Swimming and muscle structure in fish

In this series of studies the relations between swimming behaviour of fish in general and extreme swimming responses in particular (called fast starts or escape responses) and the structure and ontogeny of the muscle system was investigated. Special attention was paid to relate functional differences between anterior and posterior parts of the axial myotomal muscles of fish to differences in their structural design. In the past considerable knowledge has been accumulated concerning the muscular system. There are however still many unsolved questions. What for example is the influence of swimming in different fish species on the ontogeny of their muscles. How is the development of the muscle system reflecting functional demands (e.g. strength of fibre, elastic properties etc.) and what is the relationship between muscle development on a molecular scale and a macro scale? These and other questions will partly be addressed in this study.Initially the larval muscle system and its function was investigated in general as fish larvae swim in a different hydrodynamic environment, compared to adult fish, characterised by the importance of viscous forces which can not be neglected (Osse, 1990). In contrast to adults, the different muscles during the early stages of life of many fish species (e.g. Rutilus rutilus , Alburnus alburnus , Leuciscus cephalus , Clupea harengus, Clarias gariepinus ) have an aerobic metabolism (El-Fiky et al., 1987). In yolk-sac larvae of Clarias gariepinus for example, at a time when gill development is still insufficient and muscle rely for oxygen supply on diffusion through the body surface, both the superficial red muscle layer as well as the inner 'larval white' muscle mass are aerobic. The superficial red layer initially only consisted of a monolayer. At the moment the gills started to develop the superficial red layer acquired additional fibres along the horizontal septum, resulting in a double layer of red fibres at this location. The differences in metabolism between the red aerobic fibres and the white anaerobic fibres develop during the free-swimming larval stage of e.g. roach ( Rutilus rutilus ) and rainbow trout ( Salmo gairdneri ) and the adult pattern of muscle fibre type distribution emerges (Hinterleitner et al., 1987). As this development probably occurs in relation to gill development, it is thought that the red layer of yolk-sac larvae has a negligible role in swimming but an important role in respiratory (El-Fiky et al., 1987). Once the adult pattern of muscle fibre type distribution has developed the actual differences between the various muscles can be studied in great detail.The effects of transmission of forces on the structure and function of different muscle fibre types and at different locations along the body axis were studied during swimming of adult carp ( Cyprinus carpio L.). The connection between muscle fibres and collagen fibres, myotendinous junctions (MTJs), was investigated electron-microscopically. Especially during extreme swimming movements such as escape fast-starts large forces are imposed on the muscular system and mainly on the MTJs. During these life-saving swimming movements large sarcomere strains (relative to sarcomere slack length) occurred. Muscle fibres in the tail region (together with the connective tissue) play an important role in the transmission of force produced by more anterior fibres. Posterior fibres have a longer phase of eccentric activity (active while being stretched) than the anterior fibres and will therefore develop greater forces (van Leeuwen et al., 1990; van Leeuwen, 1995). It was therefore expected that greater forces in these posterior fibres would be accompanied by stronger MTJs (a greater membrane amplification). Posterior (80% of the fork length, FL ) muscle fibres of carp indeed had much larger myotendinous surface areas than anterior fibres (40% FL ) and consequently can transmit larger forces and 'bear' larger loads during swimming. Red muscle fibres of carp had a larger membrane amplification at the MTJs than white fibres. Red fibres are active at lower tail beat frequencies (longer cycle times) than white fibres and for longer periods of time, resulting in a longer duration of the load on the junction of red fibres. Tidball and Daniel (1986) proposed that the degree of membrane amplification at MTJs not only depends on the magnitude but also on the duration of load on the junction. Curtis (1961) and Rand (1964) showed that the mechanical behaviour of cell membranes is dependent on loading time. Cells can survive a certain shear load (caused by applying either a large load for a short time or a small load for a longer time) by reducing the stress on the membrane through an amplification of the membrane area. It was therefore suggested that the larger membrane amplification at the MTJs of carp red muscle fibres may be related to the longer duration of the load on the junction in this fibre type.Not only the MTJs were subjected to large forces during fast-starts (accompanied by large strain fluctuations). High demands will also be imposed on the muscle system itself and the series elastic elements within the sarcomere unit, such as the titin filaments (Wang et al., 1991). This may be reflected in the type and structure of the elastic elements as different isoforms of titin seem to exist (Wang et al., 1991; Granzier and Wang, 1993a,b). To help elucidate the relation between the possible occurrence of different titin isoforms and the functional properties of different fibre types, the presence of different titin isoforms in red and white anterior and posterior fibres of the axial muscles of adult carp was investigated. Titin is a striated-muscle-specific giant muscle protein that spans the distance from the Z- and M-lines of the sarcomere (Wang, 1985; Maruyama, 1986, 1994; Trinick, 1991). The elastic segment of titin in the I-band is thought to function as a molecular spring that is responsible for maintaining the central positions of the thick filaments in contracting sarcomeres and develops passive tension upon sarcomere stretch (Horowits et al., 1986; Fürst et al. 1988; Wang et al., 1991, 1993; Granzier et al., 1996).Gel-electrophoresis of single fibres of carp revealed that the molecular mass of titin was larger in red than in white fibres. For both red and white fibres the molecular mass of titin was larger in posterior than in anterior muscle fibres. Thus depending on the fibre type and its location along the body axis different titin isoforms were expressed.Furthermore the contribution of titin to passive tension and stiffness of red anterior and red posterior fibres was determined in micro-mechanical experiments. It appeared that more passive tension and stiffness was needed to stretch fibres with smaller titin isoforms (red anterior fibres) to a certain sarcomere length than in fibres with larger titin isoforms (red posterior fibres). Continuous swimming is the most frequently used swimming mode in adult carp and is driven by the activity of red muscle. During this type of swimming sarcomere strain is larger in red muscle fibres, which have larger titin isoforms, than in the three-dimensionally folded white muscle tissue, due to differences in distance between the sarcomere and the body axis and differences in fibre arrangement between both types. As during cyclic swimming local curvature increases from anterior to posterior the sarcomere strain is consequently larger in posterior fibres, which have larger titin isoforms. The finding that exactly those fibres that are exposed to the largest sarcomere strains during continuous swimming also possessed the largest titin isoforms led to the suggestion that titin isoform and sarcomere strain are correlated in order to minimise energy loss during cyclic loading of muscle fibres.However, it was still unknown how large the maximum sarcomere strains actually were during the most extreme swimming responses of adult carp. Therefore a study on the kinematics and muscle dynamics of escape fast-starts of carp was conducted. Adult carp perform escape C- or S-starts, based on the typical body curvature of the fish during these movements. During the Mauthner initiated C-starts (Eaton et al., 1977; Kimmel et al., 1980) adult carp made a large angle of turn directed away from the stimulus (approximately 150°) with a high acceleration at 0.3 FL of up to 54 m s -2. The maximum sarcomere strains (both anteriorly and posterior) were approximately 27% for red fibres and approximately 16.5% for white fibres. During escape S-starts however maximum strain in anterior fibres was more than twice that of posterior fibres with an angle of turn of approximately 70°. This large anterior peak curvature enabled the fish to control the direction of escape better but with lower accelerations at 0.3 FL (approximately 40 m s -2), although little is known about the neuronal mechanisms controlling S-starts. The largest strains occurred in red anterior fibres during S-starts (39%). It was found that during continuous and intermittent swimming the largest strains (red posterior fibres, approximately 5%) were found in fibres with the largest titin isoforms. This enabled these fibres to attain large strain amplitudes with relatively low passive tensions.It was surprising to find that in all fast-starts both red and white muscle were simultaneously active at a given longitudinal location, whereas only red muscle were active during continuous swimming. Red fibres could contribute to muscle fibre shortening at the beginning of their mechanical response for a very short period of time (before the full response was reached). This implies that red fibres hence could contribute to force generation during these extremely fast swimming modes, although little. Red and white muscle at a given longitudinal location were not necessarily active synchronously and could be uncoupled during escape S-starts. In this way mechanically sub-optimal patterns of force generation can be avoided. In both C- and S-starts both anterior and posterior muscle were active whilst lengthening at a certain moment, thus initially absorbing power which results in significant force- and performance enhancement.Fish larval swimming on the other hand is very different from adult swimming. Small carp larvae of approximately 6.5-8 mm total length are subjected to relatively low Reynolds-regimes of approximately 200≤Re≤500 and therefore require special features to overcome effects of friction. As superficial red fibres of Cyprinid larvae are mainly used as a respiratory organ (see above), larval swimming behaviour is mainly powered by the inner 'larval white' fibres (El-Fiky et al., 1987). But how exactly are these inner 'larval white' fibres able to generate enough power to overcome these friction effects and reach velocities of over 20 bodylength s -1? As small carp larvae and adults show large differences in their swimming behaviour the sarcomere strain ranges during fast swimming of larvae were investigated, together with their size of titin. During fast swimming of carp larvae all muscle fibres showed maximum sarcomere strains of approximately 20%, whereas their titin appeared to be shorter than any titin isoform found in adult muscle. Apparently the molecular structure of titin changed in the course of ontogeny. This shorter titin isoform (requiring larger stress for the same strain) is thought to help restricting form changes of the swimming larvae and to increase the elastic contribution to the tail beat. Such molecules possibly also increase the resonant frequency of the beating tail and thereby provide the required high frequency for swimming in a relatively low Reynolds-regime.The present study corroborates the idea that strong relations exist between the structural design of the muscular system, from micro- to macro-level, and its functions, also in diverse levels, in a fish's specific habitat. Starting at a structural level, differences in muscle function during swimming of fish can be used in an effort to explain and possibly predict morphological differences between the various muscle types and even within the same muscle type.