Feeding muscles scale differently from swimming muscles in sunfish (Centrarchidae)

Muscle power production during intermittent swimming in bluegill sunfish

Locomotion is essential for the survival and fitness of animals. Fishes have evolved a variety of mechanisms to minimize the cost of transport. For instance, bluegill sunfish have recently been shown to employ intermittent swimming in nature and in laboratory conditions. We focused on the functional properties of the power-producing muscles that generate propulsive forces in bluegill to understand the implications of intermittent activity. We used in vivo aerobic or red muscle activity parameters (e.g., oscillation frequency and onset time and duration of activation) in muscle physiology experiments to examine muscle power output during intermittent versus steady swimming in these fish. Intermittent propulsion involves swimming at relatively slow speeds with short propulsive bursts alternating with gliding episodes. The propulsive bursts are at higher oscillation frequencies than would be predicted for a given average swimming speed with constant propulsion. The work-loop muscle physiology experiments with red muscle demonstrated that intermittent activity allows muscle to produce sufficient power for swimming compared with imposed steady swimming conditions. Further, the intermittent muscle activity in vitro reduces fatigue relative to steady or continuous activity. This work supports the fixed-gear hypothesis that suggests that there are preferred oscillation frequencies that optimize efficiency in muscle use and minimize cost of transport.

Epaxial and hypaxial co-contraction: a mechanism for modulating strike pressure and accuracy during suction feeding in channel catfish

Most fish species use concentric epaxial and hypaxial contractions to suction feed, whereby both muscle groups produce cranial expansion and negative intraoral pressures. In contrast, channel catfish (Ictalurus punctatus) suction feed with little to no cranial elevation and epaxial shortening, generating suction power primarily with hypaxial shortening and pectoral girdle retraction. We hypothesized that channel catfish (1) actively anchor the head via isometric contraction of the epaxials and (2) vary feeding performance by modulating the absolute and relative outputs of the co-contracting muscles. We used a combination of electromyography, intraoral pressure recordings and specimen manipulation, and developed a new dual-lever model to explore this idea. We detected epaxial and hypaxial co-contraction prior to suction force development in all strikes. Our model revealed that the differential between the co-contracting muscles may be used to modulate suction pressure and strike accuracy.

Royal knifefish generate powerful suction feeding through large neurocranial elevation and high epaxial muscle power

Suction feeding in ray-finned fishes involves powerful buccal cavity expansion to accelerate water and food into the mouth. Previous XROMM studies in largemouth bass (Micropterus salmoides), bluegill sunfish (Lepomis macrochirus), and channel catfish (Ictalurus punctatus) have shown that more than 90% of suction power in high performance strikes comes from the axial musculature. Thus, the shape of the axial muscles and skeleton may impact suction feeding mechanics. Royal knifefish (Chitala blanci) have an unusual postcranial morphology, with a ventrally flexed vertebral column and relatively large mass of epaxial muscle. Based on their body shape, we hypothesized that royal knifefish would generate high power strikes by utilizing large neurocranial elevation, vertebral column extension, and epaxial shortening. As predicted, C. blanci generated high suction expansion power compared to the other three species studied to date (up to 160 W), which was achieved by increasing both the rate of volume change and the intraoral subambient pressure. The large epaxial muscle (25% of body mass) shortened at high velocities to produce large neurocranial elevation and vertebral extension (up to 41 deg, combined), as well as high muscle mass-specific power (up to 800 W kg-1). For the highest power strikes, axial muscles generated 95% of the power, and 64% of the axial muscle mass consisted of the epaxial muscles. The epaxial-dominated suction expansion of royal knifefish supports our hypothesis that postcranial morphology may be a strong predictor of suction feeding biomechanics.

Motor control in the epaxial musculature of bluegill sunfish in feeding and locomotion

Fishes possess an impressive repertoire of feeding and locomotor behaviors that in many cases rely on the same power source: the axial musculature. As both functions employ different skeletal systems, head versus body, integrating these functions would likely require modular motor control. Although there have been many studies of motor control in feeding or locomotion in fishes, only one study to date has examined both functions in the same individuals. To characterize bilateral motor control of the epaxial musculature in feeding and locomotion, we measured muscle activity and shortening in bluegill sunfish (Lepomis macrochirus) using electromyography and sonomicrometry. We found that sunfish recruit epaxial regions in a dorsal-to-ventral manner to increase feeding performance, such that high-performance feeding activates all the epaxial musculature. In comparison, sunfish seemed to activate all three epaxial regions irrespective of locomotor performance. Muscle activity was present on both sides of the body in nearly all feeding and locomotor behaviors. Feeding behaviors used similar activation intensities on the two sides of the body, whereas locomotor behaviors consistently used higher intensities on the side undergoing muscle shortening. In all epaxial regions, fast-starts used the highest activation intensities, although high-performance suction feeding occasionally showed near-maximal intensity. Finally, active muscle volume was positively correlated with the peak rate of body flexion in feeding and locomotion, indicating a continuous relationship between recruitment and performance. A comparison of these results with recent work on largemouth bass (Micropterus salmoides) suggests that centrarchid fishes use similar motor control strategies for feeding, but interspecific differences in peak suction-feeding performance are determined by active muscle volume.

Bifunctional Role of the Sternohyoideus Muscle During Suction Feeding in Striped Surfperch, Embiotoca lateralis

In ray-finned fishes, the sternohyoideus (SH) is among the largest muscles in the head region and, based on its size, can potentially contribute to the overall power required for suction feeding. However, the function of the SH varies interspecifically. In largemouth bass (Micropterus salmoides) and several clariid catfishes, the SH functions similarly to a stiff ligament. In these species, the SH remains isometric and transmitts power from the hypaxial musculature to the hyoid apparatus during suction feeding. Alternatively, the SH can shorten and contribute muscle power during suction feeding, a condition observed in the bluegill sunfish (Lepomis macrochirus) and one clariid catfish. An emerging hypothesis centers on SH muscle size as a predictor of function: in fishes with a large SH, the SH shortens during suction feeding, whereas in fish with a smaller SH, the muscle may remain isometric. Here, we studied striped surfperch (Embiotoca lateralis), a species in which the SH is relatively large at 8.8% of axial muscle mass compared with 4.0% for L. macrochirus and 1.7% for M. salmoides, to determine whether the SH shortens during suction feeding and is, therefore, bifunctional—both transmitting and generating power—or remains isometric and only transmits power. We measured skeletal kinematics of the neurocranium, urohyal, and cleithrum with Video Reconstruction of Moving Morphology, along with muscle strain and shortening velocity in the SH and epaxial muscles, using a new method of 3D external marker tracking. We found mean SH shortening during suction feeding strikes (n = 22 strikes from four individual E. lateralis) was 7.2 ± 0.55% (±SEM) of initial muscle length. Mean peak speed of shortening was 4.9 ± 0.65 lengths s⁻¹, and maximum shortening speed occurred right around peak gape when peak power is generated in suction feeding. The cleithrum of E. lateralis retracts and depresses but the urohyal retracts and depresses even more, a strong indicator of a bifunctional SH capable of not only generating its own power but also transmitting hypaxial power to the hyoid. While power production in E. lateralis is still likely dominated by the axial musculature, since even the relatively large SH of E. lateralis is only 8.8% of axial muscle mass, the SH may contribute a meaningful amount of power given its continual shortening just prior to peak gape across all strikes. These results support the finding from other groups of fishes that a large SH muscle, relative to axial muscle mass, is likely to both generate and transmit power during suction feeding.

Bluegill sunfish use high power outputs from axial muscles to generate powerful suction-feeding strikes

Suction-feeding fish rapidly expand the mouth cavity to generate high-velocity fluid flows that accelerate food into the mouth. Such fast and forceful suction expansion poses a challenge, as muscle power is limited by muscle mass and the muscles in fish heads are relatively small. The largemouth bass powers expansion with its large body muscles, with negligible power produced by the head muscles (including the sternohyoideus). However, bluegill sunfish - with powerful strikes but different morphology and feeding behavior - may use a different balance of cranial and axial musculature to power feeding and different power outputs from these muscles. We estimated the power required for suction expansion in sunfish from measurements of intraoral pressure and rate of volume change, and measured muscle length and velocity. Unlike largemouth bass, the sternohyoideus did shorten to generate power, but it and other head muscles were too small to contribute more than 5-10% of peak expansion power in sunfish. We found no evidence of catapult-style power amplification. Instead, sunfish powered suction feeding by generating high power outputs (up to 438 W kg^-1) from their axial muscles. These muscles shortened across the cranial half of the body as in bass, but at faster speeds that may be nearer the optimum for power production. Sunfish were able to generate strikes of the same absolute power as bass, but with 30-40% of the axial muscle mass. Thus, species may use the body and head muscles differently to meet the requirements of suction feeding, depending on their morphology and behavior.

Scaling of the fast-start escape response of juvenile bluegills

Morphology, size and physiological properties change markedly across fish ontogeny. This impacts locomotor performance and organismal fitness, although the effects are unpredictable due to the complexity of phenotype-function relationships. Morphological and behavioral changes with growth are often paralleled by changes in habitat use, diet and vulnerability to predators. Our goal was to quantify the changes in external morphology and escape performance throughout post-larval development in bluegill sunfish (Lepomis macrochirus), and place these changes in context with known changes in habitat use in the field. Development into adult ecomorphs is associated with phenotypic plasticity in response to habitat-specific differences in diet. On this basis, we hypothesized that variation in morphology and performance would increase during bluegill ontogeny as diversification of adult ecomorphs occurred. However, we found that variation in phenotype and escape performance decreased during early ontogeny. Phenotypic variation expanded later in development, after fish gained access to the variety of habitats and food types that may favor phenotypic plasticity. Performance is predicted to decline with growth due to the differential scaling of inertia and cross-sectional area, a major determinant of muscle force. In contrast, acceleration increased with size, and velocity and acceleration increased more rapidly with size than predicted. Post-larval maturation in bluegill featured a shift to a deeper body shape, and an increase in the relative size of the anal and caudal fins. This was a likely factor in the deviation of escape performance scaling relationships from predictions based on geometric similarity.

Morphology, Kinematics, and Dynamics: The Mechanics of Suction Feeding in Fishes

Suction feeding is pervasive among aquatic vertebrates, and our understanding of the functional morphology and biomechanics of suction feeding has recently been advanced by combining experimental and modeling approaches. Key advances include the visualization of the patterns of flow in front of the mouth of a feeding fish, the measurement of pressure inside their mouth cavity, and the employment of analytical and computational models. Here, we review the key components of the morphology and kinematics of the suction-feeding system of anatomically generalized, adult ray-finned fishes, followed by an overview of the hydrodynamics involved. In the suction-feeding apparatus, a strong mechanistic link among morphology, kinematics, and the capture of prey is manifested through the hydrodynamic interactions between the suction flows and solid surfaces (the mouth cavity and the prey). It is therefore a powerful experimental system in which the ecology and evolution of the capture of prey can be studied based on first principals.

Contractile properties of the myotomal muscle of sheepshead, Archosargus probatocephalus

Swimming in fishes is powered by myotomal red, white and pink skeletal muscle. Slow swimming is powered by the red (slow-twitch muscle), fast speeds are achieved by the white (fast-twitch) muscle and pink muscle apparently serves an intermediate function. In recent years, the physiological properties and molecular composition of red (slow) and white (fast) muscle fibers have been well studied, while the intermediate pink muscle, which falls in a thin sheet between the superficial red muscle and deeper white muscle, has received less attention. The goal of this study is to determine the contractile properties of red, pink, and white muscle and to establish the molecular basis of fiber type variations in contractile properties in a sheepshead (Archosargus probatocephalus). Isometric and isovelocity muscle mechanics experiments demonstrated a general pattern of increasing contractile speed from red to pink to white muscle, although red and pink muscle did not differ significantly for most contraction kinetics variables. As myosin heavy chain (MyHC) is the most important structural protein found in the muscle fibers, MyHC content was examined through immunohistochemistry. Myosin antibodies suggest a gradient in myosin content corresponding to differences in muscle contraction kinetics.

Thermal acclimation in rainbow smelt, Osmerus mordax, leads to faster myotomal muscle contractile properties and improved swimming performance

Rainbow smelt (Osmerus mordax) display an impressive ability to acclimate to very cold water temperatures. These fish express both anti-freeze proteins and glycerol in their plasma, liver, muscle and other tissues to avoid freezing at sub-zero temperatures. Maintenance of glycerol levels requires active feeding in very cold water. To understand how these fish can maintain activity at cold temperatures, we explored thermal acclimation by the myotomal muscle of smelt exposed to cold water. We hypothesized that cold-acclimated fish would show enhanced swimming ability due to shifts in muscle contractile properties. We also predicted that shifts in swimming performance would be associated with changes in the expression patterns of muscle proteins such as parvalbumin (PV) and myosin heavy chain (MyHC). Swimming studies show significantly faster swimming by smelt acclimated to 5°C compared to fish acclimated to 20°C when tested at a common test temperature of 10°C. The cold-acclimated fish also had faster muscle contractile properties, such as a maximum shortening velocity (V_max) almost double that of warm-acclimated fish at the same test temperature. Cold-acclimation is associated with a modest increase in PV levels in the swimming muscle. Fluorescence microscopy using anti-MyHC antibodies suggests that MyHC expression in the myotomal muscle may shift in response to exposure to cold water. The complex set of physiological responses that comprise cold-acclimation in smelt includes modifications in muscle function to permit active locomotion in cold water.

Myosin heavy chain and parvalbumin expression in swimming and feeding muscles of centrarchid fishes: the molecular basis of the scaling of contractile properties

In centrarchid fishes, such as bluegill (Lepomis macrochirus, Rafinesque) and largemouth bass (Micropterus salmoides, Lacepède), the contractile properties of feeding and swimming muscles show different scaling patterns. While the maximum shortening velocity (V(max)) and rate of relaxation from tetanus of swimming or myotomal muscle slow with growth, the feeding muscle shows distinctive scaling patterns. Cranial epaxial muscle, which is used to elevate the head during feeding strikes, retains fast contractile properties across a range of fish sizes in both species. In bass, the sternohyoideous muscle, which depresses the floor of the mouth during feeding strikes, shows faster contractile properties with growth. The objective of this study was to determine the molecular basis of these different scaling patterns. We examined the expression of two muscle proteins, myosin heavy chain (MyHC) and parvalbumin (PV), that affect contractile properties. We hypothesized that the relative contribution of slow and fast MyHC isoforms will modulate V(max) in these fishes, while the presence of PV in muscle will enhance rates of muscle relaxation. Myotomal muscle displays an increase in sMyHC expression with growth, in agreement with its physiological properties. Feeding muscles such as epaxial and sternohyoideus show no change or a decrease in sMyHC expression with growth, again as predicted from contractile properties. PV expression in myotomal muscle decreases with growth in both species, as has been seen in other fishes. The feeding muscles again show no change or an increase in PV expression with growth, contributing to faster contractile properties in these fishes. Both MyHC and PV appear to play important roles in modulating muscle contractile properties of swimming and feeding muscles in centrarchid fishes.

Energetic limitations on suction feeding performance in centrarchid fishes

Energetic analysis of ecologically relevant behaviors can be useful because animals are energetically limited by available muscle mass. In this study we hypothesized that two major determinants of suction feeding performance, the magnitudes of buccal volumetric expansion and subambient buccal pressure,would be correlated with, and limited by, available muscle mass. At least four individuals of three centrarchid species were studied: largemouth bass(Micropterus salmoides), bluegill (Lepomis macrochirus) and green sunfish (Lepomis cyanellus). Buccal pressure was measured directly via cannulation of the buccal cavity with a catheter-tipped pressure transducer. Buccal expansion was estimated from lateral high-speed video (500 or 1000 Hz) sequences and published data on internal kinematics of largemouth bass. These estimates were calibrated from silicone casts made of the buccal cavity post-mortem. Estimated work and power were found to be significantly correlated with muscle mass over all individuals. The slopes of these relationships, estimates of mass-specific muscle work and power, were found to be 11±2 J kg–1 and 300±75 W kg–1, respectively. These estimates are consistent with observations made of in vivo and in vitro muscle use and with digital particle image velocimetry measurements of water flow in feeding centrarchids. A direct trade-off between mean pressure and change in volume was observed, when the latter was normalized to muscle mass. We conclude that available muscle mass may be a useful metric of suction feeding performance,and that the ratio of muscle mass to buccal volume may be a useful predictor of subambient buccal pressure magnitude.