POLYMERIZATION OF SILICA IN ACIDIC SOLUTIONS: A NOTE OF CAUTION TO PHYCOLOGISTS1

Concentrated stock solutions of sodium metasilicate (Na₂SiO₃.9H₂O), prepared in deionized-distilled water and acidified to pH 2 as suggested by McLachlan (1973), were found to be polymerized; stocks kept at their alkaline pH of dissolution were not polymerized. Depolymerization studies using the acidified stock added to deionized-distilled water, 3% NaCl or artificial saltwater medium showed that complete depolymerization took longer than 2 days. Studies of Si uptake, using the marine diatom Thalassiosira pseudonana (Hust.) Hasle and Heimdal, showed that use of an acidified Si stock for experiments can lead to anomolous results. It is recommended that concentrated Si stocks be stored at their pH of dissolution and not be acidified prior to addition to media. In instances where the pH must be adjusted prior to addition (e.g. in some Si-uptake experiments), it may be necessary to take into account polymerization / depolymerization effects.     

Vertical habitat segregation as a mechanism for coexistence in sympatric rodents

Coexistence has been widely studied in small mammals and frequently is assumed to be facilitated by habitat segregation. Using live trapping and spool-and-line experiments, we analyzed habitat selection and segregation across multiples scales for Peromyscus leucopus and Ochrotomys nuttalli. At the habitat scale of a forest stand (∼1–100s ha) P. leucopus co-occurred at all sites where O. nuttalli was found, and we did not detect evidence of positive or negative associations or habitat segregation. However, O. nuttalli was restricted to early successional forests, and P. leucopus had significantly lower abundances in early successional forests than in other habitats. We found similar patterns at the mesohabitat scale of the study site (400 m²). O. nuttalli abundance increased with increasing shrub and tree densities, while increases in P. leucopus abundance were associated with open understories. At the microhabitat scale of the individual movement trail, we found vertical segregation. Movement trails for O. nuttalli were at significantly higher elevation (mean height = 142.93 ± 37.10 cm) than P. leucopus trails (mean height = 15.4 ± 4.98 cm; F-value = 35.29, p < 0.001). We concluded that microhabitat segregation was driven by differential use of vertical space for movement and foraging. We suggest O. nuttalli is superior to P. leucopus in its ability to acquire food especially in shrubby subcanopies where few acorns are available. However, P. leucopus is superior in its ability to forage and avoid predators.     

Intraspecific interactions influence size specific depth distribution in Cottus bairdi

Synopsis We tested depth selection by different sizes of mottled sculpin, Cottus bairdi, in a southern Appalachian stream. Field observations indicated that, during one hour periods, both small (<50 mm SL) and large (≥55 mm SL) individuals move within an area less than 0.50 m². Individuals of both sizes, placed in field enclosures, preferred deep microhabitat. When large fish were placed in cages with small fish, small fish initially spent more time in slope and shallow microhabitats. Average interfish distances were not correlated with their absolute size differences, suggesting C. bairdi interactions may involve both predation and competition. In streams, size-related differences in microhabitat depth may result more from intraspecific interactions than from size-specific depth preferences.     

Laboratory experiments on the schooling and feeding behaviour of Trichogaster fasciatus (Pisces: Perciformes)

Laboratory experiments were performed on the schooling and feeding behaviour of Trichogaster fasciatus . There was a tendency on the part of individuals of similar size to school together; the instinct being stronger in younger specimens. Unavailability of similar-sized individuals led to schooling by members of different size-groups. Column feeding was exhibited by the fish. While feeding at the bottom, the fishes formed angles varying from 30°90°. Larger individuals formed a right angle more often than younger members of the species. Optimum degree of crowding seemed necessary for active feeding as confinement of single individual, or overcrowded condition, both resulted in a decline of feeding activity.     

Propagation of the myxozoan parasite Myxobolus cerebralis by different geographic and genetic populations of Tubifex tubifex: An Oregon perspective

Tubifex tubifex are obligate invertebrate hosts in the life cycle of Myxobolus cerebralis, the myxozoan parasite that causes whirling disease in salmonid fishes. This exotic parasite is established to varying degrees across Oregon’s Columbia River system (Pacific Northwest, USA) and characteristics of local T. tubifex populations likely play a role in the pattern of disease occurrence. To better understand these patterns, we collected T. tubifex from three Oregon river basins (Willamette, Deschutes, and Grande Ronde), determined their genotype (mitochondrial 16S rDNA lineage and RAPD genotype) and exposed 10 different populations to M. cerebralis in the laboratory. Four mt lineages were identified: I, III, V and VI. Lineage III was found in all river basins but dominated both central and eastern sites. The RAPD assay further divided these lineages into geographic sub-populations; no RAPD genotype was common to all basins. There was a significant difference in prevalence of infection and level of parasite production among the populations we exposed to M. cerebralis that was attributed to genotypic composition. Only lineage III worms released actinospores and only populations dominated by this lineage amplified the parasite. These populations had the lowest survival, however, the lineage dominant before exposure remained dominant despite the high prevalence of infection. The distribution and infection dynamics of susceptible T. tubifex throughout Oregon may contribute to the differences in M. cerebralis occurrence; our studies further support the influence of oligochaete genotypes on the manifestation of whirling disease in salmonid populations.     

Differential feeding behaviour of the dominant cladocera as an explanation of zooplankton community structure in the Loosdrecht Lakes

The zooplankton of Loosdrecht Lakes is dominated byBosmina longirostris in the Spring period but is replaced by the largerB.coregoni in early summer at a time when phytoplankton populations, dominated by filamentous cyanophytes, are rapidly increasing. Throughout the summerB.coregoni coexists with the longer, but more slender,Daphnia cucullata.Different size-categories of¹⁴C labelled lake-phytoplankton were fed to natural assemblages of crustacean zooplankton to investigate effects of body length on feeding and the relative selectivities of theDaphnia andBosmina species on food particle-size.B.coregoni feeds more effectively, especially on larger sizes of food, thanB.longirostris and this may be a factor in the observed change of bosminid species dominance.B.coregoni is able to feed on a wide size range of particles and grazes and assimilates greater volumes of food thanD.cucullata. B.coregoni preferentially feeds on food-particles greater than 7 m whereasD.cucullata selects food below 7 m. The coexistence of these species appears to depend on a niche separation through different feeding behaviours and selectivities.