The fraction of the total DNA used for coding proteins in eukaryotes is much less than in prokaryotes. Many eukaryotic species have nuclear DNA with an order-of-magnitude more base pairs than are required to encode all the different proteins. Note also that the size of the genome has little correlation with the complexity of the organism. This observation is referred to as the C-value paradox. The C value is the total number of DNA base pairs in the genome (per haploid set of chromosomes). When we compare this with the complexity of the organism, there is a massive discrepancy. Some species seem to have far too much DNA for their limited complexity. Humans have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), cows have 39, carp have 52, and alligators have 16. The sizes of chromosomes also differ markedly among species. And the largest number of chromosomes is found in a flowering plant. It is clear that the position of a species in the evolutionary tree or in the food chain has little bearing on the size of its genome.
