Some of the most vibrant members of the class Cephalopoda, cuttlefish are soft-bodied invertebrates with many remarkable morphological structures and functional adaptations. They are often mistaken for squid, which also possess an internal shell and a set of anterior tentacles. In comparison, cuttlefish have more rounded bodies owing to the structure of their cuttlebone. This chambered “backbone” helps them maintain buoyancy in their natural environment.
Like squid and octopuses, cuttlefish are restricted to marine and brackish habitats. Though they may be washed into estuarine zones or coastal lakes, they are seldom in these brackish zones for lengthy periods of time. These intelligent animals are built for gliding through relatively shallow, marine waters, where they use their tentacles to catch prey and quickly evade predators. It is highly unlikely that they would voluntarily swim into freshwater areas, even in search of food, due to their physiology.
A Closer Look at Cuttlefish
There are around 120 extant species of cuttlefish distributed throughout the coastal zones and reefs of tropical to temperate regions. The common cuttlefish (Sepia officinalis) is often what first comes to mind when one is made to think about these quirky marine mollusks. Native to European and African waters, this particular species migrates between inshore and deeper coastal waters. It can actually survive in brackish water, where it may regularly spawn in spring to summer.
Researchers have looked into the effects of salinity levels on the hatching rate of common cuttlefish eggs. They found that this parameter interacted with water temperature to significantly influence hatching. Across all boards, cuttlefish eggs (of the more commonplace species, at least) are unlikely to hatch in salinities that dip to below 27 ppm. For this reason alone, it is highly unlikely that their generations can be sustained in low-salinity environments.
One of the most popular species of cuttlefish carried in pet stores is Sepia bandensis. Known as the dwarf cuttlefish, it has to be reared in salinity levels that are as close to that of seawater as possible – ideally around 32 – 34 ppt. Though slightly lower values may be tolerated, especially if all other parameters are optimized, it is highly unlikely that any form of acclimatization would allow these animals to survive in freshwater.
Cuttlefish Are Osmoconformers (Built for Marine Environments)
A major reason behind a cuttlefish’s inability to survive in freshwater lies in its inability to regulate its body osmolarity. All cuttlefish are osmoconformers. This means that the salt concentration of their internal fluids matches that of their external environment. Osmoconformers are quite common in marine environments, but they are hardly ever found in freshwater habitats, which are dominated by osmoregulators.
Cuttlefish maintain a delicate equilibrium with their surrounding waters. As osmoconformers, they passively adjust their internal salt concentrations through osmotic diffusion – meaning no excess energy is spent. This adaptation is highly effective in stable salinities. In aquatic habitats where salt levels are significantly lower, their bodies would continually lose salt and water in an effort to mirror dilute concentrations in freshwater.
What Would Happen if You Put a Cuttlefish in Freshwater?
Introducing a cuttlefish to a freshwater environment would initiate a series of cascading consequences, primarily due to the significant difference in salinity between its internal fluids and the surrounding water. Due to osmotic imbalance, water would rush into the cuttlefish’s cells and cause them to expand.
The influx of water would affect the proper functioning of vital organs. Their gills could become damaged due to swelling. Their kidneys, tasked with regulating electrolyte levels, would work overtime to expel the excess water. This could potentially lead to kidney damage and an increased need for excretion. Paradoxically, while the cells would be near-to-bursting, the cuttlefish may also experience dehydration.
Simply put, placing a cuttlefish in freshwater could shock its organs and cause its death. This should never be done, even as an experiment, as it would subject the animal to considerable stress and physiological challenges. Like other cephalopods, cuttlefish are highly specialized for marine environments and are not equipped to cope with the distinct demands of freshwater habitats.
