mimicry: prey resemble predators to deter attacks.

  • Example: comet fish with false eye spot.

D. Increasing Apparent Size

  • Triggerfish erect dorsal spine to become too large to swallow.

  • Quote: “It sticks it up… much larger mouthful.”

E. Body Deepening

  • Some species evolve or induce deeper bodies.

  • Example: Crucian carp can change body proportions when predators are present.

  • Quote: “It can change the body proportions… makes it harder for predators to handle.”