Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater. The size of the olfactory bulb varies across different shark species, with size dependent on how much a given species relies on smell or vision to find their prey. In environments with low visibility, shark species generally have larger olfactory bulbs. In reefs, where visibility is high, species of sharks from the family Carcharhinidae have smaller olfactory bulbs. Sharks found in deeper waters also have larger olfactory bulbs.