The morphology and the proximate body and fatty acid composition of the lamprey Geotria australis Gray

The colour, dentition and body shape of Geotria austvalis change markedly during adult life. The immature stages possess a smaller eye and oral disc and a longer and more slender trunk than other lampreys. Metamorphosis is generally initiated in January/February and is followed by a nocturnal downstream migration in July/August during heavy discharges of fresh water. G. australis enter metamorphosis at the relatively small length of 90 to 105 mm 00.9 g). At the commencement of the spawning run, the mean lengths and weights were approximately 640 mm and 230 g. Upstream migration in the river occurred mainly during rising water levels and on dark, rainy nights. The length remained constant throughout the 5-6 months of metamorphosis but declined markedly during the exceptionally long spawning-run period of approximately 16 months. However, the mean weight and condition factor fell during both these non-trophic periods. Although the total lipid content of ammocoetes fluctuated seasonally, it increased with body size, reaching levels of approximately 141 at the end of larval life before declining during metamorphosis to 8%. The very high lipid content in the muscle (25%) and liver (38-40%), as well as in the whole animal (21%) at the end of the marine trophic phase, reflect the need for extensive energy sources in the ensuing 16 months. Values for standard animals showed that some protein, as well as large amounts of lipid, were catabolised as an energy source during metamorphosis and the upstream migration. On the spawning-run, lipid and protein from various regions of the body were used for the massive development of the ovary which occurs during this period. The main depot lipid class of G. australis was triacylglycerol. The presence of larger proportions of polyunsaturated fatty acids in adults than in larvae reflect differences in diet. The appreciable amounts of 22:5 (n-3) and 22:6 (n-3) in the phospholipids of larvae and adults would be of advantage in low water temperatures. Exceptionally high levels of these acids were also found in ovarian triacylglycerols