The Quest For Fat

Lipids are generically known as fat. They are the main energy reserve in animals and critical to the survival, fitness, reproduction, and recruitment of fish. Since the ultimate goal of an animal is to reproduce and pass its genes onto future generations, the rest can be considered details. Lipids are the underlying fuel that all organisms use to this end. Lipids pack a large energetic punch - a large amount of calories are stored in a small amount of space. In many animals like fish lipids can broadly be considered the storage fuel needed for significant energetically costly events – the afterburner if you will. A bear needs to amass a large amount of fat – which will be slowly converted to energy throughout the winter. Similarly, a salmon needs to build up a surplus lipid reserves if it is to successfully migrate hundreds of miles without feeding, attract a mate, convert some of its energy into gametes, and spawn.


Figure 1. A lipid molecule.

Lampreys are unique in that they undergo metamorphosis. Pacific lamprey transform from larvae to juveniles, which outmigrate to the ocean, parasitically feed on other fishes, and return to freshwater as adults to spawn. Western brook lamprey transform directly from larvae to adults – which will never feed again – and spawn and die. The process of metamorphosis itself is energetically costly. Internal organs rearrange and change, external anatomy changes too. This takes energy. Larval lampreys gain their nutrition (and ultimately lipids) through filter feeding detritus. Furthermore, lampreys don’t feed during transformation; Pacific lamprey will resume feeding once transformed as they approach or enter the ocean and western brook lamprey will never feed again! In theory, one should see a buildup of lipids just prior to metamorphosis – and this should be measurable. It is plausible that western brook lamprey may need even more lipids – as they will never feed again and acquire new energy.


Figure 2. Different lifestages of lamprey.

We have been evaluating lipid extraction techniques and lipid dynamics in larval Pacific lamprey and western brook lamprey. Unfortunately the lipid extraction procedure is lethal. The larval lamprey are euthanized and then homogenized into a paste. A mixture of chloroform and methanol is used to extract the lipids. A characteristic of lipids is that they are not soluble in water (this is why fat globules float on water) but they are in other similar organic solvents (e.g., chloroform, ether, acetone). Once the lipids are extracted they are weighed and expressed as a percentage of the entire body weight.


Figure 3. Solution with extracted lipids.

Knowledge of lipid content can also give a measurement of “condition” or well-being of a fish. In the fish world, the fatter the fish is usually considered healthier. Lampreys with higher lipid content would be considered healthier. We also measure condition with non-lethal means by calculating the weight-to-length ratio of the fish. In addition, we are investigating a technique for measuring body density. We use hydrostatic weighing, which is essentially weighing a fish in water, calculating the amount of water displaced, thereby calculating density. We are comparing these different measures to see if they are related and if so, we may be able to use a nonlethal technique to measure condition in lamprey.


Figure 4. Weighing a larval lamprey in water to calculate body density.

So far we haven’t been able to detect a strong pattern of correlation among the different techniques to measure condition. We have noticed that small and large western brook lamprey have higher lipid content than do similar sized Pacific lamprey. This would support the notion that western brook lamprey need to have more lipids in preparation for metamorphosis. These lampreys were collected in the fall, presumably after the time that any larvae that were going to transform would have done so. Essentially we examined larvae that were not likely to transform that year. We are going to examine lipids from another group of lamprey that were collected in the summer. Some of these individuals should be ready to transform and we might be able to see more clear relationships of lipid content.


Figure 5. Lipid content in larval Pacific lamprey and western brook lamprey.

Submitted by Jeff Jolley