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The lower Rappahannock River, a subestuary of the Chesapeake Bay, is affected by seasonal low dissolved oxygen events that are intermittent and vary in intensity. Overall, macrobenthic abundance and species number were low, decreasing towards the channel and upriver, where dissolved oxygen concentrations were lowest. Mortality of macrobenthos in early summer was attributed to hypoxia. Species abundances and number declined with the onset of hypoxia, and did not recover in the deep channel until the fall. Tolerance of macrobenthos to hypoxia appeared to be species-specific. Recruitment of hesionid polychaetes occurred in summer during normoxia, but mortality followed during hypoxia, suggesting that macrobenthic assemblages in affected areas may be largely structured by the relationship between the timing of low dissolved oxygen events and species life histories. Laboratory experiments with dominant polychaete species (Streblospio benedicti and Lomia medusa) were conducted to determine their tolerance and behavior to hypoxia, anoxia, and hydrogen sulfide. Under severe hypoxia, S. benedicti survived for two weeks without significant mortality at two experimental dissolved oxygen concentrations (14.5 and 7% air saturation). In anoxia, all worms died within 55 hrs. Larvae survived severe hypoxia without displaying behavioral ill effects for at least 4 days. The behavior of S. benedicti was modified; feeding and burrowing activities ceased, and worms came out of their tubes onto the sediment surface. L. medusa showed higher tolerance of anoxia, surviving up to 5 days. Survival under prolonged hypoxia was significantly reduced. The behavior of L. medusa was also modified by hypoxia and anoxia. Worms ceased feeding and protruded head and abdominal tip out of their tubes. Worm occurrences on the sediment surface at time of death were treatment-dependent. Survival of both species was not significantly reduced when worms were exposed to anoxia with moderate concentrations of hydrogen sulfide. These results are discussed in terms of species distributions, presence of hydrogen sulfide in sediment, and mechanisms of resistance to sulfide toxicity.

The gross structure of the ampullae of Lorenzini and its distribution on the body of 40 species of skates (Rajoidei) and 5 species of sharks (Galeomorphii) were compared in an attempt to investigate correlations within this system to feeding mechanisms. Three general lines of morphological change are observed. A larger proportion of the ampullary pores are associated with the ventral surface of the dorsoventrally flattened skates than the more conically shaped sharks. The relative proportion of ventral pores is significantly reduced on those species inhabiting aphotic waters. Secondly, the more piscivorous rajoids possess an array of ventral pores which covers the majority of the body surface whereas those species feeding predominantly on infaunal invertebrates exhibit a comparatively reduced pattern which are primarily concentrated around the mouth. The density of these pores on the adult is inversely related to the collective mobility of each species' prey items. Similarly, the relative density of pores on the sharks is reduced in both those species inhibiting pelagic waters and those exhibiting reduced prey selectivity. Lastly, the overall size of, and the number of alveoli associated with, each ampulla is directly related to the habitat depth of each skate species. The proposed effects of each of these modifications is discussed. The overall pore distribution appears compensatory for reduced visual input whereas relative densities (resolution) further reflect major differences in feeding strategies. Increased ampullary size and complexity suggest mechanisms for increased sensitivity and signal-to-noise ratios.

Three genera are recognized in the family Potamotrygonidae of neotropical freshwater stingrays: Potamotrygon, Paratrygon, and Plesiotrygon, the latter being described as a new genus. Potamotrygon is polytypic, and both Paratrygon and Plesiotrygon are monotypic, as far as known. The family name Potamotrygonidae has priority over Paratrygonidae Gill. Nineteen previously described species of Potamotrygonidae are recognized: Paratrygon aiereba, Potamotrygon brachyura, P. castexi, P. constellata, P. dumerilii, P. falkneri, P. henlei, P. histrix, P. humerosa, P. leopoldi, P. magdalenae, P. motoro, P. ocellata, P. orbignyi, P. schroederi, P. schuemacheri, P. scobina, P. signata, and P. yepezi. One additional specific name is established as new (Plesiotrygon iwamae). Three of the eighteen recognized species of Potamotrygon are resurrected from synonymy (P. dumerilii, P. henlei, and P. orbignyi), one previously overlooked subspecific name (Trygon histrix ocellata) is erected to specific rank, and one previously overlooked specific name (Taeniura constellata) is placed in the genus Potamotrygon, and treated as a senior synonym of P. circularis. The remaining names previously assigned to the Potamotrygonidae include: eight junior synonyms (Disceus thayeri, Potamotrygon brumi, P. circularis, P. laticeps, P. menchacai, Trygon mulleri, T. reticulatus, and T. strogylopterus), five doubtful names (Elipesurus spinicauda, Potamotrygon africana, P. alba, P. humboldtii, and Trygon garrapa), two invalid names (Potamotrygon labratoris and P. pauckei), two doubtful names corresponding to one unidentified marine species of the family Dasyatididae (Raja ajereba and R. orbicularis), and one freshwater species of Dasyatis (D. garouaensis) originally described in the genus Potamotrygon. Potamotrygon and Paratrygon are regarded as sister groups, and Plesiotrygon as the primitive potamotrygonid genus, based on a cladistic analysis of stingray characters. Plesiotrygon is restricted to the upper Amazon drainage, and Paratrygon is known from most of the Amazon drainage, occurring also in Rio Orinoco. Eleven species of Potamotrygon have Amazonian distribution (including Rio Tocantins), four of which occur also in other drainages. Four species of Potamotrygon are apparently endemic to Rio Paraguay and lower Rio Parana drainages (P. brachyura, P. falkneri, P. histrix, and P. schuemacheri); P. yepezi is endemic to the Maracaibo basin in Venezuela; P. magdalenae is endemic to Rio Magdalena and Rio Atrato drainages in Colombia, and P. signata is apparently endemic to Rio Parna(')iba drainage in Brazil.

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