Some allow themselves to take up more water as seawater is diluted and even their cells increase in size to remain isotonic. The barnacle has an interesting way of avoiding freezing when exposed to cold temperatures. It encourages ice crystal formation in intercellular spaces which concentrates the salts inside cells which in turn protects them from freezing. The cells may become quite hypertonic to seawater in a sense dehydrated. Most fresh water invertebrates have osmotically low internal fluids to reduce the gradient with their environment.
Mollusks can survive very dilute body fluids.
Often these are on the gill or skin. All fresh water invertebrates are osmotic regulators. They often use excretory organs to filter body fluids and then retain salts as the fluid passes down a long tube, much like nephrons. Earthworms have two nephridia in each segment that work on this principle. The nephridia are a tube leading to the animals surface whose other end is an open ciliated funnel that collects body fluid.