Marine lakes are land-locked water bodies that maintain a marine character through narrow submarine connections to the sea. They can be considered inside-out islands, and share many analogies with terrestrial island systems: high numbers of endemic species and rapidly evolving populations.
Worldwide there are only about 200 known marine lakes. Most of these have been discovered in Vietnam, Palau and Indonesia. They are usually hidden inland, sunken in karstic landscapes, and located in remote uninhabited areas.
Marine lakes represent a continuum from highly isolated to almost open systems, from brackish to almost fully marine. Surveys of over fifteen Indonesian marine lakes by Lisa Becking and colleagues allowed the discovery of many unique and new species. They also showed that the observed species were either:
- endemic to one lake,
- shared by almost all lakes, but unknown from the open sea, or
- widespread, known from various coastal locations in Indo-Pacific reefs.
The isolated water bodies of marine lakes are, like island systems, extremely vulnerable to threats from human activities and climate change. The most dire threats to marine lakes are:
- introduction of alien species
- uncontrolled development of tourism
- use as fishponds to store recent catch or to cultivate captured fish
- use as public toilets
- introduction of sea turtles for consumption and animistic rites
