Structure and function of the stomachless digestive system in three related species of New World silverside fishes (Atherinopsidae) representing herbivory, omnivory, and carnivory

We explored possible diet-related specializations in the digestive tract of stomachless fishes by summarizing the diets, verifying the absence of a stomach, and comparing gut lengths, microvilli surface areas, and activities of five digestive enzymes in four taxa of silversides from southern California coastal waters. For the comparisons, we examined these gut features in Atherinops affinis from both estuarine and kelp-forest habitats, and Atherinopsis californiensis and Leuresthes tenuis from open coastal habitats. A. affinis was found to be primarily herbivorous in estuaries and carnivorous in kelp forests, whereas As. californiensis was shown to be somewhat omnivorous but mainly carnivorous, and L. tenuis strictly carnivorous. Estuarine A. affinis exhibited the longest gut, largest microvilli surface area, and highest amylase and maltase activities, all arguably reflecting responses to an algal diet. In contrast, kelp-forest A. affinis displayed the highest trypsin activity and generally similar microvilli surface areas and aminopeptidase, amylase, and maltase activities to the two other carnivorous taxa. All four taxa showed similar lipase activities that compared closely with published values for other fishes. Taken together, our results reveal striking differences in gut structure and function among the four taxa, but especially between the estuarine and kelp-forest populations of A. affinis. Further studies are required to assess the roles of genetic variation and phenotypic plasticity in explaining the differences in these herbivorous and carnivorous taxa.