First Draft

breed [predators predator]
breed [shells shell]
predators-own [energy]
shells-own [health]

to setup
  clear-all
  setup-patches
  create-predators number [ set color orange ]
  create-shells number [ set color white ]
  reset-ticks
end 

to setup-patches
  ask patches [ set pcolor blue ]
end 

to go
  if ticks >= 1000000 [ stop ]
  move-predators
  move-shells
  reproduce
  check-death
  ask predators [
    set energy energy - 1
    catch-shells
    reproduce
  ]
  ask shells [
    set health health - 1
    eat-grass
    reproduce
  ]
  tick
end 

to reproduce
  ask turtles [
    if energy > 10 [
      set energy energy - 5
      hatch 1 [ set energy 50 ]
    ]
  ]
end 

to check-death
  ask turtles [
    if energy <= 0 [ die ]
  ]
end 

to move-predators
  ask predators [
    right random 160
    forward 1
    set energy energy - 1
  ]
  catch-shells
end 

to move-shells
  ask shells [
    right random 160
    forward 1
    set energy energy - 1
  ]
  ifelse pcolor = blue
  [ eat-grass ]
  [ get-zapped ]
end 

to setup-turtles
  create-turtles number[ setxy random-xcor random-ycor ]
  set-default-shape predators "fish"
  set-default-shape shells "dot"
  ask turtles [ setxy random-xcor random-ycor ]
end 

to eat-grass
  ask shells [
    if pcolor = blue [
      set pcolor green
      set energy energy + 10
    ]
  ifelse show-energy?
    [ set label energy ]
    [ set label "" ]
  ]
end 

to catch-shells
  let prey one-of shells-here
  if prey != nobody
    [ ask prey [ die ]
       set energy energy + 1000 ]
end 

to get-zapped
  set health health - 10
end