To increase the likelihood and probability of finding veligers within a sample, it is often advantageous to use a sugar solution. The sugar increases the density of the solution and the organisms begin to sink at different rates. The greater the mass of the organism, the faster it will sink to the bottom. The veligers, because their mass is greater than most of the contents, will sink at a faster rate and reach the bottom first, usually within 20 min. Although this method is highly effective in gathering the majority of veligers within the sample, it is not capable of separating all of the veligers. Also, it is important to note that in samples containing a large amount of algae, this method proves ineffective. The algae interfere with the settlement of the veligers in the sugar solution.
To prepare a sugar solution, dissolve 130 g of granulated sugar in 400 ml of distilled water. Using a pipette, draw 10 ml of plankton subsample and then 15 ml of sugar solution. Wait 20 min to allow the veligers to sink to the bottom of the pipette. If using a burette, pour 15 ml of sugar solution into the burette and then add the 10 ml of plankton subsample. Again, wait 20 min to allow the veligers to settle to the bottom. Drawing 1-2 ml from the bottom should yield a concentration of veligers.
The veliger samples are examined under a dissecting microscope with ³40X magnification.