# Reality Check: Experimental Studies on Microplastics Lack Realism
## 3 Size  
### Aquatic Effects  
Particle size is a critical factor in uptake and egestion. For small organisms such as plankton, particles larger than their mouth opening cannot be ingested. However, using only particles small enough to be eaten is also unrealistic, because it maximizes organism exposure [26]. One might expect that after ingestion, smaller particles would be better able to penetrate through tissues and cause greater toxicity, but that is not always the case. [PERSON] et al. (2021) found that rainbow trout evacuated 50% of particles in 12 h for 42.7 \(\upmu\)m particles and 4 h for 1086 \(\upmu\)m particles (which is less than the time for evacuating food) [27]. In contrast, the differences between sizes for evacuation by common carp were smaller: 7 h for 42.7 \(\upmu\)m particles and 4.6 h for 1086 \(\upmu\)m particles. They concluded that it is likely that large particles in rainbow trout must be actively transported out of the stomach, since they had shorter evacuation times than food, while in carp, evacuation rates of all particle sizes were in the same range as food, suggesting a passive process.  
In _Daphnia_, accumulation of fragments was higher than that of beads, and inhibition of feeding and growth depended on the size of MP fragments [28]. Survival of _D. magna_ exposed to small- and large-sized fragments (17 vs. 34 \(\upmu\)m) was significantly lower thanthat of daphnids exposed to MP beads. Small fragments significantly reduced feeding, and body retention time in the digestive system.  
In the marine medaka (_Oryzalas melastigma_), body weight, adipocyte size and liver lipid contents were significantly increased in fish exposed to large (200 \(\upmu\)m) PS-MPs, while fish exposed to smaller (2 and 10 \(\upmu\)m) MPs had liver injury, specifically fibrosis and inflammation [29]. Since the larger particles did not enter the circulatory system, their impacts on intestinal microbes were investigated. Gut microbial diversity and composition were altered in fish exposed to PS-MPs, particularly the larger particles. However, when goldfish (_Carassius auratus_) were exposed to two sizes (0.25 and 8 \(\upmu\)m) of polystyrene MPs at different environmentally relevant concentrations, enzyme changes and histological lesions were more severe in those that were exposed to the smaller sized MPs [30].  
[PERSON] et al., 2018 exposed the benthic bivalves _Ennucula tenuis_ and _Abra nitida_ to polyethylene MPs at 1, 10, and 25 mg/kg of sediment for four weeks [31]. Three sizes (4-6; 20-25 and 125-500 \(\upmu\)m) were used. No effects were seen on survival, condition index or burrowing behavior. However, A. _nitida_ showed a significant decrease of protein in those exposed to the largest particles, at all concentrations. No changes were seen in protein, carbohydrate or lipid in _E. tenuis_, but total energy decreased in a dose-related manner in ones exposed to the largest size particles.  
In a rare (and welcome) paper comparing effects of polyethylene terephthalalate MFs of different lengths (50 and 100 \(\upmu\)m), mussels (_Mytilus galloprovincialis_) were exposed to environmental (0.5 \(\upmu\)g/L) and high (100 mg/L) MF concentrations for four days [32]. Short MFs accumulated in the lower intestinal organs, but long MFs were only observed in the upper intestinal organs. Both sized MFs affected necrosis, DNA damage, reactive oxygen species, nitric oxide, and acetylcholinesteraseinesterase. Fiber length-dependent effects occurred at environmental concentrations for DNA damage (long MFs) and AChE activity (short MFs).