Unlike the other 4 species of O‘opu, Eleotris sandwicensis does not belong to the Goby family, and instead belongs to the family Eleotridae, also known as the Sleeper Fish. What distinguishes this fish from true gobies is that it lacks the sucking disk that is characteristic of all true gobies. Instead of being fused, the pelvic fins are separated. This is a good way to tell E. sandwicensis apart from A. guamensis, which it superficially resembles. Because E. sandwicensis lacks fused pelvic fins, it also lacks the ability to cling to rocks, meaning that it cannot climb to upper reaches of streams and is restricted to lower reaches and estuaries. Like S. hawaiiensis, E. sandwicensis can be found on all the Hawaiian islands, but is especially abundant on Oahu where it can be found in both altered and unaltered streams. E. sandwicensis is a carnivore. It is adapted for both picking-biting and sediment foraging, and has the ability to ingest large prey. It is believed that this fish feeds exclusively on fishes and invertebrates. A dietary study done on E. sandwicensis in Manoa stream found that although juvenile A. guamensis, Tilapia, and topminnows were present in large numbers, the food items taken most often were thiarid snails and Asiatic clams. Thus, as in the case of A. guamensis, introduced species may be having an effect on the feeding ecology of E. sandwicensis, but more work needs to be done to verify this possibility. There is no published work on the reproductive biology of E. sandwicensis. This creates a big hole in the current existing knowledge and understanding of Hawaiian stream gobies. Studies must be done on this species to further our understanding of the mechanisms involved in spawning behavior that affect community structure by reducing competition and aggression between the 5 species of Hawaiian stream gobies.