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Inorganic and organic constituents were studied on blood serum collected from a living specimen of the coelacanth, Latimeria chalumnae.Inorganic electrolytes determined included sodium (196.7 mM/l), potassium (5.78 mM/l), magnesium (5.30 mM/l), calcium (4.94 mM/I), chloride (186.7 mM/l), bicarbonate (9.60 mM/l), phosphate (5.08 mM/l), and sulfate (4.80 mM/l). Serum urea (377 mM/l) and trimethylamine oxide (122 mM/1) were high as previously reported, and accounted for the bulk of the total non-protein nitrogen (1199 mg%)); total amino acids added a small but not insignificant fraction (21.9 mg%). High serum lactate (16.5 mM/1) and glucose (6.57 mM/1) levels were probably indicative of stress; glucose was the only carbohydrate present in appreciable amounts in the serum, although traces of glucuronic acid and rhamnose were found. Serum total cholesterol was 3.91 mM/l, organically bound phosphorus 1.99 mM/1 and total proteins 2.84 g % . Three major protein fractions were evident from cellulose acetate electrophoresis and at least 11 peaks were demonstrable by acrylamide gel electrophoresis. Latimeria serum lacks a protein component with a mobility approaching that of human serum albumin, Serum osmolarity (932 mOsm/l) was somewhat lower than that of sea water collected at the site of capture of' the specimen (1035 mOsm/l). Evolutionary implications of the similarity of Latimeria scrum chemistry to that of other marine fishes are discussed.

The previously reported hyperglycemia induced in Fvndulus heteroclitus by exposure to the subzero cold was investigated in more detail. It was found that the hyperglycemic response was elicited immediately when fish were transferred to water at -1.5"C and that the striking increase i n serum glucose was accompanied by a progressive depletion of hepatic glycogen. Muscle glycogen occasionally, but not always, showed a slight depletion after long exposure to the subzero cold. Serum glucose was not maintained at high levels indefinitely in the subzero cold. After hepatic glycogen was totally depleted, serum glucose levels began to drop. When serum glucose levels fell to normal, the fish began to die. Addition of glucose to the water elevated serum glucose levels and enhanced survival in the subzero cold. Winter fish, which produce higher levels of serum glucose in the subzero cold than do summer fish, survived the subzero cold almost twice as long as did summer fish. Serum nonglucose free carbohydrate levels were unaffected by the length of exposure to subzero temperatures. Liver protein levels were unaffected by temperatures from 20°C to -1.5"C and muscle protein levels decreased slightly only after long exposure to the subzero cold.

Physiochemical properties (serum osmolality and blood pH), serum inorganic ions (sodium, potassium, calcium, magnesium, chloride, phosphate and bicarbonate) and tissue water were studied in parallel groups of adult male Fundulus heteroclitus acclimated to various temperatures (30"C, 20°C, 10°C, 4"C, 2"C, -1°C and -1.5"C) in salt water under otherwise constant laboratory conditions. When the acclimation temperature was lowered from 20°C to -1.5"C, serum osmolality increased by 20%. This increase was not indicative of osmoregulatory failure, however, since the proportion of total serum osmolality accounted for by inorganic electrolytes dropped from 98% at 20°C to 93% at -1.5"C. Serum electrolytes such as sodium, chloride, calcium, magnesium and bicarbonate increased in the subzero cold by 12%, 17%, 30%, 33% and 11% respectively whereas serum potassium and inorganic phosphate levels were unchanged. Blood pH was significantly higher at 10°C than at any other temperature. The water content of liver, testes and muscle decreased by 8% in the subzero cold. These changes in serum electrolytes and tissue water did not indicate osmoregulatory failure, however, since the new levels, once established, were maintained as long as the fish were alive. Although osmotic and ionic regulation were not as effective in the cold, they were by n o means so poor as to cause death by osmotic imbalance.Two common causes of death jn fish at low temperatures are osmoregulatory failure and, at temperatures below freezing, the destruction of tissues by the formation of ice (Fry, '67). However, not all fish die in the cold and many can survive at subzero temperatures. For example, investigations by Scholander et al. ('57) and Umminger ('67) showed that the killifish, Fundulus heteroclitus, could survive the subzero cold in a supercooled state. To gain deeper insight into the mechanisms permitting survival of the killifish at subzero temperatures, then, at least two aspects of cold acclimation should be studied : the ability to osmoregulate in the cold and the means to avoid the formation of ice in the supercooled blood.The present paper deals with osmotic and ionic regulation by the killifish in the cold whereas a paper to follow will deal with the problem of survival in a supercooled state. However, in examining both aspects of survival at subzero temperatures, the same approach has been used: a detailed chemical analysis of the serum.J, EXP. ZOOL., 172: 283-302.In most cases, chemical analyses of both inorganic and organic constituents were made on the same serum sample. In the present paper, however, only the inorganic components of the serum are discussed since they have the greatest bearing on the problem of osmoregulation. In the paper to follow, data on the organic serum constituents are presented to complete the study of the serum chemistry at subzero temperatures. MATERIALS AND METHODSAdult male F. heteroclitus, captured at small inlets of Long Island Sound in the vicinity of New Haven, Connecticut, were maintained on an 8-ho...