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1.
When acutely transferred to diluted seawater (SW), Procephalothrix spiralis and Clitellio arenarius regulate water content (g H2O/g solute free dry wt = s.f.d.w.) via loss of Na and Cl (µmoles/g.s.f.d.w.). The present study extends these observations to a greater range of salinities and determines the effects of long-term, stepwise acclimation to diluted seawaters. Final exposure to a given experimental seawater (70, 50, 30, 15%) was 48 hours. Osmolality (mOsm/kg H2O) and Na, K, and Cl ion concentrations (mEq/l) were determined in total tissue water and in the extracellular fluid of C. arenarius. Extracellular volume was determined as the 14C-polyethylene glycol space. Both species behaved as hyperosmotic conformers in diluted seawaters. However, reduction of the osmotic gradient between worm and medium occurred in P. spiralis, but not C. arenarius, in 30 and 15% SW. In both species, osmolality and Na, Cl, and K concentrations in total tissue water decreased with increased dilution of the SW. Water content increased with dilution of the medium but was lower than that which would be predicted based on approximation of the van't Hoff relation. This indicated the occurrence of regulatory volume decrease (RVD). In P. spiralis, in 70 or 50% SW, RVD was accompanied by loss of Na and Cl contents. However, in 30 or 15% SW, Na and Cl contents increased and in worms in 15% SW K content decreased. The latter movements of Na, Cl and K are indicative of cellular hysteresis and were associated with decreased viability, indicating the lower limits of regulatory ability in this species. In comparison, RVD in C. arenarius occurred in all diluted seawaters and was accompanied by loss of Na and Cl contents. In C. arenarius, evidence for reduced viability was absent. Removal of the supra- and subesophageal ganglia of C. arenarius resulted in retention of water, Na and Cl (g H2O or µmoles/g s.f.d.w.) in worms acclimated to 70% SW. Removal of the cerebral ganglia and cephalic glands of P. spiralis did not significantly influence regulation of water content.  相似文献   
2.
Biodiversity assessments are critical for setting conservation priorities, understanding ecosystem function and establishing a baseline to monitor change. Surveys of marine biodiversity that rely almost entirely on sampling adult organisms underestimate diversity because they tend to be limited to habitat types and individuals that can be easily surveyed. Many marine animals have planktonic larvae that can be sampled from the water column at shallow depths. This life stage often is overlooked in surveys but can be used to relatively rapidly document diversity, especially for the many species that are rare or live cryptically as adults. Using DNA barcode data from samples of nemertean worms collected in three biogeographical regions—Northeastern Pacific, the Caribbean Sea and Eastern Tropical Pacific—we found that most species were collected as either benthic adults or planktonic larvae but seldom in both stages. Randomization tests show that this deficit of operational taxonomic units collected as both adults and larvae is extremely unlikely if larvae and adults were drawn from the same pool of species. This effect persists even in well-studied faunas. These results suggest that sampling planktonic larvae offers access to a different subset of species and thus significantly increases estimates of biodiversity compared to sampling adults alone. Spanish abstract is available in the electronic supplementary material.  相似文献   
3.
Nemerteans have been alleged to belong to a protostome clade called the Trochozoa that includes mollusks, annelids, sipunculids, echiurids, and kamptozoans and is characterized by, among other things, the trochophore larva. The trochophore possesses a prototroch, a preoral belt of specialized ciliary cells, derived from the trochoblast cells. Nemertea is the only trochozoan phylum for which presence of the trochophore larva possessing a prototroch had never been shown. However, so little is known about nemertean larval development that comparing it with development of other trochozoans is difficult. Development in the nemertean clade Pilidiophora is via a highly specialized planktonic larva, the pilidium, and most of the larval body is lost during a drastic metamorphosis. Other nemerteans (hoplonemerteans and palaeonemerteans) lack a pilidium, and their development is direct, forming either an encapsulated or planktonic "planuliform" larva, producing a juvenile without a dramatic change in body plan. We show that early in the development of a member of a basal nemertean assemblage, the palaeonemertean Carinoma tremaphoros, large squamous cells cover the entire larval surface except for the apical and posterior regions. Although apical and posterior cells continue to divide, the large surface cells cleavage arrest and form a contorted preoral belt. Based on its position, cell lineage, and fate, we suggest that this belt corresponds to the prototroch of other trochozoans. Lack of differential ciliation obscures the presence of the prototroch in Carinoma, but differentiation of the trochoblasts is clearly manifested in their permanent cleavage arrest and ultimate degenerative fate. Our results allow a meaningful comparison between the development of nemerteans and other trochozoans. We review previous hypotheses of the evolution of nemertean development and suggest that a trochophore-like larva is plesiomorphic for nemerteans while a pilidium type of development with drastic metamorphosis is derived.  相似文献   
4.
A checklist of benthic ribbon worm species from the Caribbean coast of Colombia is presented, including synonyms, distributions, a photographic record, and the main morphologic characters of each species for a rapid identification. This is the first research focused broadly on nemerteans in Colombia. 54 specimens of nemerteans were hand-collected from the rocky littoral of two different localities, and identified according to personal experience and specialist literature. 13 species were found; of which 11 represent new records for the country. These species belong to eight different traditionally used families: Tubulanidae, Valenciniidae, Lineidae, Amphiporidae, Cratenemertidae, Emplectonematidae, Drepanophoridae and Ototyphlonemertidae. The most common and abundant species was Dushia atra. The biodiversity of nemerteans in Colombia seems to overlap with the nemertean fauna from Florida and Brazil, explained by the convergence of the North Brazil Current, Guiana Current, Caribbean Currents and the Panama-Colombia Contracurrent in the sampled region. The results of this work suggest that the Caribbean coast of Colombia is a region with a high diversity of nemerteans, and provide important taxonomic data for environmental assessments and future biological research.  相似文献   
5.
Meiofauna represent one of the most abundant and diverse communities in marine benthic ecosystems. However, an accurate assessment of diversity at the level of species has been and remains challenging for these microscopic organisms. Therefore, for many taxa, especially the soft body forms such as nemerteans, which often lack clear diagnostic morphological traits, DNA taxonomy is an effective means to assess species diversity. Morphological taxonomy of Nemertea is well documented as complicated by scarcity of unambiguous character states and compromised by diagnoses of a majority of species (and higher clades) being inadequate or based on ambiguous characters and character states. Therefore, recent studies have advocated for the primacy of molecular tools to solve the taxonomy of this group. DNA taxonomy uncovers possible hidden cryptic species, provides a coherent means to systematize taxa in definite clades, and also reveals possible biogeographic patterns. Here, we analyze diversity of nemertean species by considering the barcode region of the mitochondrial gene Cytochrome Oxidase subunit I (COI) and different species delineation approaches in order to infer evolutionarily significant units. In the aim to uncover actual diversity of meiofaunal nemerteans across different sites in Central America, COI sequences were obtained for specimens assigned here to the genera Cephalothrix, Ototyphlonemertes, and Tetrastemma-like worms, each commonly encountered in our sampling. Additional genetic, taxonomic, and geographic data of other specimens belonging to these genera were added from GenBank. Results are consistent across different DNA taxonomy approaches, and revealed (i) the presence of several hidden cryptic species and (ii) numerous potential misidentifications due to traditional taxonomy. (iii) We additionally test a possible biogeographic pattern of taxonomic units revealed by this study, and, except for a few cases, the putative species seem not to be widely distributed, in contrast to what traditional taxonomy would suggest for the recognized morphotypes.  相似文献   
6.
Stricker, S. A., Smythe, T. L., Miller, L. and Norenburg, J. L. 2001. Comparative biology of oogenesis in nemertean worms. — Acta Zoologica (Stockholm) 82 : 213–230
In order to supplement previous analyses of oogenesis in nemertean worms, this study uses light and electron microscopy to compare the ovaries and oocytes in 16 species of nemerteans that represent various taxa within the phylum. Nemertean ovaries comprise serially repeated sacs with an ovarian wall that characteristically includes myofilament-containing cells interspersed among the germinal epithelium. Each oocyte can attach to the germinal epithelium by a vegetally situated stalk and resides in the ovarian lumen without being surrounded by follicle cells. In the ovary, oocytes arrest at prophase I of meiosis and contain a hypertrophied nucleus ('germinal vesicle') that often possesses multiple nucleoli. Intraovarian growth apparently involves an autosynthetic mode of yolk formation in most nemerteans and generates oocytes that measure ~60 µm to 1 mm. When fully developed, oocytes can be discharged through a short gonoduct and are either spawned freely or deposited within egg cases. In most species, oocytes released from the ovary possess extracellular coats and resume maturation by undergoing germinal vesicle breakdown (GVBD). Such post-GVBD specimens also form a punctate endoplasmic reticulum that may facilitate fertilization and development.  相似文献   
7.
8.
Jon L. Norenburg 《Hydrobiologia》1993,266(1-3):203-218
Riserius pugetensis gen. n., sp. n. is described from the northwest coast of Washington, U.S.A. It is probably a heteronemertine and, as such, would be the first known mesopsammic member of that order; it lives in the interstices of medium to coarse, moderately clean sands. Its morphology presents some attributes considered characteristic of mesopsammic fauna. The effect of some of these attributes is a generalized appearance of anatomical ‘simplicity’ and, as with other mesopsammic metazoans, this presents difficulties in distinguishing between primitiveness and reduction. In the new species such problematic features include the lack of subepidermal glandular cells and connective tissue, reduced proboscideal musculature, simple blood-vascular system, and the presence of a ‘secondary’, outer circular musculature in the body wall. The general appearance of this new species is very similar to the so-called palaeonemertine Hubrechtella and characteristics of both suggest relationship with the baseodiscid heteronemertines. These relationships are explored in light of a modified interpretation of proboscideal musculature, a traditional cornerstone of heteronemertmean taxonomy.  相似文献   
9.
10.
The phylogenetic relationships of selected members of the phylum Nemertea are explored by means of six markers amplified from the genomic DNA of freshly collected specimens (the nuclear 18S rRNA and 28S rRNA genes, histones H3 and H4, and the mitochondrial genes 16S rRNA and cytochrome c oxidase subunit I). These include all previous markers and regions used in earlier phylogenetic analyses of nemerteans, therefore acting as a scaffold to which one could pinpoint any previously published study. Our results, based on analyses of static and dynamic homology concepts under probabilistic and parsimony frameworks, agree in the non‐monophyly of Palaeonemertea and in the monophyly of Heteronemerta and Hoplonemertea. The position of Hubrechtella and the Pilidiophora hypothesis are, however, sensitive to analytical method, as is the monophyly of the non‐hubrechtiid palaeonemerteans. Our results are, however, consistent with the main division of Hoplonemertea into Polystilifera and Monostilifera, the last named being divided into Cratenemertea and Distromatonemertea, as well as into the main division of Heteronemertea into Baseodiscus and the remaining species. The study also continues to highlight the deficient taxonomy at the family and generic level within Nemertea and sheds light on the areas of the tree that require further refinement. © The Willi Hennig Society 2011.  相似文献   
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