All of this sea water in contact with so many parts of the animal has its price however. This means that the concentration of salts in their body is equal to the concentration of salts of the sea water in which they live. Many invertebrates have this feature, however echinoderms also have lots of tissue in contact with the water. As a result, these animals are very sensitive to changes in salinity. This is one of the most compelling reasons as to why there are no echinoderms in fresh water.
Most echinoderms are of one sex, and reproduce by shedding their gametes directly into the water.
The gametes soon fuse, and the developing embryo spends three days to three months floating in the water column. It then metamorphosises into a small replicate of the adult. Some females of certain species will retain the eggs in their body, and collect the sperm from to water. They will then brood the developing ju- venile for some time, ultimately releasing small replicates of themselves. In addition many species of starfish and brittle stars can reproduce by breaking off a small piece and having this piece generate a whole new individual.