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Flow cytometric analysis of mithramycin-stained rhizome nuclei was used to compare relative amounts of DNA among 18 populations of Spartina alterniflora Loisel (Poaceae) from the East and Gulf Coasts of the U.S.A. The DNA amount increases, significantly with increasing latitude. These findings support the general model of genome size change with latitude.  相似文献
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Knowledge of the crop-size and vertical profile with respect to sea level of dead leaf blades and other shoot parts of saltmarsh grasses is a prerequisite for projections of seawater impact upon saltmarsh grass-shoot decomposers. We measured seasonal changes in quantities of leaf blade and leaf sheath+stem of smooth cordgrass in southern temperate saltmarshes of three types: low- (LD), mid- (MD), and high-drainage (HD) marshes (i.e., with low to high densities of drainage creeks). In each type of marsh, we took samples in the three characteristic subsites, short-shoot, intermediate-height, and tall-shoot. Wholly dead (brown) and living (some green) shoots were counted and their canopy heights measured, and the mass of all shoots was measured in 10-cm height increments (above sediment), with separation of living and dead parts. Detached material was also quantified, separated into that trapped in the canopy, and that prostrate on the sediment. Extent of marsh drainage did not affect the density of wholly dead shoots, which ranged from an average of 51 m−2 in autumn to 103 m−2 in spring, and 57 m−2 in tall subsites to 89–96 m−2 in short and intermediate subsites. The top of the dead-shoot canopy was about 20 cm taller than the living-shoot canopy in winter/spring, but was below it ( ≈50 cm) in summer/autumn. The mass of standing-dead leaf blades was 4- to 5-fold greater in winter than in summer, and it was about 2.5-fold greater at tall than at short sites. Detached litter trapped among shoots above the sediment averaged 26% of total dead-shoot mass, but litter lying on the sediment averaged only 6% of total dead-shoot mass. Since export of macrodetritus from marshgrass stands is inconsequential, the small prostrate-litter mass implies that breakdown of shoots to small particles takes place within the vegetated marsh, and that standing decay predisposes fallen material to rapid disintegration. Comparison of heights of usual tidal contact on shoots at the short- and tall-shoot subsites showed that projected impacts of seawater upon leaf-blade decay would be greater at short-shoot subsites (high marsh) than at tall-shoot subsites (low marsh). This was because the vertical profile of dead-blade mass resulted in more of the short-shoot dead-blade mass residing within the vertical span most regularly contacted by flooding tides.  相似文献
3.
This study examined predation by the blue crab, Callinectes sapidus Rathbun, within intertidal Spartina alterniflora (Loisel) marshes of Dauphin Island, Alabama. Species and size preferences displayed by the predator when foraging within the marsh were investigated using nektonic, epifaunal, and infaunal prey populations including Fundulus similis Baird and Girard, Littorina irrorata Say, and Geukensia demissa Dillwyn.

Short-term field experiments involving the use of predator inclusion cages, in which the relative abundances of all prey species and the density of macrophyte vegetation were manipulated, indicated that mean mortality differed significantly among species. Blue crabs exhibited a distinct species preference for Littorina, and to a lesser extent, for Fundulus. However, the predator rarely choses infaunal individuals. Within predator inclusion cages, size selection by the crabs among three size classes of each prey was evident for Littorina and Fundulus but not for Geukensia. Blue crabs tended to select intermediate-sized snails and large fish while not exhibiting a size preference for infaunal bivalves.

In the marsh, mean percentage of the Littorina population within the 14–18 mm size class exhibited an increased mortality as compared to two other size classes, which was negatively correlated with increasing tidal height. Such a relationship may have been due to a decreasing gradient of crab predation associated with increasing tidal height. Geukensia size class distributions showed little evidence of differences along the tidal height gradient. No data are available for Fundulus, a mobile species which would not experience such differential predation along a marsh gradient.

In comparing crab predation patterns among prey species, it is apparent that Callinectes utilizes prey species differentially. Such differential utilization may be based on optimization of energy yield and minimization of energy expenditure. Thus, the preference of blue crabs for nektonic and epifaunal prey is hypothesized to be the result of a smaller energy expediture as a result of the crab's visual evaluation of these prey. Infaunal prey species (e.g., Geukensia) require a greater energy investment because of the necessity of excavating the prey item. Such prey also allow little selection by size because of being cryptic.  相似文献

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