1 Background
The coastal region of Pernambuco state is approximately 187 km in length and has the characteristic sandstone reefs that are marine outcrops that form parallel strands along the coast. These reefs are the result of consolidation or lithification of beach sand by calcareous or ferruginous cementing, and are made especially from quartz and bioclasts, mainly molluscs and algae (Sousa & Cocentino, 2004). Many of these environments are the main substratum for attachment of algae and benthic invertebrates in intertidal areas and on the coast below, therefore being an ecosystem with biological interactions that have significant importance in the structure of populations (Almeida et al., 2008, Guimaraens et al., 2011). Bivalve molluscs of the genus Brachidontes live in the intertidal zone of coves and bays and fix byssus to substrate rocky and reef surfaces, forming clusters of hundreds of individuals.
 
Numerous exotic species carried in ballast water and hull vessels have been introduced in the Brazilian coastal ecosystems (Farrapeira et al., 2011), so it is important to know aspects of the biology of these exotic species in the new environment and the potential impact on species previously established, for conservation and management of marine ecosystems invaded since invasive species can reduce biodiversity. The Isognomon bicolor (C.B. Adams, 1845) invasive bivalve colonization in the Brazilian coast has been from the State of Rio Grande do Norte to Santa Catarina (Domaneschi & Martins, 2002; Breves-Ramos et al., 2010; Zamprogno et al., 2010;. Martinez, 2012), having already been cited for the Pernambuco coast (Silva & Barros, 2011). Whereas Brachidontes solisianus (Orbigny, 1846) is distributed on the Atlantic Coast from Mexico to Uruguay (Rios, 1994).

Mussels in intertidal communities are dominant organisms on rocky shores and reefs because they are frequently superior competitors for space in these communities (Barbiero et al., 2011). Studies have shown that mussels can add complexity to benthic communities and increase species richness and diversity in the intertidal zone. Brachidontes solisianus is a small bivalve belonging to Mytilidae family. The free swimming larva of this species recruits on shallow and hard substrate and algae bases bound by byssus (Monteiro-Ribas et al., 2006). These aggregates can reduce mortality from desiccation and temperature stress. Isognomon bicolor have been found fixed by their byssus on rocky coasts and occurring together with Ulva spp. (Fernandes et al., 2004).

Given the potential risks of bioinvasion along with knowledge on the biological diversity of this area, it is necessary to study the characteristics of the animal communities that inhabit the beach of Piedade, not only to evaluate possible damage, but also to the knowledge of current biodiversity. It is extremely important to study the environment containing subsidies for the installation of invasive species. In addition to preparing a floristic survey to check the health of the reef and the interaction with individuals who also live on site during periods of dry and rainy seasons.

The objective of the study was to evaluate the population structure of Isognomon bicolor and Brachidontes solisianus in the dry and wet seasons in Piedade Beach reef  and assess the biomass Ulva spp. (Delile, 1813) on the same site and collection periods which is related to the bivalves populations recruitment and occurrence.

2 Results
In February 2012, the water temperature was 28. 3℃ and salinity of 34, while the temperature in May was 27.6℃ and salinity was 38 at Piedade Beach (Figure 1). Rainfall of the month before the first collection was 198.6 mm, and from the month before the second collection was 55.8 mm, featuring a dry period that ran during the rainy season.

Figure 1 – Sampling Site

The Chi-Square test detected a significant difference between the population structure of bivalve Brachidontes solisianus (p < 0.01) for the months of February and May. Brachidontes solisianus showed higher density of individuals in the smallest size class less than 1 mm in February 2012 (218 individuals /m²) than in May 42 individuals / m² (Table 1). Whereas 3 mm size class that was the one with the highest density in both seasons, being higher in May 2012 (1046 individuals /m²). The dominant algae in terms of biomass was Ulva spp., also characteristic of this patch of reef.

Table 1 – Size Classes abundance and Percentage for Brachidontes solisianus at Piedade Beach in 1 m2

Ulva spp. biomass was lower in May during the drought (11.92 g dry weight /m²), showing the harmful effect of desiccation on this abundant algae in the upper mesolittoral in February (107.75 grams of dry weight /m², p <0.05). This alga has been correlated with the recruitment of Brachidontes solisianus in the area (Guimaraens et al., 2013) since May and June have been a recruitment months and in the present study we found February as a recruitment month (Figure 2). Probably related to the abundance of Ulva spp. and less desiccation stress which can affect the abundance of Isognomon bicolor as well (Figure 3). The effect of drought was also showed by the lack of brown algae Dictyota and Dictyopteris in May 2012 (Simões et al., 2009, Table 2).

Figure 2 – Distribution by size class of Brachidontes solisianus the size classes are less than 1 mm (1) to 1 up to 8 mm (9) and 11 mm represented by 10 in the month of May (B) in 1200 cm2

Figure 3 – Distribution by size class of Isognomon bicolor with classes from 1 to 4 (1) up to 37 to 40 (10) in 1200 cm2 in February (A) and May (B)

Table 2 – Flora from the quadrats collected at Piedade Beach

We recorded a fauna and flora, as bivalve companion, with the presence of Tetraclita stalactifera (Lamarck, 1818) barnacles and Chthamalus bisinuatus (Pilsbry, 1916) also abundant in mesolittoral zone where the bivalves were collected. Brachidontes solisianus had its highest density in both months in the 3 mm size class and yet rated as juvenile probably a size that resists more desiccation and predation as well. The Chi-Square test also showed a difference between size classes distribution for Isognomon bicolor (p<0.05). The highest percentage was 26.78% in 13 – 16 size class for Isognomon bicolor in May with the next percentage in February for the same class (26%) which showed some similarity in the distribution of size classes (Table 3). However, numerically the largest Isognomon bicolor density was in February. As for Brachidontes solisianus, the highest percentage was in February 25.93 to 3 mm size range showing an increase in their density from February (979 individuals / m²) to May (1046 individuals / m²).

Table 3 – Size Classes abundance and Percentage for Isognomon bicolor at Piedade Beach in 1 m2

3 Discussion
The higher density of Brachidontes solisianus in May could represent the growth of the animals given that the most common form of ecological succession observed in Piedade was the closing side of the Brachidontes solisianus patch (Guimaraens et al., 2013). The drying can damage the colonization of Isognomon bicolor and Brachidontes solsianus given the positive relationship of these animals with the presence of algae such as Ulva, Sargassum and calcareous articulated algae (Fernandes et al., 2004, López & Coutinho, 2010a, Martinez, 2012, Guimaraens et al., 2013).

Individuals of Isognomon bicolor are aggregated into crevices with higher densities in these places, which was also observed in Piedade Reef. Isognomon bicolor was observed colonizing empty shells of Tetraclita stalactifera (Zamprogno et al., 2010), as well as Brachidontes solisianus (Guimaraens et al., 2013), which discloses a way to reduce drying. Considering that Brachidontes solisianus had higher density in the period of drought (May 2012), despite the higher number of Isognomon bicolor in both collections, the results suggest that Isognomon bicolor and Brachidontes solisianus can interact and coexist although Isognomon bicolor has become dominant where previously the dominance was from barnacles and mussels.

The invasive Isognomon bicolor occupies the area previously dominated by barnacles such as Tetraclita stalactifera and the bivalve Brachidontes solisianus since it has a larger size than Brachidontes and has the habit of growing over each other, which can optimize space for growth. These invasive bivalves, are less preferred by local predators, such as Stramonita haemastoma Linnaeus, 1767, than native species (López et al., 2010b). Despite the higher number of Isognomon bicolor, Brachidontes solisianus had its highest density in the month of May corresponding to a period of drought during the rainy season, but Brachidontes solisianus had fewer recruits in May 2012 possibly correlated with the decrease in biomass of Ulva spp.. The bivalves showed unimodal distribution with the same class most abundant size in two collections showing some stability despite the environmental impacts such as trampling and desiccation.

4 Materials and Methods
To assess the density of invertebrates and Ulva spp. biomass were randomly collected three quadrats of 20 x 20 cm on February 6 and May 5, 2012, in dry and wet seasons, respectively in the intertidal region of the southern end of Piedade Reef 08º09'17 "- 08º13'29" S. In the top midlittoral where both Brachidontes solisianus and Isognomon bicolor co-occur. The water temperature and salinity were measured with the aid of a digital thermometer and a refractometer. Rainfall data were obtained from the INMET website (http://www. inmet.gov.br/). The organisms were scraped and placed and labeled plastic bags and frozen for screening. After the sorting with the aid of a sieve of 300 micrometers the bivalves shell length were measured with a caliper. Aluminum baskets were previously weighed, where the algae were placed and taken to oven at 60 degrees Celsius for 24 hours. The baskets were weighed again in analytical scale to access the dry weight of algae, taking the values of those empty baskets from the dried seaweed. The Mann-Whitney test was done to evaluate the difference of Ulva between collection dates (February and May). The Chi-Square test was used to assess the difference between the frequency distribution of the size classes in the different sampled seasons. The statistical analyses were made with the software Biostat.

Author's Contributions
MAG and VD conducted the field work VD sorted and measured the bivalves and MAG analyzed the data.

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