Ambidexter symmetricus Manning & Chace , 1971 ( Decapoda , Processidae ) : first record for Rio Grande do Norte and overview of the species distribution on the Brazilian coast

We present the first record of Ambidexter symmetricus Manning & Chace, 1971 for the state of Rio Grande do Norte, northeastern Brazil, filling a distribution gap of this species in the country. We confirm the preference of this species for seagrass meadows in the Curimataú river estuary. Lastly, a brief overview and an updated distribution map for the records of this species on the Brazilian coast are provided.

Ambidexter symmetricus is the only representative of its genus with an amphi-American distribution; it occurrs in the western Atlantic from Florida (USA) to Santa Catarina (Brazil) (Manning and Chace 1971, Abele 1972, Pachelle et al., 2016), and in the eastern Pacific, known only by 1 record in the western Gulf of California (Mexico) (Ríos and Carvacho 1982).In Brazil, this species has been reported from several localities in the states of Ceará, Paraíba, Pernambuco, Espírito Santo, Bahia, Rio de Janeiro, São Paulo, and Santa Catarina (Ramos-Porto 1977, Christoffersen 1980, Ramos-Porto 1980, Santos and Coelho 1995, Santos and Coelho 1998, Coelho et al. 2006, Nascimento and Torres 2007, Riul et al. 2008, Souza et al. 2011, Almeida et al. 2012, Amaral et al. 2010, Terossi and Mantelatto 2014, Pachelle et al. 2016).However, many gaps remain throughout the distribution of A. symmetricus in Brazil.Thus, we report the first occurrence record of A. symmetricus in the state of Rio Grande do Norte, which fills 1 of these distribution gaps on the coast of Brazil.

Methods
The specimens was collected in the tropical estuarine system of the Curimataú River, which is located on the southern coast of the state of Rio Grande do Norte, near the border with the state of Paraíba, Brazil.It is within the Köppen-Geiger climate region As (Miranda et al. 2006, Rubel andKottek 2010).At sampled site near the mouth of the estuary, the width of the Curimataú River is 570 m and the maximum depth varies from 0 to 1.80 m.On both were observed large sandbanks that are exposed at low tide.The substrate of the river is clayey, with a predominance of sedimentary fractions of silt and clay over most of the area sampled, but shallower areas had predominantly of sedimentary fractions of fine sand.Throughout the year, the water temperature varyies little, approximately 26.6-28.8°C.The bottom salinity varies between 29 and 36 ppt (Miranda et al. 2006, A.B. de Moraes personal observation).
The field samplings were performed at random linear stretches of 500 m (500 m transects) were along the border of the Curimataú river estuary (between 06°19′25″S, 035°03′08″ W and 06°19′05″ S, 035°02′45″ W).The samples were collected with the aid of an artisanal shrimp trawl (12 m × 2 m, 5 mm mesh), in areas with and without occurrence of sea grass, in accordance to federal environmental laws (Collection license SISBIO-IBAMA #28314-1).Digital calipers (0.01 mm accuracy) were used to measure the length of the carapace of the specimens (LC, linear distance between the poster orbital margin and the posterior dorsal margin of the carapace).The sex of the specimens was determined by the appendix masculina on the second pair of pleopods.Finally, the specimens were fixed in 70% ethanol and deposited in the carcinological collection of the Grupo de Estudos de Ecologia e Fisiologia de Animais Aquáticos, Universidade Federal do Rio Grande do Norte (GEEFAA/ UFRN), Brazil.
To compare occurrence records of this species at our sites with the presence or absence of marine grass, the homogeneity of data variance and normality were verified by the Levene and Shapiro-Wilks tests, respectively (Zar 1996).Then, a Wilcoxon's test (non-parametric data) was performed, using the significance level of 5% (Zar 1996).
Identification.Rostrum slightly curved down, not extending beyond anterior margin of eye; apex bifid with longer lower tooth.Antennal spine present.Stylocerite rounded and laterally unarmed.Second pair of pereiopods symmetrical, with 4 meral articles and 9 or 10 carpal articles.Carpus of the fifth pair of pereiopods longer than the propodus.Fifth abdominal somite unarmed postlaterally.Abdominal sternites unarmed.
Color.Body covered with scattered red chromatophores, background semitransparent.Chromatophores arranged in a transverse band on the anterior portion of the abdomen.The color is also concentrated in the carapace, pleura and appendages (Fig. 1).

Discussion
From 56 different points sampled, the most common type of substrate was sedimentary fractions of silt and clay.Despite this, all of specimens collected of A. symmetricus occurred in areas where marine grass was present (Wilcoxon test, p <0.001).Specimens of A. symmetricus Table1.Records of Ambidexter symmetricus Manning & Chace, 1971  were also located visually in small pools formed during the low tide near these areas.These observations agree with the ones presented by Manning and Chace (1971) and Rasch and Bauer (2016), who discussed that this species is recurrent of marine grass meadows, which makes it an important component of these habitats.However, this species has also been reported from areas having substrates formed by calcareous algae (Ramos-Porto 1980), biogenic coarse sand (Coelho Filho 2006), sandstone reefs (Nascimento and Torres 2007), and intertidal rocks (Terossi and Mantellato 2014).Pachelle et al. (2016) stated that A. symmetricus is well-distributed throughout the Brazilian territory, it is quite rare and difficult to find due to its nocturnal habit.In fact, most Brazilian records in the literature are based on the capture of only 1-3 specimens (Christoffersen 1980, Coelho and Santos 1988, Coelho Filho 2006, Nascimento and Torres 2007, Riu et al. 2008, Almeida et al. 2012, Pachelle et al. 2016).
Our material represents the first record of this species for the state of Rio Grande do Norte, filling 1 of the gaps in the distribution of the species along the Brazilian coast.The new sites are located 92 km from the nearest southern record in Paraíba (Riul et al. 2008) and 375 km from the nearest northern record in Ceará (Pachelle et al. 2016) (Fig. 2).The only records of this species from the states of Espírito Santo, Rio de Janeiro, and Santa Catarina are those collected between 1970 and 1980 and reported by Christoffersen (1980).
in Brazil.Present study highlighted in bold.Geodetic datum = WGS84.* = Records with georeferenced localization estimated based on the description provided on each record.