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dcyphr | Accumulation and effects of microplastic fibers in American lobster larvae (Homarus americanus)

Abstract 

This study looks at the effect of microplastic fibers (MPF) on American lobster, Homarus americanus. The life span, molting patterns, and oxygen consumption of larval stages 1-3 and post-larval stage 4 are tested. Overall the results show that the effects of MPF on American lobster larvae are dependent upon the MPF concentration, the stage of the larvae, and the presence of other food. 


Introduction 

Microplastics are any plastic particle less than 5mm in diameter. Most are produced from the degradation of larger pieces of plastic. They are the most abundant plastic debris in the ocean. Microplastic was found in 61% of samples taken from the gulf of Maine, but microplastic fibers (MPF) were found in 91% of samples. Therefore, MPF appears to be the most common form of microplastic. 

After they reach the microscopic size, they often further fragment to reach a nanoscopic size which is less than 0.1 mm. When they are this small, they enter the scale of Plankton, which are key food items for larval fish and crustaceans living in the upper water column. The water column is just a theoretical column of water that stretches from the surface to the bottom. Ingestion of microplastic can affect all levels of marine life as marine life is unable to distinguish between microplastic and food items. According to other studies, microplastics have been found in the stomachs of all marine organisms.

    Studies report the presence of microplastic, but rarely their physiological effects. However, according to a recent study on oysters, mussels, and shore crabs they will bring microplastic into their gill cavity. Which can have negative effects on growth, digestion, and feeding. Newly hatched American lobster larvae are found higher in the water column where the MFP concentration is several orders of magnitude larger. This study investigates the effect of MPF concentrations on life time, molting, ingestion and oxygen consumption (as a stand in for individual physiological performance in the larval stage). The ubiquity of microplastic in marine environments could pose a large threat to lobster ecology, health, and development. 


Materials and Methods

Microplastic fibers characterization and quality control 

MPF was created by shearing a neon pink polyethylene terephthalate (PET) fleece. Special care was taken to ensure no environmental microplastics could enter the experiment. MPF concnetations were set to mimic those found at the near-surface level in the gulf of Maine coastal waters. Larval survival, development, ingestion, plastic accumulation, and oxygen consumption were measured independently with different experiments. 



Results 

Larval survival rates (stage I & II) 

All larval and post-larvae had MPF under their cephalothorax carapace and/or ingested them. The presence of MPF also affected survival, molting, and oxygen consumption rates differently at each larval and post-larval stage. The Highest MPF concentration of 25 MPF per mL significantly reduced larval survival. Starved larvae exposed to the highest MPF concentration started dying at a rate much quicker than those who are exposed to the highest MPF concentration and are fed. The survival rate did not differ for the two lower MPF concentrations of 1 and 10 MPF per mL. 


Microplastic fiber accumulation under the cephalothorax carapace 

     When no food was available MPF accumulated under the carapace in the first larval stage. When food was available a larger amount of microplastic was required to cause MPF accumulation. MPF accumulation under the carapace peaked at 2 days and did not increase for the highest MPF concentration. No such peak was seen in the lowest MPF concentration. The percentage of larvae with MPF under the carapace was dependent upon the large stage. Stage II had the highest MPF accumulation at both MPF concentrations, while stage IV post larvae had the least. 


Microplastic fiber Ingestion

Post-larval stage IV ingested the most MPF at both high and low concentrations. Stage III then ingested the second most amount of MPF followed by stage II and then I. This is the opposite trend seen in the amount of MPF that accumulates under the cephalothorax carapace . 


Oxygen Consumption Rates

For larval stage III and stage IV post larvae, oxygen consumption rates (OCR) decreased significantly for those exposed to MPF. 


Discussion

     Accumulation of MPF occurred under the carapace and inside the gastrointestinal tract. This likely leads to decreased survival and oxygen consumption rates. The survival of stage I and II larva decreases as exposure to MPF increases. The damaging results may be cumulative and require extended exposure to microplastic as those with temporary exposure did not experience any change in survival during exposure. Interestingly, even at low MPF, survival rate decreased further in the absence of food despite little to no MPF accumulation in the carapace. This suggests an unknown mechanism that intensifies food issues. This was seen in a similar study using microplastic beads. The reasons behind the results are still unknown.     

    The percentages of American lobsters with MPF accumulation was likely underestimated as the lobster can dislodge the contaminant (MPF) using a variety of methods. Researchers hypothesize that it can do this through special appendages, molting, or reversal of the flow of water through the carapace. However, only stage III and IV post-larval may be able to efficiently produce a current flow through their gills to dislodge the MPF which might help explain the lower amount of MPF in their carapace despite higher amounts of ingested MPF.    

    The transfer of MPF vertically and up through the food chain provides an additional route for the accumulation of MPF in American lobster guts. Current studies also indicate that the MPF concentrations are severely underestimated and could be much higher. The effects of microplastic ingestion are being investigated currently and it may have physiological, and ecological effects. However, research should not just focus on the adults in a system as they come from larval stages. The larvae ensure the continuation of a species, so microplastics effects on this population is just as critical as on the individual adult. Future studies should research the ecosystem level effects in combination with recent global warming.