Seite - 165 - in Freshwater Microplastics - Emerging Environmental Contaminants?

well as particle interactionwithother stressors, and (b)D.magna seems relatively resistant toMPexposures. The lowsensitivity ofD.magna couldbedue to its behavioral andmorpholog- ical adaptations as a generalist filter feeder. D. magna feeds nonselectively on seston components encountering multiple particle sizes, shapes, and materials. High concentrationsofSPMreduce thefiltration rates as daphnids reject collected particles before ingestion or even narrow their carapace opening to avoid large particles [44, 45]. Besides pre-ingestion adaptations to unsuitable SPM, the peritrophicmembrane protects the epitheliumof the digestive tract fromparticle- induced injury. It consists of a complexmatrix of chitinmicrofibrils, polysaccha- rides,aswellasproteinsandsurrounds thefoodbolus in thedigestive tractofmany arthropods[46,47].Poresofseveralnanometers indiameterensure the transportof digestive fluids and nutrients and protect against pathogens andmechanical dam- age. The packed food particles pass the digestive tract and are egested with the surrounding peritrophic membrane. Therefore, a direct interaction of MPs with epithelial cells in thedigestive tract and thus injuries anda transferofMPs into the surrounding tissue are unlikely.However, Rosenkranz et al. [48] observed 20 and 1,000nmparticles in theoildropletsofD.magna implyinga translocation through thegut’sepithelialcells,whereas themajorityofstudieswithnanomaterialsdidnot confirm this observation [49, 50]. 3.1.3 OtherCrustaceans Null effects were found in the amphipodGammarus pulex exposed to irregular polyethylene terephthalate (PET) fragments (0.4–4,000PmL 1, size 10–150μm; [31]).After48days,MPsdidnot induceanyeffectsonbehavior (feedingactivity), metabolism (energy reserves), development (molting), and growth. Au et al. [18] testedweathered polypropylene (PP) fibers (20–75 μm, 0–90 PmL 1) as well as laboratory-made PE fragments (10–27 μm, 0–105 P mL 1) in the amphipod Hyalella azteca. In a 10-day acute exposure, PP fibers weremore toxic than PE fragmentswithLC50valuesof71.43and46,400PmL 1, respectively.Thismight be related to the longer gut retention times of fibers versus fragments and again highlights the importance of particle shape. In the same study, a 42-day chronic exposure toPE fragments significantly decreased growth and reproduction. At present, besides the studieswithD.magna and the amphipods, there is very limited data regarding other freshwater crustaceans as the majority of research focuses on marine species. In addition to the increasing number of laboratory studies, the monitoring of wild populations of the common shrimp Crangon crangon [51] and theNorway lobsterNephrops norvegicus [52] have shown that fieldpopulations inmarineenvironmentsareexposed toMPs. Inbothstudies,MPs (predominantly fibers) were detected in 63% [51] and 83% [52] of the examined animals. A recent study byWelden andCowie [1] withN. norvegicus confirmed thatMP exposure negatively affects feeding, bodymass, metabolic activity, and energy reserves.An8-monthexposure toPPfibers via food (0.2–5mm,fivefibers Interactions ofMicroplasticswithFreshwaterBiota 165