Phylogenetic trends in the abundance and distribution of pit organs of elasmobranchs

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Journal Article
Acta Zoologica, 2005, 85 pp. 233 - 244
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Pit organs (free neuromasts of the mehcanosensory lateral line system) are distributed over the skin of elasmobranchs. To investigate phylogenetic trends in the distribution and abundance of pit organs, 12 relevant morphological characters were added to an existing matrix of morphological data (plus two additional end terminals), which was then re-analysed using cladistic parsimony methods (PAUP*Ob10). Character transformations were traced into the most parsimonious phylogenetic trees. The results suggest the following interpretations. First, the distinctive overlapping denticles covering the pit organs in many sharks are a derived freature; plesiomorphic elasmobranchs have pit organs in ope slits, with widely spaced accessory denticles. Second, the number of pit organs on the ventral surface of rays has been reduced during evolution, and third, spiracular pit organs have changed position or have been lost on several occasions in elasmobranch evolution. The concentrated-changes test in MACCLADE (version 4.05) was used to investigate the association between a pelagic lifestyle and loss of spiracular pit organs (the only character transformaion that occurred more than once within pelagic taxa). Depending on the choice o ftree, the association was either nonsignificant at P=0.06 or significant at P<0.05. Future studies, using species within more restricted elasmobranch clades, are needed to resolve this issue.
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