Mudflat snails eat the same dung again and again, days apart
On tidal mudflats, snails swallow decaying debris, digest the protein-rich microbes living on it, and pass the rest out almost untouched. A few days later, fresh microbes have colonised that dung, and the snails eat it again. This loop, called the detritus cycle, quietly recycles much of the world's dead matter.
Detritus is the decaying remains of plants and animals, along with their faeces, and it is almost always crawling with microorganisms breaking it down. On land it takes the form of leaf litter and the humus-rich soil beneath it. In rivers and lakes it is mostly fallen leaves, shredded by small invertebrates or filtered from the water. In the sea it drifts down as marine snow, a slow rain of dead plankton and other particles.
Decomposition happens in stages. Feeders quickly absorb easy compounds such as sugars, fats and proteins, while complex carbohydrates take far longer. Excreta on their own are a poor, unbalanced food, but the bacteria that multiply on them change that, which is why the snails can return profitably to their own droppings. Whatever the detritivores leave is finished off by bacteria and fungi. On land, far more material is broken down this way, as dead matter, than is ever eaten by animals while alive.
Much of the organic matter in freshwater comes from the surrounding land, and in small streams plant litter is the leading source of carbon. Water carries fine particles long distances, and plant material can settle as a silty layer called mulm or break down into tannins that stain the water. Sea anemones trap drifting particles on sticky tentacles, brittlestars and gastropods scoop them from the seabed, and buried bivalves reach up with their siphons to feed on the surface layer. Too much suspended detritus clouds the water and slows photosynthesis.
Aquarium keepers now use the same cycle. In the Berlin method, a protein skimmer lifts detritus out on air bubbles before it rots, while porous live rock hosts the bacteria and small creatures that consume what remains. Early tanks relied only on physical filters.
Source: Detritus
