Ethanol: a better preservation technique for Daphnia

Plankton ecologists often kill and fix zooplankton specimens for future study. The standard protocol used by limnologists is a sucrose-formalin solution first recommended by Haney and Hall (1973). This solution is an aqueous solution of 4% formalin (final concentration) and 40 g of sucrose per liter. Commercially available formalin is a 37% by weight, or a 40% by volume, aqueous solution. Thus, a 4% formalin solution is a 1.5% formaldehyde solution. The 4% formalin solution was recommended because it reduced morphological distortion, even of soft animals like Daphnia, while taking advantage of fixation and preservation properties of formalin. Minimum distortion is particularly desirable in studies of size-selective predation or reproductive rates, in which eggs need to stay in the brood chamber of branchiopods. Whereas there are advantages of formalin fixation, there also exists significant human health concerns for formaldehyde exposure. Acute effects of exposure—The effects of greatest interest to limnologists are caused by inhalation and contact. The greatest acute exposure occurs during analysis of samples when the limnologist looks at plankton samples through a microscope, often for hours at a time. Labs are often poorly ventilated, leading to increased exposure through inhalation of formaldehyde vapors. This inhalation is highly irritating to the upper respiratory tract and eyes.