Three general types of swimbladders were found in the eight species of myctophids studied: gas-filled, fat-invested, and atrophied or reduced. Small specimens of all species had thin-walled, gas-filled swimbladders. Large specimens of Stenobrachius leucopsarus had fat-invested swim-bladders and large Diaphus theta had either gas-filled or atrophied swimbladders, as found by other workers. Large Tarletonbeania crenularis had either gas-filled or reduced swimbladders, large Lampanyctus ritteri and L. regalis had reduced swimbladders, and large Stenobrachius nannochir had fat-invested swimbladders. Protomyctophum thompsoni and P. crockeri retained gas-filled swimbladders.High body lipid content was found in S. leucopsarus, S. nannochir, L. ritteri and D. theta, and low lipid content was found in the other four species. Myctophids with high lipid content had specific gravities close to that of sea water (1.026–1.030). Tarletonbeania crenularis with a reduced swimbladder had a specific gravity of 1.088. Lampanyctus regalis had a lower specific gravity (1.040) due to high water content of the tissue.The swimbladder to body volumes in S. leucopsarus and D. theta were inversely related to body size and lipid content, indicating that lipids assume the primary buoyancy function as the gas-filled swimbladder regresses with age. This change may eliminate the physiological constraints imposed by a gas-filled swimbladder and permit the more extensive diel vertical migrations of adults
