Marine Biology

The Biota of Estuaries

There are three types of fauna in estuaries: marine, freshwater, and brackishwater or estuarine.The m arine fauna is the largest group in terms of numbers of species and includes third  subgroups. The stenohaline marine animals are marine forms that either are unable, or barely able, to tolerate broad range of salinity changes. These organisms are usually restricted to the mouth of estuaries where salinity is generally 25 ppt or above. These animals are often the same species found in the open sea. The second subgroup is the euryhaline marine animals  capable of tolerating varying amounts of salinity reduction below 30 ppt. Such species are capable of penetrating varying distance up the estuary. Most tolerate senilities down to 15 – 18 ppt (psu), with a few hardy species tolerating levels down to 5 ppt.

The brackishwater or true estuarine species are found in the middle reaches of the estuary in salinities between 5and 18 ppt but are not found in freshwater or in full sea water. Examples of these animals include the polychaeta Nereis diversicolor , oysters ( Crassostrea, Ostrea ) clams ( Scrobicularia plana, Macoma balthica ) small gastropods ( Hydrobia ), crabs ( Callimectes ) and shrimps ( Palaemonets). Some of these estuarine genera may be limited in seaward direction not by physiological tolerances but by biological interactions, such as competition and production.

The third component is derived from freshwater. These animals are able to tolerate salinities much above 5 psu (ppt) and are restricted to the upper reaches of the estuary.

There is also a transitional component. This includes those organisms such as migratory fishes, that pan through the estuary on their way to breeding grounds either in freshwater or salt water. Common examples are the salmon ( Salmo, Omcorhynchns) and the eels ( Anguilla). Also included here are forms that spend only part of their lives in estuary. Usually it is the juvenile stage; and the adults are found at sea. Good examples of this latter group are the various shrimps of the family Panaeidae ( Peaneus setiferus, P. azetiecus, P, duorarum). The young occur in estuaries. The transitional form also includes species that enter the estuary only to feed and includes many birds and fishes.

The number of species of organisms inhabiting estuarine systems is significantly lower than the numbers inhabiting nearby marine or freshwater habitats. This is probably because freshwater organisms cannot tolerate the increased salinities and marine organisms cannot tolerate the decreased salinities of the estuary.

The true estuarine organisms are derived primarily from marine stocks and not freshwater. This is similar to the situation in the other transitional zone, the intertidal, which is also populated mainly with marine organisms, not terrestrial forms. In contrast to the intertidal, however, the number of true estuarine species is very small, and the middle reaches of estuaries are depauperate .  If  marine animals can tolerate a greater reduction in salinity than the freshwater ones can endure salinity increases, and because the true estuarine organisms are primarily derived from marine stocks, most of the estuary is inhabited by marine animals.

Why are there so few estuarine species? The most common explanation is that the fluctuating environmental conditions, mainly salinity, are of such magnitude that only a few species have been able to evolve the necessary physiological specializations to exist there. Another explanation is that estuaries have not existed long enough in geological time to permit a complete estuarine fauna to develop. A final reason may be that estuarine areas have little topographic diversity, being mainly broad expanses of mud. There are fewer niches and, therefore, fewer species. It is not possible at this point of time to say if one, all or none of the above are responsible for the presence of few estuarine  species and  apparently eurihalinity is a trait not easily acquired.