Ultrastructural study of metamorphosis in the freshwater bryozoan Plumatella fungosa (Bryozoa,Phylactolaemata)

Summary

At metamorphosis the attachment of the Plumatella larva to the substrate is effected by secretions from glandular cells in the apical plate, the leading pole during swimming. The larval mantle folds back and slides down towards the substrate. By ciliary activity an adhesive secretion is spread over the metamorphosing larva and the attachment area. Two polypides appear through the larval terminal opening. The mantle fold, together with gland cells, nerve cells, sensory cells, and muscle cells from the larva form a nutritive cell mass. Reduction of this nutritive cell mass is accomplished by autolysis and phagocytosis. An invaginated area of the nutritive cell mass is provided with a dense layer of microvilli, which seem to have an absorbtive function. The nutritive cell mass consisting of transitory larval tissues provides a significant source of nutrient for the developing polypide buds.     

A benign approach to the preparation of freshwater bryozoan statoblasts for scanning electron microscopy (SEM) imaging

Abstract

Several different species of freshwater Bryozoa, belonging to the genera Plumatella, Rumarcanella and Fredericella, were detected within the Northern Mallee Pipeline (NMP) system in Victoria, Australia, that required definitive identification. These organisms produce asexual buds called statoblasts, with valves composed of sclerotised chitin that bear minute micro-ornamentations of considerable taxonomical significance. Imaging and analysis of these distinctive micro-ornamentations using scanning electron microscopy (SEM) is often employed for species identification. Meticulous preparation of statoblast samples is therefore required that necessitates the removal of adhering debris, dehydration and drying—whilst mitigating specimen damage and distortion. This technical note describes an approach whereby each of these three steps have been individually designed to be as benign as possible, using mild detergent/sonication to remove debris, a gradual and gentle dehydration procedure using ethanol, and critical point drying. For the overall process, these methods are chosen to optimise control and to minimise the use of harsh and hazardous chemicals.     

Plumatella nitens, a new species of freshwater bryozoan from North America (Ectoprocta: Phylactolaemata), previously misidentified

Plumatella nitens is a newly recognized species of phylactolaemate bryozoan in North America. It has previously been combined with either P. repens or P. fungosa, from which it differs primarily by the floatoblast. In P. nitens the ventral floatoblast annulus is uniformly narrow with no significant widening at the poles. The dorsal floatoblast fenestra is conspicuously larger than in either P. repens or P. fungosa. Reticulated ridges on the dorsal fenestra are weak along the margins, becoming unusually prominent in the center. The sessoblast is densely covered with uneven papillae. Spotty sampling so far shows a distribution only in the northern half of North America, including Massachusetts, Michigan, Minnesota, Wisconsin, Ontario, and the northern regions of Illinois, Indiana, and Ohio.     

Intervention des populations de plumatella fungosa (Pallas) (Bryozoaire phylactoleme) dans l'autoepuration des eaux d'un etang et d'un ruisseau

A very dense population of Plumatella fungosa (Pall.) (Bryozoa Phylactolema) has been observed in a pond and its outlet, polluted by organic matter from sewers.These organisms play an prominent part in the process of water purification; they can absorb annually an average quantity of 113 kg Nitrogen/ha in the zones where the biomass of the colonies reaches 15,6 T/ha.There is moreover an obvious relation between the evolution of the biological quality of water, classified according to the Kolkwitz-Marsson system, and the localization and density of the Bryozoa populations. The greatest development was observed between the alfa mesosaprobic and beta mesosaprobic zone.

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Intervention des populations de plumatella fungosa (Pallas) (Bryozoaire phylactoleme) dans l'autoepuration des eaux d'un etang et d'un ruisseau

     

Threshold effects of food concentration on the skeletal morphology of the bryozoan Electra pilosa (Linnaeus, 1767)

Many palaeontological studies rely heavily on characteristics of the preserved phenotype, i.e. the morphology of skeletal hard parts. Although the potential for environmental influences on the phenotype is expected, rarely is the magnitude of the effects quantifiable relative to genetic factors. The clonal/colonial body plan of Bryozoa allows for the partitioning of morphological variance into its genetic and environmental factors addressing the question of, ‘how much phenotypic variation is induced in a population by changing a single environmental factor?’ The effects of variation of food concentration on whole‐colony growth rate and on zooid size/morphology can be profound in bryozoans. Here we test experimentally food effects on the skeletal phenotype of the bryozoan Electra pilosa (Linnaeus, 1767), an encrusting sheet‐like bryozoan. A threshold effect was observed for the relationship between zooecium size and food concentration. Very low concentrations resulted in stunted colonies with small zooecia, but at low to intermediate concentrations a close relationship existed with zooecium size. Maximum zooecium size occurred at submaximal food concentration and submaximum zooecium size occurred at higher food concentrations. Previous studies that have reported no effect of food availability on zooecium size assessed food concentration effects at higher concentrations than were effective in the present study. In the absence of other factors, variation in zooecium size is minimal and unchanging at moderate to high food concentrations. Greater variation in zooecium size is expected at and below threshold food concentrations. We show that the preservable phenotype of these specimens subjected to controlled and induced environmental variation also records information with genetic significance.     

Asajirella gelatinosa in Panama: a bryozoan range extension in the Western Hemisphere (Ectoprocta: Phylactolaemata)

Asajirella gelatinosa, a freshwater bryozoan known previously only from eastern Asia is now reported in the Republic of Panama. Colonies were collected during April 1992 in the broad reaches of the lower Río Chagres as it enters Gatun Lake, part of the Panama Canal system. A subsequent collection in 1998 included large colonies and numerous statoblasts from a region upstream of this site in the impounded Lago Alajuela (Madden Lake). Colonies and statoblasts resemble those from Japan in almost every detail. Human transport of statoblasts is a likely cause of this disjunct distribution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Development and molecular characterisation of the microsporidian Schroedera airthreyi n. sp. in a freshwater bryozoan Plumatella sp. (Bryozoa: Phylactolaemata)

The development of a new species of microsporidian, infecting a freshwater Plumatellid bryozoan, is described. The small-subunit rDNA, internal transcribed spacer region (ITS), and partial large-subunit rDNA genes were sequenced. Phylogenetic analysis demonstrated that the parasite clustered with Schroedera plumatellae. However, while there were morphological affinities with this species, significant differences were also observed. The infection initially appeared as a roughening of the peritoneum lining the metacoelom of the bryozoan. This roughening resolved into meront-infected syncytia, composed of interconnected cells of the body wall that detached to float in the coleomic cavity. Spores were observed to develop within these syncytia. All stages of development were diplokaryotic in contrast to S. plumatellae, which has a distinct monokaryotic merogony preceding sporogony. The infection was pathogenic to the host. Direct bryozoan-bryozoan transmission was not observed. We propose to name the microsporidian Schroedera aithreyi n. sp.     

Versatile ciliary behaviour in capture of particles by the bryozoan cyphonautes larva

Cyphonautes larvae of a bryozoan, Membranipora membranacea, used several ciliary mechanisms to capture algal cells upstream from the lateral band of cilia that produces a feeding current. (1) Lateral cilia changed beat and a backcurrent occurred at the time and place that particles were retained. (2) Algal cells were sieved and held stationary at the upstream (frontal) side of a row of laterofrontal cilia that were not beating. (3) Localized extension of cilia toward the inhalant chamber, coincident with particle captures, indicated that laterofrontal cilia flick toward the inhalant chamber. These flicks may aid transport of captured particles toward the mouth. Thus my earlier report that larvae only sieve, in contrast to the adults (which have an active ciliary response) was in error. The similar ciliary bands in adult and larval bryozoans and in other lophophorates (brachiopods, and phoronids) suggest that these animals share a core repertoire of ciliary behaviours in the capture and concentration of suspended food particles.     

Bacterial diversity in the breadcrumb sponge Halichondria panicea (Pallas)

The aim of this study was to investigate the diversity and variability of bacterial communities associated with the marine sponge Halichondria panicea with respect to tissue compartmentalization as well as seasonal and small-scale geographic variation. Diversity of microorganisms in sponges was investigated recently, but work on the variability and succession of associated bacterial communities is rare. Despite some information on Pacific and Mediterranean sponges, it is still uncertain whether bacteria and sponges are specifically associated. In this study, H. panicea specimens were sampled throughout the year at different stations around the island of Helgoland (North Sea) and investigated using molecular tools. The bacterial community associated with H. panicea was diverse, consisting of one denaturing gradient gel electrophoresis (DGGE) band occurring in most 'tissue' samples and additional variable bands. Variability was observed between different sponge fractions (i.e. the aquiferous system and the 'tissue'), sampling locations, and sampling dates. A PCR-DGGE specific for the Roseobacter group of marine Alphaproteobacteria displayed low diversity and a marked similarity between all samples. Phylogenetic analysis also pointed to specific Alphaproteobacteria of the Roseobacter group, which was predominant in most sponge 'tissue' samples. We conclude that H. panicea harbour a specific Roseobacter population with varying bacterial co-populations occurring seasonally or on a small-scale geographically, sometimes even dominating the bacterial community.     

Environmental influences on the formation and germination of hibernacula in the brackish-water bryozoan Victorella pavida Saville Kent, 1870 (Ctenostomata: Victorellidae)

Brackish-water and fresh-water bryozoans produce asexually derived dormant propagules that allow survival of unfavourable conditions and provide a potential means of dispersal. The propagules of brackish-water ctenostome bryozoans are called hibernacula. We monitored the life-cycle of the brackish-water ctenostome Victorella pavida Saville Kent, 1870 in its natural habitat and investigated, in laboratory cultures, the influence of temperature and salinity on the production and germination of hibernacula and on subsequent colony growth. V. pavida is a protected species in the UK, where its only locality is at Swanpool lagoon, Falmouth. Colonies were collected from Swanpool monthly from January 2004 to January 2005. Hibernaculum germination appeared to be triggered by increased water temperature (c. 13 °C) in the lagoon in March and April. In culture, germination was triggered by transfer from 5 °C to 19 °C in a range of salinities; subsequent colony growth was affected by salinity, with strongest growth at 13, 18 and 36 psu, and reduced growth at 5 and 9 psu. At 3.5 psu, hibernacula germinated, but there was no further development. At 36 psu there was an initial lag in growth, but after 30 d the colonies were comparable with those kept at 18 psu. Hibernaculum formation by colonies occurred from June to October, with production increasing towards October. Hibernacula appear not to have long-term viability but merely to permit survival from one year to the next. The results suggest that any changes in the hydrographic regime at Swanpool could have significant consequences for the survival of V. pavida.     

Use of the rotifer, Brachionus calyciflorus Pallas, in freshwater ornamental fish larviculture

The Brachionus calyciflorus used in this study were produced by batchculture using Chlorella spp. as feed. Larviculture experiments in indoor10-l and 200-l tanks revealed that, compared with egg yolk, the rotifersused as starter food significantly improved the growth and survival of DwarfGourami larvae (Day 2–12). These beneficial effects also extended tothe subsequent Artemia feeding phase (Day 13–32), suggesting that thequality of starter food is crucial to later development. At metamorphosis,the overall survival rate of larvae fed on rotifers in indoor tanks(65.1–74.5%) was about four times of that obtained in extensiveculture in open ponds (17.5%). In Discus, larvae are dependent on thebody slime of their parent as a nutrient during the first two weeks ofexogenous feeding. Our observation demonstrated that Brown Discus larvaecould be raised in the absence of the parent fish by using rotifers asstarter food followed by Artemia nauplii. Their growth and survival ratewere comparable to those on parental feeding. The artificial feeding wouldeliminate the risk of larvae being eaten by the parent fish and shorten thebrooding interval of the spawners, thereby leading to higher yield of fry.This feeding protocol is less tedious and more practical for use incommercial farming of Discus than the existing strategies of smuggling thebatch of larvae to foster parents or feeding the larvae with egg food. Theuse of rotifers would enable freshwater larviculture to improve larvalperformance, increase yield, and facilitate breeding of new fish specieswith small larvae.     

New observations on the skeletons of the earliest bryozoans from the Fenhsiang Formation (Tremadocian,Lower Ordovician), Yichang,China

The earliest unequivocal bryozoans occurred in the Fenhsiang Formation (upper Tremadocian, Lower Ordovician) of Yichang, South China, providing important clues about the early evolution of primitive bryozoans. In this study, SEM and EDAX were used to analyze the microstructure of the walls of two bryozoan genera, Nekhorosheviella and Orbiramus, from the Fenhsiang Formation. All walls of Nekhorosheviella and endozonal walls of Orbiramus show poor preservation in thin sections, with a granular appearance reflecting extensive neomorphism, whereas exozonal walls of Orbiramus are distinctly laminated. These preservational differences may reflect skeletal chemistry, particularly the magnesium content of the calcite. Phosphatic skeletal linings were found to be distributed unevenly in the autozooecia of Nekhorosheviella, but were absent in Orbiramus.     

Genetic variation in a freshwater bryozoan. I: Populations in the Thames basin, UK

In spite of increasing interest in metapopulation dynamics, the genetic consequences of a metapopulation structure remain poorly understood. Here we examine the metapopulation genetic structure of the colonial, facultatively sexual freshwater bryozoan Cristatella mucedo , in the Thames basin of southern England, UK. Populations from nine sites were sampled and colonies genetically characterized using random amplified polymorphic DNA (RAPD)–PCR. A total of 78 different clones was detected over all sites. Despite the large number of clones, genetic distances among clones both within and among sites were very small. Nonetheless, no clone was present at more than one site even though C. mucedo has an asexual dispersal propagule, and clones strongly clustered by sites. No consistent pattern of clonal structure was evident, with both the number and equitability of clones varying greatly among sites. Although sites were genetically distinct, population genetic regions were absent, and a Mantel test indicated that there was no relationship between geographical distances among sites and genetic distances among populations. Our results indicate that C. mucedo exists as a classical metapopulation in the Thames basin, with dispersal independent of distance and all sites contributing to the genetic diversity of the metapopulation.     

Lead toxicity,locomotion and feeding in the freshwater snail,Lymnaea stagnalis (L.)

The effects of lead (5 or 10 ppm) on the survival of the freshwater snail Lymnaea stagnalis (L.) collected from lead contaminated or uncontaminated environments were evaluated under controlled laboratory conditions. The animals from the contaminated environment had significantly greater survivability than those from the unpolluted environment to subsequent acute (up to 24 days) exposure to lead. Acute (72 h) exposure to lead inhibited several behavioural activities including locomotion, feeding, tentacle extension and emergence from the shell. Lead bioaccumulated in the snail tissues, especially the buccal mass and stomach. The freshwater snail provides a valuable system for studying the bioaccumulation and development of tolerance to environmental lead. Electronic Publication     

细菌信号分子N-酰基高丝氨酸内酯调控植物抗性的研究进展

自然界中植物与细菌长期共存,共同进化,二者之间形成由不同信号分子介导的复杂的相互作用网络。N-酰基高丝氨酸内酯（AHLs）是革兰氏阴性细菌胞间通讯的信号分子,可被植物感知,并能调控植物多种生理行为,包括植物的天然免疫、生长发育、耐逆性等。本文综述近年来的相关研究进展,有助于全面了解植物与细菌间的信息交流机制,并对农业生产提供理论指导。     

Habenariol, a freshwater feeding deterrent from the aquatic orchid Habenaria repens (Orchidaceae)

An uncommon ester, bis-p-hydroxybenzyl-2-isobutylmalate, habenariol, has been isolated, by bioassay-guided fractionation, from the organic extract of the freshwater orchid Habenaria repens. The structure of habenariol was determined by the interpretation of spectral data. Feeding bioassays showed that habenariol deters feeding by the common freshwater crayfish Procambarus clarkii. Habenariol is related to a bis-ester glycoside isolated from the non-aquatic orchid Galeola faberi.     

Fine structure of the dorsal lingual epithelium of the little tern, Sterna albifrons Pallas (Aves, Lari).

The dorsal surface of the tongue of the little tern, Sterna albifrons, has a distinctive anterior region for five-sixths of its length and a terminal posterior region. The anterior region observed by scanning electron microscopy is distinguished along its forward half by a median line from which median papillae protrude. The hind half of the anterior region has a median sulcus without papillae. The deciduous epithelium on both sides of the median line and sulcus bears scattered epithelial protrusions. The posterior lingual region has neither median papillae nor deciduous epithelium. So-called giant conical papillae are located in a transverse row between anterior and posterior regions. Delicate microridges adorn the surfaces of all outer epithelial cells in both regions. Examination of the dorsal lingual epithelium by light and electron microscopy provides histologic and cytologic criteria for distinguishing anterior and posterior regions. Basal cells are nearly alike throughout the dorsal epithelium. Intermediate layer cells of the anterior region contain numerous tonofibrils in electron-dense bundles composed of 10 nm tonofilaments. The outer layer is composed of electron-dense, well-keratinized cells, and electron-lucent epithelial protrusions are present on the exposed surface of the outermost cells. Median papillae are composed of typical keratinized cells, which are nearly filled with keratin filaments. Intermediate layer cells in the posterior region of the tongue are nearly filled with unbundled tonofilaments. There is only a very thin outer keratinized layer in this region.