Tuning of shortening speed in coleoid cephalopod muscle: no evidence for tissue-specific muscle myosin heavy chain isoforms

The contractile protein myosin II is ubiquitous in muscle. It is widely accepted that animals express tissue-specific myosin isoforms that differ in amino acid sequence and ATPase activity in order to tune muscle contractile velocities. Recent studies, however, suggested that the squid Doryteuthis pealeii might be an exception; members of this species do not express muscle-specific myosin isoforms, but instead alter sarcomeric ultrastructure to adjust contractile velocities. We investigated whether this alternative mechanism of tuning muscle contractile velocity is found in other coleoid cephalopods. We analyzed myosin heavy chain transcript sequences and expression profiles from muscular tissues of a cuttlefish, Sepia officinalis, and an octopus, Octopus bimaculoides, in order to determine if these cephalopods express tissue-specific myosin heavy chain isoforms. We identified transcripts of four and six different myosin heavy chain isoforms in S. officinalis and O. bimaculoides muscular tissues, respectively. Transcripts of all isoforms were expressed in all muscular tissues studied, and thus S. officinalis and O. bimaculoides do not appear to express tissue-specific muscle myosin isoforms. We also examined the sarcomeric ultrastructure in the transverse muscle fibers of the arms of O. bimaculoides and the arms and tentacles of S. officinalis using transmission electron microscopy and found that the fast contracting fibers of the prey capture tentacles of S. officinalis have shorter thick filaments than those found in the slower transverse muscle fibers of the arms of both species. It thus appears that coleoid cephalopods, including the cuttlefish and octopus, may use ultrastructural modifications rather than tissue-specific myosin isoforms to adjust contractile velocities

The diversity of hydrostatic skeletons

A remarkably diverse group of organisms rely on a hydrostatic skeleton for support, movement, muscular antagonism and the amplification of the force and displacement of muscle contraction. In hydrostatic skeletons, force is transmitted not through rigid skeletal elements but instead by internal pressure. Functioning of these systems depends on the fact that they are essentially constant in volume as they consist of relatively incompressible fluids and tissue. Contraction of muscle and the resulting decrease in one of the dimensions thus results in an increase in another dimension. By actively (with muscle) or passively (with connective tissue) controlling the various dimensions, a wide array of deformations, movements and changes in stiffness can be created. An amazing range of animals and animal structures rely on this form of skeletal support, including anemones and other polyps, the extremely diverse wormlike invertebrates, the tube feet of echinoderms, mammalian and turtle penises, the feet of burrowing bivalves and snails, and the legs of spiders. In addition, there are structures such as the arms and tentacles of cephalopods, the tongue of mammals and the trunk of the elephant that also rely on hydrostatic skeletal support but lack the fluid-filled cavities that characterize this skeletal type. Although we normally consider arthropods to rely on a rigid exoskeleton, a hydrostatic skeleton provides skeletal support immediately following molting and also during the larval stage for many insects. Thus, the majority of animals on earth rely on hydrostatic skeletons

Evidence That Eye-Facing Photophores Serve as a Reference for Counterillumination in an Order of Deep-Sea Fishes

Counterillumination, the masking of an animal\u27s silhouette with ventral photophores, is found in a number of mesopelagic taxa but is difficult to employ because it requires that the animal match the intensity of downwelling light without seeing its own ventral photophores. It has been proposed that the myctophid, Tarletonbeania crenularis, uses a photophore directed towards the eye, termed an eye-facing photophore, as a reference standard that it adjusts to match downwelling light. The potential use of this mechanism, however, has not been evaluated in other fishes. Here, we use micro-computed tomography, photography and dissection to evaluate the presence/absence of eye-facing photophores in three families of stomiiform fishes. We found that all sampled species with ventral photophores capable of counterillumination possess an eye-facing photophore that is pigmented on the anterior and lateral sides, thus preventing its use as a laterally directed signal, lure or searchlight. The two species that are incapable of counterillumination, Cyclothone obscura and Sigmops bathyphilus, lack an eye-facing photophore. After determining the phylogenetic distribution of eye-facing photophores, we used histology to examine the morphology of the cranial tissue in Argyropelecus aculeatus and determined that light from the eye-facing photophore passes through a transparent layer of tissue, then the lens, and finally strikes the accessory retina. Additionally, eight of the 14 species for which fresh specimens were available had an aphakic gap that aligned with the path of emitted light from the eye-facing photophore, while the remaining six had no aphakic gap. These findings, combined with records of eye-facing photophores from distantly related taxa, strongly suggest that eye-facing photophores serve as a reference for counterillumination in these fishes

Ontogeny of mantle musculature and implications for jet locomotion in oval squid Sepioteuthis lessoniana

SUMMARY We examined the relationship between mantle muscle structure and mantle kinematics in an ontogenetic series (5-85 mm dorsal mantle length) of oval squid, Sepioteuthis lessoniana. Thick filament length increased during growth in the mantle muscle fibres that power jet locomotion (i.e. the circular muscles). The thick filament length of both the superficial mitochondria-rich (SMR; analogous to vertebrate red muscle fibres) and central mitochondria-poor (CMP; analogous to vertebrate white muscle fibres) circular muscles increased significantly during ontogeny. Thick filaments in the SMR circular muscle fibres of newly hatched squid (N=5) ranged from 0.7 to 1.4 μm and averaged 1.0 μm, while the thick filaments of the SMR fibres of the largest squids (N=4) studied ranged from 1.2 to 3.4μ m and averaged 1.9 μm. The ontogeny of thick filament length in the CMP circular muscle fibres showed a similar trend. The range for hatchling CMP circular muscles was 0.7-1.4 μm, with an average of 1.0 μm, whereas the range and average for the largest squids studied were 0.9-2.2 μm and 1.5μ m, respectively. Within an individual hatchling, we noted no significant differences between the thick filament lengths of the SMR and CMP fibres. Within an individual juvenile, the thick filaments of the SMR fibres were∼ 25% longer than the CMP fibres. The change in thick filament length may alter the contractile properties of the circular muscles and may also result in a decrease in the rate of mantle contraction during jetting. In escape-jet locomotion, the maximum rate of mantle contraction was highest in newly hatched squid and declined during ontogeny. The maximum rate of mantle contraction varied from 7-13 muscle lengths per second in newly hatched squid (N=14) and from 3-5 muscle lengths per second in the largest squids (N=35) studied

Differences in scaling and morphology between lumbricid earthworm ecotypes

Many soft-bodied invertebrates use a flexible, fluid-filled hydrostatic skeleton for burrowing. The aim of our study was to compare the scaling and morphology between surface-dwelling and burrowing earthworm ecotypes to explore the specializations of non-rigid musculoskeletal systems for burrowing locomotion. We compared the scaling of adult lumbricid earthworms across species and ecotypes to determine whether linear dimensions were significantly associated with ecotype. We also compared the ontogenetic scaling of a burrowing species

Muscular tissues of the squid Doryteuthis pealeii express identical myosin heavy chain isoforms: an alternative mechanism for tuning contractile speed

The speed of muscle contraction is largely controlled at the sarcomere level by the ATPase activity of the motor protein myosin. Differences in amino acid sequence in catalytically important regions of myosin yield different myosin isoforms with varying ATPase activities and resulting differences in cross-bridge cycling rates and interfilamentary sliding velocities. Modulation of whole-muscle performance by changes in myosin isoform ATPase activity is regarded as a universal mechanism to tune contractile properties, especially in vertebrate muscles. Invertebrates such as squid, however, may exhibit an alternative mechanism to tune contractile properties that is based on differences in muscle ultrastructure, including variable myofilament and sarcomere lengths. To determine definitively whether contractile properties of squid muscles are regulated via different myosin isoforms (i.e. different ATPase activities), the nucleotide and amino acid sequences of the myosin heavy chain from the squid Doryteuthis pealeii were determined from the mantle, arm, tentacle, fin and funnel retractor musculature. We identified three myosin heavy chain isoforms in squid muscular tissues, with differences arising at surface loop 1 and the carboxy terminus. All three isoforms were detected in all five tissues studied. These results suggest that the muscular tissues of D. pealeii express identical myosin isoforms, and it is likely that differences in muscle ultrastructure, not myosin ATPase activity, represent the most important mechanism for tuning contractile speeds

Muscle specialization in the squid motor system

Although muscle specialization has been studied extensively in vertebrates, less is known about the mechanisms that have evolved in invertebrate muscle that modulate muscle performance. Recent research on the musculature of squid suggests that the mechanisms of muscle specialization in cephalopods may differ from those documented in vertebrates. Muscle diversity in the development and the evolution of cephalopods appears to be characterized by modulation of the dimensions of the myofilaments, in contrast to the relatively fixed myofilament dimensions of vertebrate muscle. In addition, the arrangement of the myofilaments may also be altered, as has been observed in the extensor muscle fibres of the prey capture tentacles of squid and cuttlefish, which show cross-striation and thus differ from the obliquely striated pattern of most cephalopod locomotor muscle fibres. Although biochemical specializations that reflect differences in aerobic capacity have been documented previously for specific layers of the mantle muscle of squid, comparison of protein profiles of myofilament preparations from the fast cross-striated tentacle fibres and slow obliquely striated fibres from the arms has revealed remarkably few differences in myofilament lattice proteins. In particular, previous studies using a variety of SDS-PAGE techniques and peptide mapping of the myosin heavy chain were unable to resolve differences in the myosin light and heavy chains. Since these techniques cannot exclude the presence of a highly conserved variant that differs in only a few amino acids, in this study semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) analysis of myosin heavy chain messenger RNAs (mRNAs) from the cross-striated tentacle and obliquely striated arm muscle fibres was conducted. This analysis showed that a previously reported alternatively spliced isoform of the squid myosin motor domain is present only in low abundance in both muscle types and therefore differential expression of the two myosins cannot explain the difference in contractile properties. It thus appears that modulation of the contractile properties of the musculature of squid and other cephalopods occurs primarily through variation in the arrangement and dimensions of the myofilaments

Scaling of the hydrostatic skeleton in the earthworm Lumbricus terrestris

The structural and functional consequences of changes in size or scale have been well studied in animals with rigid skeletons, but relatively little is known about scale effects in animals with hydrostatic skeletons. We used glycol methacrylate histology and microscopy to examine the scaling of mechanically important morphological features of the earthwor

Electromyography of the buccal musculature of octopus (Octopus bimaculoides): a test of the function of the muscle articulation in support and movement

SUMMARY The buccal mass musculature of the octopus (Octopus bimaculoides) was studied with electromyography to test the predictions of a previous morphological study in which we suggested that the muscles of the buccal mass serve as both the effectors of movement and as the joint itself, forming a new category of flexible joint termed a `muscle articulation9. The predictions of muscle function were tested by correlating muscle electrical activity in isolated buccal masses with spontaneous beak movements. Bipolar electromyography electrodes were implanted in the various beak muscles and beak position was recorded simultaneously with an electronic movement monitor (N=14). The results are consistent with the hypothesis that the lateral mandibular muscles produce opening movements of the beaks and provide the first definitive explanation of the opening mechanism. The results are also consistent with the hypothesis that the superior mandibular muscle functions primarily in closing. Co-contraction of the lateral mandibular muscles and the superior mandibular muscles was also observed, suggesting that these muscles may also stabilize the beaks during movement or provide a means of controlling the location of the pivot between the beaks. This study provides an important first test of the predictions of the role of the complex musculature found in muscle articulations such as the cephalopod buccal mass

Predicting the effects of spatiotemporal modifications of muscle activation on the tentacle extension in squid

Squid use eight arms and two slender tentacles to capture prey. The muscular stalks of the tentacles are elongated approximately 80% in 20–40 ms towards the prey, which is adhered to the terminal clubs by arrays of suckers. Using a previously developed forward dynamics model of the extension of the tentacles of the squid Doryteuthis pealeii (formerly Loligo pealeii), we predict how spatial muscle-activation patterns result in a distribution of muscular power, muscle work, and kinetic and elastic energy along the tentacle. The simulated peak extension speed of the tentacles is remarkably insensitive to delays of activation along the stalk, as well as to random variations in the activation onset. A delay along the tentacle of 50% of the extension time has only a small effect on the peak extension velocity of the tentacle compared with a zero-delay pattern. A slight delay of the distal portion relative to the proximal has a small positive effect on peak extension velocity, whereas negative delays (delay reversed along stalk) always reduce extension performance. In addition, tentacular extension is relatively insensitive to superimposed random variations in the prescribed delays along the stalk. This holds in particular for small positive delays that are similar to delays predicted from measured axonal diameters of motor neurons. This robustness against variation in the activation distribution reduces the accuracy requirements of the neuronal control and is likely due to the non-linear mechanical properties of the muscular tissue in the tentacle