Ontogenetic shifts and spatial associations in organ positions for snakes

Ultrasonographic evaluation of the liver and gallbladder and hepatic histogram of non-venomous snakes

This study aimed to describe sonographic features of the liver, gallbladder and hepatic histogram from grey-scale ultrasound in three species of healthy non-venomous snakes. Twenty-eight adult snakes were enrolled in the study, including 10 common boas (Boa constrictor), eight black-tailed pythons (Python molurus) and 10 rainbow boas (Epicrates crassus). The snakes fasted for 30 days and were manually restrained while conscious. For B. constrictor and P. molurus the liver and gallbladder were best visualized in ventral recumbency, and E. crassus in dorsal recumbency. A single elongated hepatic lobe was identified in all snakes. The gallbladder was positioned caudal and separated from the liver, with an oval shape and homogeneous anechoic content in the lumen, and thin and regular walls. A region of interest by pixel number was chosen for the liver, fat bodies, left kidney, and splenopancreas. The mean grey level (G) of the organs had significant differences within each species. Standard deviation of grey levels (SG ) had significant differences within B. constrictor and E. crassus. P. molurus had no significant difference among organs. The comparison among snakes showed that E. crassus had G of liver and splenopancreas lower than B. constrictor and P. molurus. The SG of the liver in E. crassus was lowest compared to B. constrictor and P. molurus. P. molurus showed the highest values in mean of G and SG . In conclusion, despite the liver and gallbladder having similar sonographic features, the grey-level histogram showed that liver echotexture and echogenicity differ among species.

Regionalization of the vertebral column and its correlation with heart position in snakes: Implications for evolutionary pathways and morphological diversification

Spinal regionalization has important implications for the evolution of vertebrate body plans. We determined the variation in the number and morphology of vertebrae across the vertebral column (i.e., vertebral formula) for 63 snake species representing 13 families using intracolumnar variation in vertebral shape. Vertebral counts were used to determine the position of the heart, pylorus, and left kidney for each species. Across all species we observed a conspicuous midthoracic transition in vertebral shape, indicating four developmental domains of the precloacal vertebral column (cervical, anterior thoracic, posterior thoracic, and lumbar). Using phylogenetic analyses, the boundary between the anterior and posterior thoracic vertebrae was correlated with heart position. No associations were found between shifts in morphology of the vertebral column and either the pylorus or left kidney. We observed that among taxa, the number of preapex and postapex vertebrae could change independently from one another and from changes in the total number of precloacal vertebrae. Ancestral state reconstruction of the preapex and postapex vertebrae illustrated several evolutionary pathways by which diversity in the vertebral column and heart position have been attained. In addition, no conspicuous pattern was observed among the heart, pylorus, or kidney indicating that their relative positions to each other evolve independently. We conclude that snakes exhibit four morphologically distinct regions of the vertebral column. We discuss the implications of the forebody and hindbody vertebral formula on the morphological diversification of snakes.

Macroevolution in axial morphospace: innovations accompanying the transition to marine environments in elapid snakes

Sea snakes in the Hydrophis-Microcephalophis clade (Elapidae) show exceptional body shape variation along a continuum from similar forebody and hindbody girths, to dramatically reduced girths of the forebody relative to hindbody. The latter is associated with specializations on burrowing prey. This variation underpins high sympatric diversity and species richness and is not shared by other marine (or terrestrial) snakes. Here, we examined a hypothesis that macroevolutionary changes in axial development contribute to the propensity, at clade level, for body shape change. We quantified variation in the number and size of vertebrae in two body regions (pre- and post-apex of the heart) for approximately 94 terrestrial and marine elapids. We found Hydrophis-Microcephalophis exhibit increased rates of vertebral evolution in the pre- versus post-apex regions compared to all other Australasian elapids. Unlike other marine and terrestrial elapids, axial elongation in Hydrophis-Microcephalophis occurs via the preferential addition of vertebrae pre-heart apex, which is the region that undergoes concomitant shifts in vertebral number and size during transitions along the relative fore- to hindbody girth axis. We suggest that this macroevolutionary developmental change has potentially acted as a key innovation in Hydrophis-Microcephalophis by facilitating novel (especially burrowing) prey specializations that are not shared with other marine snakes.

Heart position and pulmonary vasculature in snakes with different lung morphologies

The respiratory system of snakes, composed of a trachea and one or two lungs, shows considerable variation in terms of size and complexity, especially in terms of length and distribution of the respiratory epithelium. The importance of heart position within snakes has previously been investigated concerning gravitational stress. The relationship between respiratory gas exchange epithelium and heart position, however, has not been addressed in detail, which seems necessary, since the heart needs to pump blood through the pulmonary circulation for effective gas exchange. Herein, we analyze the morphology of the respiratory epithelium in Boa constrictor and Crotalus durissus stereologically regarding the composition of the gas exchange tissue and the distribution of blood vessels within the vascularized parts of the respiratory system. The gas exchange epithelium is composed of blood capillaries, larger vessels, trabeculae, and septa, forming an overall faveolar-type epithelium in both species. Pulmonary capillaries and respiratory surface area showed a tendency to be more concentrated in the anterior and middle portions of each lung’s respiratory epithelium, suggesting a tendency toward greater parenchymal development in these regions. Therefore, there seems to be no conclusive relationship between the position of the heart and pulmonary circulation, since in C. durissus the anterior and middle parenchymal regions are distant from the heart, whereas in B. constrictor the anterior and middle parenchymal regions are close to the heart, facilitating blood transport between the heart and the gas exchange epithelium.

Comparative and Functional Anatomy of the Ectothermic Sauropsid Heart

The heart development, form, and functional specializations of chelonians, squamates, crocodilians, and birds characterize how diverse structure and specializations arise from similar foundations. This review aims to summarize the morphologic diversity of sauropsid hearts and present it in an integrative functional and phylogenetic context. Besides the detailed morphologic descriptions, the integrative view of function, evolution, and development will aid understanding of the surprising diversity of sauropsid hearts. This integrated perspective is a foundation that strengthens appreciation that the sauropsid hearts are the outcome of biological evolution; disease often is linked to arising mismatch between adaptations and modern environments.

Interspecific variation in organ position in hydrophiine snakes is explained by modifications to the vertebral column

Interspecific disparities in the position of the internal organs of snakes have been associated with evolutionary history and cardiovascular performance, as influenced by habitat use. For snakes, the positions of internal organs are typically determined as a linear measurement relative to body length. Therefore, interspecific variation in organ position could be explained either as heterotopic shifts in organ position or by modifications to the vertebral column. Using vertebral counts from radiographs, I determined the positions of the atria and pyloric sphincter relative to the cloaca in hydrophiine sea snakes. I found interspecific variation in the number of pre-atrial vertebrae to be labile, whereas the number of vertebrae in the atria to pyloric sphincter region and in the pyloric sphincter to cloaca region was relatively constrained. Furthermore, the number of pre-atrial vertebrae was dissociated from the number of vertebrae between the atria and cloaca, indicating that these two regions of the vertebral column can evolve independently. I conclude that variation in organ position among hydrophiine sea snake species is attributable, in part, to differences in the number of vertebrae among regions of the vertebral column rather than to heterotopic shifts in the positions of the internal organs.

Variation of organ position in snakes

The complex and successful evolutionary history of snakes produced variation in the position and structure of internal organs. Gravity strongly influences hemodynamics, and the impact on structure and function of the cardiovascular system, including pulmonary circulation, is well established. Therefore, we hypothesized that interspecific variation in the position of the heart and vascular (faveolar) lung should exceed that of other internal organs that are less sensitive to gravity. We examined the position of selected internal organs in 72 snakes representing 5 families and 13 species including fully aquatic and scansorial/arboreal species, representing the extremes of gravitational influence. Tests for differences of variance and coefficients of variation largely confirm that interspecific variation in position of the heart and vascular lung generally exceed those of other organs that we measured, particularly posterior organs. The variance of heart position generally exceeded that of more posterior organs, was similar to that of the anterior margin of the vascular lung, and was exceeded by that of the posterior margin of the vascular lung (variance ratio = 0.23). The gravity-sensitive vascular lung exhibited the greatest variation of any organ. Importantly, these findings corroborate previous research demonstrating the influence of gravity on cardiopulmonary morphology. Snakes offer useful model systems to help understand the adaptation of organs to a spectrum of conditions related to diversity of behavior and habitat across a broad range of related taxa.

Ontogenetic shifts of heart position in snakes

Heart position relative to total body length (TL) varies among snakes, with anterior hearts in arboreal species and more centrally located hearts in aquatic or ground-dwelling species. Anterior hearts decrease the cardiac work associated with cranial blood flow and minimize drops in cranial pressure and flow during head-up climbing. Here, we investigate whether heart position shifts intraspecifically during ontogenetic increases in TL. Insular Florida cottonmouth snakes, Agkistrodon conanti, are entirely ground-dwelling and have a mean heart position that is 33.32% TL from the head. In contrast, arboreal rat snakes, Pantherophis obsoleta, of similar lengths have a mean heart position that is 17.35% TL from the head. In both species, relative heart position shifts craniad during ontogeny, with negative slopes = -.035 and -.021% TL/cm TL in Agkistrodon and Pantherophis, respectively. Using a large morphometric data set available for Agkistrodon (N = 192 individuals, 23-140 cm TL), we demonstrate there is an anterior ontogenetic shift of the heart position within the trunk (= 4.56% trunk length from base of head to cloacal vent), independent of head and tail allometry which are both negative. However, in longer snakes > 100 cm, the heart position reverses and shifts caudally in longer Agkistrodon but continues toward the head in longer individuals of Pantherophis. Examination of data sets for two independent lineages of fully marine snakes (Acrochordus granulatus and Hydrophis platurus), which do not naturally experience postural gravity stress, demonstrate both ontogenetic patterns for heart position that are seen in the terrestrial snakes. The anterior migration of the heart is greater in the terrestrial species, even if TL is standardized to that of the longer P. obsoleta, and compensates for about 5 mmHg gravitational pressure head if they are fully upright.

Differential growth of body segments explains ontogenetic shifts in organ position for the Diamondback Water Snake (Nerodia rhombifer)

As snakes grow, their organs move anteriorly relative to body size. We explored a developmental explanation for the ontogenetic shift in the relative position of internal organs for snakes using the Diamondback Water Snake (Nerodia rhombifer (Hallowell, 1852)). With age, this water snake’s heart, liver, small intestine, and right kidney move anteriorly by 2.5–5.0 percentage points of snout–vent length. The number of precaudal vertebrae did not vary due to size or sex. The anterior edge of the heart, liver, small intestine, and right kidney were typically aligned within a span of 4–8 vertebrae that likewise did not differ as a function of size or sex. Snakes exhibited a positive relationship between the number of precaudal vertebrae and the vertebra number aligned with each organ. Total length, centrum length, centrum width, ball width, height, and mass of eight vertebrae sampled at consistent vertebral number revealed that vertebrae in the middle region of the body grow at a greater rate than vertebrae at the anterior or distal ends of the body. For N. rhombifer, the observed forward shift in relative organ positions is the product of regional differences in the growth of body segments. Predictably, these differences arise from a developmental program generated by the differential expression of Hox genes.