ON THE PECULIARITIES OF THE EVOLUTIONARY PROCESS IN SHRIMPS (CRUSTACEA, DECAPODA)

摘要

During the evolution of shrimps, the essential aspects of their organization have remained unchanged. Approximately the same adaptations that have generated a large number of parallelisms within this relatively small group of arthropods have been used in the process of their adaptive evolution. This evolutionary process is a typical telogenesis. The ancient shrimp fauna of both Dendrobranchiata and Pleocyemata could have originated in shallow waters not later than the Jurassic. This is confirmed by the remains of Dendrobranchiata found in the shallow sediments of the Cenomanian (early Upper Cretaceous), and of Pleocyemata as inhabitants of shallow water refugia and representatives of the most primitive caridean shrimp family Procarididae. The latter group is very similar morphologically to fossil shrimps of the Jurassic, for example Udorella agassizi. Later, as a result of adaptive radiations at the generic level, the shrimps occupied both continental slope and pelagic zone. These shallow-water species could have been ousted then by a new shrimp fauna during its colonization of the shelf. The previous shrimp fauna might have shifted to and kind of bordered the continental slope. Some part of the species from various genera could have moved deeper along the slope down to the abyssal zone, while some others might have shifted to occupy the pelagic zone. This seems to have been associated with food resources being limited on the slope in the places with low levels of terrigenous sedimentogenesis. Numerous species seem to have switched from the detrital food chain to the grazing one. That process must have been repeated several times, resulting in the survivor representatives of the most ancient and its next faunas to have occurred at ever greater depths. As it follows, a pulsing type of telogenesis has been characteristic of shrimps. Therefore, the modern shrimp fauna consists of young shelf groups with intensive cladogenesis, as well as of groups that have persisted since the Cretaceous (or even Jurassic), and of remains of earlier faunas that have survived at great depths or shallow-water refugia. This must be considered by researchers who use morphological and molecular methods for studying the phylogenetic relationships within and between the two suborders of Decapoda crustaceans.