Xenostrobus Securis Research Articles

Fine-scale spatial partitioning of genetic variation and evolutionary contestability in the invasive estuarine mussel Xenostrobus securis

ABSTRACTInitial studies of Australian populations of the widely invasive mussel Xenostrobus securis raised the possibility of non-random spatial partitioning within estuaries of mitochondrial DNA (mtDNA) clades that have dispersed intercontinentally and those that have not. A fine-scale phylogeographic investigation was made to examine this possibility using cytochrome c oxidase DNA sequences and data from five microsatellite loci developed here. X. securis in the study region was found to comprise multiple robustly supported mtDNA clades, many of which were genetically very distinct. Frequently, the clades comprised numerous haplotypes that have narrow ranges and may have evolved in situ within drainages or nearby. Microsatellite data reveal significant intra- and inter-drainage differentiation but at lower levels than mtDNA. Variation in mtDNA appears to reflect not only recolonization after infrequent local extinction within drainages but also the low probability of migratory haplotypes successfully establishing during evolutionary contests, with resident haplotypes having selective or demographic advantages derived from local occupation. The patterns of mtDNA haplotype distribution suggest that central New South Wales is the source of the internationally invasive lineages of X. securis.

A rapid assessment survey of invasive species of macrobenthic invertebrates in Korean waters

Introduced species are a growing and imminent threat to living marine resources in parts of the world’s oceans. The present study is a rapid assessment survey of invasive macrobenthic invertebrate species in Korean ports. We surveyed over 40 ports around Korea during the period of May 2010~March 2013. Among the sampling sites were concrete walls, docks and associated floats, bumpers, tires, and ropes which might harbor non-native species. We found 15 invasive species as follows: one Sponge, two Bryozoans, three Mollusks, one Polychaete, four Cirripedes, and four Ascidians. Three morphologically similar species, namely X. atrata, M. galloprovincialis, and X. securis were further examined for distinctions in their morphology. Although they could be reasonably distinguished based on shell shapes, significant overlap was noted so that additional analysis may be required to correctly distinguish them. Although many of the introduced species have already spread to all three coastal areas, newly arrived invasive species showed a relatively restricted range, with a serpulid polychaete Ficopomatus enigmaticus and a mytilid bivalve Xenostrobus securis found only at a few sites on the East Coast. An exception is for Balanus perforatus, which has rapidly colonized the East coast of Korea following its introduction into the region. Successful management of invasive macrobenthic invertebrates should be established in order to contain the spread of these newly arrived species.

The recently introduced bivalve Xenostrobus securis has higher thermal and salinity tolerance than the native Brachidontes variabilis and established Mytilopsis sallei

The recently introduced bivalve Xenostrobus securis and the previously introduced Mytilopsis sallei (~30years) are dominant over the native Brachidontes variabilis in estuarine fouling communities in Hong Kong. This study tested whether these introduced species have higher thermal and salinity tolerance than the native species under local subtropical seawater conditions. Survival, attachment, clearance rate and byssal thread production of these three species were examined through 96-h acute temperature and salinity tests. The results indicated that X. securis responded normally over a wide range of temperature and salinity conditions. Though M. sallei exhibited a wide salinity tolerance, its sub-lethal responses decreased in cold-seawater conditions. Brachidontes variabilis had the narrowest tolerance to temperature and salinity. These findings may explain the dominance of the non-native bivalves over B. variabilis. The high tolerance of X. securis enables them to become highly invasive in subtropical regions across Southeast Asia, impacting natural communities and shellfish farming.

Epigenetic signatures of invasive status in populations of marine invertebrates

Epigenetics, as a DNA signature that affects gene expression and enables rapid reaction of an organism to environmental changes, is likely involved in the process of biological invasions. DNA methylation is an epigenetic mechanism common to plants and animals for regulating gene expression. In this study we show, for the first time in any marine species, significant reduction of global methylation levels during the expansive phase of a pygmy mussel (Xenostrobus securis) recent invasion in Europe (two-year old), while in older introductions such epigenetic signature of invasion was progressively reduced. Decreased methylation was interpreted as a rapid way of increasing phenotypic plasticity that would help invasive populations to thrive. This epigenetic signature of early invasion was stronger than the expected environmental signature of environmental stress in younger populations sampled from ports, otherwise detected in a much older population (>90 year old) of the also invasive tubeworm Ficopomatus enigmaticus established in similar locations. Higher epigenetic than genetic diversity found in X. securis was confirmed from F. enigmaticus samples. As reported for introduced plants and vertebrates, epigenetic variation could compensate for relatively lower genetic variation caused by founder effects. These phenomena were compared with epigenetic mechanisms involved in metastasis, as parallel processes of community (biological invasion) and organism (cancer) invasions.

Response of Two Mytilids to a Heatwave: The Complex Interplay of Physiology, Behaviour and Ecological Interactions.

Different combinations of behavioural and physiological responses may play a crucial role in the ecological success of species, notably in the context of biological invasions. The invasive mussel Xenostrobus securis has successfully colonised the inner part of the Galician Rias Baixas (NW Spain), where it co-occurs with the commercially-important mussel Mytilus galloprovincialis. This study investigated the effect of a heatwave on the physiological and behavioural responses in monospecific or mixed aggregations of these species. In a mesocosm experiment, mussels were exposed to simulated tidal cycles and similar temperature conditions to those experienced in the field during a heat-wave that occurred in the summer of 2013, when field robo-mussels registered temperatures up to 44.5°C at low tide. The overall responses to stress differed markedly between the two species. In monospecific aggregations M. galloprovincialis was more vulnerable than X. securis to heat exposure during emersion. However, in mixed aggregations, the presence of the invader was associated with lower mortality in M. galloprovincialis. The greater sensitivity of M. galloprovincialis to heat exposure was reflected in a higher mortality level, greater induction of Hsp70 protein and higher rates of respiration and gaping activity, which were accompanied by a lower heart rate (bradycardia). The findings show that the invader enhanced the physiological performance of M. galloprovincialis, highlighting the importance of species interactions in regulating responses to environmental stress. Understanding the complex interactions between ecological factors and physiological and behavioural responses of closely-related species is essential for predicting the impacts of invasions in the context of future climate change.

Barcodes of marine invertebrates from north Iberian ports: Native diversity and resistance to biological invasions

Ports are gateways for many marine organisms transported by ships worldwide, especially non-indigenous species (NIS). In this study carried out in North Iberian ports (Cantabrian Sea, Bay of Biscay) we have observed 38% of exotic macroinvertebrates. Four species, namely the barnacle Austrominius modestus, the tubeworm Ficopomatus enigmaticus, the Pacific oyster Crassostrea gigas and the pygmy mussel Xenostrobus securis, exhibited clear signs of invasiveness. A total of 671 barcode (cytochrome oxidase subunit I or 18S rRNA) genes were obtained and confirmed the species status of some cryptic NIS. Negative and significant correlation between diversity estimators of native biota and proportion of NIS suggests biotic resistance in ports. This could be applied to management of port biota for contributing to prevent the settlement of biopollutants in these areas which are very sensitive to biological invasions.

Controlling populations of invasive pygmy mussel (Xenostrobus securis) through citizen science and environmental DNA

Early detection of dangerous exotic species is crucial for stopping marine invasions. The New Zealand pygmy mussel Xenostrobus securis is a problematic species in coasts of temperate regions in the northern hemisphere. In this study we have controlled a population of this invader that recently expanded in a north Iberian estuary with both a participatory approach involving researchers and citizens, and employing a sensitive eDNA-based tool to monitor the population expansion in the estuary. Results demonstrate successful eradication of pygmy mussels in the outer part of the estuary with citizen science and the practical utility of eDNA for controlling biological invasions.

Detection and characterisation of the biopollutant Xenostrobus securis (Lamarck 1819) Asturian population from DNA Barcoding and eBarcoding

DNA efficiently contributes to detect and understand marine invasions. In 2014 the potential biological pollutant pygmy mussel (Xenostrobus securis) was observed for the first time in the Avilés estuary (Asturias, Bay of Biscay). The goal of this study was to assess the stage of invasion, based on demographic and genetic (DNA Barcoding) characteristics, and to develop a molecular tool for surveying the species in environmental DNA. A total of 130 individuals were analysed for the DNA Barcode cytochrome oxidase I gene in order to determine genetic diversity, population structure, expansion trends, and to inferring introduction hits. Reproduction was evidenced by bimodal size distributions of 1597 mussels. High population genetic variation and genetically distinct clades might suggest multiple introductions from several source populations. Finally, species-specific primers were developed within the DNA barcode for PCR amplification from water samples in order to enabling rapid detection of the species in initial expansion stages.

DNA barcoding for assessment of exotic molluscs associated with maritime ports in northern Iberia

ABSTRACTPorts are gateways for aquatic invasions. New arrivals from maritime traffic and disturbed environmental conditions can promote the settlement of exotic species. Molluscs fall into the most prevalent group of invasive species and can have a tremendous impact on aquatic ecosystems. Here we have investigated exotic molluscs in three ports with different intensities of maritime traffic in the Cantabrian Sea. DNA barcodes were employed to identify the species using BLASTn and BOLD IDS assignment. Deep morphological analysis using diagnostic criteria confirmed BLAST species assignation based on COI and 16S rRNA genes. Results confirmed the usefulness of DNA barcoding for detecting exotic species that are visually similar to native species. Three exotic bivalves were identified: Ostrea stentina (dwarf oyster), the highly invasive Crassostrea gigas (Pacific oyster) and Xenostrobus securis (pygmy mussel). This is the first record of O. stentina in the Bay of Biscay and the second of X. securis in the Cantabrian Sea. Furthermore, we report on the presence of the cryptogenic mussel Mytilaster minimus in the central Cantabrian Sea. These exotic species might have been overlooked due to their phenotypic similarity with co-occurring oyster and mussel species. This study illustrates how combining morphological and DNA taxonomic analysis can help in port and marina biosecurity surveys.

Spatio-temporal variability in recruitment pattern of the invasive mussels Xenostrobus securis

Spatio-temporal variability in recruitment pattern of the invasive mussels Xenostrobus securis

Non-indigenous bivalves (Mollusca) from the Cantabrian Sea (Bay of Biscay)

Event Abstract Back to Event Non-indigenous bivalves (Mollusca) from the Cantabrian Sea (Bay of Biscay) Irene Fernández Rodríguez1*, Andrés Arias Rodríguez1 and Nuria Anadón Álvarez1 1 University of Oviedo, Department of Organisms and Systems Biology, Spain Alien or non-indigenous species represent a potential risk to receiving ecosystems, since they may develop an invasive behavior, negatively affecting the native diversity and causing, in some cases, economic losses. On the case of introductions of marine species, the most frequent vectors of introduction are the shipping (e.g. ballast water, biofouling) and the aquaculture activities. Furthermore, Global Warming may play a role in the establishment of alien species in new areas. The Cantabrian Sea (southern Bay of Biscay) is considered a potential area for the successful establishment of alien species. It is an area influenced by the Gulf Stream with a gradient of warmer water, and delimited northwards and westwards by regions with colder waters. On the other hand, during the last decades, there has been an increasing on the average water temperature in associated to global warming (IPCC, 2007). Here we present an updated review of the alien bivalve species reported in the Bay of Biscay until the date. A total of 15 species from 9 families of class Bivalvia (Pharidae, Veneridae, Ostreidae, Mytilidae, Arcidae, Semelidae, Pectinidae, Chamidae and Kelliidae) were reported as introduced, and four of them - Ensis directus, Ruditapes phillippinarum, Crassostrea gigas and Xenostrobus securis -have showed an invasive behavior: (Arias & Anadón 2012; Pejovic et al. 2016). The manila clam, R. phillippinaru and the Pacific oyster C. gigas were introduced by means of aquaculture and the American razor clam, E. directus, and the New Zealand pigmy mussel, X, securis,by ballast water. More recently, several specimens of Anadara transversa and Ostrea stentina have been reported for the fisrt time in the Bay of Biscay, from the Villaviciosa and Eo estuaries and the Avilés Port respectively (Fernández-Rodríguez et al. 2016; Pejovic et al. 2016). Although a harmful behavior has not been reported for those species yet, they certainly have the potential to become invasive and thus monitoring studies should be undertaken in order to ascertain its evolution in the area. References Arias, A., and Anadón, N. (2012). First record of Mercenaria mercenaria (Bivalvia: Veneridae) and Ensis directus (Bivalvia: Pharidae) on Bay of Biscay, Iberian Peninsula. J. Shellfish Res. 31(1), 57–60. Fernández-Rodríguez, I., Bañón, R., Anadón, N., and Arias, A. (2016). First record of Anadara transversa (Say, 1822) (Bivalvia: Arcidae) in the Bay of Biscay. Cah. Biol. Mar. 57(3), in press. IPCC (2001). Climate Change: The Scientific Basis. Cambridge University Press. Pejovic, I., Ardura, A., Miralles, L., Arias, A., Borrell, Y.J., and García-Vázquez, E. (2016). DNA barcoding for assessment of exotic molluscs associated with maritime ports in northern Iberia. Mar. Biol. Res. 12(2), 168–176. Keywords: alien species, molluscs, Biodiversity, Spain, Iberian Peninsula Conference: XIX Iberian Symposium on Marine Biology Studies, Porto, Portugal, 5 Sep - 9 Sep, 2016. Presentation Type: Poster Presentation Topic: 2. GLOBAL CHANGES, INVASIVE SPECIES AND CONSERVATION Citation: Fernández Rodríguez I, Arias Rodríguez A and Anadón Álvarez N (2016). Non-indigenous bivalves (Mollusca) from the Cantabrian Sea (Bay of Biscay). Front. Mar. Sci. Conference Abstract: XIX Iberian Symposium on Marine Biology Studies. doi: 10.3389/conf.FMARS.2016.05.00178 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 30 Apr 2016; Published Online: 03 Sep 2016. * Correspondence: Miss. Irene Fernández Rodríguez, University of Oviedo, Department of Organisms and Systems Biology, Oviedo, Spain/Asturias, Spain, irefdezrguez@hotmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Irene Fernández Rodríguez Andrés Arias Rodríguez Nuria Anadón Álvarez Google Irene Fernández Rodríguez Andrés Arias Rodríguez Nuria Anadón Álvarez Google Scholar Irene Fernández Rodríguez Andrés Arias Rodríguez Nuria Anadón Álvarez PubMed Irene Fernández Rodríguez Andrés Arias Rodríguez Nuria Anadón Álvarez Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Ecological interactions modulate responses of two intertidal mussel species to changes in temperature and pH

Predicted climate change scenarios for the end of this century suggest a substantial warming and ocean acidification of sea surface waters. Physiological performance of marine coastal organisms will be affected leading to changes in whole communities. In particular, ocean acidification poses an important threat for shelled molluscs. Nevertheless, climate change impacts are species-specific and responses may even differ among species that play a similar role in the ecosystems. Currently, two mussel species, the native Mytilus galloprovincialis Lamarck 1819 and the non-indigenous Xenostrobus securis Lamarck 1819, share the same habitat along shores in the inner part of the Galician Rias Baixas (NW Spain) forming mixed patchy clumps. Here, we used an experimental approach in mesocosms to evaluate the effects of increasing temperature and acidification on mussel clumps of different composition (monospecific and mixed clumps). Our model systems were artificially constructed clumps resembling those found on rocky shores. Responses to environmental stressors were measured on several functioning variables at the individual (i.e. survivorship, growth, condition index and composition of shell) and clump level (i.e. respiration, ammonia and phosphate excretion rates). The decrease of pH had a lethal effect and reduced growth on the native mussels. The invader was more resilient although individual responses were very often shaped by the composition of the clump. The reduced pH and, especially the high temperature conditions, tended to cause an increase in respiration rates of all types of clumps. Interactions between the two species seemed to modulate many effects at both individual and clump levels highlighting the complexity of responses when considering multiple stressors.

Introduction of the alien Xenostrobus securis (Bivalvia: Mytilidae) into Hong Kong, China: Interactions with and impacts upon native species and the earlier introduced Mytilopsis sallei (Bivalvia: Dreissenidae)

This constitutes the first record of the Australian alien mytilid Xenostrobus securis in China. The introduction occurred prior to 2010 probably via shipping arriving at Yantian in Mirs Bay, China, close to Hong Kong. Point sources of infection could be Australia or Korea or Japan where it has similarly been introduced. Analysis of X. securis in Hong Kong shows that it tolerates a wide range of salinities from 5.4‰ to 28.7‰. Water temperatures in Hong Kong match those in its native range.In Hong Kong, X. securis co-occurs with the similarly introduced Mytilopsis sallei and a native bivalve community. The population structure of X. securis suggests a lifespan of >2years with new generations produced by conspecifics. Such a life history strategy is also exhibited by M. sallei. Now established in China, northern Asia, the Mediterranean and, most recently, the Iberian Atlantic, the invasion of other locations by X. securis seems probable.

Feeding behaviour of an intertidal snail: Does past environmental stress affect predator choices and prey vulnerability?

Predation is one of the most important factors in determining structure and dynamics of communities on intertidal rocky shores. Such regulatory role may be of special relevance in novel communities resulting from biological invasions. Non-indigenous species frequently escape natural predators that limit their distribution and abundance in the native range. However, biological interactions also can limit the establishment and spread of non-native populations. There is a growing concern that climate change might affect predator–prey interactions exacerbating the ecological impacts of non-indigenous species. However, mechanisms underlying such interactions are poorly understood in marine ecosystems. Here, we explored if past environmental stress, i.e., increasing temperature and decreasing pH, could affect the vulnerability of two mussel prey, the native Mytilus galloprovincialis and the non-indigenous Xenostrobus securis, to predation by the native dogwhelk Nucella lapillus. In addition, we evaluated the consequences on the feeding behaviour of N. lapillus. First, we exposed monospecific assemblages of each mussel species to combined experimental conditions of increasing temperature and decreasing pH in mesocosms for 3weeks. Then assemblages were placed on a rocky shore and were enclosed in cages with dogwhelks where they remained for 3weeks. Despite the lack of preference, consumption was much greater on the native than on the invasive mussels, which barely were consumed by dogwhelks. However, this trend was diverted when temperature increased. Thus, under a coastal warming scenario shifts in dogwhelks feeding behaviour may help to contain invader's populations, especially in estuarine areas where these predators are abundant.

Recruitment and population structure of the non-indigenous brackish-water mytilid Xenostrobus securis (Lamark, 1819) in the Kino River, Japan

The recruitment and population structure of the non-indigenous brackish water mytilid Xenostrobus securis (Lamark, 1819), native to Australasia, were studied in the lower reaches of the Kino River, central Japan. Post-larvae > 2 mm in shell length were found exclusively among barnacle stands of Fistulobalanus kondakovi (Tarasov and Zevina, 1957) and algal fronds of Gloiopeltis tenax (Turner) Decaisone, 1842 from January to May and from July to October. Density of the post-larvae was much greater in the latter period than in the former. Small mussels with shell lengths of 2–4 mm settled to the conspecific mussel beds in April and September. Primary and secondary settlements onto different substrates were observed in the early stages of the life cycle. Two or three cohorts were identified in conspecific mussel beds, and they grew at a rate of ca. 1 mm per month. Density in the beds fluctuated monthly and was greater in the lower than in the middle intertidal zone, suggesting greater spatial heterogeneity in density due to predation in the former.

Byssus attachment strength of two mytilids in mono-specific and mixed-species mussel beds

The mussel Xenostrobus securis is endemic to the brackish waters of New Zealand and Australia, but has successfully invaded the inner Galician Rías of NW Spain, where it coexists with the indigenous mussel Mytilus galloprovincialis. In this laboratory study, the plasticity of the byssus attachment strength of two mytilids was compared by manipulating substratum, salinity, and bed assembly. M. galloprovincialis showed stronger byssus detachment strength than X. securis, despite lower byssus coverage. Both species responded similarly to the substratum, with substantially lower byssus strength on methacrylate, which offered the lowest surface free energy. Byssus detachment values for M. galloprovincialis were lower at lower salinity. In mixed beds, a number of mussels moved upwards, eventually colonising the upper layers of the assemblage. This behaviour increased byssus strength but only for X. securis. X. securis is adapted to a wide spectrum of abiotic conditions, a trait that may promote its dissemination within estuarine environments.

Invasive and non-invasive lineages in Xenostrobus (Bivalvia: Mytilidae)

The mussel genus Xenostrobus comprises eight named extant species from the Indo-West Pacific, including Xenostrobus inconstans, Xenostrobus pulex and Xenostrobus securis from Australia, the latter two also occurring in New Zealand. Xenostrobus species are predominantly restricted to estuaries or sheltered marine habitats although X. pulex inhabits the rocky intertidal on open coasts. Xenostrobus has recently been suggested to be congeneric with the freshwater invasive species Limnoperna fortunei. Xenostrobus securis is itself invasive in the Far East and Europe. This study employed DNA sequences from cytochrome c oxidase (COI), the D1 expansion region of 28S rRNA and the internal transcribed spacers of the ribosomal cistrons to investigate species relationships and habitat transitions in Australasian Xenostrobus. It is unlikely that Xenostrobus and Limnoperna are congeneric as their COI sequences are very different. There was unexpected complexity in defining Xenostrobus species. Xenostrobus pulex from New Zealand is probably a distinct species to the Australian taxon of that name with the name X. neozelanicus available for that taxon. Xenostrobus inconstans and Australian X. pulex were not reciprocally monophyletic in COI analyses. The phylogeography of the COI haplotypes of X. securis suggests that it may be a cryptic species complex, although this possibility could not be confirmed by sequences of the other genes.

尼崎運河での優占二枚貝を活用した水中懸濁物除去手法の開発

The Amagasaki Canal has been polluted,especially eutrophication and hypoxic condition have occurred significantly yet. To restore the aqueous environment of the canal,a water purifying plant was constructed by Amagasaki Sea Blue Project of Hyogo Prefecture. This study was made to clarify the effect of removing suspended substance by the water improvement aquarium using bivalve (Xenostrobus securis) part of the plant,and method of operation and maintenance. As a result,effect of removing suspended substance,material conversion and necessity of control biomass of bivalve was clarified. Moreover,calculation formula of material conversion for control biomass was found out.

外来二枚貝コウロエンカワヒバリガイの日本海沿岸での分布

The distribution of the non-indigenous mytilid bivalve Xenostrobus securis (Lamarck, 1819), native to Australasia, was investigated through a survey of 325 sites along the Japanese coast of the Sea of Japan from 2006 to 2011. The species was found at 20 sites from Toyama Prefecture westwards (Toyama Port and Shin-minato Fishing Port in Toyama Prefecture; Kanazawa Port and the Daishoji River in Ishikawa Prefecture; the Inokuchi River, Lake Kugushi, and Lake Suigetsu in Fukui Prefecture; the Takeno River in Hyogo Prefecture; Lake Nakaumi in Tottori and Shimane Prefectures; and Dokai Bay and Hakata Bay in Fukuoka Prefecture), all of which were located in brackish-water areas such as lagoons, river mouths, and the innermost parts of large ports. Size distributions of the collected mussels suggest that breeding populations have been established at eight sites. Although X. securis was not found at 86 sites from Niigata Prefecture northwards, water temperature regimes in the native and invaded areas suggest that it may eventually spread as far north as the southern end of Hokkaido.

Cytogenetic characterization of the invasive mussel species Xenostrobus securis Lmk. (Bivalvia: Mytilidae)

The chromosomes of the invasive black-pigmy mussel (Xenostrobus securis (Lmk. 1819)) were analyzed by means of 4',6-diamidino-2-phenylindole (DAPI) / propidium iodide (PI) and chromomycin A3 (CMA) / DAPI fluorescence staining and fluorescent in situ hybridization using major rDNA, 5S rDNA, core histone genes, linker histone genes, and telomeric sequences as probes. The diploid chromosome number in this species is 2n = 30. The karyotype is composed of seven metacentric, one meta/submetacentric, and seven submetacentric chromosome pairs. Telomeric sequences appear at both ends of every single chromosome. Major rDNA clusters appear near the centromeres on chromosome pairs 1 and 3 and are associated with bright CMA fluorescence and dull DAPI fluorescence. This species shows five 5S rDNA clusters close to the centromeres on four chromosome pairs (2, 5, 6, and 8). Three of the four core histone gene clusters map to centromeric positions on chromosome pairs 7, 10, and 13. The fourth core histone gene cluster occupies a terminal position on chromosome pair 8, also bearing a 5S rDNA cluster. The two linker histone gene clusters are close to the centromeres on chromosome pairs 12 and 14. Therefore, the use of these probes allows the unequivocal identification of 11 of the 15 chromosome pairs that compose the karyotype of X. securis.