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Detritus Feeders and Scavengers

We have learned that much of the organic material produced in the pelagic zone is precipitated to the bottom in the form of living or dead bodies of the planktonic and nektonic organisms and their excreta. Added to this is detrital material resulting from disintegration of benthic plants and animals and also from influx of terrestrial material. In

regions of relatively shallow water, where there is a heavy growth of eel grass and algae, the detrital material contributed by the plankton may be of secondary significance (Jensen, 1914), but over vast stretches of the ocean the plankton must contribute the major portion. This material forms a mixture with the mud and sand on the bottom and a thin film of slime or ooze accumulates as a sort of pap at the interface of the water and the bottom. Thus, in contrast to the pelagic region, the food on the bottom can become very concentrated in a narrow horizon.

The organic material available in this mixture on the bottom is fed upon by bacteria and other microorganisms such as Protozoa, nematodes, and rotifers, and the whole mixture in turn is consumed by larger detritus feeders. Bottom organic detritus is sometimes considered the main source of nourishment for most benthic invertebrates. In a survey of Danish waters Blegvad (1914) concluded that of 90 or more species of invertebrates investigated, 69 (the most common animals) were some form of detritus eaters, while 5 were herbivorous and carnivorous and 16 were purely carnivorous. In the strictest sense only those organisms subsisting solely on detrital organic remains are detritus feeders, but it is not practicable in a general survey to draw the line so closely. So in a broader sense, we see that some detritus feeders may be nourished in part by living organisms and are not necessarily entirely scavengers in habit. In the littoral zone, especially in the eulittoral zone, part of the detrital mixture consists of photosynthetic organisms such as littoral diatoms growing naturally on the bottom and thus contributing to the organic material available in the detritus.

The concentration of detritus feeders is, of course, dependent upon the extent of production of plants and nonscavenger animals. Where this production is great, there also the scavengers must be numerous. With increasing depth, the detrital food on the bottom becomes less. The plant material diminishes until in the great depths not even sinking pelagic plants produced in the euphotic zone above ever reach the bottom, being disintegrated through bacterial action or autolysis, or eaten and converted into bodies of animals. Such mid-depth and abyssal pelagic animals serve as links in a series of changes that convey sufficient organic material, as animal detritus, from the euphotic zone downward to the bottom to support at least a sparse benthic population of detritus-feeders, and the animals that in turn live upon them. It is hardly conceivable that any plant material as such ever reaches the bottom to enrich the detritus of abyssal depths. The rate of sinking of Chaetoceros diatoms in still water is only about 1 m in 4 3/4 hours. Plankton animals such as salps sink at the rate of 4000 m in 2 days 7 hours (Hesse, Allee, and Schmidt, 1937). The low temperature of great depths is important in delaying bacterial action, thus allowing more time for sinking before complete disintegration takes place, while the rate of sinking must

become increasingly diminished as the particles become decreased in size owing to disintegration (Seiwell and Seiwell, 1938).

Various types of animals may be mentioned as detritus feeders. Their habits of feeding are varied and may combine several methods. Such burrowing worms as the lugworm Arenicola and others, and the protochordate Balanoglossus, swallow the mud indiscriminately in the process of digging and are nourished by whatever digestible material may be present in the mud and sand.

The clam Macoma nasuta, lying buried in the mud and sand of shallow water, extends its long inhalant siphon to suck up slime that has accumulated on the surface of the mud. The digestible material thus taken in is entangled in mucus and propelled by cilia to the labial palps and the mouth. The bivalves Nacula and Yoldia gather detrital material by directly extending the unusually long labial palps to pick up the food.

Among the echinoderm detrital feeders the sea cucumbers, illustrated by Stichopus, suck up large quantities of mud and detritus that has come to rest on the bottom. It has been calculated (Crozier, 1918) that in certain shallow coastal areas of Bermuda, these animals pass 6 or 7 kg (dry weight) of mud per square meter per year through their digestive tracts; in a certain enclosed area of 1.7 mi2 the mud eaten annually may be from 500 to 1000 tons. The stomach fluids are somewhat acid and may dissolve calcium carbonate. Feeding of this type is important in the biological “working over” of bottom deposits. The sea urchins, Strongylocentrotus spp., also subsist on plant and animal detritus. The mud-dwelling brittle star lies buried below the surface of the mud with several arms extended out over the surface in contact with the top slime which they collect and carry to the mouth.

Mud-dwelling tube worms collect the nutrient-rich detritus by means of extended cirri along which food material can be carried in ciliated grooves. Detritus feeders living in the mud also have their quota of filter and mucus feeders, although the material consumed doubtless also consists of typical planktonic organisms as well as suspended organic detritus stirred off the immediate bottom by currents or, in some instances, purposely stirred into suspension by the animals, for example, by Callianassa and other Crustacea.

The echiuroid worm Urechis, which inhabits U-shaped burrows in muddy sand, has a remarkable method of obtaining food through a combination of mucus secretion and filtration. Mucous glands at the anterior end of the worm secrete a funnel-shaped mass of mucus 5 to 20 cm long lining the upper end of the burrow in front of the animal. The broad end of the funnel fits against the walls of the burrow, while the narrow end fits as a snug collar around the anterior end of the animal. Water is then forcibly pumped through the burrow from front to back and in its passage through the mucous tube small particles of detritus,

bacteria, and small animals are entangled. When the tube becomes clogged, which may require an hour in relatively clear water, the animal disengages the collar, pushes forward, and eats the whole mucous tube together with its load of intercepted organic food (Fisher and MacGinitie, 1928).

An interesting method of detrital feeding is shown by the sand crab Emerita, which inhabits wave-washed sandy beaches especially in tropical and subtropical waters. Living in sand burrows in the lower part of the intertidal region, the animals face seaward while their long feathery antennae protrude from the sand to intercept fine detrital material that is washed out with the receding waves.

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