Indigenous Mussels Research Articles

Freshwater unionid mussels threatened by predation of Round Goby (Neogobius melanostomus)

Indigenous freshwater mussels (Unionidae) are integral to riverine ecosystems, playing a pivotal role in aquatic food webs and providing ecological services. With populations on the decline worldwide, freshwater mussels are of conservation concern. In this study, we explore the propensity of the invasive Round Goby (Neogobius melanostomus) fish to prey upon indigenous freshwater mussels. First, we conducted lab experiments where Round Gobies were given the opportunity to feed on juvenile unionid mussels and macroinvertebrates, revealing rates and preferences of consumption. Several Round Gobies consumed whole freshwater mussels during these experiments, as confirmed by mussel counts and x-ray images of the fishes. Next, we investigated Round Gobies collected from stream habitats of the French Creek watershed, which is renowned for its unique and rich aquatic biodiversity. We developed a novel DNA metabarcoding method to identify the specific species of mussels consumed by Round Goby and provide a new database of DNA gene sequences for 25 indigenous unionid mussel species. Several of the fishes sampled had consumed indigenous mussels, including the Elktoe (non-endangered), Creeper (non-endangered), Long Solid (state endangered), and Rayed Bean (federally endangered) species. The invasive Round Goby poses a growing threat to unionid mussels, including species of conservation concern. The introduction of the invasive Round Goby to freshwaters of North America is shaping ecosystem transitions within the aquatic critical zone having widespread implications for conservation and management.

Trends in mussel cover, density and size at exploited and unexploited intertidal reefs in eastern South Africa

The brown mussel Perna perna is the dominant indigenous mussel along the east coast of South Africa, where it is harvested by recreational and subsistence fishers. High fishing pressure near urban areas led to declining abundance and consequently to the closure of some reefs to fishing in 1998. We estimated trends in mussel population dynamics at exploited and unexploited sites, along fixed transects, over a 27-year period (1993–2019). Trends in recreational fishing effort were inferred from yearly permit sales and existing catch statistics. At high levels of fishing effort, short-term trends in mussel cover and densities were inversely related to fishing effort at three of the four sites considered, while the fourth site was influenced by intermittent breaching of a nearby estuary. Mussel size was inversely related to population densities. The effects of longer-term harvesting bans were partially obscured by sharp declines in fishing effort across the entire recreational fishery. Seasonal and interannual patterns in cover and density were partially synchronised among sites, indicating environmental forcing at similar time-scales. The long-term dataset was invaluable in disentangling the relative effects of fishing and environmental factors on mussel population dynamics and should be continued as a baseline for assessing future climate-induced effects on rocky-shore biota.

Condition Index of Mussel Mytilus galloprovincialis (Lamarck, 1819) as a Physiological Indicator of Heavy Metals contamination

The condition index (CI) is an ecophysiological indicator used to assess the response of mussels at different environmental pressures. The present paper aims to assess the heavy metals contamination of three polluted sites (Midia Port, Constanta Port, and Mangalia Port) and one reference site (2 Mai), using indigenous mussels Mytilus galloprovincialis as bioindicators of pollution. The concentrations of heavy metals (Cu, Cd, Pb, Ni, and Cr) in mussels’ tissues, seawater and sediments from the 4 studied areas were evaluated in order to find a possible correlation with the physiological index (condition index) of the mussels. For this purpose, the condition index, Bioaccumulation Factor (BAF) and Biota-Sediment Accumulation Factor (BSAF) have been assessed and related to seasons and location. The study showed that mussels can bioaccumulate elements such as Cu, Cd, Pb, Ni, and Cr, certain differences being reported between sampling sites and season. Condition index showed a very significant correlation with heavy metals concentration in mussels (Cu, Pb, and Ni), with BAF (Pb) and with BSAF (Cd, Ni, and Pb). Also, CI correlates significantly with Cd concentration in seawater and Cr content in sediments. The highest values of CI and heavy metal concentrations were highlighted in mussels collected from the most polluted sites. Thus, any significant seasonal correlation between condition index and metal concentration in mussels may be related to food availability and the level of pollution in the studied sites.

Biogeographical Patterns of Endolithic Infestation in an Invasive and an Indigenous Intertidal Marine Ecosystem Engineer

By altering the phenotypic properties of their hosts, endolithic parasites can modulate the engineering processes of marine ecosystem engineers. Here, we assessed the biogeographical patterns of species assemblages, prevalence and impact of endolithic parasitism in two mussel species that act as important ecosystem engineers in the southern African intertidal habitat, Perna perna and Mytilus galloprovincialis. We conducted large-scale surveys across three biogeographic regions along the South African coast: the subtropical east coast, dominated by the indigenous mussel, P. perna, the warm temperate south coast, where this species coexists with the invasive Mediterranean mussel, M. galloprovincialis, and the cool temperate west coast dominated by M. galloprovincialis. Infestation increased with mussel size, and in the case of M. galloprovincialis we found a significantly higher infestation in the cool temperate bioregion than the warm temperate region. For P. perna, the prevalence of infestation was higher on the warm temperate than the subtropical region, though the difference was marginally non-significant. On the south coast, there was no significant difference in infestation prevalence between species. Endolith-induced mortality rates through shell collapse mirrored the patterns for prevalence. For P. perna, endolith species assemblages revealed clear grouping by bioregions. Our findings indicate that biogeography affects cyanobacteria species composition, but differences between biogeographic regions in their effects are driven by environmental conditions.

Comparisons of life-history traits of the alien invasive Semimytilus algosus and three other mytilid mussels on the West Coast of South Africa

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 607:113-127 (2018) - DOI: https://doi.org/10.3354/meps12794 Comparisons of life-history traits of the alien invasive Semimytilus algosus and three other mytilid mussels on the West Coast of South Africa Zannè Zeeman*, George M Branch, Deena Pillay Department of Biological Sciences and Marine Research Institute, University of Cape Town, Rondebosch 7700, South Africa *Corresponding author: zannezeeman@gmail.com ABSTRACT: The invasive mytilid mussel Semimytilus algosus was first recorded on South African shores in 2009 and rapidly spread to occupy 500 km of the West Coast, where it dominates lower portions of the rocky shore. To identify mechanisms underlying the invasive success of S. algosus, the life-history parameters survivorship, reproductive output, recruitment, growth rate and shell and byssus strength were compared among S. algosus and another alien, Mytilus galloprovincialis, and the indigenous mussels Aulacomya atra and Choromytilus meridionalis. To standardise the effects of wave exposure and shore height on life history patterns, all observations and sampling were limited to the low shore at the research site, Bloubergstrand. The 4 mussel species employed very different means of maximising survival and/or space occupancy. S. algosus proved to be a typical r-selected species, with low survival due to its weak shell and byssal strength. Unexpectedly, it grew relatively slowly but had a high reproductive output and exceptionally high recruitment rate that largely account for its invasive success. This study therefore demonstrates how life-history traits influence the invasive success of alien species and allows predictions about their relative impacts on intertidal rocky shores. KEY WORDS: Invasive species · Competition · Rocky shores · Recruitment · Growth · Reproduction Full text in pdf format Supplementary material PreviousNextCite this article as: Zeeman Z, Branch GM, Pillay D (2018) Comparisons of life-history traits of the alien invasive Semimytilus algosus and three other mytilid mussels on the West Coast of South Africa. Mar Ecol Prog Ser 607:113-127. https://doi.org/10.3354/meps12794 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 607. Online publication date: December 06, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Assessment of metals bioaccumulation and bioavailability in mussels Mytilus galloprovincialis exposed to outfalls pollution in coastal areas of Casablanca

The present work aims to study the metallic contamination of four sampling sites located nearby major sewage outfalls of the Casablanca coast (Morocco), using indigenous mussels Mytilus galloprovincialis as bioindicators of pollution. This research offered the opportunity to study trace metals bioaccumulation mechanisms, which represent a major factor in assessment processes of the pollution effects in coastal ecosystem health. The bioavailability and the bioaccumulation of trace metals (Cu, Zn, Ni, Pb) were evaluated in order to compare the metallic contamination in mussels’ tissues and find a possible correlation with physiological parameters of this filter feeding species. Our results showed a significant spatiotemporal variation of bioaccumulation, compared to control. A significant correlation coefficient between metals (Zn and Pb) bioavailability and physiological index (CI) was revealed in mussels from the most polluted location. The seasonal variation of trace metal accumulation was also raised; the highest values recorded during the dry period.

Diurnal variations in personal care products in seawater and mussels at three Mediterranean coastal sites.

The presence of personal care products (PCPs) in the marine environment is of major concern. PCPs, UV filters, and musks can enter the marine environment indirectly through wastewater or directly via recreational activities. We conducted this study to document patterns in the occurrence of seven PCPs at three coastal sites impacted by recreational activities during 1day. The study focused on diurnal variations in these seven PCPs in seawater and indigenous mussels. In seawater, UV filters showed diurnal variations that mirrored variations in recreational activities at the sites. Ethylhexyl methoxycinnamate (EHMC) and octocrylene (OC) water concentrations increased from under the limit of quantification in the morning to 106 and 369ng/L, respectively, when recreational activities were the highest. In mussels, diurnal variations in OC were observed, with the lowest concentrations recorded in the morning and then increasing throughout the day. As Mytilus spp. are widely used as sentinels in coastal pollution monitoring programs (mussel watch), our findings on diurnal variations could enhance sampling recommendations for recreational sites impacted by PCPs.

Susceptibility of two co-existing mytilid species to simulated predation under projected climate change conditions

Properties of the shells and byssus filaments secreted by marine mussels are affected by environmental and biotic factors. In this study, we investigated the effects of pH and temperature on shell and byssus in artificially created monospecific and mixed aggregations of the indigenous mussel Mytilus galloprovincialis and the invasive mussel Xenostrobus securis. The variability in the response of the mussels was mainly explained by species-specific interactions derived from the type of aggregation. In the mixed groups, acidic conditions caused a decrease in byssus strength in M. galloprovincialis, but an increase in byssus strength in X. securis. Increased temperature positively affected shell strength in X. securis, but only in mixed aggregations. Interactive effects of acidification and warming were only detected in the organic matter of shells, the strength of which decreased in M. galloprovincialis in mixed aggregations. Although the invasive mussel may be able to take advantage of changed conditions by enhancing byssal attachment, the effects that acidification has on shells may make this species more vulnerable to some predators. The study findings provide some insight into the responses of protective and attachment structures of mussels to biotic and abiotic stressors, highlighting how species interactions may shape the future of mytilid populations.

Trophic signatures of co-existing invasive and indigenous mussels: selective feeding or different metabolic pathways?

Species invasions can have profound ecological impacts, including changes in patterns of distribution, abundance and diversity of native species, altering food-web dynamics and ecosystem functioning. In the marine benthic environment temperature, predation, competition for space and wave action exposure are some of the main factors that determine invasion success; however, the importance of food availability for invasive species has been underestimated. The present study investigates the diets of two mussel species, Mytilus galloprovincialis and Perna perna, that occur in mixed-species mussel beds, using fatty acid (FA) and stable isotope (SI) approaches over two periods of time in 2012 and 2014. With the exception of one site, we found no significant differences in the SI signatures between species, while strong dissimilarities were present in their FA signatures. Both species showed signatures of exposure to good food quality, however, neither showed links to specific dietary trophic markers. The FA differences observed between species could be related to food selectivity by one or both species or more likely by different metabolic pathways. Further investigations are needed to clarify the pattern observed; however, the results suggested that the food environment was not an important factor in determining the success of the invasive species.

Assessment of polycyclic aromatic hydrocarbons (PAHs) in mussels (Mytilus galloprovincialis) of Prince Islands, Marmara Sea

In this study, PAH analyses have been conducted on indigenous mussels. Mussel samples (Mytilus galloprovincialis) have been collected from seven stations of Prince Islands during September 2015. Concentrations of total determined PAHs (sum of 16 compounds) ranged between 664 and 9083ngg−1. The origin of PAHs has been found to be pyrolytic according to the PHE/ANT and FA/PYR ratios in Büyükada. For other islands, PAH origins have been observed as pyrolytic and petrogenic together according to the PHE/ANT, FA/PYR and BaA/CHR ratios.

Cheating the Locals: Invasive Mussels Steal and Benefit from the Cooling Effect of Indigenous Mussels.

The indigenous South African mussel Perna perna gapes during periods of aerial exposure to maintain aerobic respiration. This behaviour has no effect on the body temperatures of isolated individuals, but when surrounded by conspecifics, beneficial cooling effects of gaping emerge. It is uncertain, however, whether the presence of the invasive mussel Mytilus galloprovincialis limits the ability of P. perna for collective thermoregulation. We investigated whether varying densities of P. perna and M. galloprovincialis influences the thermal properties of both natural and artificial mussel beds during periods of emersion. Using infrared thermography, body temperatures of P. perna within mixed artificial beds were shown to increase faster and reach higher temperatures than individuals in conspecific beds, indicating that the presence of M. galloprovincialis limits the group cooling effects of gaping. In contrast, body temperatures of M. galloprovincialis within mixed artificial mussel beds increased slower and exhibited lower temperatures than for individuals in beds comprised entirely of M. galloprovincialis. Interestingly, differences in bed temperatures and heating rates were largely dependent on the size of mussels, with beds comprised of larger individuals experiencing less thermal stress irrespective of species composition. The small-scale patterns of thermal stress detected within manipulated beds were not observed within naturally occurring mixed mussel beds. We propose that small-scale differences in topography, size-structure, mussel bed size and the presence of organisms encrusting the mussel shells mask the effects of gaping behaviour within natural mussel beds. Nevertheless, the results from our manipulative experiment indicate that the invasive species M. galloprovincialis steals thermal properties as well as resources from the indigenous mussel P. perna. This may have significant implications for predicting how the co-existence of these two species may change as global temperatures continue to rise.

Reproductive Features of Indigenous and the Invasive Chinese Freshwater Mussels (Mollusca, Bivalvia, Anodontinae) in Ukraine

Propagation processes of indigenous mussels of water reservoirs and water currents of Ukraine proceed inadequately. In Anodonta anatinа (Linnaeus, 1758), 100 % of female mussels reproduced in about 60 % of collection points, in the rest of collection points only 17-90 % of female mussels appeared to be “pregnant”, and in A. cygnea (Linnaeus, 1758) this number was about 50 %. In almost 70 % of collection points, A. cygnea and Pseudanodonta complanata Rossmassler, 1835 female mussels’ marsupia were not completely filled with reproductive products. On the other hand, invasive Chinese mussel, Sinanodonta woodiana (Lea, 1834), had multiple ovipositions during the whole year, its hemibranchs were completely filled with reproductive products.

Spilled Oils: Static Mixtures or Dynamic Weathering and Bioavailability?

Polynuclear aromatic hydrocarbons (PAHs) from sequestered MV Selendang Ayu oil were biologically available in 2008, 3.6 y after it was spilled along Unalaska Island, Alaska. Thermodynamically driven weathering was the most probable mechanism of organism exposure to PAHs. Alkane and PAH composition in oil changed over time as smaller constituents were preferentially lost, indicative of weathering. In contrast, composition of the largest compounds (biomarkers) including triterpanes, hopanes, and steranes remained unchanged. Smaller molecules (the PAHs) lost from stranded oil were observed in indigenous mussels and passive samplers deployed in July 2008. Concentration and composition of PAHs were significantly different than in a non-oiled reference area and patterns observed in mussels were repeated in passive samplers deployed in three zones (intertidal, subtidal, and water). Thus, hydrocarbons lost from one compartment (sequestered whole oil) were detectable in another (mussels and passive samplers) implying aqueous transfer. Quantities of mobile oil constituents were small, yielding uptake concentrations that are likely inconsequential for mussels, but the sensitivity provided by bioaccumulation and passive sampler uptake ensured that dissolved hydrocarbons were detectable.

Changes in habitat complexity resulting from sequential invasions of a rocky shore: implications for community structure

Worldwide, marine rocky shores are being modified by alien species, but their successive impacts are rarely recorded. We documented sequential invasions of Marcus Island on the west coast of South Africa by comparing communities from 1980 (pre-invasion), 2001 (after invasion by the mussel Mytilus galloprovincialis) and 2012 (following invasions by another mussel, Semimytilus algosus, and the barnacle Balanus glandula). Their influence on habitat complexity was measured with a novel technique enabling retrospective calculation of historical complexity. In 1980, habitat complexity, invertebrate abundance and species richness decreased from the low-shore to the high-shore, but homogenised in 2001 after M. galloprovincialis elevated habitat complexity across most of the shore. In 2012, these variables returned to pre-invasion patterns, after M. galloprovincialis declined in the high-shore and was replaced there by B. glandula. With the first mussel invasion, several indigenous species extended up the intertidal, but retreated once M. galloprovincialis receded. Community composition differed significantly among nearly all years and zones, irrespective of whether the alien species were included in the analyses or not. Some once-dominant native species were negatively affected by the invasions: one indigenous mussel, Choromytilus meridionalis, disappeared by 2012, and another, Aulacomya atra, declined. The abundance of recruits of the limpet Scutellastra granularis rose and fell with the arrival and recession of M. galloprovincialis, but adults were adversely affected. Changes to habitat complexity induced by sequential invasions supported hypothesised changes in invertebrate abundance and species richness, but could not alone predict changes in community composition, which were also influenced by zonation.

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.

Assessment of trace elements and legacy contaminant concentrations in California Mussels (Mytilus spp.): Relationship to land use and outfalls

NOAA’s Mussel Watch Program funded a regional pilot project in California that characterized contaminants associated with various land uses in conjunction with state, federal and private partners. Herein we assess the magnitude and distribution of trace elements and persistent organic contaminants in indigenous mussels with respect to land use, presence of outfalls and a subset of California Areas of Special Biological Significance (ASBS). We detected significant differences among the land use categories for the majority of trace elements and legacy contaminants measured. There was no significant difference between sites with and without outfalls. PCBs and PAHs were significantly lower in sites within ASBS boundary compared to other sites. The findings of this study will help fine tune future regional and national assessments as well as guide development of resource management and remediation activities and programs.

Narrow valve gaping in the invasive mussel Limnoperna securis: implications for competition with the indigenous mussel Mytilus galloprovincialis in NW Spain

The black pygmy mussel Limnoperna securis (Lamarck 1819) is endemic to the brackish waters of New Zealand and Australia, but over the past decade, it has successfully invaded the inner Galician Rias of NW Spain. There is growing concern that L. securis will expand its range to the outer zones of the Rias, where it would pose a threat to the intensive raft culture of the indigenous mussel Mytilus galloprovincialis (Lamarck 1819). In this paper, we compare the valve-opening behaviour of the two mytilids under simulated raft conditions, i.e. full-strength seawater (35 g l−1) and a low current flow regime (2–5 cm s−1). Modes of valve-opening amplitudes that were most frequently observed in both species were in the range of 60–90 %, indicating a tendency towards full valve openness. Both species displayed circadian periodicity (τ = 24 h): maximal gaping was generally observed during periods of darkness and minimum gaping during daylight hours. The only prominent difference in behaviour between the two species was related to the degree of valve opening. The maximum recorded gape angle was 8.2° (SE = 0.9) for L. securis versus 14.8° (SE = 1.4) for M. galloprovincialis. This difference may place L. securis at a competitive disadvantage on substrates, where the two species coexist, such as over rocky shores or potentially mussel culture ropes.

Deja vu? A second mytilid mussel, Semimytilus algosus, invades South Africa's west coast

A second marine mussel is shown to have invaded South Africa's west coast. Molecular techniques, based on intraspecific gene sequence divergences, prove its identity as Semimytilus algosus, a member of the family Mytilidae, native to Chile. The identity of an older introduced population found in Namibia is also confirmed. The present geographic range of S. algosus in South Africa extends some 500 km, from Groenriviersmond in the north to Bloubergstrand in the south. Together with Mytilus galloprovincialis, another previously established invasive mussel, S. algosus has become a dominant intertidal organism on wave-exposed rocky shores across this region. Both species are now much more abundant intertidally than either of the indigenous mussels Aulacomya ater and Choromytilus meridionalis, which have become largely confined to sublittoral and sand-inundated habitats respectively. The two invasive mussels display strong spatial segregation, with M. galloprovincialis dominating the midshore and S. algosus blanketing the lower shore. Through a combination of its small size and high abundance, S. algosus is likely to greatly increase food availability for a range of intertidal predators, many of which cannot consume mussels above a threshold size.

Selection of Target Mussel Tissue for Application of Cellular Energy Allocation as a Physiological Biomarker in Native MusselsMytilus galloprovincialis(Lamarck, 1819)

Three selected mussel tissues (digestive gland, mantle, and gills) were studied to determine which was the most suitable for the potential use of the cellular energy allocation (CEA) methodology in indigenous mussels Mytilus galloprovincialis. In addition, the applicability of CEA in the assessment of natural stress caused by salinity fluctuations in stratified estuary was tested in selected tissues. It was important to identify the mussel gender to reliably assess the changes in organism energy budget. CEA value was calculated as a ratio between available energy (Ea) and energy consumption (Ec). Mantle tissue was under the strongest influence of the differences in protein and lipid content between male and female mussels, and therefore reflected physiological changes in the organism itself, rather than those caused by natural environmental stress. CEA in gills had lower values than in mantle and digestive gland, and was similar at two selected sampling sites, so the changes in CEA caused by natural stress could not be detected in the gill tissue. Greater Ec in mussels from the estuarine site than from the coastal site was detected only in the digestive gland tissue, and can probably be attributed to the energetically costly maintenance of osmotic balance. Last, using digestive gland tissue in CEA analysis demonstrated a clear difference between coastal and estuarine sampling sites, providing the measure of the natural stress posed by variations in salinity.

Mechanisms of habitat segregation between an invasive (Mytilus galloprovincialis) and an indigenous (Perna perna) mussel: adult growth and mortality

The invasive mussel Mytilus galloprovincialis and the indigenous mussel Perna perna coexist intertidally on the south coast of South Africa through partial vertical habitat segregation: M. galloprovincialis dominates the upper shore and P. perna the lower shore. Recruitment patterns can explain the zonation of P. perna, but not the invasive species. We examined the role of post-recruitment interactions by measuring spatial and temporal differences in adult growth and mortality rates of the two species. Specifically, we tested the hypothesis that interspecific differences in growth and mortality reflect adult distribution patterns. The two study locations, Plettenberg Bay and Tsitsikamma, are 70 km apart with two sites (separated by 300–400 m) per location, each divided into three vertical zones. Growth was measured seasonally using different marking methods in 2001 and 2003. Cumulative adult mortality was measured through summer in 2003/2004. Both species generally grew more slowly upshore, but they showed different effects of season. For P. perna, growth was significantly reduced in winter in the low zone, but unaffected by season in the high zone. For M. galloprovincialis, growth was either unaffected by season or increased in winter, even in the high zone. Thus, growth of P. perna and M. galloprovincialis was reduced under cool winter and warm summer temperatures, respectively; and while growth was more similar between species in summer, M. galloprovincialis grew much faster than P. perna in winter. Mortality of P. perna increased upshore. For M. galloprovincialis, mortality was not zone-dependent and was significantly greater than for P. perna on the low-shore and (generally) across the shore in Tsitsikamma. Both species had higher growth and mortality rates in Plettenberg Bay than in Tsitsikamma. Thus, P. perna seems able to maintain spatial dominance on the low-shore and at certain sites because of higher mortality of M. galloprovincialis. We conclude that seasonality in growth of the two species reflects their biogeographic affinities and that coexistence is possible through pre-recruitment effects that limit the vertical distribution of P. perna and post-recruitment effects that limit M. galloprovincialis.