New Insights About the Behavioral Ecology of the Coelacanth Latimeria chalumnae Video Recorded in the Absence of Humans Off South Africa

Graphical Abstract

Coelacanth discoveries in Madagascar, with recommendations on research and conservation

The presence of populations of the Western Indian Ocean coelacanth (Latimeria chalumnae) in Madagascar is not surprising considering the vast range of habitats which the ancient island offers. The discovery of a substantial population of coelacanths through handline fishing on the steep volcanic slopes of Comoros archipelago initially provided an important source of museum specimens and was the main focus of coelacanth research for almost 40 years. The advent of deep-set gillnets, or jarifa, for catching sharks, driven by the demand for shark fins and oil from China in the mid- to late 1980s, resulted in an explosion of coelacanth captures in Madagascar and other countries in the Western Indian Ocean. We review coelacanth catches in Madagascar and present evidence for the existence of one or more populations of L. chalumnae distributed along about 1000 km of the southern and western coasts of the island. We also hypothesise that coelacanths are likely to occur around the whole continental margin of Madagascar, making it the epicentre of coelacanth distribution in the Western Indian Ocean and the likely progenitor of the younger Comoros coelacanth population. Finally, we discuss the importance and vulnerability of the population of coelacanths inhabiting the submarine slopes of the Onilahy canyon in southwest Madagascar and make recommendations for further research and conservation.

Mandibular musculature constrains brain–endocast disparity between sarcopterygians

The transition from water to land by the earliest tetrapods in the Devonian Period is seen as one of the greatest steps in evolution. However, little is understood concerning changes in brain morphology over this transition. Here, we determine the brain–braincase relationship in fishes and basal lissamphibians as a proxy to elucidate the changes that occurred over the fish–tetrapod transition. We investigate six basal extant sarcopterygians spanning coelacanths to salamanders ( Latimeria chalumnae, Neoceratodus, Protopterus aethiopicus, P. dolloi, Cynops, Ambystoma mexicanum ) using micro-CT and MRI and quantify the brain–braincase relationship in these extant taxa. Our results show that regions of lowest brain–endocast disparity are associated with regions of bony reinforcement directly adjacent to masticatory musculature for the mandible except in Neoceratodus and Latimeria . In Latimeria this deviation from the trend can be accounted for by the possession of an intracranial joint and basicranial muscles, whereas in Neoceratodus difference is attributed to dermal bones contributing to the overall neurocranial reinforcement. Besides Neoceratodus and Latimeria, regions of low brain–endocast disparity occur where there is less reinforcement away from high mandibular muscle mass, where the trigeminal nerve complex exits the braincase and where endolymphatic sacs occupy space between the brain and braincase wall. Despite basal tetrapods possessing reduced adductor muscle mass and a different biting mechanism to piscine sarcopterygians, regions of the neurocranium lacking osteological reinforcement in the basal tetrapods Lethiscus and Brachydectes broadly correspond to regions of high brain–endocast disparity seen in extant taxa.

Evolution and domestication of Tc1/mariner transposons in the genome of African coelacanth (Latimeria chalumnae)

Here, we comprehensively analysed the abundance, diversity, and activity of Tc1/mariner transposons in African coelacanth (Latimeria chalumnae). Fifteen Tc1/mariner autonomous transposons were identified and grouped into six clades: DD34E/Tc1, DD34D/mariner, DD35D/Fot, DD31D/pogo, DD30-31D/pogo-like, and DD32–36D/Tigger, belonging to three known families: DD34E/Tc1, DD34D/mariner, and DD×D/pogo (DD35D/Fot, DD31D/pogo, DD30-31D/pogo-like, and DD32-36D/Tigger). Thirty-one miniature inverted-repeat transposable element (MITE) transposons of Tc1/mariner were also identified, and 20 of them display similarity to the identified autonomous transposons. The structural organization of these full Tc1/mariner elements includes a transposase gene flanked by terminal inverted repeats (TIRs) with TA dinucleotides. The transposases contain N-terminal DNA binding domain and a C-terminal catalytic domain characterized by the presence of a conservative D(Asp)DE(Glu)/D triad that is essential for transposase activity. The Tc1/mariner superfamily in coelacanth exhibited very low genome coverage (0.3%), but it experienced an extraordinary difference of proliferation dynamics among the six clades identified; moreover, most of them exhibited a very recent and current proliferation, suggesting that some copies of these transposons are putatively active. Additionally, at least four functional genes derived from Tc1/mariner transposons were found. We provide an up-to-date overview of Tc1/mariner in coelacanth, which may be helpful in determining genome and gene evolution in this living fossil.

Lung anatomy and histology of the extant coelacanth shed light on the loss of air-breathing during deep-water adaptation in actinistians

Lungs are specialized organs originated from the posterior pharyngeal cavity and considered as plesiomorphic for osteichthyans, as they are found in extant basal actinopterygians (i.e. Polypterus ) and in all major groups of extant sarcopterygians. The presence of a vestigial lung in adult stages of the extant coelacanth Latimeria chalumnae is the result of allometric growth during ontogeny, in relation with long-time adaptation to deep water. Here, we present the first detailed histological and anatomical description of the lung of Latimeria chalumnae , providing new insights into its arrested differentiation in an air-breathing complex, mainly represented by the absence of pneumocytes and of compartmentalization in the latest ontogenetic stages.