alligator beneath cayman croc crocodile danger fear fresh gator reptile salt saltwater submerged sveres teeth underneath underwater water armor armored armorment armour armoured armourment Australian-freshwater-crocodile backstraps bony-ridges crocodylus-johnsoni dorsal-surface exotic fauna fresh-water-crocodile freshie hide Johnstone's-crocodile leather leathery lethargic omnivorous opportunistic opportunistic-predator osteoderms patterns powerful predator reptilian ridges rough rugged scales scutes semi-submerged skin tail tail-scutes territorial texture textured textures