首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到10条相似文献,搜索用时 61 毫秒
1.
A new genus and species of monostiliferoidean enoplan nemertean from Alaska is described and illustrated. The nemertean, Alaxinus oclairi gen. et sp. nov. , was found on the egg mass of a red king crab, Paralithodes camtschatica.  相似文献   

2.
A new species of monostiliferoidean nemertean, Tetrastemmafozensis , from the mantle cavity of the bivalve mollusc Scrobicularia plana , in the Ría de Foz, north-western Spain, is described and illustrated. Other nemerteans from the mantle cavity of Scrobicularia collected in Poole Harbour, southern England, are provisionally identified as conspecific with the Ria de Foz material, but show certain anatomical differences whose taxonomic significance cannot at present be assessed.  相似文献   

3.
4.
The phenomenon of codon usage bias is known to exist in many genomes and it is mainly determined by mutation and selection. To understand the patterns of codon usage in nemertean mitochondrial genomes, we use bioinformatic approaches to analyze the protein-coding sequences of eight nemertean species. Neutrality analysis did not find a significant correlation between GC12 and GC3. ENc-plot showed a few genes on or close to the expected curve, but the majority of points with low-ENc values are below it. ENc-plot suggested that mutational bias plays a major role in shaping codon usage. The Parity Rule 2 plot (PR2) analysis showed that GC and AT were not used proportionally and we propose that codons containing A or U at third position are used preferentially in nemertean species, regardless of whether corresponding tRNAs are encoded in the mitochondrial DNA. Context-dependent analysis indicated that the nucleotide at the second codon position slightly affects synonymous codon choices. These results suggested that mutational and selection forces are probably acting to codon usage bias in nemertean mitochondrial genomes.  相似文献   

5.
Four authors (A. A. W. Hubrecht, J. M. Macfarlane, D. D. Jensen and E. N. Willmer) have argued that nemerteans are the proximal invertebrate ancestors of vertebrates. While all these authors accept certain homologies (i.e. vertebrate pituitary and notochord with nemertean proboscis invagination and proboscis sheath or mesoderm), there is considerable variation among them in the derivation of the vertebrate nervous system, sense organs and foregut apertures. For example, Hubrecht suggests that the vertebrate nervous system developed from a solid dorsal nerve found in some nemerteans; Macfarlane suggests it developed by fusion of the left and right solid brains of nemerteans; Jensen suggests that the vertebrate tubular nervous system developed by the backward extension of the nemertean frontal organ and fusion with the nemertean dorsal and ventral brain ganglia. Modern EM, histochemical and biochemical methods can test empirically the four theories. Research testing these theories will also test the more popular acraniate theory, since that theory implies that vertebrate head structures evolved de novo in chordates and that no homologies should exist between vertebrate and nemertean head structures.  相似文献   

6.
Ray Gibson  Hongzhu Wang 《Hydrobiologia》2002,489(1-3):185-196
A new genus and species of freshwater monostiliferous hoplonemertean, Limnemertes poyangensis gen. et sp. nov., from Poyang Lake, People's Republic of China, is described and illustrated. The taxon is compared and contrasted with previously described freshwater hoplonemerteans. This is the fourth species of freshwater nemertean to be described from China and the first recorded from Poyang Lake.  相似文献   

7.
Wang H  Sun S  Li Q 《The Biological bulletin》2008,214(2):166-175
The feeding behavior of the nemertean Procephalothrix simulus and the effects of extrinsic factors on the feeding rates of this nemertean were experimentally studied. Procephalothrix simulus showed a typical macrophagous feeding strategy and consumed various food items. When capturing freshwater Tubifex sp., the nemerteans successfully ingested prey in all attacks, and they did not evert the proboscis in 25% of capture events. When capturing marine Saccocirrus gabrillae, the nemerteans failed to consume prey in more than half of the attacks, and they always everted the proboscis. There was a positive relationship between nemertean body weight and the rate of successful attacks, and a negative relationship between nemertean body weight and the duration of feeding events. The feeding rate of P. simulus increased when the temperature was raised from 5 degrees C to 30 degrees C but was significantly inhibited at 32 degrees C. Food intake was significantly reduced in media diluted to a practical salinity of 20 and 10 and in medium with the salinity elevated to 45. Dark conditions induced higher food intake, but prey density had no significant effect on feeding rate. These results suggest that P. simulus is a predator successfully adapted to the variable environmental conditions of the intertidal habitat.  相似文献   

8.
A marine bacterium provisionally identified as Vibrio alginoliticus, which is associated withtetrodotoxin production, was isolated from severalspecies of marine nemerteans. The presence of thisbacterium strongly supports the potential use oftetrodotoxins by nemerteans for protection againstingestion by predators or as an agent in their ownpredatory activities. Tetrodotoxins may facilitatethe ability of the nemertean proboscis to subdue preyduring capture. One commensal nemertean speciesappears not to possess the bacterium. This may bedue, in part, to its non-predatory life style.  相似文献   

9.
We estimated the phylogenetic relationships of 15 nemertean (phylum Nemertea) species from the four subclasses Hoplo-, Hetero-, Palaeo-, and Bdellonemertea with 18S rDNA sequence data. Three outgroup taxa were used for rooting: Annelida, Platyhelminthes, and Mollusca. Parsimony and maximum-likelihood analyses supported the monophyletic status of the Heteronemertea and a taxon consisting of hoplonemerteans and Bdellonemertea, while indicating that Palaeonemertea is paraphyletic. The monophyletic status of the two nemertean classes Anopla and Enopla is not supported by the data. The unambiguous clades are well supported, as assessed by a randomization test (bootstrapping) and branch support values.  相似文献   

10.
Abstract. The reproductive cycle of the entocommensal nemertean Malacobdella arrokeana inhabiting Panopea abbreviata from San José Gulf, Patagonia, Argentina, and the nemertean–host relationship were investigated. The mature nemertean population showed a sex ratio of 1:0.64, with females predominating. The number of individuals of M. arrokeana per host varied from 1–60 individuals (mean±SD=3.7±9.4 mm). Different stages of gonadic development were observed in individual nemerteans. Spawning peaks occurred during late summer and early spring. Sexually mature individuals of both sexes were present year round, possibly indicative of a continuous breeding period. Our results suggest that M. arrokeana has no cannibalistic habits and confirm its entocommensal symbiosis with P. abbreviata .  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号