Structure and function of the median finfold in larval teleosts.

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.

Flow patterns of larval fish: undulatory swimming in the intermediate flow regime.

Fish larvae, like many adult fish, swim by undulating their body. However, their body size and swimming speeds put them in the intermediate flow regime, where viscous and inertial forces both play an important role in the interaction between fish and water. To study the influence of the relatively high viscous forces compared with adult fish, we mapped the flow around swimming zebrafish (Danio rerio) larvae using two-dimensional digital particle image velocimetry (2D-DPIV) in the horizontal and transverse plane of the fish. Fish larvae initiate a swimming bout by bending their body into a C shape. During this initial tail-beat cycle, larvae shed two vortex pairs in the horizontal plane of their wake, one during the preparatory and one during the subsequent propulsive stroke. When they swim ;cyclically' (mean swimming speed does not change significantly between tail beats), fish larvae generate a wide drag wake along their head and anterior body. The flow along the posterior body is dominated by the undulating body movements that cause jet flows into the concave bends of the body wave. Patches of elevated vorticity form around the jets, and travel posteriorly along with the body wave, until they are ultimately shed at the tail near the moment of stroke reversal. Behind the larva, two vortex pairs are formed per tail-beat cycle (the tail beating once left-to-right and then right-to-left) in the horizontal plane of the larval wake. By combining transverse and horizontal cross sections of the wake, we inferred that the wake behind a cyclically swimming zebrafish larva contains two diverging rows of vortex rings to the left and right of the mean path of motion, resembling the wake of steadily swimming adult eels. When the fish larva slows down at the end of a swimming bout, it gradually reduces its tail-beat frequency and amplitude, while the separated boundary layer and drag wake of the anterior body extend posteriorly to envelope the entire larva. This drag wake is considerably wider than the larval body. The effects of the intermediate flow regime manifest as a thick boundary layer and in the quick dying-off of the larval wake within less than half a second.

A study on cell lineage, especially the germ cell line, in embryos of the teleost fish,Barbus conchonius.

Lucifer Yellow-Dextran labelling of lower layer cells (LLC), sometimes together with upper layer cells (ULC), of the 64-cellBarbus conchonius embryo resulted in labelled primordial germ cells (PGCs) at 12 h after fertilization (a.f.) in about 25% of cases. The presence of labelled PGCs was independent of the location of the injected blastomere with respect to the later orientation of the embryonic axis. After injection of an ULC alone, however, labelled PGCs were never found. Also, the distribution of labelled somatic cells differed between the ULC- and LLC-injected embryos. When we found fluorescent PGCs, only a few of them were labelled, suggesting that either a single predecessor exists earlier than the 64-cell stage or that the formation of germ cells is a polyclonal process. Tracing the fluorescent cells at successive stages of development shows an extensive mixing with unlabelled cells during the epiboly stage, which might well be the cause of partly unpredictable cell lineages. The chance of being committed to a specific fate is different for the ULC and LLC descendants. This might be due to relatively limited cell mixing between these two cell populations.