What is the main reason for using formalin in the preservation of aquatic specimens?

Formalin is primarily used for the preservation of aquatic specimens due to its ability to prevent decay and decomposition. This chemical, a solution of formaldehyde in water, effectively penetrates tissues and cross-links proteins, which helps to stabilize them and inhibit enzymatic and microbial actions that would normally lead to deterioration. By doing so, formalin maintains the structural integrity and appearance of the specimens for extended periods, making it invaluable in biological research and education where accurate representation of specimens is crucial.

Other choices do not align with the main purpose of formalin. For instance, enhancing color is not a primary function; in fact, the use of formalin can sometimes dull the colors of specimens. Similarly, promoting growth of microorganisms is directly counter to the goal of preservation, as the aim is to eliminate such growth rather than enhance it. Lastly, while shipping weight may be a concern in some contexts, the use of formalin does not primarily serve to increase weight for shipping purposes.