1. Vaquita (_Phocoena sinus_): This small, critically endangered porpoise is found only in the Gulf of California, Mexico. As of 2022, the population is estimated to be around 10 individuals. Vaquitas face threats from illegal fishing, particularly gillnets used for capturing totoaba, a fish with high demand in China.
2. Blue whale (_Balaenoptera musculus_): Once the largest animal on Earth, blue whales were pushed to the brink of extinction by commercial whaling in the 20th century. While the species has been protected internationally since the 1960s, its population still faces challenges from ship strikes, pollution, and limited food availability.
3. Northern white rhinoceros (_Ceratotherium simum cottoni_): Although not strictly a marine species, this subspecies of rhinoceros is threatened by poaching and habitat loss. In 2018, only two individuals remained, both females, making the northern white rhinoceros the most endangered mammal on Earth.
4. Leatherback turtle (_Dermochelys coriacea_): The largest of all sea turtles, leatherbacks are listed as vulnerable due to several factors, including overharvesting of eggs, entanglement in fishing gear, and loss of nesting beaches.
5. Atlantic Bluefin tuna (_Thunnus thynnus_): This large tuna species is highly sought after for its meat, leading to overfishing. In 2021, the IUCN uplisted Atlantic Bluefin tuna from endangered to critically endangered.
6. Hawksbill turtle (_Eretmochelys imbricata_): Hawksbills are one of the most critically endangered sea turtles. They are threatened by habitat loss, illegal trade for their beautiful shells, and accidental capture in fishing gear.
7. Coral reefs: While not a single species, coral reefs worldwide are facing catastrophic decline due to rising ocean temperatures, ocean acidification, pollution, and destructive fishing practices. Many coral species are listed as endangered.
8. Seahorses and pipefishes: Many species of seahorses and pipefishes are threatened by habitat loss, overfishing for traditional medicine and aquarium trade, and pollution.
9. Dolphins: Several species of dolphins are endangered due to habitat destruction, bycatch in fishing gear, pollution, and hunting for meat or body parts.
10. Manatees: Manatees are large, gentle marine mammals found in tropical and subtropical waters. They are vulnerable due to habitat loss, hunting, and boat collisions.
It's important to note that the IUCN's Red List is constantly being updated, and the conservation status of species can change over time due to conservation efforts or ongoing threats.