TAURINE IN MARINE INVERTEBRATES
235
species organic nitrogenous substances are present in much the same
concentration as in Carcinus (Shaw, 1958b). It seems likely that
Carcinus utilizes a situation which already exists. The reason for the
presence of organic nitrogenous compounds is possibly morc obscure
than a t first sight. The ionic composition of the muscles of marine
animals is fairly similar t o that found in many terrestrial and frcshwatcr animals, and it may be that there is an optimum ionic conccntration for striated muscle fibre. I n marine animals therefore, thc high
osmolar concentration is made up by the addition of organic nitrogen
compounds and the development of the ability to regulate these compounds makes it possible for a marine animal that is unable to osmoregulate its internal environment (e.g. Arenicola) t o penetrate into brackish
watcr. I n animals like Carcinus which osmoregulate the blood, the two
processes possibly supplement each other (Shaw, 1958b).
The remarkable efficiency of the regulation in Carcinus was shown
by Duchiiteau et al. (1959) studying the response t o the change in
osmotic pressure produced by moulting. At emergence the blood
osmotic pressure is suddenly decreased, yet the decrease of intracellular
amino acids in response to this is sufficiently rapid to prevent any
hydration of the muscles. They too conclude that this regulation must
be to maintain the inorganic composition of the cells in the presence of
changcs in the osmotic pressure of the surrounding fluid. Shaw (1959)
studied the effects of changing the salinity of the surrounding medium
of an animal which would not be expected t o possess this intracellular
osmotic regulation. He adapted the freshwater crab Potamon niloticus
(Milne-Edwards) t o different concentrations of sca water and found
that up to 50% salinity there was not as much dehydration of the
muscles as would be expected by simple osmosis. This implies some
small addition of osmotically active substances to the muscle and in
fact there is a slight increase in intracellular amino acid concentration
up t o 75% salinity. Above this salinity, however, no more amino acid is
added. Osmotic regulation of the internal medium has been shown t o
occur in Astacus (Duchiiteau and Florkin, 1961), Nereis (Jeuniaux et al.,
1961a), Leader (Jeuniaux et al., 1961b) and Eriocheir (BricteaxGrBgoire et al., 1962) and complete poikilosmosity, i.e., no regulation
of the internal medium, in Arenicola (Duchiiteau et al., 1961), Perinereis
(Jeuniaux et al., 1961a), Mytilus (Bricteax-GrBgoire et al., 1964a) and
Gryphuea (Bricteax-GrBgoire et al., 1964b). I n all these animals aminoacid concentration increases with increasing salinity, the increase bcing
greater than would be accounted for by changes in hydration. The
amino acids which show the greatest changes in concentration are
alanine, glycine, glutamine and glutamic acid, proline and arginine,
235
species organic nitrogenous substances are present in much the same
concentration as in Carcinus (Shaw, 1958b). It seems likely that
Carcinus utilizes a situation which already exists. The reason for the
presence of organic nitrogenous compounds is possibly morc obscure
than a t first sight. The ionic composition of the muscles of marine
animals is fairly similar t o that found in many terrestrial and frcshwatcr animals, and it may be that there is an optimum ionic conccntration for striated muscle fibre. I n marine animals therefore, thc high
osmolar concentration is made up by the addition of organic nitrogen
compounds and the development of the ability to regulate these compounds makes it possible for a marine animal that is unable to osmoregulate its internal environment (e.g. Arenicola) t o penetrate into brackish
watcr. I n animals like Carcinus which osmoregulate the blood, the two
processes possibly supplement each other (Shaw, 1958b).
The remarkable efficiency of the regulation in Carcinus was shown
by Duchiiteau et al. (1959) studying the response t o the change in
osmotic pressure produced by moulting. At emergence the blood
osmotic pressure is suddenly decreased, yet the decrease of intracellular
amino acids in response to this is sufficiently rapid to prevent any
hydration of the muscles. They too conclude that this regulation must
be to maintain the inorganic composition of the cells in the presence of
changcs in the osmotic pressure of the surrounding fluid. Shaw (1959)
studied the effects of changing the salinity of the surrounding medium
of an animal which would not be expected t o possess this intracellular
osmotic regulation. He adapted the freshwater crab Potamon niloticus
(Milne-Edwards) t o different concentrations of sca water and found
that up to 50% salinity there was not as much dehydration of the
muscles as would be expected by simple osmosis. This implies some
small addition of osmotically active substances to the muscle and in
fact there is a slight increase in intracellular amino acid concentration
up t o 75% salinity. Above this salinity, however, no more amino acid is
added. Osmotic regulation of the internal medium has been shown t o
occur in Astacus (Duchiiteau and Florkin, 1961), Nereis (Jeuniaux et al.,
1961a), Leader (Jeuniaux et al., 1961b) and Eriocheir (BricteaxGrBgoire et al., 1962) and complete poikilosmosity, i.e., no regulation
of the internal medium, in Arenicola (Duchiiteau et al., 1961), Perinereis
(Jeuniaux et al., 1961a), Mytilus (Bricteax-GrBgoire et al., 1964a) and
Gryphuea (Bricteax-GrBgoire et al., 1964b). I n all these animals aminoacid concentration increases with increasing salinity, the increase bcing
greater than would be accounted for by changes in hydration. The
amino acids which show the greatest changes in concentration are
alanine, glycine, glutamine and glutamic acid, proline and arginine,