OPTIMAL FORAGING IN THE THALASSINIDEAN SHRIMP CALLIANASSA-SUBTERRANEA- IMPROVING FOOD QUALITY BY GRAIN-SIZE SELECTION

The grain size distributions and organic content of habitat sediment, stomach content and faecal pellets of the endobenthic shrimp C. subter ranea were analyzed to study food selection and its nutritional yield. Sub-samples of sediment from the shrimps' habitat and the stomach con tent were fractioned into seven grain size classes. The stomach contai ned a significantly larger mass fraction of grains smaller than 30 mu m, whereas the habitat sediment contained mainly grains larger than 70 mu m. A more detailed look shows that grains smaller than 70 mu m 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 pas sage rate of C. subterranea was derived from the cumulative faecal pro duction rate of 49 animals. The average faecal production of animals w hich were put into quarantine after they were removed from their burro ws stayed initially high, levelling off to almost zero after 2 h and s taying 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 2.3 8+/-0.05 mm(3)/h, which could be converted to an average consumption o f organic matter(OCR) of 4.2+/-1.4 mgDM/day. From all data of the indi vidual animals, relations of GPR and OCR vs, animal length resulted. A Length-Mass relation derived from 152 C. subterranea enabled conversi on of the OCR-Length relation to a function relating OCR to the body a sh-free dry mass (AFDM(b)): OCR, = 0.133 AFDM(b)(0.91), with AFDM(b) = 8.35 10(-4) * L-3.15 (OCRM in mgDM/day, L in mm and AFDM(b) in mg ). From these results the yearly organic consumption of the C. subterr anea population in the central North Sea was estimated. (C) 1998 Elsev ier Science B.V. All rights reserved.