Composition of fluid from the notochordal canal of the coelacanth, Latimeria chalumnae

Buoyancy and hydrostatic balance in a West Indian Ocean coelacanth Latimeria chalumnae

Background

Buoyancy and balance are important parameters for slow-moving, low-metabolic, aquatic organisms. The extant coelacanths have among the lowest metabolic rates of any living vertebrate and can afford little energy to keep station. Previous observations on living coelacanths support the hypothesis that the coelacanth is neutrally buoyant and in close-to-perfect hydrostatic balance. However, precise measurements of buoyancy and balance at different depths have never been made. 

Results

Here we show, using non-invasive imaging, that buoyancy of the coelacanth closely matches its depth distribution. We found that the lipid-filled fatty organ is well suited to support neutral buoyancy, and due to a close-to-perfect hydrostatic balance, simple maneuvers of fins can cause a considerable shift in torque around the pitch axis allowing the coelacanth to assume different body orientations with little physical effort.

Conclusions

Our results demonstrate a close match between tissue composition, depth range and behavior, and our collection-based approach could be used to predict depth range of less well-studied coelacanth life stages as well as of deep sea fishes in general.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01354-8.

Bone histology and microanatomy of Edaphosaurus and Dimetrodon (Amniota, Synapsida) vertebrae from the Lower Permian of Texas

Here we describe the histology and microanatomy of vertebral centra of the iconic pelycosaur-grade synapsids Edaphosaurus boanerges and Dimetrodon spp. Vertebrae from different axial positions and, in the case of Dimetrodon, from different ontogenetic stages were selected. For the histological description, we produced histological petrographic thin sections ground to a thickness of 50-80 μm of the vertebrae in the sagittal and transversal cutting planes. After the preparation process, the thin sections were examined under transmitted and cross-polarized light in a polarized microscope. The analyzed vertebrae reveal similar bone tissues, where both taxa have cortical parallel-fibered bone (PFB). PFB and lamellar bone (LB) forms in the cancellous part. However, in juvenile Dimetrodon, woven-fibered bone (WFB) is also deposited and shows a high degree of vascularity. This suggests that Dimetrodon had slightly faster bone growth than Edaphosaurus, which is mainly made of PFB and LB and shows poorly developed vascular canals. In addition, one specimen of Dimetrodon displays the preservation of an ossified notochord, which can be assumed to be indicative of how the intervertebral tissues were developed. Historically, evidence of how the joint between Dimetrodon vertebral centra was built was lacking until this specimen appeared. If the notochord ran persistently through the vertebrae, it would have possibly increased the stiffness of the vertebral column and would have affected the limbs and locomotion. Furthermore, the organization of trabeculae and relative thickness of the vertebral cortex gives insights into how the animals were adapted to their habitat.

Identification of a new mineralized tissue in the notochord of reared Siberian sturgeon (Acipenser baerii)

In a study aiming to improve knowledge on the mineralization of the axial skeleton in reared Siberian sturgeon (Acipenser baerii Brandt, 1869), we discovered a new mineralized tissue within the notochord. To our knowledge, such a structure has never been reported in any vertebrate species with the exception of the pathological mineralization of the notochord remains in degenerative intervertebral disks of mammals. Here, we describe this enigmatic tissue using X-ray microtomography, histological analyses and solid state NMR-spectroscopy. We also performed a 1-year monitoring of the mineral content (MC) of the notochord in relation with seasonal variations of temperature. In all specimens studied from 2-year-old juveniles onwards, this mineralized structure was found within a particular region of the notochord called funiculus. This feature first appears in the abdominal region then extends posteriorly with ageing, while the notochord MC also increases. The mineral phase is mainly composed of amorphous calcium phosphate, a small amount of which changes into hydroxyapatite with ageing. The putative role of this structure is discussed as either a store of minerals available for the phosphocalcic metabolism, or a mechanical support in a species with a poorly mineralized axial skeleton. A pathological feature putatively related to rearing conditions is also discussed.

Conservation of shh cis-regulatory architecture of the coelacanth is consistent with its ancestral phylogenetic position

Background

The modern coelacanth (Latimeria) is the extant taxon of a basal sarcopterygian lineage and sister group to tetrapods. Apart from certain apomorphic traits, its morphology is characterized by a high degree of retention of ancestral vertebrate structures and little morphological change. An insight into the molecular evolution that may explain the unchanged character of Latimeria morphology requires the analysis of the expression patterns of developmental regulator genes and their cis-regulatory modules (CRMs).

Results

We describe the comparative and functional analysis of the sonic hedgehog (shh) genomic region of Latimeria menadoensis. Several putative enhancers in the Latimeria shh locus have been identified by comparisons to sarcopterygian and actinopterygian extant species. Specific sequence conservation with all known actinopterygian enhancer elements has been detected. However, these elements are selectively missing in more recently diverged actinopterygian and sarcopterygian species. The functionality of the putative Latimeria enhancers was confirmed by reporter gene expression analysis in transient transgenic zebrafish and chick embryos.

Conclusions

Latimeria shh CRMs represent the ancestral set of enhancers that have emerged before the split of lobe-finned and ray-finned fishes. In contrast to lineage-specific losses and differentiations in more derived lineages, Latimeria shh enhancers reveal low levels of sequence diversification. High overall sequence conservation of shh conserved noncoding elements (CNE) is consistent with the general trend of high levels of conservation of noncoding DNA in the slowly evolving Latimeria genome.

Comparative ultrastructure of the cellular components of the unconstricted notochord in the sturgeon and the lungfish

Unconstricted notochords are found in only a few extant vertebrates. Within the bony fishes the notochord is present throughout life in the sturgeons and the lungfishes. To better understand the function of this organ, the cellular ultrastructure of the notochord was examined in the shortnose sturgeon, Acipenser brevirostratus, and in the African lungfish, Protopterus annectens. In both species, the medulla of the notochord consists of vacuolated cells connected by desmosomes. The vacuoles are surrounded by a dense cytoplasmic network of intermediate filaments. In Acipenser, the cuboidal to columnar basal cells are avacuolate and contain an extensive network of rough endoplasmic reticulum. In Protopterus, the cytoplasm of the basal cells lacks rough endoplasmic reticulum and often contains a large central vacuole surrounded by a network of intermediate filaments. Cells resembling fibroblasts are found in the fibrous sheath of the notochord of the lungfish, but the fibrous sheath of the sturgeon is acellular. On the basis of the structures observed in the two species, the medulla of the notochord has many of the characteristics of a keratinized epithelium, and the fibrous sheath may be derived from different cellular sources. J. Morphol. 236:75-104, 1998. © 1998 Wiley-Liss, Inc.