The cartilaginous fishes which excrete urea are often the salt water fishes. They will often invest some energy to convert the ammonia into urea, as urea is much less toxic compared to ammonia. Though they use energy for this process, urea has several advantages which compensates for the energy loss. Because it is less toxic, it can be allowed to accumulate in the blood to some extent, and many organisms have specialized organs to remove urea and other wastes from the blood and excrete them. Urea is commonly used as an excretory product in vertebrates, and is rarely used in invertebrates. Some organisms, such as sharks and snails, allow urea to accumulate in their blood to help with overall osmotic balance. Whales, for instance, use urea in the blood to make them hyperosmotic in relation to seawater, thus they tend to gain water from the ocean and do not have to worry about dehydration.