Megascopic Symmetrical Metazoans from the Ediacaran Doushantuo Formation in the Northeastern Guizhou,South China

In South China, various megascopic symmetrical metazoan fossils were found in the upper Doushantuo （陡山沱） Formation of the Neoproterozoic Ediacaran. The worm-like fossil is characterized by modern taxological annelid, for many metameres, parapodia, one possible tentale, an alimentary canal, and a dorsal vessel. The triradiate discoidal fossils belong to Trilobozoa, and the octaradiate discoidal fossil might be Ctenophora. All these fossils indicate that the megascopic metazoans have appeared in the Doushantuoian of Eidacaran and imply that the symmetrical metazoans must have originated at least 550 Ma ago.

Exceptionally preserved brachiopods from the Chengjiang Lagerst(a|¨)tte(Yunnan,China):Perspectives on the Cambrian explosion of metazoans

The Cambrian explosion was coined to describe the geologically sudden appearance of numerous bilaterian body plans(Phyla)around the Ediacaran-Cambrian transition,around 565-520million years ago.Many explanations and conjectures have been postulated in order to explain the pattern and duration of this explosive radiation of many different phyla of early metazoans.Here,we focus on the evolution of a phylum of marine suspension-feeding animals—the brachiopods,as exemplified by the exceptionally preserved taxa from the celebrated Chengjiang Konservat Lagerst(a|¨)tte(Yunnan,China).The abundant soft-bodied preservation at these fossil quarries gives us the only firm insights into what brachiopods looked like and how they functioned and lived when they first appeared on the Earth.Studies of Chengjiang brachiopods demonstrate that the early animals developed a remarkably varied organization of tissues and organs shortly after the onset of Cambrian explosion.In the marine suspension-feeding brachiopods,most importantly the tentaculate feeding structure of early brachiopods is already differentiated into two shapes of lophophore,anteriorly coiled(spiralled)and posteriorly arching tentacle crowns and the unique latter type was previously not documented from fossil and living brachiopods.Also unlike any known Recent brachiopod,all the known Cambrian brachiopods from Chengjiang have an open digestive tract that was disposed either as a Ushaped gut in linguliform and stem group brachiopods,or straight gut with a posterior anus in some calcareous-shelled stocks.Moreover,in contrast toliving lingulids,all the Cambrian brachiopods have an epibenthic lifestyle either cemented by a ventral valve or attached by variable pedicles to establish complex ecological community encompassing primary tierers and variable secondary tierers.It is therefore assumed that brachiopods were the first benthic metazoan that achieved their success in ecological stratification and tiering complexity by late Atdabanian.The setae are also important for the brachiopod suspension-feeding life style,and in the Chengjiang braehiopods they include two types cilia-like and spine-like setae.The mantle canals of different braehiopod species are also distinctly variable in arrangement,mainly disposed in pinnate,baeulate and peripheral conditions.Of these,the peripheral disposition of mantle canals is for the first time proposed here so as to differentiate from the bifurcate condition in recent lingulids in that the former is devoid of posteriorly extending main trunks of sinus,but possesses a diverging dorsal vascula media in dichotomy.

LTR-mediated retroposition as a mechanism of RNA-based duplication in metazoans

Subject Code:C03With the support by the National Natural Science Foundation of China,the Chinese Academy of Sciences,and the National Key Basic Research Program of China,Prof.Zhang Yong(张勇)and colleagues at the Institute of Zoology,Chinese Academy of Sciences uncovered a general mechanism mediated by LTR retrotransposons to create transcribed RNA-based duplicates in metazoans,which

Reefs in the Early Paleozoic Taebaek Group,Korea:A Review

Various early Paleozoic （Cambrian Series 3-Middle Ordovician） reefs are found in the Taebaek Group, eastern Korea, located in the eastern margin of the Sino-Korean Block. They occur in every carbonate-dominant lithostratigraphic unit of the group, but their morphology and composition differ markedly. The Daegi Formation （middle Cambrian： Cambrian Series 3） contains siliceous sponge-Epiphyton reefs formed in a shallow subtidal environment, which is one of the earliest metazoan-bearing microbial reefs after the archaeocyath extinction. The Hwajeol Formation （upper Cambrian： Furongian） encloses sporadic dendrolites consisting of Angulocellularia, which developed in a relatively deep subtidal environment, representing a rare deeper water example. The onset of the Ordovician radiation resulted in the formation of microbialite-Archaeoscyphia-calathiid patch reefs in shallow subtidal deposits of the Lower Ordovician Dumugol Formation. Subsequent late Early Ordovician relative sea-level fall established extensive peritidal environments, forming microbial mats and stromatolites of the Lower-Middle Ordovician Makgol Formation. Ensuing Ordovician radiation resulted in one of the earliest metazoan skeletal reefs of the Middle Ordovician Duwibong Formation, constructed by stromatoporoid Cystostroma and bryozoan Nicholsonella, and developed around shallow shoals. These reefs reflect ongoing evolution and sea-level change during the early Paleozoic, and exemplify a rare glimpse of peri-Gondwanan records of reef evolution, which warrant detailed investigations and comparison with their counterparts in other regions.

A Fresh Look at Dickinsonia:Removing It from Vendobionta

The Ediacaran Dickinsonia is well-known for being the only fossil to be assigned to many phyla, ranging from lichens, Cnidaria, Piatyheiminthes, Annelida, and a phylum of its own to a nonmetazoan kingdom. A new specimen from the Ediacaran fine-grained sandstone on the Winter Coast of the White Sea in northern Russia, which has an age of -555 million years ago, preserved convincing internal anatomies of definite animals, comparable with meridionai canals of extant ctenophores （comb jellies）. Additionally, we reconsidered Dickinsonia as a biradiaily symmetrical animal rather than a bilateral one as previously thought. The animal nature of Dickinsonia is, thus, well established and its affinities are most probably allied to ctenophores. This research is not only removing Dickinsonia from Vendobionta, but also bringing the fossil record of ctenophores forward to 20 million years before the Cambrian ＂explosion＂.

The naked planet Earth:Most essential pre-requisite for the origin and evolution of life

Our blue planet Earth has long been regarded to carry full of nutrients for hosting life since the birth of the planet.Here we speculate the processes that led to the birth of early life on Earth and its aftermath, finally leading to the evolution of metazoans.We evaluate：（1） the source of nutrients,（2） the chemistry of primordial ocean,（3） the initial mass of ocean,and（4） the size of planet.Among the life-building nutrients,phosphorus and potassium play a key role.Only three types of rocks can serve as an adequate source of nutrients：（a） continent-forming TTG（granite）,enabling the evolution of primitive life to metazoans;（b） primordial continents carrying anorthosite with KREEP（Potassium,Rare Earth Elements, and Phosphorus） basalts,which is a key to bear life;（c） carbonatite magma,enriched in radiogenic elements such as U and Th,which can cause mutation to speed up evolution and promote the birth of new species in continental rift settings.The second important factor is ocean chemistry.The primordial ocean was extremely acidic（pH = 1-2） and enriched in halogens（CI,F and others）,S,N and metallic elements（Cd,Cu,Zn,and others）,inhibiting the birth of life.Plate tectonics cleaned up these elements which interfered with RNA.Blue ocean finally appeared in the Phanerozoic with pH = 7 through extensive interaction with surface continental crust by weathering,erosion and transportation into ocean.The initial ocean mass was also important.The birth of life and aftermath of evolution was possible in the habitable zone with 3-5 km deep ocean which was able to supply sufficient nutrients. Without a huge landmass,nutrients cannot be supplied into the ocean only by ridge-hydrothermal circulation in the Hadean.Finally,the size of the planet plays a crucial role.Cooling of massive planets is less efficient than smaller ones,so that return-flow of seawater into mantle does not occur until central stars finish their main sequence.Due to the suitable size of Earth,the dawn of Phanerozoic witnessed the initiation of return-flow of seawater into the mantle,leading to the emergence of huge landmass above sea-level,and the distribution of nutrients on a global scale.Oxygen pump also played a critical role to keep high-PO_2 in atmosphere since then,leading to the emergence of ozone layer and enabling animals and plants to invade the land. To satisfy the tight conditions to make the Earth habitable,the formation mechanism of primordial Earth is an important factor.At first,a ＇dry Earth＇ must be made through giant impact,followed by magma ocean to float nutrient-enriched primordial continents（anorthosite ＋ KREEP）.Late bombardment from asteroid belt supplied water to make 3-5 km thick ocean,and not from icy meteorites from Kuiper belt beyond cool Jupiter.It was essential to meet the above conditions that enabled the Earth as a habitable planet with evolved life forms.The tight constraints that we evaluate for birth and evolution of life on Earth would provide important guidelines for planetary scientists hunting for life in the exosolar planets.

Behavioral Diversity and Complexity Indicated by Metazoan Traces at a Topological Level since Proterozoic

On the basis of topological criteria, metazoan traces can be classified as four ichnoorders (clusters) and twenty two ichnofamilies (kinds), which consist of nine basic and three combined topological configurations. At a topological level, the behavioral diversity and complexity indicated by metazoan traces have remained quite stable since the Early Cambrian. Dramatic changes of these traits took place in the Neoproterozoic, in which 75% (12/16) of the topoichnotaxa and all nine basic topological configurations of metazoan traces occurred, but the most complex and highest level topological configuration occurred in the Early Cambrian of the coelomate animal explosive evolution, showing that in the Neoproterozoic and Early Cambrian gestated not only the Ediacarian fauna and all the basic types of multi cellular animals known today, but also all the basic and the most complex metazoan traces through geological time.

Extinctions,Morphological Gaps,Major Transitions,Stem Groups,and the Origin of Major Clades,with a Focus on Early Animals

Systematic extinctions can leave major morphological gaps between living crown-group clades.Such morphological gaps would be perceived,from a neontological point of view,as major evolutionary transitions.In order to fill these morphological gaps and to map the evolutionary steps toward major evolutionary transitions,we need to integrate extinct stem-group taxa in phylogenetic studies.However,the recognition of stem group has not been widely adopted in the study of early animal fossils,despite that all fossils are stem groups at one level or another.Part of the difficulty is that stem groups may not have all features that collectively diagnose the respective crown group,and they can have unique(autapomorphic)features,making them tantalizingly similar to and frustratingly different from the crown group(e.g.,stemgroup eukaryotes can be prokaryotic and stem-group animals can be protistan).The need to embrace stem groups and to implement the PhyloCode,in order to achieve phylogenetic clarity and to offer key paleontological insights into the origin and early animal evolution,is illustrated in debates on several controversial Ediacaran and Cambrian fossils.

Host-parasite interactions in perpetual darkness:Macroparasite diversity in the cavefish Astyanax mexicanus

Astyanax mexicanus has repeatedly colonized cave environments,displaying evolutionary parallelisms in many troglobitic traits.Despite being a model system for the study of adaptation to life in perpetual darkness,the parasites that infect cavefish are practically unknown.In this study,we investigated the macroparasite communities in 18 cavefish populations from independent lineages and compared them with the parasite diversity found in their sister surface fish populations,with the aim of better understanding the role that parasites play in the colonization of new environments.Within the cavefish populations,we identified 13 parasite taxa,including a subset of 10 of the 27 parasite taxa known for the surface populations.Parasites infecting the cavefish belong to five taxonomic groups,including trematodes,monogeneans,nematodes,copepods,and acari.Monogeneans are the most dominant group,found in 14 caves.The macroparasites include species with direct life cycles and trophic transmission,including invasive species.Surprisingly,paired comparisons indicate higher parasite richness in the cavefish than in the surface fish.Spatial variation in parasite composition across the caves suggests historical and geographical contingencies in the host-parasite colonization process and potential evolution of local adaptations.This base-line data on parasite diversity in cavefish populations of A.mexicanus provides a foundation to explore the role of divergent parasite infections under contrasting ecological pressures(cave vs.surface environments)in the evolution of cave adaptive traits.

Metazoan Parasites of the Mediterranean Garfish <i>Belone belone gracilis</i>(Teleostei: Belonidae) as a Tool for Stock Discrimination

The aim of the present study was to provide a current survey of metazoan parasites of the Mediterranean garfish Belone belone gracilis Lowe, 1839 off Tunisian coast and their potential use as biological tags for discriminate stocks. Parasitological analysis of 453 specimens of B. b. gracilis along the eastern Tunisian coast revealed the presence of ten species of metazoan parasites: one monogenean Axine belones Abildgaard, 1794;four digeneansLecithostaphylus retroflexus (Molin, 1859), Tergestia acanthocephala (Stossich, 1887) Stossich, 1899, Aponurus laguncula Looss, 1907 and Condylocotyla pilodora(metacercaria) Pearson et Prévot, 1985;one copepod Bomolochus bellonesBurmeister, 1835;two isopods Irona nana Schioedte et Meinert, 1884 andNerocila orbignyi (Guérin-Méneville, 1832);1 acanthocephalan Telosentis exiguus (Von Linstow, 1901) and one nematod Anisakid larvae. Discriminant analysis using parasites as biological tags, allowed identifying two stocks ofB. b. gracilis. Digenea L. retroflexus, A. laguncula and Anisakid larvae were the most important species in determining the position of sampled fishes from the central coast off Tunisia. Metacercaria C. pilodora characterized specimens from the Southern coast off Tunisia. These results were corroborated by comparing parameters of prevalence and mean abundance between zones. Discriminant analysis used for the separation of B. b. gracilis between seasons in both localities showed that a seasonal variation affected mainly specimens from the center suggesting seasonal move of fishes.

Effect of Locality, Host Species and Sex on the Metazoan Parasitic Infestation of Two, Species of <i>Scarus</i>Fish from the Red Sea Coast at Jeddah and Rabigh in Saudi Arabia

A total number of 60 specimens of two different species of Scarus fish caught from the Red Sea coast at Jeddah (30) and Rabigh (30) cities in Saudi Arabia were examined to determine their metazoan parasitic fauna, their prevalences, and mean intensities. Collected parasites were identified as, Isopoda (praniza larva of Gnathia sp.), Copepoda (Hatschekia sp.) Monogenea (Diplectanum sp.) and Digenea (Lecithoclaster and Bucephalus spp). Out of 30 specimens of fish caught from Jeddah coast 13 (43.4%) were infested by 90 parasites comprising of two parasite species, praniza larva and Hatschekia sp., with prevalence of infestation 40% and 3.33%, and mean intensity 7 and 6 parasites per fish respectively. 19 (63.3%) of Scarus fish caught from Rabigh coast were infested by 205 parasites representing 4 parasite spp. Diplectanum sp. represents the most commonly encountered parasite with prevalence (53.3%) and mean intensity (10.94%), followed by (Lecithoclaster sp. then praniza larva, prevalances (16.66% and 13.3%) and mean inentisities (3.4. and 1.75) respectively. Bucephalus sp. showed the minor, prevalence (3.33%) and mean intensity (6). Generally Scarus fish species from Rabigh had higher prevalence and mean intensity (63.3% and 7.59%) than Scarus fish species from Jeddah (43.4% and 6.92%). Female fish from Jeddah showed no parasitic infestation, while males were infested (prevalence 65% and mean intensity 6.92). Female fish from Rabigh had higher prevalence (84.21) and lower intensity (6.43) than males which showed prevalence (27.27%) and intensity (19). Relationship between parasitic infestation and fish species and sex were also studied in the two different localities and discussed.

Metazoan Parasites of Clariid Fishes, Lake Victoria: Reflection of the Original Fauna in the Lake?

A total of 32 parasite species were documented from 658 clariid fishes examined. Thirteen species were nematodes, eleven digeneans, three cestodes, two crustaceans, and the Monogenea, Acanthocephala and Hirudinea were each represented by one species. Of the parasites recovered 16 were identified to species level, 12 to genus and 4 parasites were not identified. Most of the parasites recorded in the present study have been reported in fish ofTanzaniaand elsewhere, however, there were a few first records, and these included Neogoezia sp., Comephoronema sp., Travnema sp., Quimperia sp. and Gendria tilapiae. It is stipulated that these parasites represent the original fauna of the lake prior to introduction of the alien species. The clariid fish species investigated were a good model for studying the parasites of fish in the lake because of having a variety of parasites some of which being specific to certain clariid fishes, for instance Diplostomum and Tylodelphys species infecting only C. gariepinus and the camallanids being typical of the clariid fishes.

There is no such thing as the ‘Ediacara Biota'

The term ＇Ediacara Biota＇ （or many variants thereof） is commonly used to refer to certain megascopic fossils of Precambrian and early Palaeozoic age - but what does the term actually mean？ What differ- entiates a non-Ediacaran ＇Ediacaran＇ and an Ediacaran ＇Ediacaran＇ from an Ediacaran non-＇Ediacaran＇？ Historically, the term has been used in either a geographic, stratigraphic, taphonomic, or biologic sense. More recent research and new discoveries, however, mean that the term cannot actually be defined on any of these bases, or any combination thereof. Indeed, the term is now used and understood in a manner which is internally inconsistent, and unintentionally implies that these fossils are somehow distinct from other fossil assemblages, which is simply not the case. Continued use of the term is a historical relic, which has led in part to incorrect assumptions that the ＇Ediacara Biota＇ can be treated as a single coherent group, has obscured our understanding of the biological change over the Precambrian-Cambrian boundary, and has confused research on the early evolution of the Metazoa. In the future, the term ＇Ediacaran＇ should be restricted to purely stratigraphic usage, regardless of affinity, geography, or taphonomy; sufficient terminology also exists where reference to specimens on a geographic, tapho- nomic, or biologic basis is required. It is therefore time to abandon the term ＇Ediacara Biota＇ and to instead treat ecmallv all of the fossils of the Ediacaran System.

Dimorphism in Calceola sandalina (Linneé, 1771)

Based on studying the ontogenetic development of Calceola sandalina in the Early Devonian Yukiang Formation from Guangxi Zhuang Autonomous Region, South China, it is for the first time to issue the evidence indicating the existence of sexual dimorphism in Calceola sandalina. The new discovery will add important data on sexual dimorphism throughout the non-colonial fossil metazoan, and it also may throw lights on studying the taxonomy and phylogeny of Calceola sandalina and its relevant taxa.

Oxygen Requirements for the Cambrian Explosion

Hypoxic tolerance experiments may be helpful to constrain the oxygen requirement for animal evolution. Based on literature review, available data demonstrate that fishes are more sensitive to hypoxia than crustaceans and echinoderms, which in turn are more sensitive than annelids, whilst mollusks are the least sensitive. Mortalities occur where O_2 concentrations are below 2.0 mg/L, equivalent to saturation with oxygen content about 25% PAL（present atmospheric level）. Therefore, the minimal oxygen requirement for maintaining animal diversity since Cambrian is determined as 25% PAL. The traditional view is that a rise in atmospheric oxygen concentrations led to the oxygenation of the ocean, thus triggering the evolution of animals. Geological and geochemical studies suggest a constant increase of the oxygen level and a contraction of anoxic oceans during Ediacaran-Cambrian transition when the world oceans experienced a rapid diversification of metazoan lineages. However, fossil first appearances of animal phyla are obviously asynchronous and episodic, showing a sequence as： basal metazoans〉lophotrochozoans〉ecdysozoans and deuterostomes. According to hitherto known data of fossil record and hypoxic sensitivity of animals, the appearance sequence of different animals is broadly consistent with their hypoxic sensitivity： animals like molluscs and annelids that are less sensitive to hypoxia appeared earlier, while animals like echinoderms and fishes that are more sensitive to hypoxia came later. Therefore, it is very likely that the appearance order of animals is corresponding to the increasing oxygen level and/or the contraction of anoxic oceans during Ediacaran-Cambrian transition.

Systematic analysis of microfauna indicator values for treatment performance in a full-scale municipalwastewater treatment plant

The indicator values of microfauna functional groups and species for treatment performancewere systematically evaluated based on the continuous monitoring of the entire microfauna communities including both protozoa and metazoa over a period of 14 months, in two parallel full-scale municipalwastewater treatment systems in a plant in Beijing, China. A total of 57 species of ciliates, 14 species （units） of amoebae, 14 species （units） of flagellates and4 classes of small metazoawere identified,with Arcella hemisphaerica, Vorticella striata, Vorticella convallaria, Epistylis plicatilis and small flagellates （e.g. Bodo spp.） as thedominant protozoa, and rotifers as thedominant metazoa. The abundance of the sessile ciliateswas correlatedwith the removals of BOD 5 （Pearson＇s r = 0.410, p 〈 0.05） and COD Cr （r = 0.397, p 〈 0.05）while the testate amoebaewas significantly positively related to nitrification （r = 0.523, p 〈 0.01）. At the same time, some other associationswere also identified： the abundances of the large flagellates （r = 0.447, p 〈 0.01）, the metazoa （r = 0.718, p 〈 0.01） and species Aspidisca sulcata （r = 0.337, p 〈 0.05）were positively related to nitrification; the abundance of Aspidisca costatawas correlated to the TN （total nitrogen） removal （r = -0.374, p 〈 0.05 ）; the abundances of the sessile species Carchesium polypinum （r = 0.458, p 〈 0.01） and E. plicatilis （r = 0.377, p 〈 0.05）were correlatedwith the removal of suspended solids.

Advances in phylogenetic studies of Nematoda

Nematoda is a metazoan group with extremely high diversity only next to Insecta. Caenorhabditis elegans is now a favorable experimental model animal in modern de-velopmental biology, genetics and genomics studies. However, the phylogeny of Nematoda and the phylogenetic position of the phylum within animal kingdom have long been in debate. Recent molecular phylogenetic studies gave great challenges to the traditional nematode classification. The new phylog-enies not only placed the Nematoda in the Ecdysozoan and divided the phylum into five clades, but also provided new insights into animal molecular identification and phyloge-netic biodiversity studies. The present paper reviews major progress and remaining problems in the current molecular phylogenetic studies of Nematoda, and prospects the devel-opmental tendencies of this field.

Rare horizontal transmission does not hide long-term inheritance of SINE highly conserved domains in the metazoan evolution

Transposable elements （TEs） are self-replicating, mobile DNA sequences which constitute a significant fraction of eukaryotic genomes. They are generally considered selfish DNA, as their replication and random insertion may have deleterious effects on genome functionalities, although some beneficial effects and evolutionary potential have been recognized. Short interspersed elements （SINEs） are non-autonomous TEs with a modular structure： a small RNA-related head, a body, and a long interspersed element-related tail. Despite their high turnover rate and de novo emergence, the body may retain highly conserved domains （HCDs） shared among divergent SINE families： in metazoans, at least nine HCD-SINEs have been recognized. Data mining on public molecular databases allowed the retrieval of 16 new HCD-SINE families from cnidarian, molluscs, arthropods, and vertebrates. Tracking the ancestry of HCDs on the metazoan phylogeny revealed that some of them date back to the Radiata-Bilateria split. Moreover, phylogenetic and age versus divergence analyses of the most ancient HCDs suggested that long-term vertical inheritance is the rule, with few horizontal transfer events. We suggest that the evolutionary conservation of HCDs may be linked to their potential to serve as recombination hotspots. This indirectly affects host genomes by maintaining active and diverse SINE lineages, whose insertions may impact （either positively or negatively） on the evolution of the genome.

Fossilized metazoan embryos——the historical testimony of the Cambrian explosion

IN zoology, the developmental stage of individual from zygote to hatching was called embryo. The modem embryology is a branch of study on the organism’ s embryos. The palaeoembryology is a new field in palaeontology, and it has not been considered an important field before, because the invertebrate’s eggs or embryos are minute size and without mineralized envelope. Until now, fossilized eggs of invertebrates in Cambrian have been rarely reported and most of them are small globular structures without recognizable features. Other reported fossilized embryos found from Middle Cambrian are spherical with a

Influence of season,size and sex on the dynamic of gill metazoan parasite infesting the Balistes capriscus(Teleostei:Balistidae)of the Gulf of Gabès(Tunisia)

Objective:To investigate the effects of season,size and sex on the dynamic of gill metazoan parasite infesting the Balistes capriscus(Gmelin,1788)(Teleostei:Balistidae)(B.capriscus)of the Gulf of Gabès(Southern Tunisia,Central Mediterranean Sea).Methods:A parasitological survey of the grey triggerfish B.capriscus from the Gulf of Gabès(Southern Tunisia,Central Mediterranean Sea)was conducted monthly from June 2011 to June 2012.A total of 1080 fish were collected from commercial catches by pelagic trawl net at different fishing ports at Chebba(34°14'N,11°06'E),Kerkennah(34°45'N,11°17'E)and Zarzis(33°41'N,11°48'E).The weight,size,sex,date and area of capture of each specimen were recorded.Then,B.capriscus was examined to search for ectoparasites.For each parasite species,parasitological indices were calculated.Results:The parasite species are indentified as two copepods:Naobranchia variabilis,Taneacanthus ballistae and a monogenean:Ancyrocephalus balisticus.The parasitological indices depend significantly on seasonality;the highest prevalence of Naobranchia variabilis,Taneacanthus ballistae and Ancyrocephalus balisticus(28.89%,35.93%and 55.56%respectively)was recorded during summer season(June–August),while the lowest prevalence of each(6.3%,4.44%,8.15%)recorded during winter season(December–February).Furthermore the parasitological indices depend significantly on the host size but not on host sex.Conclusions:Our data suggest that the dynamic of gill metazoan parasite infesting B.capriscus is a result of a complex of biotic and abiotic factors.It is the first study on the effects of season,size and sex on the dynamic of gill metazoan parasite infesting B.capriscus(Teleostei:Balistidae)of the Gulf of Gabès(Southern Tunisia,Central Mediterranean Sea).