I feed all my fish KOI pellets, have been for years. Every now and then I feed them shrimp but that's rare.
Summary
1. Among vertebrates, herbivores have longer digestive tracts than animals at higher trophic levels, a pattern thought to reflect a trade-off between digestive efficiency and tissue maintenance costs. However, phylogenetic influences on this pattern have rarely been considered. Taxa that have undergone diversification accompanied by dietary shifts provide a powerful opportunity to examine the relationship between diet and intestine length while accounting for phylogeny.
2. In this paper we assess the relationship between diet and intestine length in the cichlid fishes of Lake Tanganyika, which are renowned for their diversity of species and trophic strategies.
3. First, we test the effect of trophic position on intestine length across 32 species, while controlling for phylogeny. Trophic position was inferred from nitrogen stable isotopes, which provide a temporally integrated, quantitative perspective on the complex diets of tropical fish. Second, we examine patterns of intraspecific variation in intestine length of an algivorous cichlid (Tropheus brichardi) along a natural spatial gradient in algal nitrogen content.
4. Trophic position explains 51% of size-standardized variation in intestine length after accounting for phylogeny. Accounting for phylogeny does not substantially alter the relationship between trophic position and intestine length, despite the existence of phylogenetic signal in both traits. Thus, diet is a strong predictor of variation at the interspecific level.
5. There is a striking inverse relationship between intestine length and algal nutrient content among populations of T. brichardi, suggesting substantial plasticity in response to food quality, and thus a strong dietary influence on patterns of intraspecific variation.
6. Diet is a strong predictor of intestine length at both intra- and interspecific scales, indicating that fish adjust their phenotype to balance nutritional needs against energetic costs. Furthermore, functional explanations for trophic diversification of cichlid fishes in the African Great Lakes have long focused on jaw structures, but our results indicate that intestinal plasticity in response to diet quality may also be an important mechanism for accommodating trophic shifts during evolutionary radiations.
Thus, we believe that the observed variation in T. brichardi intestine length is a largely plastic response to differences in the nutrient content of their algal diet. Both our broad phylogenetic survey and our intraspecific comparisons suggest that the intestine length of Tanganyikan cichlids is determined in large part by diet quality.
Ecological stoichiometry theory provides a framework for evaluating diet quality based on the degree of imbalance between the demand for a nutrient by a consumer and the nutrient content of its food resources (Sterner et al. 2002). Tropheus brichardi consumes algae that is very low in nitrogen (C : N > 18; Fig. 3) compared with its own body (C : N < 6, n = 4, P. McIntyre, unpublished data). In the light of this imbalance,the inverse relationship between algivore intestine length and algal nitrogen content suggests that longer intestines aid in extracting nutrients from low-quality foods. The overall inverse relationship between trophic position in the food web and gut length among 32 species of Tanganyikan cichlids (Fig. 2) also matches broad predictions from ecological stoichiometry.
The nutrient imbalance between fishes and their food resources increases from piscivores (no imbalance) to invertivores (moderate imbalance) to algivores (extreme imbalance), and we found that cichlid intestine lengths increase in the same order. The inverse relationship between intestine length and diet quality matches expectations from the general trade-off between maximizing the extraction of nutrients and energy from the diet and minimizing the maintenance costs of digestive tissues.
Digestive tissues are notoriously plastic in their responses to dietary change (Starck 1999). Plasticity in gut morphology has been observed in response to fasting, increases in food intake and changes in diet (e.g. Starck 1999; Naya, Karasov & Bozinovic 2007; Olsson et al. 2007)
BTW - your fish always look great, Duane! This isn't a case of a right or wrong way to feed these fish, I just don't want to see people stuffing their fish with low nutrient food such as they would consume in the wild, and sparing the higher quality nutrients that they need to thrive in captivity. Obviously you don't, but some of the more inexperienced members might if they felt that feeding lots of greens from their fridge was the ideal diet for their fish.