During certain stages in an animal's life cycle, energy requirements may exceed energy intake from the diet. The spawning migration of temperate eels is a textbook example of negative energy balance, forcing these fish to rely on stored fats (triacylglycerides) to provide their muscles with energy for swimming and their growing oocytes with the nutrients needed to develop and support healthy offspring. We predicted broad implications of this great need for endogenous triacylglycerides in terms of their packaging, transport, and ovarian uptake. To test this, serum lipid concentrations and transcript abundances of intestinal and hepatic triacylglyceride packagers and ovarian triacylglyceride modifiers and receivers were investigated throughout previtellogenesis (feeding phase) and into early vitellogenesis (fasting phase) in short-finned eels. A switch from exogenous to endogenous triacylglyceride packaging was seen as the liver upregulated transcript levels of apolipoprotein B and microsomal triacylglyceride transport protein and downregulated those of apolipoprotein E and lipoprotein lipase. In the intestine, the reverse response was observed. Furthermore, ovarian transcript abundances of triacylglyceride modifiers and receivers increased (apolipoprotein E, lipoprotein lipase, and vitellogenin receptor), indicative of increased triacylglyceride uptake during previtellogenesis. We propose that increased hepatic apolipoprotein B production is a conserved vertebrate response to prolonged periods of negative energy balance.

A total of 691 specimens (114 males, 538 females, and 39 immature specimens) were collected from Bushehr coastal waters of the Persian Gulf from February 2007 to February 2008 to study the reproduction of this species along with its histological gonadal development. Mean absolute and relative fecundity was 263,162 ± 31,046 and 273 ± 27, respectively. The oogonia and oocyte diameter ranged between 6 µm and 875 µm with a mean of 318 µm. The relationship between fecundity and total weight was F = 192.85W + 69,291. Monthly changes in the gonadosomatic index exhibited a higher value in May and October in both sexes (P ≤ 0.05). Observations on the seasonal distribution of maturity stages and seasonal fluctuations in the gonadosomatic index confirmed recent findings that the spawning periods have 2 peaks, a higher peak in May and a lower peak in October. The hepatosomatic index and gonadosomatic index fluctuations were similar in females but different in males. Males and females of Saurida tumbil reach first sexual maturity at 25.5 and 27 cm, respectively. The sex ratio was 1M:5F (P ≤ 0.001). The simultaneous presence of postovulatory follicles and yolk globules in some ovaries indicated that this species is a multiple spawner.

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