A Saltwater Infusorian

The marine protozoan Rhopalophrya salina is not well known to aquarists. Because they are much smaller than baby brine shrimp, are prolific and easy to culture, they make a desirable live food for many species of crustaceans which are in turn used as live food for larger fishes.

Natural history

Rhopalophrya salina is a free-living ciliated marine protozoa that grows to about fifty-five microns with a cylindrical and slightly curved body, narrower at the end where the mouth is located. Because of their small size, these features can be seen only with the aid of a microscope.

They are commonly found in saline ponds, tide pools, and salt marshes along the West Coast of the U.S.A., tolerating a range of salinity from that of seawater to the more concentrated salinity of salt marshes. Their natural food is bacteria, which are especially abundant among decomposing debris. R.salina are usually found in company with the larger and more pear-shaped  Fabrea salina, which is also an excellent food for baby fishes. R. salina reproduces by cell division, dividing into two individuals two or three time’s daily. This accounts for a tremendous population in a very short time. Because R. salina exists in temporary salt marshes, we know that they are capable of producing cysts, which resist prolonged drying and freezing – a rare characteristic among marine protozoa’s.

Culturing

Starter cultures of R. salina may be available from amateur culturists, or your first culture may have to be started with live specimens from saltwater ponds or marshes, or with encysted animals usually present in the mud of dry salt marshes or from unwashed brine shrimp eggs. Such a starting culture may also contain cysts of other organisms. When this happens it is usually necessary to subculture R. salina.

The easiest way to do this is to place a bright light on one side of the culture jar until a cloud of organisms gathers. Use an eyedropper to remove as many R.salina as possible to a new culture jar. The chances are that a few undesirable organisms are going to be transferred, but if several subcultures are made, a virtually pure culture of R. salina can be obtained.

A wide mouth gallon glass jar is a good culture container. It can be filled with artificial seawater and the salinity adjusted to 1.022 to 1.030, pH can be adjusted to 8.0 to 9.0 with sodium bicarbonate. Provide moderate aeration, illumination and a temperature of about 70 degrees F. One or two backup cultures should be maintained as insurance in case one culture goes bad.

Feed sparingly with cultured marine bacteria. If a marine bacteria culture is not available, the protozoa will subsist on a variety of dietary products such as malted milk, kelp tablets desiccated liver tablets. All forms should be powdered and fed sparingly.

Marine bacteria are cultured separately in gallon jars filled with adjusted artificial sea water and inoculated with the following concoction:  Mix in a little hot water; 1 teaspoon each chopped dry kelp and seashore debris. ½ teaspoon each pulverized dried shrimp and powdered malted milk. Add ¼ teaspoon cooked and mashed brown rice.

Put the concoction in the culture jar and place it in a warm dark place for two days with mild aeration. Feed just enough of the bacteria culture to slightly cloud the protozoa culture water, and repeat the feeding only when the water has cleared. The bacteria cultures should be subcultured weekly using the same formula in fresh artificial seawater. When starting new bacteria cultures, inoculate them with part of an established culture when possible.

R. salina can be forced to encyst by saturating a thin layer of pulverized vermiculate or peat moss and allowing the layers to slowly dry. The impregnated substrate can be mailed in small plastic envelopes and revived at its destination.  

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