G. van den Thillart

Eel migration to the Sargasso: remarkably high swimming efficiency and low energy costs

SUMMARY One of the mysteries of the animal kingdom is the long-distance migration (5000–6000 km) of the European eel Anguilla anguilla L. from the coasts of Europe to its spawning grounds in the Sargasso Sea. The only evidence for the location of the spawning site of the European eel in the Sargasso Sea is the discovery by Johannes Schmidt at the beginning of the previous century of the smallest eel larvae (leptocephali) near the Sargasso Sea. For years it has been questioned whether the fasting eels have sufficient energy reserves to cover this enormous distance. We have tested Schmidts theory by placing eels in swim tunnels in the laboratory and allowing them to make a simulated migration of 5500 km. We find that eels swim 4–6 times more efficiently than non-eel-like fish. Our findings are an important advance in this field because they remove a central objection to Schmidts theory by showing that their energy reserves are, in principle, sufficient for the migration. Conclusive proof of the Sargasso Sea theory is likely to come from satellite tracking technology.

Behavioral strategy and the physiological stress response in rainbow trout exposed to severe hypoxia.

In higher vertebrates, two opposite behavioral coping strategies can be distinguished that are associated by a typical neuroendocrine pattern. Little is known about the individual variation in the stress response in lower vertebrates such as teleosts. In the present study, rainbow trout were fitted with an indwelling aortic catheter for repeated blood sampling and exposed to severe hypoxia and subsequent recovery and their behavior was characterized semiquantitatively during hypoxia. Blood levels of catecholamines, cortisol, glucose, FFA, lactate, and electrolytes were measured. About 60% of the fish survived the experiment whereas the others died during the recovery period. Behavioral strategy appeared to be highly related to survival since nonsurviving fish displayed strenuous avoidance behavior involving burst type activity whereas surviving fish did not panic and remained quiet. These behavioral differences were associated with marked differences in plasma catecholamine levels, which were 4- to 5-fold higher in nonsurviving fish as compared to survivors whereas the cortisol response tends to be lower in nonsurviving fish. Plasma lactate levels in nonsurvivors were 4- to 5-fold higher as compared to survivors while a severe hyperkalemia developed during recovery indicating the loss of intracellular homeostasis. The individual differences in behavioral concepts and neuroendocrine activation observed in rainbow trout during stress show great similarity with the active and passive coping strategies distinguished in higher vertebrates and may be determinant for survival during hypoxia.

Eel fat stores are enough to reach the Sargasso

It has long been assumed that the European eel (Anguilla anguilla) migrates to the Sargasso Sea — a region of the Atlantic Ocean between the Azores and the West Indies — to spawn1,2,3. During the past decade, however, the number of glass eels has inexplicably dropped4, and it has been suggested that a shortage of fat stores in adults, resulting from diminished food resources for juveniles in inland waters, may prevent the starving silver eels from reaching the spawning grounds4,5,6. But we find that the energetic cost of the 6,000-km migration is actually quite low, with 60% of the fat store remaining available for the developing gonads.It has long been assumed that the European eel (Anguilla anguilla) migrates to the Sargasso Sea — a region of the Atlantic Ocean between the Azores and the West Indies — to spawn. During the past decade, however, the number of glass eels has inexplicably dropped, and it has been suggested that a shortage of fat stores in adults, resulting from diminished food resources for juveniles in inland waters, may prevent the starving silver eels from reaching the spawning grounds. But we find that the energetic cost of the 6,000-km migration is actually quite low, with 60% of the fat store remaining available for the developing gonads.

Substrate mobilization and hormonal changes in rainbow trout (Oncorhynchus mykiss, L) and common carp (Cyprinus carpio, L) during deep hypoxia and subsequent recovery

Common carp (at 20°C) and rainbow trout (at 15°C) were fitted with an indwelling cannula in the dorsal aorta. The fish were exposed to a controlled decline of waterpO2 followed by 90 min deep hypoxia at 0.3 kPa (carp) or 4.8 kPa (trout). Thereafter, normoxic recovery was monitored in both species for 48 h. At regular intervals blood samples were analysed for glucose, lactate, free fatty acids, adrenaline, noradrenaline and cortisol. The oxygen restriction was maximal in both species and resulted in a significant increase of plasma lactate levels. In carp, adrenaline, noradrenaline and cortisol levels increased to 2, 50, and 753 ng·ml-1 respectively during anoxia, whereas in trout these hormones increased to 12, 8 and 735 ng·ml-1 respectively during hypoxia. In hypoxic trout, the plasma levels of glucose (3 mol·l-1) were increased modestly whereas levels of free fatty acids (0.25 mmol·l-1) were decreased to 0.15 mmol·l-1. In carp, however, a marked and prolonged hyperglycaemia (from 5 to 10 mmol·l-1) and a significant continuous depression of plasma levels of free fatty acids (from 0.4 to 0.2 mmol·l-1) were observed indicating a difference in metabolic organization. It is suggested that hyperglycaemia is likely to be the result of hepatic glycogenolysis, stimulated by circulating catecholamines and a stimulation of gluconeogenesis by cortisol during recovery. The mechanism for the decline of plasma levels of free fatty acids is most probably a reduction of lipolytic activity, which appears to be an adaptation to hypoxia.

Anaerobic energy-metabolism of goldfish, Carassius auratus (L.): Influence of Hypoxia and Anoxia on Phosphorylated Compounds and Glycogen

SummaryGoldfish, acclimated to 20°C and normal

Silvering of European eel ( Anguilla anguilla L.): seasonal changes of morphological and metabolic parameters

P_{O_2 }

The anaerobic energy metabolism of goldfish determined by simultaneous direct and indirect calorimetry during anoxia and hypoxia

= 130 mmHg) and low (