Aspects of the taxonomy, ecology and hydrodynamics of the Australian Psephenidae (Coleoptera)

Psephenid larvae (waterpennies) are well known members of the freshwater benthos, possessing a characteristic dorso-ventrally flattened shield. They are found clinging to rocks in rivers and streams that experience fast and turbulent flow regimes for at least part of the year. However, despite the fact that they occur in many of the rivers and streams of eastern Australia, and throughout Tasmania, very little study has been made of the family here. Sclerocyphon is the only Australian genus of the Psephenidae. A brief review of the history of the Psephenidae, with particular emphasis on this genus and the subfamily encompassing it, the Eubriinae, is given. Sclerocyphon comprises 12 species, six of which are described as new. Descriptions of six larval types representing discrete species but not, as yet, associated with adults, are also given. Re-descriptions of the genus and all previously described species are included as earlier workers possessed only limited material and features now considered to be of taxonomic importance, in particular, the male and female genitalia, were not described. Keys for the identification of both adults and larvae are given. A major emphasis of the taxonomic study was on the association of adults and larvae and this was achieved mainly by laboratory breeding. Larvae are the dominant life history phase and fairly common while adults are much rarer. The relative apomorphy or plesiomorphy of a number of larval characters is discussed and a phylogenetic classification of Sclerocyphon, based on larval characters, is proposed. Hinton's (1966) view of the Eubriinae as a subfamily (encompassing Sclerocyphon) of the Psephenidae, rather than Bertrand's (1972) concept of the Eubriinae as a separate family, is supported. The distribution of each species and larval type is described and discussed with respect to present and some past environments in Australia. The complete endemism of the Tasmanian species of Sclerocyphon is considered to be a reflection of the low vagility of the genus and appears to be best explained by the vicariance model of historical biogeography. A Gondwanaland origin of Sclerocyphon is suggested as it (or its nearest relative) occurs in Chile, as well as Australia, but nowhere else in the world. This disjunct southern distribution in a genus with low vagility is considered to be best explained by the vicariance of an ancestral Gondwanaland fauna. Multivariate statistical techniques were used to investigate the shape of larvae of the Tasmanian species of Sclerocyphon. The most common Tasmanian species, S. secretus, was found to be the most variable, exhibiting a west-east trend in larval shape. Variation in larval shape was continuous with the widest, almost circular forms occurring on the west coast and the narrowest,most elongate forms on the east coast. Larval shape was found to be correlated with rainfall distribution, substrate and stream order. Contrary to expectations the narrowest, most elongate larvae were found to occur in streams of low mean velocity while wider forms occurred in streams of higher velocity. Three possible explanations for this paradox are given. The second river and stream dwelling species in Tasmania, S. aquaticus, was found to be less variable in shape with larvae being predominantly wide and almost circular in form. The occurrence, and therefore the shape, of S. aquaticus larvae was found to be highly correlated with high stream order. Flow visualisation techniques were used to investigate the hydrodynamics of larval Sclerocyphon. Dye trails revealed that all larvae are streamlined and inhabit a region of reduced flow immediately adjacent to t.he substrate known as the laminar, or viscous, sublayer. The viscous sublayer exists within a thicker region of reduced flow assoc~ iated with the substrate, the boundary layer, which, under most stream conditions, is turbulent. Drag forces upon the larva are minimal while it remains within the viscous sublayer however respiratory exchange across this vii is limited to ·the slow rate of molecular diffusion. Larval Sclerocyphon overcome the cons.traints of this situation by creating their own respiratory current with a pair of anal, retractible, tracheal gills. During ventilation actively pumping gills create a turbulent area at ·the rear of the body which greatly enhances respiratory processes and waste removal. The gills also act as vortex generators, decreasing drag forces by controlling the larva 1 s wake. At high Reynolds number the thickness of the sublayer is reduced and larvae appear to employ a method of boundary layer control known, in fluid mechanics theory, as suction. The spaces between the lateral laminae act as slots through which a small amount of boundary layer fluid passes, creat.ing a thinner but. more stable boundary aver the larva.. This delays separation and therefore decreases the likelihood of the larva being swept off the substrate. For larvae of the Psepheninae and Eubrianacinae, which possess passive ventral tracheal gills, suction through slots between the lateral laminae may be the only means of main-taining adequate water: flow over the gills. Streamlining and associated boundary layer control allow larvae to move across substra·tes in high energy flows and to exploit the food sources there that are unavailable to other benthic invertebrates not possessing these hydrodynamic advantages.