Evolution of axial patterning in elongate fishes

Within the ray-finned fishes, eel-like (extremely elongate) body forms have evolved multiple times from deeper-bodied forms. Previous studies have shown that elongation of the vertebral column may be associated with an increase in the number of vertebrae, an increase in the length of the vertebral centra, or a combination of both. Because the vertebral column of fishes has at least two anatomically distinct regions (i.e. abdominal and caudal), an increase in the number and relative length of the vertebrae could be region-specific or occur globally across the length of the vertebral column. In the present study, we recorded vertebral counts and measurements of vertebral aspect ratio (vertebral length/width) from museum specimens for 54 species representing seven groups of actinopterygian fishes. We also collected, from published literature, vertebral counts for 813 species from 14 orders of actinopterygian and elasmobranch fishes. We found that the number of vertebrae can increase independently in the abdominal and caudal regions of the vertebral column, but changes in aspect ratio occur similarly in both regions. These findings suggest that abdominal vertebral number, caudal vertebral number, and vertebral aspect ratio are controlled by separate developmental modules. Based on these findings, we suggest some candidate developmental mechanisms that may contribute to vertebral column patterning in fishes. Our study is an example of how comparative anatomical studies of adults can generate testable hypotheses of evolutionary changes in developmental mechanisms.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 97–116.     

Comparative morphology of the myomeres and axial skeleton in four genera of centrarchid fishes

We used X-rays and dissection of myotomes to quantify the axial morphology of four species of centrarchid fishes (Micropterus salmoides, Ambloplites rupestris, Pomoxis nigromaculatus, and Lepomis macrochirus). Proceeding from dorsal to ventral, we designated the two epaxial and two hypaxial protions of the myomeres AB, BC, CD, and DE, respectively. For each of 11 myomeres, spaced at 10% increments along the length of the fish, a total of 14 variables described the length and orientation of each portion, the dorsalventral symmetry, and the overall height and longitudinal span of the entire myomere. Nine variables described the lengths, orientation, and symmetry of the vertebral centra, neural and hemal spines, and ribs. Analysis of variance revealed that, with one exception, all 23 morphological variables varied significantly both among species and among longitudinal locations within a species. However, the extent of longitudinal and interspecific variance differed considerably among different variables. Maximal myomeric height ranged from about 45% of the standard length (SL) in Lepomis to 27% SL in Micropterus. Longitudinal and interspecific increases in overall height of the trunk myomeres resulted primarily from greater lengths of CD. Compared to other portions of the myomere, the length of BC was most conservative both longitudinally and interspecifically. Dorsal-ventral symmetry of the myomeres and axial skeleton was greater in the caudal region than in the trunk in all species, and the myomeric morphology diverged least among species in the posterior caudal region. The overall longitudinal span of superficial myomeric landmarks varied from 6% to 18% SL, and, including the deep portions of the myomeres, the longitudinal span varied from about 7 to 10 vertebrae. Within each of the species, myomeric and skeletal variables were often not significantly correlated, but for the pooled data of all species there were usually highly significant correlations between myomeric and skeletal morphology. For example, strong correlations existed between BC and the underlying neural spines, and between CD and the underlying ribs and hemal spines. In contrast, the longitudinal spans of entire myomeres and underlying axial skeletal segments were only weakly associated. © 1994 Wiley-Liss, Inc.     

Development of the axial,paired and unpaired fin skeleton in three species of Patagonian zoarcids

Zoarcids are abundant in the intertidal zone of the Argentine Patagonian coast. However, their osteological development has not yet been described. Thus, the aim of this work is to describe the ontogeny of the axial skeleton and that of the paired and unpaired fins of the zoarcids Iluocoetes elongatus, Phucocoetes latitans and Dadyanos insignis. The three species studied showed a high degree of development at hatching and meristic counts coincide with those of the adults. At hatching, most of the structures showed that the appearance of bones was in progress. In the three species studied, this process occurred in a cephalo-caudal direction and, in most of the structures, the appearance of bones is observed from the middle to the rim.     

Evolution of the axial skeleton in armadillos (Mammalia,Dasypodidae)

Intraspecific and interspecific variation in cervical, thoracic, and lumbar region of the vertebral column of Dasypodidae were examined in a phylogenetic framework. The number of vertebrae for each region were recorded for 86 specimens and metric data for each vertebra (centrum length, high, and width) were recorded for 72 specimens, including eight of the nine living genera. The number of vertebrae and degree of fusion between them were used to define four characters which were plotted on two alternative phylogenies of Dasypodidae. The ratio between centrum height and width is similar across all taxa analyzed except for Chlamyphorus, which exhibits a deviation in the last two lumbars. Tolypeutes matacus is unique among the taxa examined in having a second co-osified bone called postcervical bone, which is a fusion of the seventh cervical and first thoracic vertebrae. The thoraco-lumbar numbers of dasypodids are reduced when compared with other xenarthrans and are more diverse than those of some other mammalian clades of similar geological age and higher ecomorphological diversity. Changes in size are somewhat coupled with changes in the number of body segments. Independent of the phylogenetic framework taken, changes in size are accompanied with small changes in numbers of thoracolumbar vertebrae within each genus. There are functional and phylogenetic correlates for changes in number of thoraco-lumbar vertebrae in dasypodids.     

Notochordal and elastic components of the axial skeleton of fishes and their functions in locomotion

Structural details of the notochord and elastic longitudinal ligaments (dorsal and ventral) of fish are presented with discussions on their possible contribution to the speed, power and modes of swimming by conferring an automatic spring-like resilience to the vertebral axis as a whole. The notochord is also believed to function as a series of ring-like hinges placed intervertebrally which dictates that the centra must be biconcave (amphicoelous) to support and house them. Examination of about 100 species shows that, whilst the dorsal ligament is always present, the ventral is found in primitive teleosts only. The phylogenetic significance of this in relation to the different efferent branchial systems will be submitted (with diagrammatic recording of dissections) for publication in the near future. The dorsal and ventral ligaments are suitably situated to assist the circulation of lymph and blood respectively in the small lateral vessels associated with the main longitudinal one of the appropriate system. Experimental work is required to test the hypotheses presented.     

Homology of fin lepidotrichia in osteichthyan fishes

Lepidotrichia are dermal elements located at the distal margin of osteichthyan fins. In sarcopterygians and actinopterygians, the term has been used to denote the most distal bony hemisegments and also the more proximal, scale-covered segments which overlie endochondral bones of the fin. In certain sarcopterygian fishes, including the Rhizodontida, these more proximal, basal segments are very long, extending at least half the length of the fin. The basal segments have a subcircular cross section, rather than the crescentic cross section of the distal lepidotrichial hemisegments, which lack a scale cover and comprise short, generally regular, elements. In rhizodonts and other sarcopterygians, e.g. Eusthenopteron, the basal elements are the first to appear during fin development, followed by the endochondral bones and then the distal lepidotrichia. This sequence contradicts the 'clock-face model' of fin development proposed by Thorogood in which the formation of endochondral bones is followed by development of lepidotrichia. However, if elongate basal 'lepidotrichia' are not homologous with more distal, jointed lepidotrichia and if the latter form within a distal fin-fold and the former outside this fold, then Thorogood's 'clock-face' model remains valid. This interpretation might indicate that the fin-fold has been lost in early digited stem-tetrapods such as Acanthostega and Ichthyostega and elongate basal elements, but not true lepidotrichia, occur in the caudal fins of these taxa.     

Mineralized cartilage in the skeleton of chondrichthyan fishes

The cartilaginous endoskeleton of chondrichthyan fishes (sharks, rays, and chimaeras) exhibits complex arrangements and morphologies of calcified tissues that vary with age, species, feeding behavior, and location in the body. Understanding of the development, evolutionary history and function of these tissue types has been hampered by the lack of a unifying terminology. In order to facilitate reciprocal illumination between disparate fields with convergent interests, we present levels of organization in which crystal orientation/size delimits three calcification types (areolar, globular, and prismatic) that interact in two distinct skeletal types, vertebral and tessellated cartilage. The tessellated skeleton is composed of small blocks (tesserae) of calcified cartilage (both prismatic and globular) overlying a core of unmineralized cartilage, while vertebral cartilage usually contains all three types of calcification.     

Canaliculi in the tessellated skeleton of cartilaginous fishes

The endoskeletal elements of sharks and rays are comprised of an uncalcified, hyaline cartilage‐like core overlain by a thin fibro‐ceramic layer of mineralized hexagonal tiles (tesserae) adjoined by intertesseral fibers. The basic spatial relationships of the constituent tissues (unmineralized cartilage, mineralized cartilage, fibrous tissue) are well‐known – endoskeletal tessellation is a long‐recognized synapomorphy of elasmobranch fishes – but a high‐resolution and three‐dimensional (3D) understanding of their interactions has been hampered by difficulties in sample preparation and lack of technologies adequate for visualizing microstructure and microassociations. We used cryo‐electron microscopy and synchrotron radiation tomography to investigate tessellated skeleton ultrastructure but without damage to the delicate relationships between constituent tissues or to the tesserae themselves. The combination of these techniques allowed visualization of never before appreciated internal structures, namely passages connecting the lacunar spaces within tesserae. These intratesseral ‘canaliculi’ link consecutive lacunar spaces into long lacunar strings, radiating outward from the center of tesserae. The continuity of extracellular matrix throughout the canalicular network may explain how chondrocytes in tesserae remain vital despite encasement in mineral. Extracellular fluid exchange may also permit transmission of nutrients, and mechanical and mineralization signals among chondrocytes, in a manner similar to the canalicular network in bone. These co‐adapted mechanisms for the facilitated exchange of extracellular material suggest a level of parallelism in early chondrocyte and osteocyte evolution.     

Supernumerary median fin-rays in teleostean fishes

Supernumerary fin-rays in the dorsal and anal fins are those that articulate directly with the head of the first proximal radial; they lie in front of the ray serially associated with the first radial. The argument presented here is that just as the number of fin-rays per dorsal and anal radial has decreased in actinopterygian history, so the number of dorsal and anal supernumeraries has decreased in teleosts. It is proposed that D > 3 and A > 3 (more than three dorsal and anal supernumeraries) is the condition primitive for teleosts, and that D2 and A3 are primitive for acanthomorphs.     

Mechanical properties of the axial skeleton in gorgonians

Axial skeletons of thirteen species representing a wide range of genera of Gorgonians were investigated using Young's modulus as a measure of stiffness and Torsion modulus as a measure of resistance to shear or twist. Atomic absorption spectroscopic determination of magnesium and calcium content as measures of mineralization were done. Relative quantities of calcareous material in the axial skeletons showed a strong linear correlation with Young's modulus and suggests an important role for calcareous material in the modulation of the mechanical properties of axoskeleton. Torsion moduli also showed a mathematical but non-linear relationship to calcareous content. Axis stiffness correlated well with zonation-related water movement. Stiffest axes occur in deeper water with no wave surge, most flexible in shallower water with moderate surge and intermediate stiffness in shallow, high energy habitats. An extremely high MgCO₃ containing carbonate that may be a previously unreported biological structural material was found in the Plexauridae.     

Functional morphology of the caudal skeleton in teleostean fishes

The basic function of the caudal skeleton in teleostean fishes is to support the caudal fin, but its parts contribute to this function in somewhat different ways. The main axis for this support is the upturned terminal end of the vertebral column, which ends at the base of the uppermost principal rays. The uroneural struts just ahead of this axis provide support for it. The parts of the caudal skeleton behind and below this upturned axis, the hypurals and parhypural, not only support the caudal rays but also provide a means for differential movements between the upper and lower parts of the fin base. This basic caudal skeleton varies with the position of the fish in the sequence of teleosten evolution, the way in which the fish uses its caudal fin, and to some extent with the shape of the fin.     

Functional morphology of the fin rays of teleost fishes

Ray‐finned fishes are notable for having flexible fins that allow for the control of fluid forces. A number of studies have addressed the muscular control, kinematics, and hydrodynamics of flexible fins, but little work has investigated just how flexible ray‐finned fish fin rays are, and how flexibility affects their response to environmental perturbations. Analysis of pectoral fin rays of bluegill sunfish showed that the more proximal portion of the fin ray is unsegmented while the distal 60% of the fin ray is segmented. We examined the range of motion and curvatures of the pectoral fin rays of bluegill sunfish during steady swimming, turning maneuvers, and hovering behaviors and during a vortex perturbation impacting the fin during the fin beat. Under normal swimming conditions, curvatures did not exceed 0.029 mm^?1 in the proximal, unsegmented portion of the fin ray and 0.065 mm^?1 in the distal, segmented portion of the fin ray. When perturbed by a vortex jet traveling at approximately 1 ms^?1 (67 ± 2.3 mN s.e. of force at impact), the fin ray underwent a maximum curvature of 9.38 mm^?1. Buckling of the fin ray was constrained to the area of impact and did not disrupt the motion of the pectoral fin during swimming. Flexural stiffness of the fin ray was calculated to be 565 × 10^?6 Nm². In computational fluid dynamic simulations of the fin‐vortex interaction, very flexible fin rays showed a combination of attraction and repulsion to impacting vortex dipoles. Due to their small bending rigidity (or flexural stiffness), impacting vortices transferred little force to the fin ray. Conversely, stiffer fin rays experienced rapid small‐amplitude oscillations from vortex impacts, with large impact forces all along the length of the fin ray. Segmentation is a key design feature of ray‐finned fish fin rays, and may serve as a means of making a flexible fin ray out of a rigid material (bone). This flexibility may offer intrinsic damping of environmental fluid perturbations encountered by swimming fish. J. Morphol. 274:1044–1059, 2013. © 2013 Wiley Periodicals, Inc.     

Structure of supporting elements in the dorsal fin of percid fishes

The dorsal fin is one of the most varied swimming structures in Acanthomorpha, the spiny‐finned fishes. This fin can be present as a single contiguous structure supported by bony spines and soft lepidotrichia, or it may be divided into an anterior, spiny dorsal fin and a posterior, soft dorsal fin. The freshwater fish family Percidae exhibits especially great variation in dorsal fin spacing, including fishes with separated fins of varying gap length and fishes with contiguous fins. We hypothesized that fishes with separated dorsal fins, especially those with large gaps between fins, would have stiffened fin elements at the leading edge of the soft dorsal fin to resist hydrodynamic loading during locomotion. For 10 percid species, we measured the spacing between dorsal fins and calculated the second moment of area of selected spines and lepidotrichia from museum specimens. There was no significant relationship between the spacing between dorsal fins and the second moment of area of the leading edge of the soft dorsal fin.     

Correlated evolution of body and fin morphology in the cichlid fishes

Body and fin shapes are chief determinants of swimming performance in fishes. Different configurations of body and fin shapes can suit different locomotor specializations. The success of any configuration is dependent upon the hydrodynamic interactions between body and fins. Despite the importance of body–fin interactions for swimming, there are few data indicating whether body and fin configurations evolve in concert, or whether these structures vary independently. The cichlid fishes are a diverse family whose well‐studied phylogenetic relationships make them ideal for the study of macroevolution of ecomorphology. This study measured body, and caudal and median fin morphology from radiographs of 131 cichlid genera, using morphometrics and phylogenetic comparative methods to determine whether these traits exhibit correlated evolution. Partial least squares canonical analysis revealed that body, caudal fin, dorsal fin, and anal fin shapes all exhibited strong correlated evolution consistent with locomotor ecomorphology. Major patterns included the evolution of deep body profiles with long fins, suggestive of maneuvering specialization; and the evolution of narrow, elongate caudal peduncles with concave tails, a combination that characterizes economical cruisers. These results demonstrate that body shape evolution does not occur independently of other traits, but among a suite of other morphological changes that augment locomotor specialization.     

The axial skeleton of the labyrinthodont Eogyrinus attheyi

An articulated length of vertebral column is used as a basis for the reconstruction of the salient features of the axial skeleton of the embolomerous anthracosaur Eogyrinus attheyi Watson, together with other material, including the holotype, in the Hancock Museum, Newcastle upon Tyne.
The trunk vertebrae are typically emboloinerous, with disoshaped notochordal pleuro-centra, firmly attached by broad facets to this neural arches, and much thinner intercentra. Regional variation is chiefly concerned with the span of the transverse processes, which diminishes posteriorly, and the associated separation of the two heads of each rib. A longitudinal series of trunk ribs, of diminishing length from the mid-trunk backwards, is reconstructed.
Eogyrinus has a normal tetrapod sacrum with one characteristic sacral rib. The first few caudal vertebrae bear ribs of unusual form, (of which four are preserved in sequence in the articulated specimen. The fifth caudal intercentrum bears the first and largest haemal arch and the pleurocentrum of the seventh caudal is distinguished by marked muscle origins presumably for the caudifemoral muscles.
The probability that Eogyrinus, like the few other embolomeres known, had an unusually long vertebral column for a labyrinthodont, is supported by an orthometric comparison using Romer's data on the American form Archeria.     

Dual motor innervation in the axial musculature of fishes

A systematic survey of motor innervation in the myotomal muscle fibres in several groups of fishes was conducted to observe the extent of dual innervation, a phenomenon found within those myotomal muscle fibres with terminal innervation. It is concluded that the dually innervated myotomal muscle fibre is a primitive vertebrate feature. Furthermore, it is suggested that this particular form of polyneuronal innervation is retained in the course of neural evolution within the homogeneous white myotomal muscle fibre region in these fish groups in order to insure synchrony of firing during sudden, rapid locomotion. The findings are examined in light of current ideas regarding the direct adrenergic innervation in addition to cholinergic innervation of striated muscle fibres.     

Nomenclature of cartilaginous elements in the caudal skeleton of teleostean fishes

Nomenclature and abbreviations are proposed for the cartilaginous elements of the caudal skeleton of teleostean fishes. These were developed on the basis of examination of 510 species within 198 families of 31 orders and the determination of the positional relationship between these structures and the bony elements. A review of the most important relative literature is also provided.     

Non-lethal measurement of pectoral fin aspect ratio in coral-reef fishes

This study describes a novel method for measuring pectoral fin aspect ratio (AR) on live coral-reef fishes and tests the method against traditional measurements taken from a dissected fin. No significant differences were detected among repeated fin measurements, which validates the accuracy (intact v. dissected) and precision (repeatability over several days) of fin AR measurements on live fishes. One exception highlighted issues that may arise when working with species prone to fin damage.