Development of the caudal-fin skeleton reveals multiple convergent fusions within Atherinomorpha

Abstract

Background The caudal fin of teleosts is a highly diverse morphological structure and a valuable source of information for comparative analyses. Within the Atherinomorpha a high variation of conditions of the caudal-fin skeleton can be found. These range from complex but basal configurations to simple yet derived configurations. When comparing atherinomorph taxa, it is often difficult to decide on the homology of skeletal elements if only considering adult specimens. However, observing the development of caudal-fin skeletons allows one to evaluate complex structures, reveal homologies and developmental patterns, and even reconstruct the grundplan of the examined taxa. Results We studied the development of the caudal-fin skeleton in different atheriniform, beloniform and cyprinodontiform species using cleared and stained specimens. Subsequently we compared the development to find similarities and differences in terms of 1) which structures are formed and 2) which structures fuse during ontogeny. For many structures, i.e., the parhypural, the epural(s), the haemal and neural spines of the preural centra and the uroneural, there were either no or only minor differences visible between the three taxa. However, the development of the hypurals revealed a high variation of fusions within different taxa that partly occurred independently in atheriniforms, beloniforms and cyprinodontiforms. Moreover, comparing the development of the ural centra exposed two ways of formation of the compound centrum: 1) in atheriniforms and the beloniforms Oryzias and Hyporhamphus limbatus two ural centra develop and fuse during ontogeny while 2) in cyprinodontiforms and Exocoetidae (Beloniformes) only a single ural centrum is formed during ontogeny. Conclusions We were able to reconstruct the grundplan of the developmental pattern of the caudal-fin skeleton of the Atheriniformes, Beloniformes and Cyprinodontiformes as well as their last common ancestors. We found two developmental modes of the compound centrum within the Atherinomorpha, i.e., the fusion of two developing ural centra in atheriniforms and beloniforms and the development of only one ural centrum in cyprinodontiforms. Further differences and similarities for the examined taxa are discussed, resulting in the hypothesis that the caudal-fin development of a last common ancestor to all atherinomorphs is very much similar to that of extant atheriniforms.

Morphological characters in light of new molecular phylogenies: the caudal-fin skeleton of Ovalentaria

“…Leuresthes tenuis has two ural centra (U1 + U2) in early ontogeny that later fuse to form the CC [ 68 , 70 ]. In Atherinella eriarcha the PH is fused to the LHP.…”

“…Hemiramphidae —3 out of 8 genera examined [ 46 , 70 , 72 – 74 ], e.g. Hyporhamphus limbatus ( figure 3 e ).…”

“…Ontogenetic stages of Aplocheilus lineatus suggest that one elongated UC develops anterior to the hypural plates [ 70 ]. It cannot be determined if this UC represents UC1, UC2 or if it is a product of evolutionary fusion of these two UC.…”

“…Cichlidae —19 out of 251 genera examined [ 45 , 46 , 70 , 93 , 129 , 130 ], e.g. Coptodon zillii ( figure 5 a ).…”

“…Ontogenetic data of different species (e.g. Astatotilapia burtoni , Hemichromis bimaculatus ) indicate the development of one elongated UC anterior to the hypurals [ 70 , 130 ]. It cannot be determined if this UC represents UC1, UC2 or if it is a product of evolutionary fusion of these two UC.…”