Freshwater crab sister group relationships with marine eubrachyuran families were investigated. A morphology‐based cladistic analysis was conducted on representatives of the freshwater crab families Deckeniidae, Gecarcinucidae, Parathelphusidae, Potamidae, Potamonautidae, Pseudothelphusidae, and Trichodactylidae using a disparate assemblage of marine heterotreme and thoracotreme brachyurans as possible sister groups. The monophyly of the freshwater crabs sensu lato is falsified. The family Trichodactylidae and the marine portunid subfamily Carcininae form basal groups within the superfamily Portunoidea. The monophyly of the Pseudothelphusidae and the Paleotropical freshwater crab families is supported, and this clade is the sister group of the Thoracotremata (Gecarcinidae, Grapsidae s.l., and Ocypodoidea). The origin, groundplan, and diversification of freshwater crabs are discussed in the context of previously published scenarios of their evolution.

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ABSTRACT. Pseudothelphusidae is a well diversified group of Neotropical freshwater crabs currently comprising 40 genera and at least 255 species and subspecies. The biology of these crabs has been an active field of research in the last 20 years. The aim of the present contribution is to discuss the significance of the new knowledge on the biology of these freshwater crabs after September 1992, to stress the interconnection of the diverse lines of research and at the same time to suggest promising new lines of investigation. All taxa described from

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A revision of the cladistic analysis presented by Rodriguez and Pereira (1992) for the species of the genus Fredius Pretzmann, 1967 (Pseudothelphusidae), was carried out using additional species and new homologies to obtain a more robust cladogram. The tree obtained differs from the previous one in the clustering of species, but the area cladogram confirms a previous theory on the plausible evolution of the group in the Guianan lowlands of South America, between the Orinoco and the Amazon rivers. A new species is described from Colombia, extending considerably the range of the genus into the upper Amazon basin. A key to all species in the genus is given.

Text-figs, l-i 1)Palaemon elegans Rathke on South Wales coasts, unlike P. serratus (Pennant), does not migrate offshore in winter but stays in intertidal pools throughout the year. Both species occur in rock pools in summer, with P. elegans extending farthest up the shore and often occurring in pools above M.H.W.N. Each shows a spontaneous but short-lived tidal rhythm of locomotor activity in constant laboratory conditions, the rhythm persisting longer in isolated prawns than in groups of animals recorded together. Peak activity occurred on the 'expected' ebb, and light/dark changes, including those associated with tidal rise and fall, appear to be important in synchronizing the rhythm. The tidal rhythm of Palaemon elegans above M.H.W.N. quickly rephases to a semi-diurnal dusk/dark rhythm during the period of neap tides, the rhythm again becoming tidal when spring tides return. The possible significance of ebb activity is discussed.

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