Abstract
The coelacanth, Latimeria chalumnae Smith, 1939 [1] (Sarcopterygii: Actinistia), together with the closely related L. menadoensis Pouyaud et al ., 1999 [2], remains the only living representative of one of the most basally-branching primary radiations of lobe-finned fishes (Sarcopterygii). Even though extant species cannot be considered ‘primitive’ due to the inherent logic of phylogenetic theory, the coelacanth nonetheless is invaluable for understanding evolutionary transformations in basal sarcopterygians as it can help in the determination of character polarity. The appearance of one novelty during early vertebrate evolution that had major implications for the success of a huge number of species is the origin of lungs. The conventional interpretation is that lungs evolved in basal bony fishes (Osteichthyes or Osteognathostomata), were maintained in the lobe-finned fishes, and eventually were transformed into a swimbladder among the ray-finned fishes (Actinopterygii) (e.g. [3]). However, the currently available data do not rule out separate origins of lungs and swimbladders from a common ‘respiratory pharynx’, even though this would require a slightly less parsimonious course of evolution [4,5]. The coelacanth is a key species in addressing this question and for this reason the data recently provided by Cupello and colleagues [6] are a very welcome addition to the discussion. Here, I would like to add a few points pertinent to lung evolution that appear to be a consequence of these exciting data. One of the most interesting aspects of the coelacanth is that it apparently exhibits an unpaired structure of putative homology with lungs [6–8]. In the Polypteriformes (bichir and reed fish), the lungs are paired [5,9,10], as are those of the lungfishes (Dipnoi) [11], except the Australian lungfish, Neoceratodus forsteri (Krefft, 1870) …
Highlights
One of the most interesting aspects of the coelacanth is that it apparently exhibits an unpaired 2 structure of putative homology with lungs [6,7,8]
Is the unpaired condition the result of a direct development or does the coelacanth exhibit a secondary reduction of one of its lungs? It has been speculated that the dual presence of an esophagial diverticulum and a fatty organ in coelacanths could indicate a paired lung homologue, one of which became fat-filled whereas the other one degenerated to the small diverticulum [5]
The ‘main lung’ could be represented by the fatty organ in which fat accumulation dominated the developmental process, resulting in a singular ‘solid’ organ with a buoyancy function. Such a scenario would be in agreement with the palaeontological data on the pulmonary complex for early actinistians [20] and the hypothesis that a fatty organ became the dominant buoyancy regulator once the habitat was shifted towards deeper water depths [6,8]
Summary
One of the most interesting aspects of the coelacanth is that it apparently exhibits an unpaired 2 structure of putative homology with lungs [6,7,8]. If one of the lungs persists in a vestigial form in the adult, it presents an arrested early developmental stage [17].
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