Most fish species have, in association with the epidermis, an outer cuticle layer comprised of a mucopolysaccharide coat that appears to be secreted by both Malphigian and goblet cells (Bullock and Roberts, 1974; Whitear, 1970; Whitear and Mittal, 1984). This mucoid coat is useful for escaping capture, and protection against bacterial and parasitic infections. This protection is both mechanical: by providing a slimy barrier to the epidermis; and biochemical: due to the presence of inhibitory substances in the mucous that are anti-parasitic and anti-microbial. It has also been suggested that these mucoid secretions, when dissolved in the water, decrease the friction of turbulent water, thereby increasing locomotory efficiency by reducing drag (Bullock and Roberts, 1974). For all these reasons, loss of this protective coat through handling, intensive crowding, and poor health will be detrimental to the fitness of the fish, and the exposed epidermis susceptible to attack by a variety of fish parasites and bacteria.

There are several commercially available products that have been developed to serve as an artificial source of mucous protection. These products claim to bind to the epidermis, replacing the natural mucous in areas where it has been removed. Such water treatments are presently being used by both hobbyists and aqua culturists. However, due to increased stringency in federal regulations imposed on the aquaculture industry, it has now become necessary for the manufacturers of fish skin therapeutics to prove the efficacy of their products. Two such protective treatments, Polyaqua and Novaqua were subjected to a series of in-vivo experiments using the channel catfish, Ictalurus punctatus in an effort to demonstrate product binding to the epidermal layer. These products are designed to mimic the natural mucous layer of the fish skin, and as a result, are difficult to distinguish from natural fish exudates.

Polyaqua and Novaqua were mixed with various markers, including polystyrene latex beads, and iodine, and applied to the water according to manufacturer's recommendation. Detection of the tagged products was attempted by examination of the catfish epithelium using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray microanalysis. The channel catfish was chosen because it lacks scales and pigment granules that may make examination and interpretation more difficult. Removal of the natural mucous layer was imperative in order to determine if the products had bound to the epithelium, and was attempted by several methods including physical wiping of the skin (Shiomi, et al., 1988; Marshall, 1978; Hart and Oglesby, 1979), and application of 70% ethanol. Polystyrene latex beads mixed with the product appeared to be the most reliable marker for the detection of the product, but the difficulties encountered during experimentation raised the question: How does one accurately approach proving the efficacy of fish skin protective products? An in-vitro approach using Fat Head Minnow (FHM) epithelial cell cultures and covalently linked fluorescent markers that can be visualized in-situ using fluorescent microscopy may be the answer.

References

1.  Bullock, A.M. and Roberts, R.J., 1974. The Dermatology of Marine Teleost Fish. I. The Normal Integument. Oceanogr. Mar. Biol. Ann. Rev., 13: 383-411.

2.  Hart, R.K. and Oglesby, G.B., 1979. Toxin Induced Changes in the Gill Epithelium of Rainbow Trout (Salmo gairdneri Scanning Electron Microscopy lilt SEM Inc., AMF O'Hare, IL 60666, USA.

3.  Marshall, W.S., 1978. On the Involvement of Mucous Secretion of Teleost Osmoregulation. Can. J. Zool., 56 (5-8): 1088-1091.

4.  Shiomi, K., Uematsu, H., Yamanaka, H., and Kikuchi, T., 1989. Purification and Characterization of a Galactose-Binding Lectin from the Skin Mucus of the Conger Eel, Conner myriaster. Comp. Biochem. Physiol., 92B (2): 255-261.

5.  Whitear, M., 1970. The Skin Surface of Bony Fishes. J. Zool., London., 160: 437-454

6.  Whitear, M. and Mittal, A.K., 1984. Surface Secretions of the Skin of Blennius (Lipophrvs) pholis L. J. Fish. Biol., 25: 317-331.