Axial elongation in fishes: using morphological approaches to elucidate developmental mechanisms in studying body shape

One of the most notable features in looking across fishes is their diversity of body shape and size. Extant actinopterygian fishes range in shape from nearly spheroidal in pufferfishes to extremely elongate in snipe eels with nearly every shape in-between. One extreme along the body-shape continuum is a highly elongate form, which has evolved multiple times independently in Actinopterygii. Thus, comparison of these separate (independent) radiations provides a unique opportunity for examining the anatomical traits underlying elongation as well as the similarities and differences in the evolutionary pathways followed. Body elongation generally evolves via an increase in region-specific vertebral number, although certain lineages elongate via an increase in vertebral length. In this study, we describe how anatomical characters related to feeding and locomotion are correlated with elongation of the body across Actinopterygii. In addition to modifications of the postcranial axial skeleton, elongation in fishes is often accompanied by an increase in head length, loss of the pelvic fins, reduction of the pectoral fins, and expansion of the median fins. Based on anatomical studies and on recent studies of developmental control of the body axis in different species, we hypothesize how an axial trait might change at the genetic level. Overall, we discuss the evolution of body elongation in fishes in light of an understanding of the underlying anatomical modifications, developmental control, ecology, and locomotion.     

Shared extremes by ectotherms and endotherms: Body elongation in mustelids is associated with small size and reduced limbs

An elongate body with reduced or absent limbs has evolved independently in many ectothermic vertebrate lineages. While much effort has been spent examining the morphological pathways to elongation in these clades, quantitative investigations into the evolution of elongation in endothermic clades are lacking. We quantified body shape in 61 musteloid mammals (red panda, skunks, raccoons, and weasels) using the head‐body elongation ratio. We also examined the morphological changes that may underlie the evolution toward more extreme body plans. We found that a mustelid clade comprised of the subfamilies Helictidinae, Guloninae, Ictonychinae, Mustelinae, and Lutrinae exhibited an evolutionary transition toward more elongate bodies. Furthermore, we discovered that elongation of the body is associated with the evolution of other key traits such as a reduction in body size and a reduction in forelimb length but not hindlimb length. This relationship between body elongation and forelimb length has not previously been quantitatively established for mammals but is consistent with trends exhibited by ectothermic vertebrates and suggests a common pattern of trait covariance associated with body shape evolution. This study provides the framework for documenting body shapes across a wider range of mammalian clades to better understand the morphological changes influencing shape disparity across all vertebrates.     

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.     

Elongation of the body in eels

The shape of the body affects how organisms move, where they live, and how they feed. One body plan that has long engaged the interest of both evolutionary biologists and functional morphologists is axial elongation. There is a growing interest in the correlates and evolution of elongation within different terrestrial and aquatic vertebrate clades. At first glance, Anguilliformes may appear to exhibit a single cylindrical form but there is considerable diversity underlying this seemingly simplified body plan. Here, we explore evolution of the axial skeleton in 54 anguilliform taxa and some close relatives. We describe the diversity of axial elongation as well as investigate how characters such as head length, branchial-arch length, and shape of the pectoral fins correlate with vertebral number to possibly facilitate changes in absolute diameter of the body. Overall, we find that precaudal vertebral numbers and caudal vertebral numbers are evolving independently across elopomorph fishes. We also find that precaudal and caudal vertebral aspect ratios are evolving together across elopomorph fishes. When focusing within Anguilliformes we find striking diversity in the mechanisms of elongation of the body, including almost every trend for axial elongation known within actinopterygian fishes. The three major clades of eels we examined have slightly different mechanisms of elongation. We also find a suite of morphological characters associated with elongation in anguilliform fishes that appears to coincide with a more fossorial lifestyle such as high elongation ratios, a more posteriorly extended-branchial region, and a reduction in the size of the pectoral fins. Lastly, we point out that a diverse range of derived behaviors such as head- and tail-first burrowing, rotational feeding, and knotting around prey are only found in long cylindrical vertebrates.     

Evolution of skull and body shape in Triturus newts reconstructed from three‐dimensional morphometric data and phylogeny

To explore the relationship between morphological change and species diversification, we reconstructed the evolutionary changes in skull size, skull shape, and body elongation in a monophyletic group of eight species that make up salamander genus Triturus. Their well‐studied phylogenetic relationships and the marked difference in ecological preferences among five species groups makes this genus an excellent model system for the study of morphological evolution. The study involved three‐dimensional imagery of the skull and the number of trunk vertebrae, in material that represents the morphological, spatial, and molecular diversity of the genus. Morphological change largely followed the pattern of descent. The reconstruction of ancestral skull shape indicated that morphological change was mostly confined to two episodes, corresponding to the ancestral lineage that all crested newts have in common and the Triturus dobrogicus lineage. When corrected for common descent, evolution of skull shape was correlated to change in skull size. Also, skull size and shape, as well as body shape, as inferred from the number of trunk vertebrae, were correlated, indicating a marked impact of species' ecological preferences on morphological evolution, accompanied by a series of niche shifts, with the most pronounced one in the T. dobrogicus lineage. The presence of phylogenetic signal and correlated evolutionary changes in skull and body shape suggested complex interplay of niche shifts, natural selection, and constraints by a common developmental system. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 243–255.     

Interactions between locomotion,feeding, and bodily elongation during the evolution of snakes

Information from lizard lineages that have evolved a highly elongate (snake‐like) body form may clarify the selective forces important in the early evolution of snakes. Lizards have evolved bodily elongation via two distinct routes: as an adaptation to burrowing underground or to rapid locomotion above ground. These two routes involve diametrically opposite modifications to the body plan. Burrowing lizards have elongate trunks, small heads, short tails, and relatively constant body widths, whereas surface‐active taxa typically have shorter trunks, wider heads, longer tails, and more variable body widths. Snakes resemble burrowing rather than surface‐active (or aquatic) lizards in these respects, suggesting that snakes evolved from burrowing lizards. The trunk elongation of burrowing lizards increases the volume of the alimentary tract, so that an ability to ingest large meals (albeit consisting of small individual prey items) was present in the earliest snakes. Subsequent shifts to ingestion of wide‐bodied prey came later, after selection dismantled other gape‐constraining morphological attributes, some of which may also have arisen as adaptations to burrowing through hard soil (e.g. relatively small heads, rigid skulls). Adaptations of snake skulls to facilitate ingestion of large prey have evolved to compensate for the reduction of relative head size accompanying bodily elongation; relative to predator body mass, maximum sizes of prey taken by snakes may not be much larger than those of many lizards. This adaptive scenario suggests novel functional links between traits, and a series of testable predictions about the relationships between squamate morphology, habitat, and trophic ecology. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 293–304.     

Body shape transformation along a shared axis of anatomical evolution in labyrinth fishes (Anabantoidei)

Major morphological transformations, such as the evolution of elongate body shape in vertebrates, punctuate evolutionary history. A fundamental step in understanding the processes that give rise to such transformations is identification of the underlying anatomical changes. But as we demonstrate in this study, important insights can also be gained by comparing these changes to those that occur in ancestral and closely related lineages. In labyrinth fishes (Anabantoidei), rapid evolution of a highly derived torpedo‐shaped body in the common ancestor of the pikehead (Luciocephalus aura and L. pulcher) occurred primarily through exceptional elongation of the head, with secondary contributions involving reduction in body depth and lengthening of the precaudal vertebral region. This combination of changes aligns closely with the primary axis of anatomical diversification in other anabantoids, revealing that pikehead evolution involved extraordinarily rapid change in structures that were ancestrally labile. Finer‐scale examination of the anatomical components that determine head elongation also shows alignment between the pikehead evolutionary trajectory and the primary axis of cranial diversification in anabantoids, with much higher evolutionary rates leading to the pikehead. Altogether, our results show major morphological transformation stemming from extreme change along a shared morphological axis in labyrinth fishes.     

Prey capture in lizards: differences in jaw–neck–forelimb coordination

Fish body muscles are arranged along the vertebral column in three‐dimensional W‐shaped blocks, called myomeres. Each myomere is separated from its neighbours by a collagenous sheet, the myoseptum, and embedded in these myosepta and in positions that are conserved throughout gnathostome evolution are distinct tendons. Within teleosts these tendons often ossify. Ossification is usually intramembranous but cartilaginous structures within the tendons have also been reported. Ossified myoseptal tendons are homologous to intermuscular bones and appear only in teleosts. The phylogenetic signal of myoseptal tendon ossfication has not been tested previously, although the presence and morphology of intermuscular bones have been used to infer phylogenetic relationships. We sample over a broad phylogenetic range of teleost fishes to test for (1) the effects of phylogenetic history on the presence of intermuscular bones and (2) morphological correlations with the presence of intermuscular bones. Body shape and fin position as well as vertebral number and aspect ratio are characters that are likely to affect the distribution of stresses along myoseptal tendons, and are therefore good functional predictors of myoseptal tendon ossification. We use the summary information by Patterson & Johnson for a list of species with intermuscular bones and reanalyse the homology of intermuscular bones to myoseptal tendons. We find that there is a phylogenetic signal in the distribution of four out of six ossified tendons, but that after correcting for phylogenetic relationships there are still morphological predictors for the presence of all ossified tendons. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 607–622.     

The giraffe (Giraffa camelopardalis) cervical vertebral column: a heuristic example in understanding evolutionary processes?

The current study considers the osteological morphology of the giraffe (Giraffa camelopardalis) vertebral column, with emphasis on evaluating both the adaptive and constraining features compared with other ungulates as a heuristic example in understanding evolutionary processes. Vertebral columns of giraffes varying in age from calf to adult were studied in order to understand the potential evolutionary scenarios that might have led to the modern phenotype. Data from the giraffe sample were then compared with the results from several other ungulate species, including the okapi and two species of camelids that also have visibly elongated necks. Our results show that the elongated neck of the modern giraffe appears to specifically result from evolutionary changes affecting the seven cervical vertebrae, independent of the remainder of the vertebral column. The cervical vertebrae comprise over half of the length of the total vertebral column in the giraffe. The increases in cervical vertebrae lengths also appear to be allometrically constrained, with alterations in the overall length of the neck resulting from the elongation of the entire cervical series, rather than from a single vertebra or subset of vertebrae. We place our results in the context of hypotheses concerning the origin and evolution of the giraffe neck, and the evolution of long necks in a broader sense. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155 , 736–757.     

Morphological and ecological convergence in two natricine snakes

Similar morphologies between species may be due to shared ancestry or convergent evolution . Understanding instances of morphological and ecological convergence is central to evolutionary ecology because they help us understand the fit between organism and environment. Two species of stream-dwelling natricine snakes, Thamnophis rufipunctatus and Nerodia harteri present a model system for studying ecological and morphological convergence and adaptation. The species are allopatric and both live in shallow riffles in streams and forage visually for fish. We studied morphological similarity, trait evolution and functional significance of ecologically relevant traits in these and related species, and used mitochondrial DNA sequences for the ND4 gene to estimate their phylogenetic relationships. Character mapping of head length and head width supported the hypothesis of independent evolution of head shape in T .  rufipunctatus and N .  harteri . The elongate snout is a derived trait in these two taxa that is associated with reduced hydrodynamic drag on the snakes' heads when in a swift current, compared to other species with the ancestral blunt snout. We hypothesize that lower hydrodynamic drag facilitates prey capture success in these species that are known to forage by holding their position in currents and striking at fish prey. The elongate snout morphology has also resulted in a diminished binocular vision field in these snakes, contrary to the hypothesis that visually orientated snakes should exhibit relatively greater binocular vision. Convergent evolution of the long snout and reduced hydrodynamic drag in T. rufipunctatus and N. harteri are consistent with the hypothesis that the long snout is an adaptation to foraging in a swift current.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 363–371.     

Is dietary niche breadth linked to morphology and performance in Sandveld lizards Nucras (Sauria: Lacertidae)?

The functional characteristics of prey items (such as hardness and evasiveness) have been linked with cranial morphology and performance in vertebrates. In lizards particularly, species with more robust crania generally feed on harder prey items and possess a greater bite force, whereas those that prey on evasive prey typically have longer snouts. However, the link between dietary niche breadth, morphology, and performance has not been explicitly investigated in lizards. The southern African genus Nucras was used to investigate this link because the species exhibit differing niche breadth values and dietary compositions. A phylogeny for the genus was established using mitochondrial and nuclear markers, and morphological clusters were identified. Dietary data of five Nucras species, as reported previously, were used in correlation analyses between cranial shape (quantified using geometric morphometrics) and dietary niche breadth, and the proportion of hard prey taken and bite force capacity. Dietary niche breadth and the proportion of hard prey eaten were significantly related to cranial shape, although not once phylogeny was accounted for using a phylogenetic generalized least squares regression. The proportion of evasive prey eaten was a significant predictor of forelimb length when phylogeny was taken into account. We conclude that, in Nucras, the percentage of evasive prey taken co‐evolves with forelimb morphology, and dietary niche breadth co‐evolves with cranial shape. However, although head width is correlated with the proportion of hard prey eaten, this appears to be the result of shared ancestry rather than adaptive evolution. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 674–688.     

Turning maneuvers in sharks: Predicting body curvature from axial morphology

Given the diversity of vertebral morphologies among fishes, it is tempting to propose causal links between axial morphology and body curvature. We propose that shape and size of the vertebrae, intervertebral joints, and the body will more accurately predict differences in body curvature during swimming rather than a single meristic such as total vertebral number alone. We examined the correlation between morphological features and maximum body curvature seen during routine turns in five species of shark: Triakis semifasciata, Heterodontus francisci, Chiloscyllium plagiosum, Chiloscyllium punctatum, and Hemiscyllium ocellatum. We quantified overall body curvature using three different metrics. From a separate group of size‐matched individuals, we measured 16 morphological features from precaudal vertebrae and the body. As predicted, a larger pool of morphological features yielded a more robust prediction of maximal body curvature than vertebral number alone. Stepwise linear regression showed that up to 11 features were significant predictors of the three measures of body curvature, yielding highly significant multiple regressions with r² values of 0.523, 0.537, and 0.584. The second moment of area of the centrum was always the best predictor, followed by either centrum length or transverse height. Ranking as the fifth most important variable in three different models, the body's total length, fineness ratio, and width were the most important non‐vertebral morphologies. Without considering the effects of muscle activity, these correlations suggest a dominant role for the vertebral column in providing the passive mechanical properties of the body that control, in part, body curvature during swimming. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Intraspecific and interspecific variation of female genitalia in two species of watersnake

The morphological differences in female genitalia within and between species are little studied and poorly understood, yet understanding patterns of variation in female genitalia can provide insights into mechanisms of genital evolution. The present study aimed to explore the patterns of intraspecific and interspecific variation in female genitalia in two sister taxa of watersnake (Nerodia sipedon and Nerodia fasciata) that have similar genital shape. We used a geometric morphometric (GM) approach to study variation in shape of the vagina between and within two sister species. We examined genital shape in female watersnakes ranging from small, sexually immature females to large reproductive females that had recently given birth. We found that shape variation of genitalia is strongly correlated with body size, where larger but not smaller females have a bifurcation in the vagina. However, we also found significant shape variation in the structure of the vagina between the two species, where N. fasciata has narrower genitalia with more prominent bifurcation, whereas N. sipedon has wider genitalia with less marked bifurcation. Using GM allowed us to detect significant differences in genital shape that were not apparent upon visual examination alone, suggesting that shape variation in female genitalia may be greater than previously assumed. Additional study of morphological differences in male reproductive organs for these species would help to determine whether there has been genital co‐evolution, and potentially mechanical reproductive isolation, in these two closely‐related and occasionally sympatric species. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 183–191.     

Adaptive radiation in miniature: the minute salamanders of the Mexican highlands (Amphibia: Plethodontidae: Thorius)

The small size and apparent external morphological similarity of the minute salamanders of the genus Thorius have long hindered evolutionary studies of the group. We estimate gene and species trees within the genus using mitochondrial and nuclear DNA from nearly all named and many candidate species and find three main clades. We use this phylogenetic hypothesis to examine patterns of morphological evolution and species coexistence across central and southern Mexico and to test alternative hypotheses of lineage divergence with and without ecomorphological divergence. Sympatric species differ in body size more than expected after accounting for phylogenetic relationship, and morphological traits show no significant phylogenetic signal. Sympatric species tend to differ in a combination of body size, presence or absence of maxillary teeth, and relative limb or tail length, even when they are close relatives. Sister species of Thorius tend to occupy climatically similar environments, which suggests that divergence across climatic gradients does not drive species formation in the genus. Rather than being an example of cryptic species formation, Thorius more closely resembles an adaptive radiation, with ecomorphological divergence that is bounded by organism‐level constraints. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 622–643.     

Morphological adaptations to marine life in snakes

We investigated morphological adaptations to aquatic life within animals that exhibit a structurally simple, elongate body form, i.e., snakes. This linear body plan should impose different biomechanical constraints than the classical streamlined body shape associated with propulsion by fins, feet, or wings. Our measurements of general body shape of terrestrial, amphibious, and marine snakes (all from the same phylogenetic lineage, the Elapidae) show that seasnakes display specialized morphological attributes for life in water. Most notably, the cross‐sectional body shape is circular in terrestrial snakes but dorso‐ventrally elongated in seasnakes (due to a prominent ventral keel); amphibious species (sea kraits) exhibit an intermediate shape. The tail of amphibious and marine species (a major propulsive structure during swimming) is higher and thinner than in terrestrial snakes (i.e., paddle‐shaped) but shorter relative to body length. The evolution of a laterally compressed shape has been achieved by an increase in body height rather than a decrease in body width, possibly reflecting selection for more effective propulsive thrust, and for an ability to maintain hydrodynamic efficiency despite the minor bodily distension inevitably caused by prey items and developing offspring. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc     

Influence of body mass on the shape of forelimb in musteloid carnivorans

In the majority of mammals, the limbs are positioned under the body and play an important role in gravitational support, allowing the transfer of the load and providing stability to the animal. For this reason, an animal's body mass likely has a significant effect on the shape of its limb bones. In the present study, we investigate the influence of body mass variation on the shape of the three long bones of the forelimb in a group of closely‐related species of mammals: the musteloid carnivorans. We use geometric morphometric techniques to quantify forelimb shape; then estimate phylogenetic signal in the shape of each long bone; and, finally, we apply an independent contrasts approach to assess evolutionary associations between forelimb shape and body mass. The results obtained show that body mass evolution is tightly coordinated with the evolution of forelimb shape, although not equally in all elements. In particular, the humeral and radial shapes of heavier species appear better suited for load bearing and load transmission than the ulna. Nevertheless, our results also show that body mass influences only part of forelimb long bone shape and that other factors, such as locomotor ecology, must be considered to fully understand forelimb evolution. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 91–103.     

A revised metric for quantifying body shape in vertebrates

Vertebrates exhibit tremendous diversity in body shape, though quantifying this variation has been challenging. In the past, researchers have used simplified metrics that either describe overall shape but reveal little about its anatomical basis or that characterize only a subset of the morphological features that contribute to shape variation. Here, we present a revised metric of body shape, the vertebrate shape index (VSI), which combines the four primary morphological components that lead to shape diversity in vertebrates: head shape, length of the second major body axis (depth or width), and shape of the precaudal and caudal regions of the vertebral column. We illustrate the usefulness of VSI on a data set of 194 species, primarily representing five major vertebrate clades: Actinopterygii, Lissamphibia, Squamata, Aves, and Mammalia. We quantify VSI diversity within each of these clades and, in the course of doing so, show how measurements of the morphological components of VSI can be obtained from radiographs, articulated skeletons, and cleared and stained specimens. We also demonstrate that head shape, secondary body axis, and vertebral characteristics are important independent contributors to body shape diversity, though their importance varies across vertebrate groups. Finally, we present a functional application of VSI to test a hypothesized relationship between body shape and the degree of axial bending associated with locomotor modes in ray-finned fishes. Altogether, our study highlights the promise VSI holds for identifying the morphological variation underlying body shape diversity as well as the selective factors driving shape evolution.     

Evolution of ontogenetic allometry shaping giant species: a case study from the damselfish genus Dascyllus (Pomacentridae)

The evolution of body size, the paired phenomena of giantism and dwarfism, has long been studied by biologists and paleontologists. However, detailed investigations devoted to the study of the evolution of ontogenetic patterns shaping giant species are scarce. The damselfishes of the genus Dascyllus appear as an excellent model for such a study. Their well understood phylogeny reveals that large‐bodied species have evolved in two different clades. Geometric morphometric methods were used to compare the ontogenetic trajectories of the neurocranium and the mandible in both small‐bodied (Dascyllus aruanus and Dascyllus carneus; maximum size: 50–65 mm standard length) and giant (Dascyllus trimaculatus and Dascyllus flavicaudus; maximum size: 90–110 mm standard length) Dascyllus species. At their respective maximum body size, the neurocranium of the giant species is significantly shorter and have a higher supraoccipital crest relative to the small‐bodied species, whereas mandible shape variation is more limited and is not related to the ‘giant’ trait. The hypothesis of ontogenetic scaling whereby the giant species evolved by extending the allometric trajectory of the small‐bodied ones (i.e. hypermorphosis) is rejected. Instead, the allometric trajectories vary among species by lateral transpositions. The rate of shape changes and the type of lateral transposition also differ according to the skeletal unit among Dascyllus species. Differences seen between the two giant species in the present study demonstrate that giant species may appear by varied alterations of the ancestor allometric pattern. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 99–117.     

Morphological convergence of shell shape in distantly related scallop species (Mollusca: Pectinidae)

Morphological convergence is a central concept in evolutionary biology, but convergent patterns remain under‐studied in nonvertebrate organisms. Some scallop species exhibit long‐distance swimming, a behaviour whose biomechanical requirements probably generate similar selective regimes. We tested the hypothesis that shell shape similarity in long‐distance swimming species is a result of convergent evolution. Using landmark‐based geometric morphometrics, we quantified shell shape in seven species representing major behavioural habits. All species displayed distinct shell shapes, with the exception of the two long‐distance swimmers, whose shells were indistinguishable. These species also displayed reduced morphological variance relative to other taxa. Finally, a phylogenetic simulation revealed that these species were more similar in their shell shape than was expected under Brownian motion, the model of character evolution that best described changes in shell shape. Together, these findings reveal that convergent evolution of shell shape occurs in scallops, and suggest that selection for shell shape and behaviour may be important in the diversification of the group. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163 , 571–584.     

Locomotion in elongate fishes: A contact sport

Despite the physical differences between water and air, a number of fish lineages are known to make terrestrial excursions on land. Many of these fishes exhibit an elongate body plan. Elongation of the body can occur in several ways, the most common of which is increasing the number of vertebrae in one or both regions of the axial skeleton – precaudal and/or caudal. Elongate species are often found in three-dimensionally complex habitats. It has been hypothesized that elongate fishes use this structure to their locomotor advantage. In this study, we consider how elongation and differences in vertebral regionalization correspond with the use of wooden pegs, which are provided as analogs to vertically oriented substrate, structures that protrude above the ground. We compare aquatic and terrestrial locomotor behaviors of Polypterus senegalus, Erpetoichthys calabaricus, and Gymnallabes typus as they move through a peg array. When considering axial elongation we find that the highly elongate species, E. calabaricus and G. typus, contact more pegs but on average move slower in both environments than P. senegalus. When considering axial regionalization, we find that the precaudally elongate species, P. senegalus and E. calabaricus, differ in the patterns of peg contact between the two environments whereas the caudally elongate species, G. typus, exhibits similar peg contact between the two environments. Our study highlights the importance of incorporating body shape and vertebral regionalization to understand how elongate fishes move in water and on land.