Deep, cold sea water has a density of 1.028 g/cm-3 where as cell cytoplasm has a density of 1.02-1.10 g/cm-3 combined with bones and skeletal tissue means that the majority of marine organism sink. (Willmer 2005) All marine organisms have adaptations that enable them to either resist pressure when they sink or stay afloat, as the depths of the deep sea is fatal for most marine species with adaptations for staying up is common. In the deep sea there is no light however and so what is up, is often a vague concept for most deep sea species, and for species that reside in the bathypelagic zone going too far down is fatal however they have adaptations to help with this.
Many species of copepod larvae have large trailing threads that help resist water flow (Willmer 2005) this is an adaptation that to extends to other animals however, many species of cnidarians have much longer tentacles or extensions that provide the same effect of increasing water resistance. Trochophore larva have extensive chaetae spines that provide increased water resistance as well as other crustacean larvae that often become flattened or elongated.
Many species simply have variance in chemical composition to achieve neutral buoyancy which is the desired outcome as once an individual has become neutrally buoyant they are free to move without sinking or rising that can cause decompression which can also be fatal. There are 3 main chemical methods that are employed: Reduction or substitution of heavy ions, losing ions without substitution and reduction of heavy minerals.
Species such as Aurelia spp. show replacement of heavy ions where they are found with unusually low sulphate levels and instead are found with high levels of chloride being as a substitute. Other animals have been found to also do this, such as ctenophores, tunicates and molluscs (Willmer 2005) an extreme example of this is Heliocranchia spp. a deep sea squid that has 65% of its body mass in a large fluid filled cavity called the pericardial cavity, of this almost every catatonic component such as magnesium and sodium has been replaced almost exclusively with ammonium ions. (Willmer 2005). Vertebrates rather than substitute ions which is commonly an invertebrate adaptations will rather exclude certain ions altogether becoming hyposmotic, this is a common method used in teleost eggs, however extreme adaptations usually involve reducing heavy materials such as calcium carbonate and phosphate in skeletal tissue. This has led to many pelagic gastropods reducing their entire shells and become very fragile, these are commonly known as “sea butterflies” or Thecosomata.