Population ecology of bivalve mollusks in beach sediments on the southern coast of Espírito Santo, BrazilAbstract
This study examined the population ecology of bivalve mollusks on Central Beach in Piúma, southern Espírito Santo, Brazil, over one year (August 2018 to July 2019). Sampling was conducted along four transects perpendicular to the shoreline, with sediment characteristics and benthic community structure analyses. 2,104 individuals were identified, representing 5 orders, 8 families and 13 species. The most abundant species were Anomalocardia flexuosa (39.78%), Strigilla carnaria (28.90%), Donax gemmula (11.93%), and Tivela mactroides (11.07%). The consistent presence of juveniles and recruits indicated that the area serves as a natural nursery for these species. Transect A, closest to the mouth of the Novo River, showed the highest organic matter content in the sediment and the most significant dominance of A. flexuosa, which was positively correlated with organic matter levels (p< 0.0001). Conversely, alpha diversity was negatively correlated with organic matter content (p= 0.0497). Sediment granulometry was uniform across transects, mainly of moderately sorted fine sand. Rainfall did not significantly affect ecological indices or the abundance of dominant species, suggesting that the bivalve community is resilient to seasonal climatic variations. These results highlight the environmental significance of Central Beach in Piúma as a vital estuarine habitat, particularly for A. flexuosa, and emphasize the need for integrated coastal management strategies that account for both marine and continental influences.
Keywords:
invertebrates; seafood; Brazilian coast; population study
Resumo
Este estudo examinou a ecologia populacional de moluscos bivalves na Praia Central em Piúma, sul do Espírito Santo, Brasil, durante um período de um ano (agosto de 2018 a julho de 2019). A amostragem foi realizada ao longo de quatro transectos perpendiculares à linha de costa, com análises das características do sedimento e da estrutura da comunidade bentônica. 2.104 indivíduos foram identificados, representando 13 espécies, 8 famílias e 5 ordens. As espécies mais abundantes foram Anomalocardia flexuosa (39,78%), Strigilla carnaria (28,90%), Donax gemmula (11,93%) e Tivela mactroides (11,07%). A presença consistente de juvenis e recrutas indicou que a área serve como berçário natural para essas espécies. O transecto A, mais próximo da foz do Rio Novo, apresentou o maior conteúdo de matéria orgânica no sedimento e a dominância mais significativa de A. flexuosa, que foi positivamente correlacionada com os níveis de matéria orgânica (p< 0,0001). Por outro lado, a diversidade alfa apresentou correlação negativa com o teor de matéria orgânica (p= 0,0497). A granulometria dos sedimentos foi uniforme em todos os transectos, principalmente de areia fina moderadamente selecionada. A precipitação pluvial não afetou significativamente os índices ecológicos ou a abundância das espécies dominantes, sugerindo que a comunidade de bivalves é resiliente às variações climáticas sazonais. Esses resultados destacam a importância ambiental da Praia Central em Piúma como um habitat estuarino vital, particularmente para A. flexuosa, e enfatizam a necessidade de estratégias integradas de manejo costeiro que levem em conta as influências marinhas e continentais.
Palavras-chave:
invertebrados; frutos do mar; litoral brasileiro; estudo de população
1. Introduction
Bivalve mollusks play essential ecological roles in coastal and estuarine ecosystems, functioning as suspension feeders and reliable bioindicators of environmental quality (Jorcin and Nogueira, 2008). In tropical regions, characterized by high primary productivity and strong continental influence, both biotic and abiotic factors, such as sediment granulometry, organic matter content, and salinity, significantly affect the composition, distribution, and structure of bivalve assemblages (Christo et al., 2016).
Exploiting bivalve mollusks constitutes a key socioeconomic activity for many coastal communities in Brazil, particularly in estuarine and brackish transitional zones. On the southern coast of Espírito Santo, Brazil, in the municipality of Piúma, bivalve mariculture has gained increasing prominence, directly engaging local families and contributing to the diversification of household incomes (Lavander et al., 2025).
The estuary of the Novo River influences the coastal zone of Espírito Santo, especially Central Beach in Piúma. This river delivers sediments and nutrients that alter the substrate's physicochemical properties and, in turn, influence the spatial distribution of benthic organisms. In this context, studies on the population ecology of bivalves are essential to understanding biodiversity patterns and informing management and conservation efforts, particularly in environments exposed to anthropogenic stressors (Silveira et al., 2016).
Given this scenario, the present study aims to characterize the bivalve mollusk community of Central Beach in Piúma over a complete annual cycle. It seeks to elucidate the relationships among ecological, sedimentary, and climatic parameters and population structure, thereby contributing to scientific knowledge and supporting environmental management of coastal ecosystems.
2. Materials and Methods
2.1. Study area and sampling design
The biological samples were obtained with authorization from the Brazilian government (CEUA 23185.001448/2028-44). The study was conducted on Central Beach in Piúma, Espírito Santo (ES), southeastern Brazil, near Ilha do Gambá, at coordinates 20°50'36.42”S and 40°43'29.32” W (Figure 1). Four transects (A, B, C, and D), parallel to each other and spaced 100 m apart, were established perpendicular to the coastline. Sampling was carried out along each transect from the lower limit of the supralittoral zone toward the sea at 50 m intervals. Transects A and B included five sampling points each, transect C had four, and transect D had three.
At each sampling point, three sediment samples were collected using cylindrical polyvinyl chloride (PVC) cores with a 25 cm diameter corresponding to an area of approximately 500 cm2. The cores were inserted 5 cm into the sediment. The extracted material was sieved through a 2 mm mesh, and all retained biological material was stored in plastic bags for transport and later identification.
2.2. Sediment analysis and bivalve collection
Sedimentary analyses were conducted for each transect one-time monthly following standardized protocols to assess granulometry, organic matter content, calcium carbonate concentration, moisture, pH, and salinity. Seventeen samples underwent granulometric analysis based on the Wentworth scale (1922). Organic matter was removed with 10% hydrogen peroxide, and particles were dispersed using an ultrasonic bath before laser diffraction analysis using a SHIMADZU SALD 3101 Laser Particle Size Analyzer. Data were processed using SysGran 3.0 software (Camargo, 2006), and classical statistical parameters were calculated according to Folk and Ward (1957) for sediment classification.
Following Carver (1971), calcium carbonate and organic matter contents were quantified using hydrochloric acid (HCl) and hydrogen peroxide (H2O2), respectively. Moisture content was determined using an oven-drying method by the ABNT NBR 6457/2016 standard. Substrate pH was measured using an analog pH meter (Environmental OFA), and the salinity of interstitial water was determined using an ATC K52-100 refractometer.
Bivalves were sampled monthly from August 2018 to July 2019, continuously at low tide during spring tide. Specimens were transported to the Malacoculture Laboratory at the Federal Institute of Espírito Santo (IFES, Piúma Campus), where they were identified using specialized literature (Rios, 2009). Morphometric measurements were taken using a 0.05 mm precision caliper, following Quayle and Newkirk (1989), based on the anteroposterior (length) and dorsoventral (height) axes.
2.3. Data analysis
Population density was calculated as the number of individuals per square meter (ind./m2) at each sampling point. Total density at each point was obtained by summing the densities of all recorded species. The following ecological indices were calculated: relative abundance, species richness (S), Shannon-Wiener diversity index (H'), Simpson’s dominance index (D), and Pielou’s evenness index (J').
Monthly shell length frequency histograms were constructed for the most abundant species using size classes defined by Sturges' rule. Six class intervals were defined: C2–7, C8–13, C14–19, C20–25, C26–31, and C32–37 mm. Recruitment patterns were evaluated by identifying species with the highest proportion of juveniles.
Pearson correlation analyses (p < 0.05) were performed to assess relationships between diversity indices (richness, abundance, dominance, and evenness) and environmental parameters, specifically rainfall and sediment organic matter content.
3. Results
The bivalve collection area was characterized based on sediment parameters, including organic matter content, calcium carbonate concentration, moisture, pH, and salinity, as presented in Table 1. Transect A differed significantly from the others by exhibiting a higher organic matter content (2.56%). No statistically significant differences were found among transects for the other physicochemical variables. Granulometric analysis classified the sediment across all transects as sandy, predominantly fine, moderately sorted sand.
Environmental variables (mean ± standard deviation) of the collection area on Piúma Central beach (ES), near Gambá Island.
Figure 2 shows rainfall data for the study period, with the highest precipitation recorded in May 2019 (214.5 mm). A total of 2,104 individuals of the phylum Mollusca were collected, all belonging to the class Bivalvia. These were classified into five orders, eight families, and thirteen species. Veneridae and Tellinidae were the most diverse, represented by three species (Table 2).
Average monthly precipitation in the study area between August 2018 and July 2019. Source: INCAPER.
Frequency of occurrence of bivalve mollusc species on Piúma beach, near Ilha do Gamba, Espírito Santo, Brazil, from August/2018 to July/2019.
The most abundant species was Anomalocardia flexuosa, with 837 individuals recorded during the sampling period, accounting for 39.78% of the total. This was followed by Strigilla carnaria (608 individuals; 28.90%), Donax gemmula (251 individuals; 11.93%), and Tivela mactroides (233 individuals; 11.07%) (Figure 3). June and July recorded the highest total abundances, whereas February and March exhibited the lowest. Only two species, A. flexuosa and S. canaria, were present in all months of the study (Figure 4).
Ecological indices—including alpha diversity, species richness, evenness (Pielou’s J′), and dominance (Simpson’s D)—did not vary significantly over the study period (August 2018 to July 2019). However, when comparing transects, Transect A showed distinct values for dominance and evenness, attributed to the high abundance of A. flexuosa in this area (D= 0.743) (Figure 5).
No significant correlations were observed between ecological indices (diversity, abundance, dominance, and richness) and monthly rainfall (p> 0.05). Similarly, analyses focusing on the most abundant species (A. flexuosa, T. mactroides, S. carnaria, and D. gemmula) also showed no correlation between abundance and rainfall. However, a significant negative correlation was found between sediment organic matter content and the alpha diversity index (p= 0.0497). Additionally, the abundance of A. flexuosa showed a positive correlation with organic matter content in the sediment (p< 0.0001).
Monthly shell length frequency histograms (Figure 6) revealed that most individuals fell within the C20–25 and C26–31 mm size classes. Nonetheless, temporal variation in the average size of individuals was observed. Furthermore, as shown in Figure 7, a substantial number of juvenile specimens (seedlings) were recorded, highlighting the ecological role of Central Beach as a nursery area for some bivalve species.
Juvenile specimens of Anomalocardia flexuosa, Tivela macroides, Strigilla carnaria and Donax gemula observed between August/2018 and June/2019.
4. Discussions
The Praia Central de Piúma (ES), the site of the present study, is directly influenced by the mouth of the Novo River, a hydrographic basin with a drainage area of approximately 780 km2. This river discharges into the coastal region, contributing to the fluvial load of sediments and organic matter (Rocha, 2024). This continental input may partially explain the elevated organic matter levels observed, particularly in Transect A, closer to the river mouth. The river discharge carries suspended particles, organic residues, and nutrients from urban and agricultural areas within the watershed, significantly influencing the physicochemical properties of coastal sediments (Alves et al., 2012; Travassos Neto, 2019). These inputs can favor the colonization of tolerant and opportunistic species, such as A. flexuosa, whose abundance positively correlates with sediment organic matter content. Thus, the influence of the Novo River plays a crucial role in structuring the local benthic community, underlining the importance of considering continental inputs in managing and planning coastal zones.
The bivalve mollusk community surveyed over the year was highly diverse, comprising 13 species across 8 families, with A. flexuosa being the most abundant species. Such patterns are typical in tropical and subtropical regions (Rodrigues et al., 2010; Carneiro et al., 2021), where opportunistic species like A. flexuosa often dominate benthic communities due to their broad environmental tolerance (Lopes et al., 2023), continuous reproduction (Lavander et al., 2011), and rapid growth (Lima et al., 2018).
The dominance of individuals in the C20–25 and C26–31 size classes suggests that adults constitute a large portion of the population. However, many recruits from various species were also observed, highlighting the study area as a natural bivalve nursery. The continuous presence of A. flexuosa and Strigilla carnaria throughout the study period supports this hypothesis, emphasizing the area's ecological value as a recruitment zone vital for maintaining local populations (Lavander et al., 2011; Maia et al., 2018).
Environmental analyses indicated that Transect A had a higher organic matter content in the sediment, which was directly reflected in the ecological structure of the community. The strong positive correlation between organic matter content and the abundance of A. flexuosa (p < 0.0001) suggests that the species thrives in organic-rich substrates, likely due to greater availability of particulate food or its ability to adapt to low-oxygen conditions. Conversely, the negative correlation between organic matter content and alpha diversity (p= 0.0497) implies that high organic matter levels may reduce diversity by favoring dominant species at the expense of community evenness. In line with these findings, Stark and Johnson (2025) noted that increased organic matter results in lower dissolved oxygen levels and the accumulation of toxic substances, leading to decreased diversity in benthic mollusk communities.
The homogeneous granulometry (fine, moderately sorted sand) across transects indicates that sediment structure had minimal influence on the spatial distribution of the community (Soares-Gomes and Pires-Vanin, 2003). However, the notable differences in the equitability and dominance indices in Transect A suggest that biotic factors (Fujibayashi et al., 2019), such as the overabundance of a single species (A. flexuosa), can also significantly influence local community composition.
Rainfall did not correlate significantly with ecological indices or the abundance of the most abundant species. This may suggest that the bivalve community is ecologically resilient to seasonal rainfall fluctuations, a pattern observed in tropical regions, where benthic organisms can adapt to periodic environmental changes (Boehs et al., 2008; Rodrigues et al., 2013; Silva-Cavalcanti et al., 2018).
Notably, species such as A. flexuosa are often targets of anthropogenic harvesting, which can impact population structure and the sustainability of natural stocks. Identifying areas with high juvenile densities highlights the need for management strategies to protect recruitment habitats, especially in light of pressures from over-exploitation and global environmental changes.
This study underscores the ecological importance of Praia Central de Piúma (ES) as a key estuarine environment for the bivalve mollusk community, particularly for A. flexuosa. The influence of the Novo River, through the input of organic matter and sediments, was a key factor in shaping the local benthic fauna, creating favorable conditions for opportunistic species. The positive correlation between organic matter and A. flexuosa abundance and the negative correlation with alpha diversity highlights the role of sediment physicochemical characteristics in driving community dynamics. The presence of areas with high juvenile densities emphasizes the region's role as a recruitment and nursery area, which requires targeted conservation and management strategies, particularly in the context of extractive pressures and environmental change.
5. Conclusions
Therefore, this work contributes to our understanding of bivalve ecology in estuarine coastal areas and stresses the importance of integrated management between continental and marine aquatic systems. Expanding research in this area is essential for better understanding the dynamics of coastal environments and their organisms in the face of anthropogenic pressures and natural changes.
Acknowledgements
To IFES and IFAM. The Fundação de Amparo à Pesquisa do Estado do Amazonas (Notice No. 020/2024 - Productivity in Statements and Investment).
Data Availability Statement
The research data analyzed in this study are not publicly available by any means.
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e-mail: adriano.oliveira@ifam.edu.brEditor:
