The lipids of some lantern fishes (family Myctophidae)

Detection without deflection? A hypothesis for direct sensing of sound pressure by hair cells

It is widely thought that organisms detect sound by sensing the deflection of hair-like projections, the stereocilia, at the apex of hair cells. In the case of mammals, the standard interpretation is that hair cells in the cochlea respond to deflection of stereocilia induced by motion generated by a hydrodynamic travelling wave. But in the light of persistent anomalies, an alternative hypothesis seems to have some merit: that sensing cells (in particular the outer hair cells) may, at least at low intensities, be reacting to a different stimulus - the rapid pressure wave that sweeps through the cochlear fluids at the speed of sound in water. This would explain why fast responses are sometimes seen before the peak of the travelling wave. Yet how could cells directly sense fluid pressure? Here, a model is constructed of the outer hair cell as a pressure vessel able to sense pressure variations across its cuticular pore, and this 'fontanelle' model, based on the sensing action of the basal body at this compliant spot, could explain the observed anomalies. Moreover, the fontanelle model can be applied to a wide range of other organisms, suggesting that direct pressure detection is a general mode of sensing complementary to stereociliar displacement.

Wax ester biosynthesis in the liver of myctophid fishes

The biosynthetic properties of wax esters in the liver were compared between two types of myctophid fishes having different body lipid composition, i.e., three triglyceride-rich species (Lampanyctus jordani, Diaphus theta, and Symbolophorus californiensis) and three wax ester-rich species (L. regalis, Stenobracius nannochir, and Stenobracius leucopsarus). n-Heptadecanol (17:0-ALC) and/or 10-cis-heptadecenoic acid (17:1-ACID) was incubated with liver homogenate of the six myctophid fishes and with co-factors such as NADPH and ATP for 2 to 5 h. Considerable amounts of wax esters with odd-numbered fatty acids and/or alcohols were produced in the liver homogenate of the wax ester-rich species. Stenobracius nannochir and L. regalis, which exclusively contained wax esters as neutral lipids, showed the highest activity of wax ester synthesis, followed by S. leucopsarus, which contained triglyceride as the minor constituent. Only trace amounts at most of odd-numbered fatty acids and alcohols were incorporated into the wax esters after incubation with the liver homogenates of the triglyceride-rich fishes. Active interchange between the fatty acids and the alcohols occurred during wax ester biosynthesis in the wax ester-rich fishes. The chain elongation and shortening of acyl moieties were also observed during incubation. These results suggested that the deposition of lipids in myctophid fishes is mainly due to their biosynthetic activities.

Lipid content and energy density of forage fishes from the northern Gulf of Alaska

Piscivorous predators can experience multi-fold differences in energy intake rates based solely on the types of fishes consumed. We estimated energy density of 1151 fish from 39 species by proximate analysis of lipid, water, ash-free lean dry matter, and ash contents and evaluated factors contributing to variation in composition. Lipid content was the primary determinant of energy density, ranging from 2 to 61% dry mass and resulting in a five-fold difference in energy density of individuals (2.0-10.8 kJg(-1) wet mass). Energy density varied widely within and between species. Schooling pelagic fishes had relatively high or low values, whereas nearshore demersal fishes were intermediate. Pelagic species maturing at a smaller size had higher and more variable energy density than pelagic or nearshore species maturing larger. High-lipid fishes had less water and more protein than low-lipid fishes. In some forage fishes, size, month, reproductive status, or location contributed significantly to intraspecific variation in energy density. Differences in quality are sufficient to potentially affect diet selection of breeding seabirds, especially when transporting food for their young to the nest site.

The high content of monoene fatty acids in the lipids of some midwater fishes: family Myctophidae

The total lipids of eleven species of Myctophids caught at depths between 20 and 700 m in the northern Pacific Ocean were analyzed using silicic acid column chromatography (lipid classes) and capillary gas chromatography (fatty acid and fatty alcohol composition). The major components in the lipid classes were triacylglycerols or wax esters; triacylglycerols were the dominant acyl neutral lipids (68.1-96.1%) in eight species, and wax esters were found as the dominant lipid (85.5-87.9%) in three species. The major fatty acids and alcohols contained in the wax esters of the three fishes were 18:1n-9, 20:1n-9, 20:1n-11, and 22:1n-11 for fatty acids, and 16:0, 18:1, 20:1 and 22:1 for fatty alcohols. Fatty acids in the triacylglycerols ranging from C14 to C22 were predominantly of even chain length. The major components were 16:0, 16:1n-7, 18:1n-9, 20:1n-11, 22:1n-11, 20:5n-3 (icosapentaenoic acid), and 22:6n-3 (docosahexaenoic acid). In both the triacylglycerols and the wax esters, the major fatty components were monoenoic acids and alcohols. It is suggested from the lipid chemistry of the Myctophids that they may prey on the same organisms as the certain pelagic fishes such as saury and herring, because the large quantities of monoenoic fatty acids are similar to those of saury, herring, and sprats whose lipids originate from their prey organisms such as zooplanktons which are rich in monoenoic wax esters.

The endemic faunae of Lake Baikal: their general biochemistry and detailed lipid composition

The biochemical composition of the important endemic faunal groups in Lake Baikal, Siberia, have been examined. The major biochemical components were determined and detailed analyses of the lipid fractions are presented. The lipids are characterized by high levels of long-chain omega 3 polyunsaturated fatty acids in the glyceride-esters, the presence of large amounts of triglyceride in pelagic and bathy-pelagic species with a complete absence of wax esters, and a very simple sterol composition dominated almost exclusively by cholesterol. The results are discussed in relation to present knowledge concerning both marine and freshwater lipids and the possible origin and evolution of the Baikalian fauna. Present theories concerning the likely role of long-chain polyunsaturates and wax esters in lipids are considered in the light of these present results and a link is suggested between sterol diversity and type of evolution.

Occurrence of wax esters in the tissues of the orange roughly (Hoplostethus atlanticus)

The skin, skeleton and a fat-filled swim bladder of the orange roughly (Hoplostethus atlanticus) each contained greater than 20% lipid by wet weight which was almost entirely wax esters. These had carbon numbers of 34-40 consistent with the major fatty acid being 18:1 and the major fatty alcohols being 16:0, 18:1, 20:1 and 22:1. In contrast, the liver and the roe contained appreciable quantities of glycerolipids with 18:1 and 22:6 as the major fatty acids.

Buoyancy adaptations in a swim-bladderless Antarctic fish

The endemic Antarctic teleosts of the suborder Notothenioidei are bottom dwellers. They lack swim bladders, are heavier than seawater, and feed on or near the bottom. The midwaters surrounding the Antarctic continent are productive and underutilized by fishes. There is an evolutionary trend toward pelagism in some notothenioids. We discovered that the largest Antarctic fish, Dissostichus mawsoni, was neutrally buoyant. Attainment of neutral buoyancy was associated with specializations of the skeletal, integumentary, muscular, and digestive systems. The skeleton had a low mineral content and contained considerable cartilage. Scales were also incompletely mineralized. Static lift was obtained from extensive lipid (mostly triglyceride) deposits. A 2-8 mm subcutaneous lipid layer accounted for 4.7% of the body weight. White muscle also contained much lipid-23% on a dry weight basis, or 4.8% of the body weight. Microscopic examination suggested that the liver was active in lipid metabolism, although it was not an organ of buoyancy. Stellate (perisinusoidal) cells with many lipid droplets were a very prominent cytological component of the liver. These specializations made Dissostichus neutrally buoyant and capable of inhabiting the food-rich Antarctic midwaters.