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Anisakis is a parasitic nematode (roundworm) found in the flesh of many marine fish and squid. Humans become accidental hosts by eating raw or undercooked seafood — sushi, sashimi, ceviche, and lightly cured fish are common sources. The larvae cannot complete their lifecycle in humans but burrow into the stomach or intestinal wall, causing a painful condition called anisakiasis. Marine mammals such as dolphins and seals are the definitive hosts.

Cymothoa exigua is one of the most remarkable parasites known to science — the only parasite known to functionally replace a host organ. This marine isopod (a crustacean related to pill bugs) enters a fish through the gills as a juvenile male, attaches to the tongue, severs its blood supply, and causes the tongue to atrophy. The parasite then takes the tongue's place, functioning as a replacement tongue while feeding on the fish's blood and mucus. The fish can still eat with the parasite in place. It poses no known risk to humans.

Sea lice are parasitic copepods (tiny crustaceans) that attach to the skin, scales, and mucus of marine fish, particularly salmon. They are one of the most economically damaging parasites in the global aquaculture industry. Sea lice feed on skin, mucus, and blood, causing lesions, stress, and secondary infections. Heavy infestations can be fatal. Salmon farms create dense fish populations that allow sea lice to multiply rapidly and spread to wild salmon populations nearby — a major environmental controversy.

Cryptocaryon irritans, commonly called marine ich or white spot disease, is a ciliated protozoan parasite that infects the skin, fins, and gills of saltwater fish. It is the marine equivalent of freshwater ich (Ichthyophthirius multifiliis) and is one of the most common and devastating diseases in saltwater aquariums. Infected fish develop characteristic white spots (trophonts) visible on the body and fins. The parasite is highly contagious and can wipe out an entire aquarium if untreated. It does not infect humans.

Gnathia are tiny parasitic isopods (crustaceans) that are among the most prevalent blood-feeding parasites on coral reefs. Only the juvenile stage (zuphea/praniza) is parasitic — they attach to fish, feed on blood and tissue fluid, then drop off to molt and mature. Adults are free-living and do not feed. Despite their small size (1–7 mm), gnathia can cause significant blood loss, stress, and disease transmission in heavily infested fish. They are a primary food source for cleaner wrasses and cleaner shrimp, making them a key part of reef ecosystem dynamics.

Amyloodinium ocellatum, commonly called marine velvet or coral fish disease, is a parasitic dinoflagellate (a type of single-celled organism related to algae) that infects the skin and gills of saltwater fish. It is considered even more dangerous than marine ich — it spreads faster, is harder to see in early stages, and can kill fish within 24–72 hours of heavy infestation. Infected fish develop a characteristic gold or rust-colored dusty coating (like velvet fabric) on their body and fins. It is one of the leading causes of fish death in marine aquariums.

Pennella are large parasitic copepods (crustaceans) that embed deeply into the flesh of large marine fish and marine mammals. Unlike most copepods, Pennella burrow their anterior (head) end into the host's muscle or blubber, leaving a long, feathery egg-bearing tail dangling externally — sometimes reaching 30 cm in length. They are among the largest known parasitic copepods. While visually dramatic, single parasites rarely cause serious harm to large hosts. Heavy infestations can cause significant tissue damage, inflammation, and secondary infections. They are commonly seen on swordfish, marlin, and whales.

Neobenedenia are parasitic flatworms (monogenean flukes) that live on the skin, fins, and eyes of marine fish. They are among the most economically damaging parasites in marine aquaculture, causing massive losses in sea bass, grouper, snapper, and other farmed species. Neobenedenia are hermaphroditic, reproduce rapidly, and can complete their entire lifecycle on a single fish host — making population explosions possible in the confined conditions of fish farms and aquariums. They feed on skin cells and mucus, causing lesions, eye damage (sometimes blindness), and secondary infections.