Micro-anatomy of the pectoral fin in blennies (Blenniini, Blennioidea, Teleostei)

Blennioid fishes show a highly differentiated pectoral fin, which they use to cling to the substrate. The lower part of the pectoralis, comprising about four to six fin rays, forms a hook-field with specific anatomical features: (1) the rim of the fin web has a saw-like appearance, because it extends from the tip of a fin ray to the shaft ofthe upper of two neighbouring fin rays, (2) the outer half of the bony fin ray carries a lepidotrichal cord composed of fibrocytes, collagen, elastic fibres and acidic GAGS, (3) the epidermis overlying the lepidotrichal cord is differentiated in terms of cyto-architecture and forms a conspicuous cuticle. The upper part of the pectoral fin does not show any obvious specializations and is used for swimming and undulation. The vascularization of the fin originates from a stem vessel which gives rise to five branches, each supplying two or three neighbouring fin rays. Each fin ray is accompanied by a single arterial vessel at its upper edge. No vessels are found in the space between the bony fin ray halves. The morphology of the shoulder girdle and pectoral fin shows only little variation among the four species of Blenniini studied. Most remarkable is the fusion of the coracoid with the cleithrum, loss of one element of the suspensorium and the absence of branched fin rays. The possible relevance of the Blennioid pectoral fin as a model for the origin of morphological novelties in connection with functional specializations is discussed.