Dr. Caulton described the digestive physiology of T. rendalli for com-
parison. The most important aspect of this is the rasping of plant material by
the pharyngeal teeth. Within one hour from the commencement of feeding
the entire gut of T. rendalli can be found packed with plant material of
which virtually none is digested, despite pharyngeal teeth disruption. At this
time, the stomach pH is about 4. This suggests that T. rendalli wastes a lot of
food, but it should be noted that not only does it normally have an over-
abundant food supply but also it can consume very large quantities very
quickly, e.g., 3 g in 10 minutes even for very small fish. After about one
hour of feeding, the stomach pH falls to about 1.4, lysis begins and assimila-
tion efficiency increases. The overall assimilation efficiency is probably
around 50%.
The stomach residence time for food is very important in T. rendalli
not only because gastric acid assists lysis of the plant cells, but also because
denaturation of the protein by acid seems to be a necessary pretreatment for
its digestion by enzymes in the intestine. It is well known that trypsins,
which are present in tilapias, act better on previously denatured proteins so
the acids here could be assisting proteolytic digestion. This is possibly
analogous to the action of renin coagulating milk protein in the mammalian
gut before digestion.
Detritus is a very complex mixture of living and non-living components.
It is erroneous to regard the food value of detritus (including plant wastes,
such as straw and grass clippings) as merely the production of microbial
protein built from the waste substrate. Non-living organic matter plays
the principal role in nutrition of many detritivores.
The methodology available to sort out the various components of detri-
tus was discussed. Light microscopy is useful to give rough estimates of
bacterial numbers and sizes, especially with epifluorescence techniques.
Bacteria and blue-green algae can be easily counted when they are made to
fluoresce against the non-living matrix. Electron microscopy (EM) has
revealed the complexity of detritus. Bacterial slime layers and capsules are
visible using EM and include proteinaceous material (stainable with osmium
tetroxide), polysaccharides (stainable with Ruthenium Red) and lipids.
These are all presumably of some nutritional value to detritivores. Bacterial
populations can also be estimated by determining the murarnic acid content
of detritus. However, even simple analysis into li~ng and non-living material
is difficult. ATP measurements will give some idea of this but say nothing of
the types of organisms present. It is well known that plant material decom-
posing in aquatic ecosystems first suffers a drop in N-content for a few days
and then the N-content increases. This has always been assumed to be
entirely due to the obvious colonization of the material by microorganisms.
As the material ages, however, some of its N-content derives from chemical
processes, e.g., precipitation/complex@g and is found as refractory nitro-
genous compounds in the detrital aggregate.
Dr. Bowen's work in Lake Valencia shows that well-established detritus