Optimal foraging in the thalassinidean shrimp Callianassa subterranea: Improving food quality by grain size selection

Abstract

The grain size distributions and organic content of habitat sediment, stomach content and faecal pellets of the endobenthic shrimp C. subterranea were analyzed to study food selection and its nutritional yield. Sub-samples of sediment from the shrimps' habitat and the stomach content were fractioned into seven grain size classes. The stomach contained a significantly larger mass fraction of grains smaller than 30 μm, whereas the habitat sediment contained mainly grains larger than 70 μm. A more detailed look shows that grains smaller than 70 μm are significantly preferred by the shrimp, and larger grains are avoided. The stomach content of C. subterranea showed a tenfold higher organic content compared to habitat sediment. This increase is mainly due to differences in total area of small grains compared to large grains per unit of mass, assuming the same amount of organic matter per unit of area. The pellets contained half as much organic matter as the stomach content, indicating an digestion efficiency of about 50%. The gut passage rate of C. subterranea was derived from the cumulative faecal production rate of 49 animals. The average faecal production of animals which were put into quarantine after they were removed from their burrows stayed initially high, levelling off to almost zero after 2 h and staying low thereafter. Feeding of the shrimps after 12 h of quarantine resulted in an increase in faecal production after another 2 h. From these results the average gut passage rate (GPR) was derived to be 2.38±0.05 mm 3/h, which could be converted to an average consumption of organic matter (OCR) of 4.2±1.4 mgDM/day. From all data of the individual animals, relations of GPR and OCR vs. animal length resulted. A Length–Mass relation derived from 152 C. subterranea enabled conversion of the OCR–Length relation to a function relating OCR to the body ash-free dry mass (AFDM b): OCR M =0.133 * AFDM b 0.91, with AFDM b =8.35 * 10 −4 * L 3.15 (OCR M in mgDM/day, L in mm and AFDM b in mg). From these results the yearly organic consumption of the C. subterranea population in the central North Sea was estimated.