Differences In Total Area Research Articles

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. 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.

Measurement of Host Curvature by The Parasitoid Wasp Trichogramma Minutum, and Its Effect on Host Examination and Progeny Allocation

ABSTRACT The parasitoid wasp Trichogramma minutum Riley (Hymenoptera: Chalcidoidea: Trichogrammatidae) adjusts its progeny allocation and host examining behaviour to the exposed volume of its host – an insect egg. The female wasp examines a potential host by walking over its surface while drumming its antennae against it, spending a longer time examining larger hosts. If wasps of different sizes are presented with glass beads of the same diameter, examining time increases significantly the larger the bead is relative to the wasp. Since the wasps’ velocities do not differ with wasp size, the total distance travelled during host examination also increases with relative host diameter. When large and small wasps are placed on large and small hosts such that the relative curvature is constant, examining time and total path length are the same. Smaller wasps take more steps to travel the same distance, therefore the duration of the examination walk, rather than step number or relative distance, is adjusted to host curvature. This adjustment of examination time may ensure that the surface of hosts of different sizes are examined adequately, despite differences in total area. Progeny allocation, usually dependent only on exposed host surface area, can be affected by the wasps’ measure of curvature. When hosts are mounted on the tips of thin wires rather than on the substrate, no finite cues dependent on surface area are available to the wasps. Such hosts receive only 25 % more progeny, either in the light or in total darkness, than hosts which are resting on the substrate, demonstrating that an upper limit to clutch size is set by cues dependent on curvature. The structure of the compound eyes suggests that the visual acuity of Trichogramma is insufficient to account for their ability to discriminate between small differences in host curvature. By placing large and small wasps on point-mounted hosts of fixed diameter, we show that the clutch size limit is set according to the relative size of the host. These observations are discussed in terms of a mechanosensory mechanism of curvature measurement.