The epidermis of the sea horse Hippocampus kuda is characterised by flame cone cells, each of which protrudes 20-40 μm above the surface and is covered by a prominent mucous cap. Unlike normal surface cells, the mucoid caps can support epiphytic microbial growth.
Histochemically the mucous cap is a neutral mucopolysaccharide-protein complex possessing 1,2 glycol groups and SH-groups; acid mucopolysaccharides are absent. The acid mucopolysaccharide glycocalyx of unmodified surface cells is absent from the mucous cap surface.
Ultrastructurally two types of vesicle can be distinguished in flame cells. Type I is oval (0·3 × 0·6 um) with contents of medium electron density and occurs principally in mature flame cells. Type II, seen only in developing cells, is spherical (0·4 μm) and contains rod-like subunits characteristic of the cap mucous. Secretion is mediated by channels formed by smooth endoplasmic reticulum. Unmodified surface cells also secrete their vesicular contents, which resemble the Type I vesicles of flame cells and are similar to the goblet mucous vesicles of other fish, through similar channels.
Flame cell mucous caps, by virtue of their histochemistry, possible provide a suitable substratum for the adhesion and growth of epiphytes which in turn may afford protection against cnidarian nematocysts.