The Role of Methylamines in Maintenance of the Osmotic Homeostasis of Cyclostomes and Bony Fishes

This article is the second part of the review of existing literature considering the roles methylamine compounds play mainly in osmoregulation and also in some other adaptations (such as thermal protection and adaptation to depth) of fishes. This part focuses on cyclostomes (hagfishes and lampreys) and bony fishes. Methylamines considered are: trimethylamine oxide (TMAO), betaine and glycerophosphocholine (GPC). The survey of wide range of methylamines contents across the examined taxa was made. Changes of those contents and corresponding enzyme activities in fish transferred to different salinities were analyzed. The common tendencies are: methylamines are usually used by marine but not freshwater organisms; methylamines are the main osmolyte class in organisms accumulating urea, i. e. ureotelic. Hagfish use TMAO as intracellular osmolyte. There is fairly no data on methylamines in lampreys in literature but general considerations suggest they do not accumulate any methylamines. Methylamines are not the main osmolyte class in bony fishes but TMAO and betaine play secondary role in some marine teleosts: Gadidae, Scorpaenidae, herring, mummichog, three-spined stickleback and others. Many marine and euryhaline teleosts can synthesize TMAO from trimethylamine (TMA) not only in their livers but also in osmoregulatory organs — kidney and gills. Freshwater teleosts do not use methylamines as osmolytes except several cases when small concentrations of TMAO or betaine emerged in their tissues after transfer to saline water. There are controversial results regarding betaine as part of FinnStim feed supplement used for commercial fish rearing: some investigators consider betaine in FinnStim enhancing salinity adaptation of fish, others assume it to be just an appetite stimulant. GPC plays minor role as osmolyte in some bony fishes as tilapia and eel.