Microstructure of Attachment Mechanisms of Newly Hatched Larvae of Four Cyprinid Species with Comments on Terminology

An adhesive organ is a prominent, protruding mucus secreting gland that is used by newly hatched tadpoles and larvae of some fishes to attach to aquatic vegetation. The objective of this research is to test the hypothesis that newly hatched cyprinid larvae of Hybognathus hankinsoni, Notemigonus crysoleucas, Cyprinus carpio and Gila atraria contain cephalic adhesive organs. Newly hatched larvae of Semotilus atromaculatus, which do not attach to submerged aquatic vegetation, were used as the control. SEM examination of newly hatched larvae indicate there were no adhesive organs on the control species (S. atromaculatus) or test species (H. hankinsoni, N. crysoleucas, C. carpio and G. atraria). Rather, newly hatched larvae of test species contain a localized highly modified epidermis (i.e., primarily on the ventral cephalic and anterioventral yolk sac surfaces of H. hankinsoni, N. crysoleucas, C. carpio, and G. atraria, and sometimes on dorsal cephalic epidermal cells of H. hankinsoni, C. carpio, and G. atraria). This modified epidermis is composed of epidermal cells with unculi-like projections, elevated microridges at peripheries of epidermal cells, and mucus from apical pores of goblet cells that probably are responsible for attachment of test species to substrates. We hypothesize that the unculi-like projections at centers of epidermal cells in newly hatched larval test cyprinids are true unculi. There is a need to define and clarify the meanings of words and phrases (i.e., cement gland, cement gland apparatus, cement gland-like structure, casquette, temporary adhesive glands, adhesive apparatus, adhesive gland, adhesive organ, attachment organ, and glue secretion and adhesion) for structures used by newly hatched larvae to attach to substrates. Definitions should be based on homologies, crucial in phylogenetic reconstructions of species relationships and in identifying developmental homologues of cells, tissues, glands, and organs that have been described as mechanisms for attachment by newly hatched larvae of various species to substrates. We proposed the phrase “attachment mechanism” as a broad definition for the ways in which newly hatched larvae attach and adhere to substrates during early development. This broad definition, however, should be modified to define specific methods of attachment (e.g. attachment mechanism of unculi, elevated epidermal microridges, and mucus) to assist in defining homologies.