This paper offers a structural and mechanical analysis of the median finfold in larval teleosts. The median finfold is strengthened by bundles of collagen fibres, known as actinotrichia. We demonstrate that these structures contribute to increase the mass of backward accelerated water during swimming. The amount, dimensions, orientation and growth of actinotrichia were measured at various locations along the finfold in several developmental stages of common carp (Cyprinus carpio) and zebrafish (Danio rerio). Actinotrichia morphology, using light microscopy (e.g. diameter, orientation) and electron microscopy (which revealed their anchoring at proximal and distal ends), correlated with expected lateral forces exerted on the water during swimming. An analytical model is proposed that predicts the extent of camber from the oblique arrangement of the actinotrichia and curvature of the body. Camber of the finfold during swimming was measured from high-speed video recordings and used to evaluate the model predictions. Based on structural requirements for swimming and strain limits for collagen, the model also predicts optimal orientations of actinotrichia. Experimental data confirm the predictions of the model.