Evolutionary trajectories of a redundant feature: lessons from bivalve gill abfrontal cilia and mucocyte distributions
Peter G. Beninger, Suzanne C. Dufour, 2000. "Evolutionary trajectories of a redundant feature: lessons from bivalve gill abfrontal cilia and mucocyte distributions", The Evolutionary Biology of the Bivalvia, E. M. Harper, J. D. Taylor, J. A. Crame
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Recent data on the distributions of cilia and mucocytes on the bivalve gill abfrontal surface are analysed with respect to evolutionary relationships of the principal autobranch gill types. From the primitive function as a mucociliary cleaning surface in the protobranchs, two evolutionary trajectories are evident: (1) progressive reduction of both cilia and mucocytes with resultant loss of surface function, seen in the homorhabdic filibranchs studied; (2) reduction of cilia but retention or increase in acid mucopolysaccharide-secreting (AMPS) mucocyte density in the eulamellibranchs, corresponding to the assumption of a new function, probably in the reduction of frictional resistance to flow in the water canals. Heterorhabdic gill abfrontal surfaces present a mixture of these characteristics: reduction of cilia and mucocytes on the ordinary filaments, and retention of both on the principal filaments. The retention of AMPS mucocytes on the abfrontal surface of the pseudolamellibranchs may be related to the degree of interlamellar fusion, reducing frictional resistance to water flow as in the eulamellibranchs. The gill abfrontal surface thus constitutes an excellent candidate for the study of the different evolutionary options and trajectories of a redundant feature.
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The Evolutionary Biology of the Bivalvia
Bivalves are key components of Recent marine and freshwater ecosystems and have been so for most of the Phanerozoic. Their rich and long fossil record, combined with their abundance and diversity in modern seas, has made bivalves the ideal subject of palaeobiological and evolutionary studies. Despite this, however, topics such as the early evolution of the class, relationships between various taxa and the life habits of some key extinct forms have remained remarkably unclear.
In the last few years there has been enormous expansion in the range of techniques available to both palaeontologists and zoologists and key discoveries of new faunas which shed new light on the evolutionary biology of this important class.
This volume integrates palaeontological and zoological approaches and sheds new light on the course of bivalve evolution. This series of 32 original papers tackles key issues including: up to date molecular phylogenies of major groups; new hard and soft tissue morphological cladistic analyses; reassessments of the early Palaeozoic radiation; important new observations on form and functional morphology; analyses of biogeography and biodiversity; novel (palaeo)ecological studies