Possible occurrence of character displacement in the sympatric skates Raja erinacea and R. ocellata (Pisces: Rajidae)

Synopsis Raja erinacea and R. ocellata are sibling species which are positively correlated with each other by occurrence and numerical abundance. In sympatry the species undergo interactive segregation; R. erinacea feeds on a higher percentage of epifauna and R. ocellata feeds on a higher percentage of infauna.An isolated allopatric population of R. ocellata occurs in the Gulf of St. Lawrence which is phenotypically intermediate between the sympatric populations of R. erinacea and R. ocellata in characters related to feeding e.g. size, number of tooth rows in the upper jaw, and shape of the upper jaw. It appears probable that the allopatric population represents the morphological state of R. ocellata before it became sympatric with R. erinacea; divergence in size, number of tooth rows, and shape of the upper jaw between the two species developed after establishment of sympatry. These divergences in character traits, related to feeding, reduced competition between the two sympatric species and permitted the present wide overlap in their ranges.Character displacement is evidently rare in demersal fishes inhabiting the flat and soft bottoms of the northwestern Atlantic because the three other pairs of sibling species that occur there are parapatrically distributed and thus would not compete for resources. Raja erinacea and R. ocellata may have been restrained from establishing parapatry by another species pair of skates (R. senta and R. radiata) which have a complementary distribution and similar feeding habits of R. erinacea and R. ocellata but which occur in deeper water.     

High frequency of natural hybrids between Atlantic salmon, Salmo salar L., and brown trout, S. trutta L., in a Swedish river

Among 332 parr from the Swedish River Grönån examined by electrophoresis, 44 (13%) were hybrids between Atlantic salmon and brown trout. The hybrid frequencies in three sections of Grönån were significantly different (23. 8 and 2%). All hybrids are evidently of natural origin. and possible factors promoting hybridization are irregular overlapping spawning times. lack of separate spawning grounds, and involvement of sneakers.     

The benthic deep-water siphonophore Rhodalia miranda and other coelenterates in the south-west Atlantic: ecological and oceanographical implications

The benthic deep-water siphonophore Rhodalia miranda was collected for the first time in 1876 by H.M.S. Challenger off the Rio de la Plata estuary beneath the Subtropical Convergence at about 1000 m depth. Rhodalia was reported again about 100 years later from certain distant localities in the subantarctic region of the south-west Atlantic. Hydrographic and topographic features that may be involved in creating this peculiar distribution pattern are discussed together with a likely mode of dispersal of benthic coelenterates. The disjunct distribution of Rhodalia and other benthos of the upper Argentine Slope suggests the subsurface oceanographic régime to be quite different from what we know of the surface layers.     

Seasonality and the scheduling of life history at different latitudes

For many ectotherms, the annual cycle is partitioned into 'growing' (summer) and 'non-growing' (winter) seasons, and the lengths of these seasons are inversely related across a latitudinal gradient. This pattern of variation has the potential to affect diverse life-history traits profoundly. A key selective agent is size-dependent winter mortality which, with increasing latitude, places an increasing premium on attainment of large body size before the first winter of life. Winter body size is determined primarily by two factors: (1) birth date, which defines the beginning of the first growing season and (2) somatic growth rate within the first season. Using examples drawn from the Atlantic silverside, Menidia menidia (L.), and other fishes, I show how latitudinal variation in the scheduling of the spawning season, countergradient variation in the capacity for growth and reproduction, and mode of sex determination (environmental v. genetic) represent adaptations to seasonality.     

The use of genetic marking to assess the reproductive success of mature male Atlantic salmon parr (Salmo salar, L.) under natural spawning conditions

The reproductive success of mature male Atlantic salmon parr ( Salmo salar ) under natural spawning conditions was estimated using the polymorphism at the MEP - 2 * locus as a genetic marker. The percentage of eggs per redd fertilized by parr varied considerably over the five redds examined (0·9–27.7%, mean 10·8%), but a gametic contribution from mature parr was detected in each case. Parr reproductive success has important implications for the population structure and evolution of the Atlantic salmon through its effect on gene flow.     

Territory size and distribution of newly-emerged Atlantic salmon (Salmo salar)

Newly emerged Atlantic salmon (Salmo salar) were observed from May to August 1981 at six isolated redds in Washington County, Maine, USA. Territorial size and distribution were measured. At the end of the emergence period (12 to 28 May), fish maintained positions (stations) at redds where water velocity did not exceed 52 cm s^–1 By 12 June, most salmon (80–96%) had moved off the redds of origin and had established territories 1 to 5 m from the redd. The area defended increased substantially after mid-June, but territorial aggression diminished by 15 July, and the fry dispersed downstream. All fish observed were territorial, and the percentage of time during which stations were held decreased from 89 in mid-May to as low as 40% in mid-June.Cooperators are the Maine Department of Inland Fisheries and Wildlife, University of Maine, U.S. Fish and Wildlife Service, and the Wildlife Management Institute     

A phytosociological study of weed communities on the southwestern Coastal Plain of North Carolina

Summary The vegetation of cultivated, fallow and ‘old’ fields of Scotland County, N.C. was both randomly and subjectively sampled throughout the growing season of 1973. Relevés were placed in each of the above management units, and the data collected in this relevé analysis were sorted into phytosociological charts, with the aid of the digital computer. Weed associations delimited in the charts reflected the importance of edaphic factors, time (aspect), and man's agricultural activities as causal factors in the determination of characteristic assemblages of weeds. Several character and differential species were denoted, in a tentative classification of the weed associations. The cultivated and fallow stands in the spring aspect were noted to be dominated by a group of spring annuals. Summer annuals were more prevalent in homologous situations in the summer. It was determined that perennials, appearing mainly as seedlings in cultivated and fallow fields, attained dominance in ‘old’ fields, one to five years in age. Within each of the major spheres of management, and in each season, associations of weeds mainly reflected differences in edaphic factors. Many character species were noted to maintain affinities to an assortment of soil types that ranged from moist loams to xeric, sterile sands. A study of roadsides, hedgerows, field margins and other ecotonal areas inhabited by weeds was carried throughout the growing season of 1973, also. Species presence was documented in each of these habitat types and reported in Sharp (1974). The weed flora of Scotland County was found to be rich in species. A total of 506 species representing 248 genera and 80 families were encountered throughout the study. Of the total number of species listed, approximately 25 percent were of foreign origin, and 28 percent represented additions to the flora of Scotland County. This paper is the short version of a masters thesis for the Department of Botany, University of North Carolina at Chapel Hill. The author would like to extend his appreciation and gratitude to Professors H. Lieth, A. E. Radford and J. Hardin for their generous advice and encouragement extended through the duration of the study.