The subterranean tidal biome includes coastal pools and subterranean voids with a partially or entirely submerged connection to marine waters. Like all other subterranean ecosystems, sunlight is absent or too dim to sustain photosynthesis.

Marine shelf ecosystems (M1), terrestrial aquifers (SF1), and surface coastal systems (T1, T2, T3, T4, T5, T6, MT1) connected to these subterranean systems are their sources of allochthonous energy, nutrients, and oxygenation. Food and energy availability are influenced by in situ microbial processing (biogeochemical transformation) of these allochthonous organic matter inputs.

The marine interface, a typical feature of coastal aquifers and subterranean estuaries, also generates a marked salinity gradient in the primary zone of biogeochemical cycling. In carbonate and volcanic geologies, the salinity gradient can often be observed in the flooded pools, voids, and caves as a halocline (a sharp salinity gradient in the water column), which is not present in other subterranean environments.

In comparison to other subterranean ecosystems, diverse assemblages of chemoautotrophic and heterotrophic microbes, as well as scavengers, filter feeders, and predators. Physiological traits enabling osmotic regulation allow some species to transit across haloclines between the fresh- and saline waters. In dark sections of the subterranean marine systems where photoautotrophs are absent, trophic webs are truncated. Some of the subterranean marine biota belong to lineages otherwise restricted to the deep sea floor (M3) and share traits with those in other low-productivity, dark biomes including depigmentation, reduced visual organs, increased tactile and chemical sensitivity, low fecundity, long lifespans, and slow metabolism and growth rates.

Tides are an important means of hydrological mixing, resource flux, biotic dispersal, and perturbation. In subterranean tidal systems with more direct connections to the sea, marine suspension feeders, particularly sponges and other sessile invertebrates, are dominant. Farther into marine and anchialine caves where tidal flushing and water exchange diminishes or disappears, the fauna consists of stygobitic crustaceans, annelids and several other faunal groups (e.g. strictly subterranean aquatic fauna that complete their entire life in this environment).