Production of benthic macroinvertebrate communities along a southern Appalachian river continuum

1. Annual production was estimated for macroinvertebrate communities of principle habitats along a first- to seventh-order river continuum in the southern Appalachian Mountains (U.S.A.). Annual production was relatively low in depositional habitats, pebble/gravel substrata, and on cobble devoid of plant biomass (mosses and hydrophytes). Production was greater in bedrock habitats and greatest on hydrophyte-covered cobble, with estimates reaching 364 g AFDM (ash-free dry mass)m^–2 yr^–1 in a sixth-order river reach. Annual production in depositional habitats was correlated to standing crops of benthic organic material (BOM) in low-order stream reaches but not in higher-order reaches, indicating differences in BOM availability with stream size. In cobble, pebble/gravel and bedrock habitats production was significantly correlated to standing crops of aquatic plants, which can stabilize substrata and enhance access of collector-filtering invertebrates to entrained food resources. 2. By accounting for proportional availability of habitats along the continuum, estimates of total production ranged from 5 to 154 g AFDM m^–2 yr^–1, and increased significantly with stream size. Annual production estimated for sixth- and seventh-order reaches of the continuum were amongst the highest reported thus far for lotic systems. Organization of the benthic community along the continuum, based on production estimates for individual functional feeding-groups, generally supported predictions of the River Continuum Concept (RCC): shredder contributions were greatest in low-order reaches and declined downstream; scraper percentages were greatest in the middle of the continuum; collector-filterer contributions increased with increasing stream size. Longitudinal trends for collector-gatherers and predators did not support RCC predictions; these groups appeared to be influenced by localized changes in habitat availability and occurrence of vertebrate predators along the continuum.     

THE STIMULATORY PROPERTIES OF SOME ORGANIC SUBSTANCES ON CYSTS OF THE BEET EELWORM, HETERODERA SCHACHTII SCHMIDT

Techniques for determining the influence of various substances on the rate of larval emergence from cysts of the beet eelworm are described. There is no significant difference between the rates of larval emergence in glutamic acid, galactinol, inositol and water; the rate of larval emergence in beet diffusate, on the other hand, was significantly higher. Studies of larval emergence in carbohydrates suggest that the cyst population used in the experiments was heterogeneous, consisting of two types of cyst which have different reactions to stimulation at different concentrations. The significance of root exudates in relation to beet eelworm is discussed.     

FIRST RECORD OF THE INDO‐PACIFIC REEF CORAL GENUS ISOPORA IN THE CARIBBEAN REGION: TWO NEW SPECIES FROM THE NEOGENE OF CURAÇAO,NETHERLANDS ANTILLES

Abstract: The coral genus Isopora, a sister group of the modern dominant Acropora until now only known from the Pliocene to Recent of the Indo‐Pacific, is recorded in the Caribbean for the first time. Two new species, Isopora ginsburgi and Isopora curacaoensis, are described from the Neogene Seroe Domi Formation of Curaçao, Netherlands Antilles. Study of large collections made systematically through the sequence indicates that Isopora first occurred in the Caribbean during the Mio–Pliocene, at approximately the same time as the origination of many modern Caribbean reef coral dominants including Acropora cervicornis. It last occurred in the region during the late Pliocene as part of a pulse of extinction, in which several genera that live today in the Indo‐Pacific became extinct in the Caribbean. Throughout its Caribbean duration, Isopora co‐occurred with the two abundant modern Caribbean species of Acropora, A. cervicornis and A. palmata. Comparisons with Neogene collections made elsewhere in the Caribbean indicate that Isopora was restricted in distribution to the southern Caribbean. Isopora species are viviparous, while Acropora are oviparous, and this difference in reproductive strategy may have played a role in the extinction of Isopora in the Caribbean. The occurrences of Isopora reported in this study are the oldest records to date of Isopora worldwide, and are important for understanding the biogeographic separation between reef coral faunas in the Caribbean and Indo‐Pacific regions.     

A MOLECULAR PHYLOGENY OF NORTH AMERICAN PLEUROCERIDAE (GASTROPODA: CERITHIOIDEA) BASED ON MITOCHONDRIAL 16S rDNA SEQUENCES

The Pleuroceridae Fischer, 1885, is one of three freshwatergastropod families currently recognized in the superfamily CerithioideaFérussac, 1819 (Mollusca: Caenogastropoda Cox, 1960).Despite considerable literature justifying various proposedgeneric names of North American pleurocerids, no study has beenconducted examining phylogenetic relationships of the recognizedgenera. In an effort to expand our understanding of evolutionaryrelationships of North American pleurocerid genrea, we examineda large portion of the mitochondrial 16S rRNA gene among 32extant North American taxa. Multiple sequence alignment of theamplified region for our taxa resulted in a matrix consistingof 900 nucleotides including insertions and deletions. Basedon analysis of nucleotide substitution patterns, we employedtwo approaches in our phylogenetic analysis: (1) all substitutionsreceived equal weighting and (2) transversions were weighted2X and 4X transitions to compensate for transition saturationamong distantly related taxa. The molecular phylogeny basedon the mitochondrial 16S rDNA sequences supports the monophylyof Pleurocera Rafinesque, 1819, Elimia H. & A. Adams, 1854,and Juga H. & A. Adams, 1854, but depicts the genera LithasiaHaldeman, 1840, and Leptoxis Rafinesque, 1819, as polyphyletic.The genus Pleurocera is sister to Elimia, which in turn is sisterto a paraphyletic assemblage including representatives of Leptoxis,Lithasia, and the monotypic genus Io Lea, 1831. Juga, a genusrestricted to west of the North American continental divideis the basal-most clade and is sister to all the aforementionedgenera found east of the continental divide. (Received 26 January 1999; accepted 14 October 1999)     

Analysis of segment number and enzyme variation in a centipede reveals a cryptic species, Geophilus easoni sp. nov., and raises questions about speciation

Geophilomorph centipedes provide an unusual opportunity to examine patterns of variation in segment number within extant species. (Most other arthropod species are invariant in this respect.) Recent work on several geophilomorph species has revealed latitudinal clines in segment number, but has generally not called into question the identities of the species themselves. However, analysis of a unique pattern of variation in British populations of Geophilus carpophagus Leach, coupled with other morphological and enzyme data, reveals a cryptic species, Geophilus easoni sp. nov. This species has not yet been detected on the European mainland. Four distinct hypotheses are advanced regarding the speciation event through which G. easoni arose and the phylogeographic processes that resulted in the present distributions of the two species in Britain. Further study will be necessary to test these hypotheses. In any event, G. carpophagus and G. easoni provide something that has until now been absent: a case study of the genetic differences between closely-related geophilomorph species.     

MOVEMENT OF EELWORMS

Experiments on vertical migration through saturated soil fractions, horizontal migration through soil fractions at different pressure deficiencies and migration in single layers of particles showed that the beet eelworm, Heterodera schachtii Schmidt, attained maximum speed when the pore diameters were between 30–60 μ. Speed of the eelworms increased as lateral displacement of the body was restricted by external resistances acting perpendicularly to the body axis; at the maximum speed there was no lateral movement, each part of the body following the part immediately in front of it. The speed of beet eelworm larvae in water films of various thickness was measured; maximum speed occurred in a film 2–5 μ thick. Four arbitrarily classified types of progression were observed in the pore spaces. It is suggested that the 'moisture characteristic' supplies most of the information required about the physical properties of the soil in relation to eelworm movement. By examining such a curve the pore size distribution can be ascertained and the probable behaviour of beet eelworm larvae in the medium predicted.