Moringua edwardsi (Moringuidae: Anguilliformes): Cranial specialization for head-first burrowing?

Burrowing constrains patterns of skull shape evolution in wrasses

The evolution of behavioral and ecological specialization can have marked effects on the tempo and mode of phenotypic evolution. Head-first burrowing has been shown to exert powerful selective pressures on the head and body shapes of many vertebrate and invertebrate taxa. In wrasses, burrowing behaviors have evolved multiple times independently, and are commonly used in foraging and predator avoidance behaviors. While recent studies have examined the kinematics and body shape morphology associated with this behavior, no study to-date has examined the macroevolutionary implications of burrowing on patterns of phenotypic diversification in this clade. Here, we use three-dimensional geometric morphometrics and phylogenetic comparative methods to study the evolution of skull shape in fossorial wrasses and their relatives. We test for skull shape differences between burrowing and non burrowing wrasses and evaluate hypotheses of shape convergence among the burrowing wrasses. We also quantify rates of skull shape evolution between burrowing and non burrowing wrasses to test for whether burrowing constrains or accelerates rates of skull shape evolution in this clade. We find that while burrowing and non burrowing wrasses exhibit similar degrees of morphological disparity, for burrowing wrasses, it took nearly twice as long to amass this disparity. Furthermore, while the disparities between groups are evenly matched, we find that most burrowing species are confined to a particular region of shape space with most species exhibiting narrower heads than many non-burrowing species. These results suggest head-first burrowing constrains patterns of skull shape diversification in wrasses by potentially restricting the range of phenotypes that can perform this behavior.

A descriptive and comparative neurocranium morphology of Anguilliformes fishes in Taiwan waters

Taiwan is one of the richest in the world in terms of eel fauna. In this study, we examined the osteological and morphological characteristics of eels under order Anguilliformes. Furthermore, we focused on the neurocranium of total of 30 Anguilliformes fishes under family Congridae (10), Muraenesocidae (1), Muraenidae (7), Nemichthyidae (1), Nettastomatidae (2), Ophichthidae (5), Synaphobranchidae (4), which are caught in Taiwanese waters. This paper shows the results of a comparative study on osteological characters of the neurocranium including the ratio of seven length characters using its NCL (neurocranium length), NCW (neurocranium width), OBL (orbit length), MFW (maximum frontal width), NCDB (neurocranium depth at basisphenoid), PEVW (premaxilla-ethmovomer width) and mPOBL (mid pre-orbital length), and 20 morphological diagnostic characters for 30 eel species. Results shows that species under family Nemichthyidae and Nettastomatidae have the highest values on the ratio of NCL/MFW, NCL/NCDB, and NCW/mPOBD. In morphological characters, it shows that species of the same family mostly share similar formation of the PEV plate and frontal structure. The usage of the length measurements and morphological diagnostic characters of neurocranium allowed for a more in depth understanding of how similar or different these eels can be. The neurocranial description and morphological characters may prove valuable for identification purposes and might be necessary tool for further studies on the status of order Anguilliformes.

Peculiar relationships among morphology, burrowing performance and sand type in two fossorial microteiid lizards

Associations among ecology, morphology and locomotor performance have been intensively investigated in several vertebrate lineages. Knowledge on how phenotypes evolve in natural environments likely benefits from identification of circumstances that might expand current ecomorphological equations. In this study, we used two species of Calyptommatus lizards from Brazilian Caatingas to evaluate if specific soil properties favor burrowing performance. As a derived prediction, we expected that functional associations would be easily detectable at the sand condition that favors low-resistance burrowing. We collected two endemic lizards and soil samples in their respective localities, obtained morphological data and recorded performance of both species in different sand types. As a result, the two species burrowed faster at the fine and homogeneous sand, the only condition where we detected functional associations between morphology and locomotion. In this sand type, lizards from both Calyptommatus species that have higher trunks and more concave heads were the ones that burrowed faster, and these phenotypic traits did not morphologically discriminate the two Calyptommatus populations studied. We discuss that integrative approaches comprising manipulation of environmental conditions clearly contribute to elucidate processes underlying phenotypic evolution in fossorial lineages.

Trade-offs between burrowing and biting force in fossorial scincid lizards?

Trade-offs are thought to be important in constraining evolutionary divergence as they may limit phenotypic diversification. The cranial system plays a vital role in many functions including defensive, territorial, predatory and feeding behaviours in addition to housing the brain and sensory systems. Consequently, the morphology of the cranial system is affected by a combination of selective pressures that may induce functional trade-offs. Limbless, head-first burrowers are thought to be constrained in their cranial morphology as narrow heads may provide a functional advantage for burrowing. However, having a wide and large head is likely beneficial in terms of bite performance. We used 15 skink species to test for the existence of trade-offs between maximal push and bite forces, and explored the patterns of covariation between external head and body morphology and performance. Our data show that there is no evidence of a trade-off between bite and burrowing in terms of maximal force. Species that generate high push forces also generate high bite forces. Our data also show that overall head size covaries with both performance traits. However, future studies exploring trade-offs between force and speed or the energetic cost of burrowing may reveal other trade-offs.

Structural and environmental constraints on reduction of paired appendages among vertebrates

Burrowing habits or complex environments have generally been considered as potential drivers acting on reduction and loss of the appendicular skeleton among vertebrates. Herein, we suggest that this might be the case for lissamphibians and squamates, but that fin loss in fishes is usually prevented by important structural constraints, because pectoral fins are commonly used to control rolling and pitching. We provide an overview of the distribution of paired appendage reduction across vertebrates while examining the ecological affinities of finless and limbless clades. We analysed the correlation between lifestyle and fin or limb loss using the discrete comparative analysis. The resulting Bayesian factors indicate strong evidence of correlation between: (1) pectoral-fin loss and coexistence of anguilliform elongation and burrowing habits or complex habitat in teleost fishes; and (2) limb loss and a burrowing or grass-swimming lifestyle in squamate reptiles and lissamphibians. These correlations suggest that a complex environment or a fossorial habit is a driving force leading to appendage loss. The only style of locomotion that is functional even in the absence of paired appendages is the undulatory one, which is typical of all elongated reptiles and lissamphibians, but certainly less common in teleost fishes.

A digital dissection of two teleost fishes: comparative functional anatomy of the cranial musculoskeletal system in pike (Esox lucius) and eel (Anguilla anguilla)

Advances in X-ray computed tomography (CT) have led to a rise in the use of non-destructive imaging methods in comparative anatomy. Among these is contrast-enhanced CT scanning, which employs chemical stains to visualize soft tissues. Specimens may then be 'digitally dissected', producing detailed, three-dimensional digital reconstructions of the soft- and hard-tissue anatomy, allowing examination of anatomical structures in situ and making accurate measurements (lengths, volumes, etc.). Here, we apply this technique to two species of teleost fish, providing one of the first comprehensive three-dimensional (3D) descriptions of teleost cranial soft tissue and quantifying differences in muscle anatomy that may be related to differences in feeding ecology. Two species with different feeding ecologies were stained, scanned and imaged to create digital 3D musculoskeletal reconstructions: Esox lucius (Northern Pike), predominantly a suction feeder; and Anguilla anguilla (European eel), which captures prey predominantly by biting. Muscle cross-sectional areas were calculated and compared between taxa, focusing on muscles that serve important roles in feeding. The adductor mandibulae complex - used in biting - was larger in Esox than Anguilla relative to head size. However, the overall architecture of the adductor mandibulae was also very different between the two species, with that of Anguilla better optimized for delivering forceful bites. Levator arcus palatini and sternohyoideus - which are used in suction feeding - are larger in Esox, whereas the levator operculi is larger in Anguilla. Therefore, differences in the size of functionally important muscles do not necessarily correlate neatly with presumed differences in feeding mode.

Burrowing and subsurface locomotion in anguilliform fish: behavioral specializations and mechanical constraints

Fish swimming is probably one of the most studied and best understood locomotor behaviors in vertebrates. However, many fish also actively exploit sediments. Because of their elongate body shape, anguilliform fishes are not only efficient swimmers but also very maneuverable. Consequently, many species live in complexly structured environments near the bottom and many are known to burrow into the sediment. To better understand burrowing and subsurface locomotion in anguilliform fish we provide descriptive kinematic data on subsurface locomotion in a burrowing eel (Pisodonophis boro) using videofluoroscopy. We also measured the maximal forces that can be exerted by this species during head-first and tail-first burrowing, and explored the implications of head-first burrowing on mechanical stress distribution in the skull. Our data show that P. boro uses lateral undulation to penetrate and move in sandy sediments under water. The kinematics of subsurface locomotion are different from those observed during swimming and are characterized by a very high slip factor. These observations differ considerably from recently published data in terrestrial sand-swimming lizards, and suggest that the sediment behaves like a solid rather than a frictional fluid. Finally, our finite element models show that the cranial shape and structure in the head-first burrowing P. boro is mechanically more suited for head-first burrowing than that of an obligate tail-first burrowing species, Heteroconger hassi.

Cephalic morphology of Pythonichthys macrurus (Heterenchelyidae: Anguilliformes): specializations for head-first burrowing

The Heterenchelyidae, a family of Anguilliformes, are highly specialized fossorial eels. This study was conducted to evaluate the cranial specialization in relation to head-first burrowing behavior in the heterenchelyid, Pythonichthys macrurus. Thereby, detailed descriptions are provided of the cranial myology and osteology of P. macrurus and its differences with that of representatives of three families: the Moringuidae (Moringua edwardsi), a head-first burrower; the Anguillidae (Anguilla anguilla), a nonburrowing representative and the Ophichthidae (Pisodonophis boro), a head and tail-first burrower. This comparison may help to get a better understanding of the cranial specialization of head-first burrowers in heterenchelyids and moringuids. We recognize as morphological adaptations to burrowing: reduced eye size, a caudoventral orientation of the anteromedial section of the adductor mandibulae muscle complex, the posterior position of the quadrate-mandibular joint, a solid conical skull, large insertion sites of epaxial and hypaxial muscle on the neurocranium, a widened cephalic lateral line canals extending into the dermal cavities, and a ventral position of the gill opening.

Morphological specializations in heterocongrinae (Anguilliformes: Congridae) related to burrowing and feeding

The remarkable lifestyle of heterocongrines has drawn the attention of many authors in the past, though no or little attention has been paid to the morphology of the tail and the head of these species. In order to examine the true nature of possible morphological specializations of the head and tail and their relation to their tail-first burrowing habit and/or feeding mode, a detailed myological and osteological study of Heteroconger hassi and Heteroconger longissimus was performed. The osteological similarities of the cranial skeleton between H. hassi and H. longissimus are striking. Most of the cranial muscles show no variation in presence, insertion or origin between these two species except for the adductor mandibulae complex, the adductor hyomandibulae and the intermandibularis. The adductor mandibulae complex is small, compared to that of other anguilliform species, and is probably related to their suction-dominated feeding mode and a diet, comprising mainly small, soft prey items. Heterocongrinae have undergone several morphological specializations in the tail for their tail-first burrowing lifestyle. The skeleton and musculature of the tail of H. hassi and H. longissimus are similar. In both species the caudal skeleton is highly reduced and fortified, forming a firm, pointed burrowing tool. Intrinsic caudal musculature is reduced and some muscles (interradials, supracarinalis) are even absent.

Pisodonophis boro (ophichthidae: anguilliformes): Specialization for head-first and tail-first burrowing?

The rice paddy eel, Pisodonophis boro (P. boro), is of special interest because of its peculiar burrowing habits. P. boro penetrates the substrate tail-first, a technique common for ophichthids, but it is able to burrow head-first as well. P. boro exhibits three feeding modes: inertial feeding, grasping, and spinning. Rotational feeding is a highly specialized feeding mode, adopted by several elongate, aquatic vertebrates and it is likely that some morphological modifications are related to this feeding mode. The detailed morphology of the head and tail of P. boro is examined with the goal to apportion the anatomical specializations among head-first burrowing, tail-first burrowing, and rotational feeding. The reduced eyes, covered with thick corneas may be beneficial for protection during head-first burrowing, but at the same time decreased visual acuity may have an impact on other sensory systems (e.g. cephalic lateral line system). The elongated and pointed shape of the skull is beneficial for substrate penetration. The cranial bones and their joints, which are fortified, are advantageous for resisting high mechanical loads during head-first burrowing. The aponeurotic connection between epaxial and jaw muscles is considered beneficial for transferring these forces from the body to the head during rotational feeding. Hypertrophied jaw muscles facilitate a powerful bite, which is required to hold prey during spinning movements and variability in the fiber angles of subdivisions of jaw muscles may be beneficial for preventing the lower jaw from being dislodged or opened. Furthermore, firm upper (premaxillo-ethmovomerine complex) and lower jaws (with robust coronoid processes) and high neurocranial rigidity are advantageous for a solid grip to hold prey during rotational feeding. The pointed shape of the tail and the consolidated caudal skeleton are beneficial for their tail-first burrowing habits. It is quite likely that the reduction of the caudal musculature is related to the tail-first burrowing behavior because the subtle movements of the caudal fin rays are no longer required.

The erectile cheek-spine apparatus in the bristlenose catfish Ancistrus (Loricariidae, Siluriformes), and its relation to the formation of a secondary skull roof

In the South American catfish family Loricariidae, the opercle has been decoupled from the lower jaw, and has also lost its function in expiration. While many loricariid species have a small and slightly mobile opercle with reduced opercular musculature, within the hypostomine subfamily a novel opercular mechanism has developed that erects a tuft of enlarged odontodes anterior to the opercle. This defensive mechanism is examined in Ancistrus cf. triradiatus. The opercle has a prominent anterior process and the orientation of the reinforced articulation hinge to the hyomandibular bone has shifted. The opercular musculature is well developed, with a hypertrophied dilatator operculi that extends deep inside the skull roof bones and toward the midline, over the brain, but below the superficial skull roof. Hence the frontal, sphenotic, parieto-supraoccipital and compound pterotic bones consist of a dorsal, superficial part and a deeper part separating the brain from the muscle: two functional skull roofs are thus formed. The impact on the path of the cranial sensory canals is substantial, moving canals away from the skull surface. Hypertrophy of cranial muscles is known from many teleosts, but the invasion of such large muscles into the skull, which is drastically modified and literally hollowed out, has never been described before. These cranial modifications are greater in males than in females, related to the territorial behavior of the former, in which the erectile spines are usually used.