Fish

Cleaner wrasse set up stations on reefs where larger fish queue up to have parasites, dead tissue, and bacteria removed. The cleaner fish even calms stressed clients by gently stroking them with its fins.

The dragonfish's fangs are made of nanocrystalline hydroxyapatite structured to be transparent, making them invisible to prey even in bioluminescent light. This stealth dentistry is unlike any other known predator.

Pacific salmon navigate thousands of kilometers of open ocean to return to their exact birth stream using Earth's magnetic field as a compass and the unique chemical scent of their home river, which they memorize as juveniles.

Pufferfish contain tetrodotoxin — a neurotoxin 1,200 times more deadly than cyanide — concentrated in their liver, skin, and ovaries. Despite this, fugu (pufferfish sashimi) is a prized delicacy in Japan prepared by licensed chefs.

When a tiny male anglerfish finds a female, he bites into her skin and fuses permanently. His organs dissolve; he becomes a sperm-producing parasite attached to her body. One female can carry up to six males at once.

Turritopsis dohrnii can revert from adult back to its juvenile polyp stage through transdifferentiation — where mature cells transform into different cell types. Theoretically, it can repeat this cycle indefinitely, making it biologically immortal.

Parrotfish use their beak-like teeth to scrape algae from coral, accidentally ingesting coral rock. They digest the organic matter and excrete the calcium carbonate as fine white sand — producing hundreds of kilograms per fish per year.

Unlike most fish, bluefin tuna are partially warm-blooded, using a heat exchange system to keep their muscles and brain warmer than the surrounding water. This allows them to swim faster and hunt in cold deep waters.

The mimic octopus dynamically alters its body shape and behavior to impersonate lionfish, flatfish, and sea snakes — often choosing which animal to mimic based on the specific predator threatening it.

Whale sharks can grow up to 18 meters and weigh 20 tonnes, making them the largest fish alive. Despite their size, they filter-feed exclusively on plankton, small fish, and fish eggs — posing no danger to humans.