Assimilation efficiency, gut morphology and pH, and digestive enzyme activity of Atherinops affinis (Teleostei: Atherinopsidae), a stomachless omnivore feeding on macroalgae

Atherinops affinis (Atherinopsidae) lives in various coastal habitats in the eastern North Pacific. Its small, dorsally-positioned mouth seems suited for picking invertebrates but not for browsing on macroalgae. A. finis has a short, simple gut and no obvious stomach or physical mechanisms for breaking down algal cells. However, its diet in estuaries is mainly green macroalgae. I investigated this seeming paradox by studying the digestive physiology of A. finis and its capacity for herbivory. My results showed that A. finis from an estuary had (1) assimilation efficiencies from a green algal diet of 89% (protein), 84% (nitrogen), 74% (carbon) and 80% (energy), (2) an alkaline gut and no pepsin activity, and (3) relatively high amylase activity and low trypsin and lipase activities. Based on its digestive mechanisms, A. finis appears to capable of functional herbivory and to fit the Type III category in a classification of herbivorous fishes.