A Eutropical subsea is a low-latitude

Latitude

In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

, circumequatorial body of water within a eutropical, that is genuinely tropical rather than sub-tropical ecozone

Ecozone

An ecozone is the broadest biogeographic division of the Earth's land surface, based on distributional patterns of terrestrial organisms.Ecozones delineate large areas of the Earth's surface within which organisms have been evolving in relative isolation over long periods of time, separated from...

. It is applied to marine waters considered truly tropical, in which the temperature does not drop below 20 degrees Celsius (68 Fahrenheit). Generally the fauna

Fauna

Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...

 are derived from Tethys Ocean

Tethys Ocean

The Tethys Ocean was an ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean.-Modern theory:...

 lineages of the Eocene

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

.

Eutropical subseas are distinguished by the presence of coral

Coral

Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...

 and accompanying carbonate

Carbonate

In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

 beds in the form of lagoon

Lagoon

A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...

s. Seagrass

Seagrass

Seagrasses are flowering plants from one of four plant families , all in the order Alismatales , which grow in marine, fully saline environments.-Ecology:...

 is present with mangrove

Mangrove

Mangroves are various kinds of trees up to medium height and shrubs that grow in saline coastal sediment habitats in the tropics and subtropics – mainly between latitudes N and S...

 forests supporting mollusks of horned snails

Potamididae

Potamididae, common name potamidids , are a family of small to large brackish water snails living in mangroves, gastropod molluscs of the clade Sorbeoconcha. Traditionally, potamidids and batillariids have been confused with each other for having similar shells and living in a similar environment...

, Conch

Conch

A conch is a common name which is applied to a number of different species of medium-sized to large sea snails or their shells, generally those which are large and have a high spire and a siphonal canal....

, sea snails of the families Xenophoridae

Xenophoridae

Xenophoridae, common name carrier shells, are a taxonomic family of medium-sized to large sea snails, marine gastropod mollusks in the clade Littorinimorpha....

, and Turbinellidae

Turbinellidae

Turbinellidae are a family of sea snails, marine gastropod mollusks in the order Neogastropoda.-Distribution:Species in this family are found world-wide, mostly in tropical shallow waters but some in deep waters.- Subfamilies :...

. The taxa are usually geographically bound and cannot exist at higher or lower, that is equatorial latitudes.