Many biologists and students are mislead to believe formaldehyde is good for fixation but not for subsequent preservation because this typical museum practice for many wet collections. I believe the preserving of formaldehyde-fixed specimens (e.g., fish) in alcohol solutions is a practice imposed primarily to avoid unnecessary exposure of museum personnel to formaldehyde fumes and solutions (I'm not very fond of alcohol fumes either!). However, formaldehyde is a perfectly fine preservative as well as fixative for many (most?) specimens and tissues. Once specimens are fixed they can be transferred to formaldehyde solutions buffered to near neutral if decalcification or erosion of bony structures is a concern. Approximately 4% formaldehyde in water (10% formalin) is generally recommended for fixation. Once fixed, the specimens can be preserved in the same solution as long as desired (decades, probably centuries); however, only enough formaldehyde in solution is required to prevent reversal fixation (uncross-link proteins?), as would happen in water without sufficient formaldehyde. For well fixed fish larvae, we find dilute, 1-2% formaldehyde solutions (2-5% formalin) are quite adequate.

For large fish, subsequent preservation in alcohol (50% isopropanol or 70% ethanol are commonly used) is fine expect perhaps when the tissues are to be studied in a more-or-less natural state. However for delicate, soft-bodied organisms such as fish larvae and even small juvenile fish, alcohol dehydrates, deforms, and shrinks specimens and decreases their flexibility. These effects are significantly reduced if specimens are stepped through a graded series of alcohol concentrations, but this is a very time consuming procedure. In very high concentrations (e.g., >90%), specimens also become quite opaque and rigid to brittle and can be very difficult to work with and may be easily damaged. I strongly prefer to retain the specimens for long-term (permanent) storage in formaldehyde solutions (either the fixative concentration or a more dilute concentration--3% formalin, phosphate buffered if future skeletal study is a likely consideration).

The only notable exceptions are fish larvae for which we intend to do otolith analysis (e.g., to determine age in days) or DNA analysis for identity verification. These we "fix?" or preserved directly to straight (95-100% ethanol).

Recently formed otoliths are very sensitive to erosion in acid solutions such as unbuffered formaldehyde which is initially required to properly "fix" tissues. Fish larvae can be removed to a dilute buffered formaldehyde after just a few hours but that may be enough to significantly damage or erode recently formed otoliths. I suspect that if fixative solutions of formaldehyde are prepared with very hard water (e.g., water with a very high calcium content) we may be able to initially fix specimens for otolith study in formaldehyde them transfer the specimens to dilute, buffered formaldehyde solutions for long-term storage (preservation). Except for some promising preliminary experiments, I haven't been able to pursue this question yet.

There are variations of techiques for DNA study suitable for formalin preserved specimens (tissues) but our specialist on DNA analysis has yet to experiment with them and for the present we know we can get good results with specimens initially and subsequently stored in straight alcohol. I expect that we soon will not need alcohol to assure our ability to determine identity by DNA analysis.

For persons interested in reading up on fixation and preservation (especially in regard to fish larvae and other zooplanton), I suggest the book edited by H. F. Steedman (1976. Zooplankton fixation and preservation. Monographs on Oceanographic Methodology 4, The Unesco Press, Paris) and two papers by M. F. DeLeon, R. O. Reese, and W. J. Conley (1991. Effects of fixation and dehydration on shrinkage and morphology in common snook yolk-sac larvae. NOAA Technical Report NMFS 95:121-128) (Reese et. al. 1991. Histological effects from long-term storage of common snook yolK-sac larvae in fixatives and alcohol. NOAA Technical Report NMFS 95:129-137).