An osmoconformer is an organism whose body fluid solute concentration conforms to or is the same as the solute concentration of the external medium in which the organism lives. This conformity occurs because the organism cannot regulate or keep the solutes of its body fluid at a concentration that is different from that of the external medium. The organism therefore has a solute concentration in its body fluids that is equal to the solute concentration of the external medium, most commonly seawater. Since the solutes of the organism's body fluids are the same concentration as those of the external medium, the organism is said to be isoosmotic. Organisms that are isoosmotic are at osmotic equilibrium with the external medium and do not gain or lose solutes or water. In this case, homeostasis of the solute concentration in the organism's body fluids depends on a relatively constant solute concentration in the external medium. In some habitats, however, environmental solute (salt) concentrations change regularly. For example, in an estuarine tidal marsh, periods of low tide tend to have lower salt concentrations whereas high tides have higher solute concentrations, so a sessile osmoconformer in this environment will have body fluid solute concentrations that change accordingly.
Osmoconformers cannot regulate the solutes in their body fluids at a concentration different from that of the external medium because their bodies are permeable and they do not have kidneys or other organs that allow them to produce a urine with lower or higher solute concentrations than the external medium. Having body fluids with higher solute concentrations than the external medium makes them hyperosmotic, whereas having body fluids with lower solute concentrations than the external medium makes them hypoosmotic.
If the solute concentration of the external medium is slowly increased and the solute concentration of the organism's body increases in exactly the same way, the organism is said to be an "osmometer", and the relationship between the solute concentration of the external medium and the osmometer creates an isoosmotic line on a graph. However, most osmoconformers are not osmometers because the solute concentrations of their body fluids are slightly above or below the isoosmotic line but parallel to it. The key feature of an osmoconformer is that the solute concentration of its body fluids shows a parallel relationship to the isoosmotic line as the solute concentration of the external medium slowly changes.
In contrast, if an osmoconformer is placed in an external medium and the solute concentration of that medium is quickly increased, the organism will not have enough time to conform to the external medium; the organism will then be hypoosmotic to the external medium and will gain solutes from the medium by diffusion and/or lose water to the medium by osmosis, causing its body to shrink. On the other hand, if an osmoconformer is placed in an external medium and the solute concentration of that medium is quickly decreased, the osmoconformer will not have enough time to conform to the external medium, will be hyperosmotic to the external medium, and will lose solutes to the medium by diffusion and gain water from the medium by osmosis and will bloat and potentially burst.
Soft-bodied aquatic animals such as worms and mollusks are osmoconformers. Some of them are capable of behavioral osmoregulation by regulating how fast or slowly the solute concentration of their body fluids encounters the solute concentration of the external medium.