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THE MARINE BIOSYSTEM

The successful maintenance of a marine biosystemrequires an understanding of certain basic scientific

principles. The more deeply one considers the problems

of designing and running a life-support system for themore delicate creatures of the coral reef, such as living

corals, certain plants, and the Moorish Idol (Zanclus

cornutus), the deeper becomes ones involvement in

fundamental physics, chemistry and biology.

PHYSICAL FACTORS

SalinityThe density of sea water is greater than that ofpure water that is, the weight of a given volume of

sea water is greater than that of the same volume of pure

waterbecause it contains dissolved salts. The higher

the salt concentration of the water the greater is itsdensity; measurement of water density thus provides an

indication of salinity. The density of a substance is

usually expressed as its specific gravity the ratio ofthe weight of a fixed volume of the substance to the

weight of the same volume of pure water. The specific

gravity can be measured simply and directly with ahydrometer. For sea water typical values of specific

gravity are in the region of 1.020.

Salinity is important because it influences the

exchange of salts and water which takes place between

marine animals and their surroundings. Most marinefishes have body fluids which are less concentrated than

the sea water in which they live. The intake of salt waterthrough the mouth and the loss of fluid through the skinand kidney creates a tendency for the fishs body fluid

to become as concentrated as the sea water.

It is obvious that the intake of sea water will raisethe fishs salt content. The loss of fluid through the skin

and kidney results in a net loss of water because these

organs are selective barriers to the movement of salt and

water: molecules of water move across them withrelatively greater ease than do the molecules of salts and

other substances in the body fluids. At the skin,

therefore, water moves out of the fish leaving saltsbehind in preference to an inward movement of salts

from the sea. This movement of water from a region of

low solute concentration into a region of high soluteconcentration across a barrier which selectively restricts

movement of solute is called osmosis. One can

alternatively think of the fishes body fluid as having ahigher concentration of water than the surrounding sea.

At the kidney the restriction to salt movement is

relatively less severe; the urine is nevertheless a weaker

solution than the blood from which it is formed andrepresents a further net loss of water. In order to balan

the tendency towards increased salt concentrat

marine fishes have salt-secreting cells located in gills. In effect fishes take in salt water and remove

excess salt from their body fluids.

The energy expended by the fish in maintaining

concentration of its body fluids at an appropriate levedirectly related to the difference in concentrat

between the fish and its environment, the higher the s

concentration of sea water, the heavier the load on

salt-secreting cells. There is a limit to the ability of fish to hold its internal environment constant and i

most important not to allow the specific gravity of

water to rise significantly above the preferred levMost coral fishes should be kept in water of speci

gravity 1.020. For fishes from the Red Sea the speci

gravity should be 1.022. In either case the limits

acceptability are 0.002.Any genuine deviation from acceptability

probably due to failure to make good evaporation los

or to topping up the water incorrectly. As waevaporates from the tank leaving the salts behind,

salinity, and hence the density, of the aquarium wa

rises. The loss should therefore be made good with p

water to bring the specific gravity back down to acceptable level. Topping up with salt water will

work. Readings outside the preferred range may res

from incorrect use of the hydrometer or from the usea faulty hydro- meter. Both the hydrometer and the

water must be clean; the hydrometer must be calibra

at the water temperature of the aquarium; hydrometer must be a good instrument accurate

within 0.001.

Temperature The fishes and the invertebrinhabitants of a marine aquarium have little ability

regulate their own body temperature as mammals a

birds do. They are poikilothermic and their botemperature tends to follow that of their environme

The rates of all metabolic processes are hig

dependent upon temperature and the creatures inparticular marine environment are adapted to life wit

a narrow temperature range. The sea is a therma

stable environment, much less subject to temperatvariation than fresh water. Typical water temperatu

range from 75-78F (24-26C) for tropical mar

species.

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Lighting Correct lighting of the marine aquarium isessential to healthy plant growth. The presence of

photosynthesizing algae is an important feature in the

chemical cycle of the system. Algae use up carbon

dioxide dissolved in the water and convert it intooxygen. They also take up nitrates from the water to

build their own cellular protein, which may in turn be

eaten by the animals in the aquarium.

CHEMICAL FACTORS

Oxygen All animals and many of the beneficialmicrobes in a marine biosystem require oxygen as a

condition of life. It is essential to keep the oxygenconcentration high by thorough aeration of the water in

order to encourage the growth and well being of the

organisms present. Flourishing plant growth and

suitable lighting also help to oxygenate the tank.Organic pollutants often exert their chief effect by

driving vital oxygen out of solution.

pH The acidity of the marine environment, like thetemperature and oxygen concentration, is a determinant

of the metabolism of micro-organisms. Too great anacidity encourages the activity of harmful anaerobic

bacteria and this will ultimately affect all the inhabitants

of the aquarium. In physical terms the acidity is afunction of the concentration of hydrogen ions. This

concentration is most conveniently expressed on the

logarithmic pH scale. The pH of 7.0 obtained for

distilled water is taken as neutral; a lower value is acidic

while a higher one is alkaline. Natural sea water variesfrompH 7.8 topH 8.4 depending upon locality, time of

day, season of year and depth. The aquarium should bekept at this slightly alkaline pH. Methods of measuringpH with indicator dyes are dealt with in the fresh water

section.

Toxicity Apart from the introduction of uncured ordirty gravel, rocks, corals or shells, all organic materialentering the aquarium does so as food or as living

creatures which eventually die. Organic matter is

decomposed by bacteria into chemically simplercompounds, some of which are extremely toxic.Nitrogenous waste (food and excreta) is rapidly attacked

by certain bacteria known as gelatine liquefiers and

converted into ammonium compounds. These arepoisonous to marine life even in small quantities.

Nitrosomonas bacteria are responsible for oxidizing

ammonium compounds to nitrites still toxic toanimal life. In the next stage of the sequence

Nitrobacter bacteria effect the further oxidation ofnitrites to nitrates. Nitrates are relatively harmless and

can be taken up by green plants, incorporated into

proteins and eventually recycled when the plant is eaor dies.

The detoxification of nitrogenous waste is

aerobic processall three groups of bacteria involv

require oxygen. The reactions which they bring abcan be reversed by bacteria which flourish in anaerob

acid conditions. Promotion of the activity of benefic

micro-organisms is essential to good wamanagement.

In a clinical system aerobic bacteria will focolonies on the strands of filter wool and on

granulated charcoalmuch of the protein waste wilany case be dealt with by the protein skimmer and nev

become subject to decomposition. In a natural syst

the bacteria colonize the available surfaces of aquariumwalls, rocks and other objects.

STABILITY AND MATURATIONIt will be evident upon consideration of what has be

said already, that the integrity of a well-kept mar

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aquarium depends not simply on a number of separatefactors acting independently but on an interaction of

conditions affecting and affected by the activities of

living organisms. Because temperature alters metabolic

rates of activity it may also alter the levels of biologicaldecay products and ultimately the pH and oxygen

concentration. There is in any case a direct relation

between oxygen tension and temperature since oxygenis less soluble in warmer water. Similarly lighting may

have far reaching effects through its influence growth.The delicate balance existing in a stable system is

not arrived at suddenly. The accompanying graph showsthe kind of changes which occur in a newly established,

maturing aquarium. The time to maturity will obviously

vary considerably: it may even be as rapid as 26 days ifthe starting conditions (stocking, feeding, filtration rate,

temperature, etc.) coincide particularly well. Normamaturation will be a matter of several weeks and t

entails some risk to the animals present in view of

high levels of ammonium and nitrite compoun

obtaining in the earlier stages. Biological maturation cbe speeded by maintaining fierce aeration and ra

filtration turnover, by keeping stocking and, the amo

of food down to a minimum, by keeping the temperatas high as the animals are accustomed to, and

stocking with hardy species.It is above all important never to disturb

conditions in the aquarium suddenly. The effects sharp change on such a complex balanced system

unpredictable and most likely to be harmful. Even

conditions are clearly abnormal the situation must rectified carefully and slowly.