Digestion theory and applications to deposit feeders
Penry, Deborah Lynn, 1957-
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A general theory of digestion can be derived from principles of chemical reactor theory, and three ideal reactor models, the batch reactor, plug-flow reactor (PFR), and continuous-flow, stirred-tank reactor (CSTR), are the basic components of models of animals' guts. These models express the extent to which an animal digests ingested food as a function of food parameters, digestive reaction kinetics, and gut throughput time. If digestive reaction kinetics are specified, the best gut-reactor in which to carry out the given reactions can be determined. Two basic predictions result. If digestive reactions follow Michaelis-Menten enzyme kinetics, an animal's gut should operate as a PFR, and if an animal's digestive strategy incorporates fermentation of refractory organics, its gut should operate (except for very long throughput times) as a CSTR-PFT series (foregut fermenters) or a PFR-CSTR (hindgut fermenters).Michaelis-Menten kinetics are postulated as the general case for digestive reactions in deposit feeders. It is predicted, and observed, that many deposit feeders have simple, tubular guts that operate as PFRs, but experimental tests reveal necessary model modifications. The modified PFR model represents an extreme case of segregated flow: flow through most of the gut is characterized by the absence of axial and radial mixing of sediment. Deposit feeders with simple, tubular guts are small in size and predominate in environments with fine-grained sediments of relatively low permeability. Ontogenetic changes in gut architecture suggest that digestion may be diffusion-limited in many of these species.Terebellimorph polychaetes are exceptions to the PFR model. Their guts must be modeled as CSTR-PFR series with a ventral bypass stream that shunts particles from the CSTR through the PFR for rapid elimination from the gut. Post-ingestion sorting may remove relatively indigestible particles and thus increase the proportion of gut volume available for processing higher-quality particles.Gut architecture varies with diet quality. Carnivorous polychaetes have less gut volume per unit of body volume than do deposit feeders, and deep-sea deposit feeders tend to have longer guts than closely-related, shallow-water species. As individuals within a deposit-feeding species increase in size, throughput time and thus extent of digestion should increase.
- Oceanography