Consumption and digestion of suspended microbes by juvenile Pacific white shrimp Litopenaeus vannamei

Abstract

Microalgae hold a key role in the diet of the world's highest-valued seafood species, Pacific white shrimp Litopenaeus vannamei, but ingestion, digestion, and feeding mechanisms employed for consuming suspended microbes are not well studied. The goal of this work was to determine whether juvenile L. vannamei could consume and digest several microbes of varying sizes and structures in suspended monocultures. Diatoms Thalassiosira weissflogii and Amphiprora sp., eustigmatophyte Nannochloropsis salina, and cyanobacterium Synechococcus bacillarus were chosen based upon their common occurrence in intensive shrimp aquaculture systems. Juvenile 2 g L. vannamei, previously starved for 24 h, were individually stocked in aerated flasks containing axenic monocultures of T. weissflogii, Amphiprora sp., N. salina or S. bacillarus for 15, 30, 60, and 90 min. After designated immersion times, shrimp were dissected for collection of stomach and intestine content samples. Fluorometry was used to measure chlorophyll a (μg/g) within the stomach and intestine contents at each time increment. Counts of intact cells (cells/g) within the stomach and intestine contents were completed for shrimp that had been immersed for 60 min. Shrimp demonstrated a sweeping behavior of the third maxillipeds during submersion in some algal cultures, which led to examination through scanning electron microscope photography. Counts of intact cells and chlorophyll a levels in the stomach and intestine indicated that juvenile shrimp actively ingested and digested T. weissflogii and Amphiprora sp. from suspended monocultures. Diatom consumption and digestion were evident across all immersion times, with increasing levels of chlorophyll a in stomach samples up to but not significantly beyond 60 min, indicating that the experimental time period represented a continuous feeding period and spanned the gut passage time. Shrimp were able to ingest S. bacillarus and N. salina to a very limited degree, but there was no evidence of digestion of these cells. Examination under the scanning electron microscope revealed that the third maxillipeds have net-like setae which could potentially select for microorganism > 10 μm in size. Since this cell size reflected the difference between those cells consumed or not consumed in this study, we hypothesize that these appendages may be used by juvenile L. vannamei to filter food particles. These observations provide valuable insight regarding this shrimp's ability to capture and digest microalgae both within its natural environment and in aquaculture systems.