Marine bioinvasions: bivalve molluscs introduced in northeast Brazil Bioinvasões marinhas: moluscos bivalves introduzidos no nordeste do Brasil

Invasive alien species are those that, once introduced from other environments, adapt, starting to reproduce and proliferate in places they did not previously inhabit. The introduction of exotic marine species as a result of anthropic activity, whether intentionally or not, is a process that has been remote for centuries, which makes it difficult to assess the impact of these activities, making studies that address the status of knowledge about such organisms always important. Thus, this research aims to gather information about bivalve molluscs introduced into marine ecosystems in northeastern Brazil, in addition to discussing the knowledge of this invasive fauna for the Northeast, Southeast and South regions of Brazil. For that there was a search for bibliography in 5 databases SciELO, Web of Science, Google Scholar, ScienceResearch.com and I3N Brazil Database of the Horus Institute for Environmental Conservation and Development. Three invasive bivalves for the northeastern region of Brazil were counted. When compared to the South and Southeast regions, the Northeast presents a significant amount of research on the ecology of the species Isognomon bicolor (C. B. Adams, 1845) and Mytilopsis leucophaeata (Conrad, 1831), which is very important, because with these results it is possible to track and understand the pathways that make bioinvasions possible. Regarding Perna Perna (Linnaeus, 1758), although there is a discussion about its supposed introduction in Brazilian waters, the species can be considered invasive in Northeastern Brazil.


INTRODUCTION
Species can be classified into three generic categories: exotic, native and cryptogenic (LOPES;VILLAC, 2009). Species classified as exotic can be included in four specific categories: contained, detected in a natural environment, established and invasive, assuming an increasing gradient of invasion potential. Native or wild species are those native to a given ecosystem or region. Finally, cryptogenic species are those of unknown or uncertain biogeographic origin (LATINI et al., 2016).
Invasive alien species are those that, once introduced from other environments, adapt, starting to reproduce and proliferate in places they did not previously inhabit.
They are characterized by having fast maturation, great reproductive capacity and a tendency for populations to persist after colonizing these new environments (LATINI et al., 2016;DARRIGRAN;DRAGO, 2000).
The introduction of species in a new location can occur intentionally or unintentionally and is most often the result of anthropic activities that overcome natural barriers which, normally, prevented the entry of these new species in environments previously not accessible by natural paths CAMPBELL, 2007). In Brazil, currently, the national database of invasive alien species of the Hórus Institute registers on its platform a total of 468 invasive species (INSTITUTO HÓRUS, 2020).
The establishment of these species in environments that are not of their origin can cause loss of local biodiversity, with major negative impacts, being able to change properties or processes at the ecosystem level, when the invaders differ from the natives in the acquisition or use of resources, modifying the trophic structure of the area (VITOUSEK, 1990).
Marine and freshwater ecosystems are particularly vulnerable to bioinvasions (SALA et al., 2000). Minchin et al. (2009) provide a comprehensive review of the vectors that contribute to the invasion of aquatic exotic species and point out eight main paths for a non-native organism to reach a new location. Among these vectors, navigation and cultivation of non-native species have been considered the most critical pathways for marine invasions worldwide (MOLNAR et al., 2008).
In the northeast of Brazil, some of the most serious cases of biological invasion in aquatic ecosystems are the consequences of voluntary introductions. Two emblematic cases are the tucunaré Cichla ocellaris and the tilapia Oreochromis niloticus in rivers, lakes and dams, which certainly resulted in several local species extinctions, especially, with loss of biodiversity on a regional scale (ROSA; GROTH, 2004).
Among the main species introduced in the different ecosystems, those belonging to the phylum Mollusca stand out (CARLTON; GELLER, 1993;CARLTON, 1992). In aquatic environments, bivalve mollusks represent a threat to native species, due to their successful colonization (invasion) of an environment, as it has short longevity, rapid growth, phenotypic plasticity, dominance in its natural habitat and wide environmental tolerance, being able to colonize several types of habitats because they have great flexibility of physiological adaptations (BARBOSA, 2009).
Although marine bioinvasion is a recurring problem in Brazilian waters, it is noticeable that society has only recently begun to understand and study its ecological

METHODOLOGICAL PROCEDURE
The search for the bibliography took place between the months of May and August 2020, and was carried out in five databases -SciELO, Web of Science, Google There are some problems and differences in the indexing processes in the bibliographic databases; thus, it was decided to search for free terms, without the use of controlled vocabulary, that is, descriptors. Using this strategy, a greater number of references was recovered, guaranteeing the detection of most of the published works within the pre-established criteria.
Regarding the inclusion and exclusion criteria, master's dissertations and doctoral theses, books, conference proceedings and original indexed articles were included, with research on taxonomy, reproduction, biological form, dispersion and its vectors, diet and preferred environments for invasion. Articles related to the cultivation of bivalves for economic purposes were excluded, as they did not directly assist in achieving the objective of this article.

MARINE BIVALVES INTRODUCED IN THE NORTHEAST
A review study carried out by Freire and Marafon (2018) FERNANDES et al., 2020;RIZZO et al., 2014). Figure 1 shows the distribution of these 3 species in northeastern Brazil from data obtained in secondary databases.

Página | 513
states of the northeast, which may be related, using the argument of Haddad and Nogueira Júnior (2006), to the fact that the main ports in Brazil are located in the south and southeast regions, and by the presence of a greater number of researchers working in large research centers in São Paulo, Rio de Janeiro and Paraná.

Isognomon bicolor (C.B. Adams) (Isognomonidae)
According to Breves-Ramos et al. (2010), there are two possible vectors that made the unintentional introduction of bivalve I. bicolor feasible. The first would be the oil platforms that are brought into Brazilian waters from different regions, and the second would be the ballast water from ships. In addition to these two, Oliveira and Creed (2008) also point to gas platforms as a possible vector.
This bivalve can be found in hard substratum and its presence has been observed in tide pools up to the shallow infralittoral. On rocky shores with high wave energy, it can be found occupying crevices and in less hit places. Its distribution in broad densities, forming clearly differentiated occupation ranges, it has already been observed by Domaneschi and Martins (2002).
Regarding its population structure, a survey conducted by Bezerra (2010), in Ceará, shows that this species has a high density, limiting the ability that many native species could have to obtain food, in addition to preventing its fixation. This result was also obtained in surveys carried out on the coast of Rio de Janeiro ( the conditions to which this bivalve is exposed in the Northeast.

Perna perna (Linnaeus, 1758) (Mytilidae)
There is a discussion about whether P. Perna is an invasive species in Brazil. Souza et al. (2003Souza et al. ( , 2004Souza et al. ( , 2005 after comparing the richness of the bivalve Pinctada imbricata in the sambaquis and in rocky shores of the south and southeast regions, found that this species was abundant in the sambaquis and rare in the nearby rocky shores. Such an event could indicate that this bivalve was seen in greater numbers in prehistoric times than in current times. Regarding P. Perna, its presence has not been confirmed for any sambaqui, however, currently, its presence is abundant in rocky shores. Thus, the authors suggested that P. imbricata was a food resource well used by prehistoric populations, being replaced, later, by the arrival of P. Perna, brought from

Página | 515
South Africa. Its possible bioinvasion on the Brazilian coast was also studied from a genetic point of view, showing a high genetic identity with populations in Africa (MOURA-NETO, 2003;SILVA, 2008). This factor would be consistent with a model of continent-islands (WRIGHT, 1978) in which, possibly, one population, from Africa, served as the founder of several other populations, in this case, Brazilians. Silva et al. (2018) studied the content of the sambaquis found in the entire distribution range of P. Perna along the Brazilian coast (578 sites), and dated some shells of this species. The results of these studies describe the same scenario proposed by Souza et al. (2003Souza et al. ( , 2004Souza et al. ( , 2005, that is, evidence that supports the hypothesis of its bioinvasion. In contrast, Pierri et al. (2016)  After studying the effect of ballast water discharged by merchant ships from Arraial do Cabo, Rio de Janeiro, Silveira (2005) recorded, for the first time, the presence of P. perna in Areia Branca, in the state of Rio Grande do Norte, probably introduced by ballast water. Thus, in any case, P. Perna is considered invasive in northeastern Brazil, because it is a species typical of the south and southeast regions (JHERING, 1900, SOUZA et al. 2004).
The species was registered in low densities in Areia Branca (RN), with young individuals that do not have well-developed populations. Its presence in this region is strong evidence of the transport of organisms from the southeast to the northeast of Brazil. Furthermore, this mussel is a typical organism that can be embedded in vessels, having probably been introduced in the Northeast via ballast water by vessels from the southeast of Brazil (SILVEIRA, 2005).
Regarding its ecological relations, in the southern region, a study by Freitas and Velastin (2010) on the ichthyofauna associated with a cultivation of P. perna in the north of Santa Catarina, showed that this bivalve has great relevance in the places they inhabit, structuring communities on rocky coastlines, in addition to being a diversity bioatractor.
To the northeast, there is still no research on the ecology, interactions or possible impacts that the mussel P. Perna can cause on native fauna. The absence of studies may be due to the fact that it was registered only in Rio Grande do Norte and in low densities. Thus, it is suggested that more research be done in regions where ballast water is discharged by ships, in order to verify whether this species occurs in other states in the northeast region.

Mytilopsis leucophaeata (Conrad, 1831) (Dreissenidae)
The  shows that the research centers of the universities are concerned with understanding the biology of this bivalve, being able, in this way, to present important information for any measure that is taken by the control organs.
In Rio de Janeiro, Fernandes et al. (2020) carried out extensive field research to evaluate the spread of M. leucophaeata, whose only site invaded by the species was the Rodrigo de Freitas lagoon (RIZZO et al., 2014;FERNANDES et al., 2018). In this study, a new record of this mussel is made for the coastal lagoon of Marapendi, where the diversity of haplotypes of this invasive species is high. The fauna associated with this species was also studied at the site.
The rapid expansion of M. leucophaeta requires more research on population dynamics and genetic variability of introduced populations, as well as their impacts on native biota (FERNANDES et al., 2020).

FINAL CONSIDERATIONS
Northeast Brazil has three invasive bivalves. In comparison with the south and southeast regions, the northeast presents a significant amount of research on the ecology of the species I. bicolor and M. leucophaeta, which is very important, because with these results it is possible to track and understand the pathways that make bioinvasions viable. Regarding P. perna, although there is a discussion about its supposed introduction in Brazilian waters, the species can be considered invasive in northeastern Brazil. With its registration in the region, research and monitoring must be carried out, in order to understand its population growth and its possible impacts on the local biota.
Since the effects of the introduced bivalves may be different according to the invaded location, special attention is needed by the Brazilian scientific community, not only in the south and southeast, but especially in the northeast region. Página | 518