The digestion of microbial and detrital resources by an omnivorous shrimp, Penaeus vannamei Boone

Abstract

Amorphous detritus, composed of organic precipitates and microbial extracellular polysaccharide (EPS), is suspected to be a more important food source for marine animals than is the familiar morphous detritus formed by fragmentation of plant remains. Omnivorous juvenile stages of many marine species occupy shallow habitats where amorphous detritus is abundant. Although these animals ingest aggregates containing amorphous detritus and microbes, only the latter have been investigated as potential foods for omnivorous consumers. I tested whether a representative omnivorous shrimp, Penaeus vannamei, could use an abundant type of amorphous detritus, bacterial EPS, as a food source. For comparison, I measured the ability of P. vannamei to digest bacteria and microalgae contained in natural aggregates of amorphous detritus and sediment. Detrital aggregates and juvenile P. vannamei used in experiments were obtained from earthen aquaculture ponds. Fluorescent microspheres were coated with [14C] EPS produced by Alteromonas atlantica and then attached to detrital aggregates fed to shrimp. Two independent methods showed that 16% of this EPS was digested and assimilated. Amorphous detritus, unlike morphous detritus, is therefore a potentially valuable food of penaeids and other omnivores. Ingestion of this material by abundant consumers may constitute an important microbial-macrofaunal trophic link in coastal ecosystems. Phospholipid fatty acid analysis showed that all groups of bacteria present in aggregates were digested with approximately 60% efficiency. Previous reports of 90-99% digestion of bacteria by penaeids may be based on methodological artifacts. Photosynthetic pigments, analyzed by HPLC, indicated that 63% of the diatoms in detrital aggregates were digested. When abundant in aggregates, diatoms could support growth of juvenile penaeids. Microbes contain high proportions of protein, but EPS is mostly carbohydrate. Given the relative digestion efficiencies and the probable abundances of EPS and microbes in detrital aggregates, penaeids ingesting aggregates exclusively could encounter food energy limitation but not protein malnutrition. Detrital aggregates are nutritionally equivalent to dilute prey, and would supplement, rather than complement, prey ingested as part of an omnivorous diet. While foraging for benthic prey, penaeids consuming encountered detrital aggregates would increase their rate of food intake with little or no additional energetic expenditure for search or capture.