Processidae, currently has 6 known genera and 69 recognized species, consists of small shrimp, which are mostly nocturnal and inhabit a wide variety of substrates such as sandy flat bottoms, sharing burrows with other invertebrates such as annelid worms and other crustaceans (^{Levitt, De Grave, & Shenkar, 2014}). They are particularly abundant in shallow waters with sea grass and coral rubble as dominant substrates (^{Manning, 1992}; ^{Manning & Chace, 1971}). The genus Ambidexter^{Manning & Chace, 1971}, composed of only 3 species, is distributed in the western Atlantic Ocean and central Pacific Ocean (^{Abele, 1972}), with 1 report in the Gulf of California (^{Ríos & Carvacho, 1982}). There are only 3 previous reports of the presence of A. symmetricus^{Manning & Chace, 1971} in Mexico, 2 on the Atlantic coast: ^{Manning and Chace (1971)} reported their presence in Tamaulipas, and ^{Álvarez, Villalobos, Rojas, and Robles (1999)} suggested their presence in Veracruz; and 1 on the Pacific coast: ^{Ríos and Carvacho (1982)} report the presence of this species in the Gulf of California.

With the aim of studying the abundance of juveniles of the pink shrimp Farfantepenaeus duorarum in the communities of Seybaplaya, Champotón and Isla Arena, in the state of Campeche, Mexico (Fig. 1), from May to November 2014, on 1 night per month and during the new moon, we conducted 50 m transects with an artisan net used for fishing pink shrimp juveniles consisting of a triangle that carries a trawl net with a mesh aperture of 0.6 cm.

Figure 1 Distribution map of Ambidexter symmetricus in the Gulf of Mexico, previous records (black circles; based on literature and biodiversity databases: ^{GBIF, 2015}; ^{OBIS, 2015}), new record (dark triangle) and locations where monitoring was performed (open circles). 

All captured shrimp were fixed in 70% alcohol, deposited in plastic bags and transported back to the laboratory. The taxonomic determination was based on the keys of ^{Abele (1972)}. We measured their post-orbital carapace length (POCL) with a digital caliper (±1 mm) and determined their sex by the presence or absence of the appendix masculina, and in the case of females, whether they were ovigerous. We took pictures of live specimens to record color patterns. The specimens collected were deposited in the Crustacean Collection of the Unidad Académica Sisal of the Universidad Nacional Autónoma de México.

In the 7 months of surveys we collected 13 specimens of Ambidexter symmetricus at the Seybaplaya location (19°38′50.58″ N, 90°41′53.16″ W) (Fig. 1). On May 27, 2014, we found 4 ovigerous females (YUC-CC-255-11-001726); on June 25, 2 ovigerous females (YUC-CC-255-11-001727); on July 24, 1 male and 1 ovigerous female (YUC-CC-255-11-001728); and on November 20, 5 ovigerous females (YUC-CC-255-11-001729). The specimens were collected at night at an average depth of 1 m on a substrate consisting of sea grass and muddy seabed, with POCL ranging from 4.0 to 6.3 mm. The 12 ovigerous females found from May to November indicate that A. symmetricus continuously bred during this period.

Taxonomic identification of these specimens was based on the presence of chela on the first pair of pereiopods and a stylocerite without a lateral spinule, which is considered the principal diagnostic characteristic for this species. Further characteristics were compared to the original description of the species, such as bifid tip of rostrum, presence of antennal spine, symmetrical second pereiopods, number of articles on carpus of second pereiopod (9 articles in 7 specimens, 10 articles in 6 specimens), fifth abdominal somite not armed, nor the other abdominal somites (^{Abele, 1972}; ^{Manning & Chace, 1971}). ^{Manning and Chace (1971)} described the color pattern of this species as light-cream, white, or colorless background with the chromatophores arranged in a band across the anterior portion of the abdomen with color also concentrated on pleura and appendages (there are no published pictures of living specimens in the literature). We found that all collected specimens of A. symmetricus exhibited the same color pattern, consisting of a translucent white with a red belt across the anterior portion of abdomen (Fig. 2).

Figure 2 Lateral and dorsal views of adult female of Ambydexter symmetricus^{Manning & Chace, 1971} from Seybaplaya, Campeche, Mexico. 

Specimens of A. symmetricus were found at Seybaplaya; no specimens were found at Champotón despite their proximity and environmental similarity; nor were specimens found in the northern location of Isla Arena. The absence of A. symetricus in these locations is not conclusive; it is necessary to perform specific monitoring to search for A. symmetricus in these 2 locations. The finding of this shrimp in Seybaplaya, extends the known distribution area of this species about 915 km south into the Gulf of Mexico, from its nearest report in Tamaulipas (^{Manning & Chace, 1971}). This is the first report of this species in the Yucatán Peninsula and an addition to the 35 species of caridean shrimp reported for Campeche (^{Hernández-Aguilera, Gracia, & Illescas, 2010}). ^{Álvarez et al. (1999)} assumed the presence of this species on the coast of Veracruz, and considering the present report we infer the possible presence of A. symmetricus in the entire Gulf of Mexico.

We are grateful to the technicians from the Centro Regional de Investigación Pesquera- Lerma, to the fishermen from the visited communities for in-field support, to Diana Ugalde-García for reviewing and giving us information about specimens, and to Sammy De Grave for his valuable English revision and corrections on manuscript. We also appreciate the comments of Alejandro M. Maeda Martínez and an anonymous reviewer for their valuable suggestions. This paper was financed by the Instituto Nacional de Pesca through the Project Análisis y seguimiento del recurso camarón rosado (Farfantepenaeus duorarum) en la sonda de Campeche.