Minute tubercles on the skin surface of larvae in the Korean endemic bitterling, Rhodeus pseudosericeus (Pisces, Cyprinidae)

Abstract

The morphology and distribution of the minute tubercles on the skin surface of larvae in Korean bitterling, Rhodeus pseudosericeus, were observed during larval development. Just after hatching, the epidermis of the larvae consists of a thin single cell layer having smaller basophilic flat or round-flattened basal cells. As the larvae grow, the epidermis contains more small flat cells and large epidermal cells that are round or hemisphere-shaped. These large unicellular epidermal cells, called minute tubercles, consist of more or less homogeneous cytoplasm that is PAS (Periodic acid-Schiff method) positive. They are more densely distributed in the wing-like yolk sac projection. Vestigial minute tubercles occur in the body region and the caudal fin-fold region. These minute tubercles grow in number and height from 6 to 8 days after hatching onward. However, they become reduced in height and number as the larvae develop. At day 31 after hatching (i.e. free-swimming stage), minute tubercles no longer exist on the larval skin. The sequence of occurrence and gradual disappearance of these cell structures are described and histologically documented for comparative purposes of beta, taxnomomic and environmental studies.