Bull Mar Sci. 92(2):181–190. 2016 research paper http://dx.doi.org/10.5343/bms.2015.1033 Density and depth distribution of the sympatric species Squilla chydaea and Squilla empusa (Stomatopoda: Squillidae) in the southern Gulf of Mexico Departamento de Recursos Marco Antonio May-Kú * del Mar, Cinvestav, Carretera antigua a Progreso, km 6. Apdo. J Gabriel Kuk-Dzul Postal 73-Cordemex, 97310 Teresa Herrera-Dorantes Mérida, Yucatán, Mexico. Pedro-Luis Ardisson * Corresponding author email: <[email protected]. mx>, <mayku_antonio@ hotmail.com>, telephone: ABSTRACT.—The present study analyzed the density 01(999)9429400, ext. 2295. and depth distribution of Squilla chydaea (Manning, 1962) and Squilla empusa (Say, 1818) along the continental shelf in Campeche Sound during the rainy season months (July, August, October) of 2012 and 2013. Samples were collected during both day and night at 26 stations in 2012 and 34 stations in 2013. In total, 234 stomatopods were collected: 45% were S. chydaea and 55% were S. empusa. The mean densities of each species varied significantly by year (2012 > 2013) and sampling time (night > day). The highest mean density (4.7 ind ha−1) was observed for S. empusa during night in 2012, which was 47 times higher than the lowest density of S. chydaea, found during the day in 2013. The highest densities for S. chydaea were observed offshore of Terminos Lagoon and for S. empusa in the vicinity of Terminos Lagoon and adjacent to Grijalva-Usumacinta and San Pedro–San Pablo rivers. Results indicated that for S. chydaea, the relationship between density and water depth was quadratic in form, with the highest densities occurring between 60 and 120 m. The density ofS. empusa decreased linearly with depth; the highest densities were observed at <30 m. For both S. chydaea and S. empusa, differences in depth distribution varied little between years. These results provide new information about stomatopod species in the Date Submitted: 19 May, 2015. Date Accepted: 23 February, 2016. southern Gulf of Mexico and are of potential interest for Available Online: 5 April, 2016. fisheries management and environmental monitoring. Stomatopods (order Stomatopoda), commonly known as mantis shrimps, are represented by 449 recorded species, 45 of which are found in the Gulf of Mexico (Reaka et al. 2009). Stomatopods prey on fishes, molluscs, annelids, and other invertebrates (Cronin et al. 2006), and are important food items for certain fishes, including commercially important species such as the red grouper, Epinephelus morio (Valenciennes, 1828) (Brulé and Rodríguez-Canché 1993). Some stomatopods have commercial value, such as Squilla mantis (Linnaeus, 1758) and Oratosquilla oratoria (De Haan, 1844), fished in the Mediterranean Sea and off Japan, respectively (Hamano et al. 1987, Maynou et al. 2004). Stomatopods can also serve as biological Bulletin of Marine Science 181 © 2016 Rosenstiel School of Marine & Atmospheric Science of the University of Miami 182 Bulletin of Marine Science. Vol 92, No 2. 2016 indicators of petroleum hydrocarbons and heavy metals in the marine environment (Risk et al. 2001, Blasco et al. 2002). Campeche Sound, in the southern Gulf of Mexico, is an area of ecological and economic importance due to its high biodiversity and the ongoing exploitation of its natural resources, especially through petroleum extraction and shrimp fisheries (Soto et al. 2014). In this area, stomatopods of the family Squillidae are represented by six genera and 15 species. Five of these belong to the genus Squilla: Squilla chydaea Manning, 1962; Squilla deceptrix Manning, 1969; Squilla edentata edentata Lunz, 1937; Squilla empusa Say, 1818; and Squilla rugosa Bigelow, 1893 (Reaka et al. 2009). With regard to the western Atlantic Ocean, there is a wide body of literature that mentions the spatial distribution of some of these species—e.g., S. deceptrix, S. e. edentata, S. empusa, and S. rugosa (Holthuis 1959, Werding and Müller 1990, Tavares 2002, Reaka et al. 2009). However, in the southern Gulf of Mexico, patterns of spatial distribution and density of squillid stomatopods are poorly known. In Campeche Sound, information on this group comes from studies by Vázquez- Bader and Gracia (1994), Ruiz (2008), and Ruiz et al. (2013), which indicated that S. chydaea (nearly endemic to the Gulf of Mexico) and S. empusa (widely distributed in the western Atlantic Ocean) are the most abundant species of Squilla on the continental shelf. Squilla empusa has been recognized as a potential fishery resource in the Gulf of Mexico (Tavares 2002, Wortham 2009), but presently no stomatopod species are commercially exploited in this area, although they are frequently caught as bycatch by the industrial shrimp fleet and discarded at sea. Analysis of the spatial distribution of sympatric congeneric species is fundamental for understanding ecological relationships between coexisting populations and can inform proper management. The aim of the present study was to assess the population density and depth distribution of the sympatric stomatopods, S. chydaea and S. empusa, on the continental shelf of Campeche Sound during the rainy season. These data were obtained as part of an ongoing environmental monitoring program in the southern Gulf of Mexico. Methods Campeche Sound is located in the south-southwest sector of the Gulf of Mexico and forms part of the continental shelf situated between the Mexican states of Tabasco and Campeche. Campeche Sound covers approximatley 65,000 km2, with depths ranging from 0 to 200 m. The western part of the sound (Campeche Bay) re- ceives substantial inputs of organic material from river runoff, while the eastern part (Campeche Bank) receives a minimal level of continental influence and the sediments have high carbonate content (Hernández-Arana et al. 2005). Three weather seasons prevail in Campeche Sound: dry (March–May); rainy (June–October); and the sea- son of cold fronts or “nortes” (November–February). In the present study, sampling was conducted during only the rainy season months (July, August, and October) of 2012 and 2013. Sixty sampling sites were distributed across an area between 18°N–21°N and 91°W–93°W, with depths ranging from 6 to 170 m. These sites are part of the sampling grid of the Environmental Monitoring Program of the southern Gulf of Mexico, carried out jointly by PEMEX Exploración y Producción-Regiones Marinas and the Centro de Investigación y de Estudios Avanzados (Cinvestav) Unidad Mérida, an ongoing interdisciplinary program focused on understanding May-Kú et al.: Squilla stomatopods in Campeche Sound 183 the environmental condition of the southern Gulf of Mexico. A commercial shrimp trawl net with a mouth aperture of 14 m and 38-mm cod-end mesh size was used for collection of benthic fauna aboard the RV Justo Sierra from the Universidad Autónoma de México and the commercial shrimp vessel Campeche I. At each site, one bottom trawl was performed for 25–30 min at a mean trawling speed of 4345 m hr−1, and sweeping an area of about 2.5 ha. Otter trawls are commonly used for col- lection of benthic fauna, providing reliable quantitative estimates of population den- sity. However, they likely underestimate the density of some invertebrate taxa, such as stomatopods, due to their burrowing behavior (Syuhada 2011). Vessel time is ex- pensive; thus, the sites were visited at varying times throughout the day and night to maximize efficiency. In 2012, 22 sites were sampled during the day (06:00–18:00 hrs) and 4 at night (18:00–06:00 hrs), while in 2013, 23 and 11 sites were sampled during day and night, respectively. All benthic fauna collected were frozen at −20 °C and transported to the laboratory, where stomatopods were separated from other inver- tebrates and individually identified to species using the taxonomic keys of Manning (1969) and Manning and Heard (1997). For each stomatopod species, we used a two-way analysis of variance (ANOVA) to test whether density (ind ha−1) varied with year (2012, 2013) and sampling time (day, night). Density data were log-transformed [log (x+1)] prior to analysis to ful- fill ANOVA assumptions of normality and homogeneity of variance. When ANOVA test detected significant differences, post hoc comparisons among means were per- formed using Bonferroni test with corrected P value for the number of comparisons made (e.g., 0.1 / 6 = 0.016). To evaluate differences in depth distribution between S. chydaea and S. empusa, we used a Student’s t-test to test for differences in mean depth. This analysis was performed for each year separately. The relationship -be tween density and water depth, evaluated separately by stomatopod species and year, was analyzed using Poisson and negative binomial general linear models (NB1 and NB2 forms; i.e., linear negative binomial and quadratic negative binomial, respec- tively) in the R software package (R Development Core Team 2008). The best fitting model was selected on the basis of the smallest Akaike information criterion (AIC) value among the models tested (Akaike 1973). An alpha level of 0.1 was used as the criterion for statistical significance for statistical analyses. Statistical analyses were performed in the statistical package InfoStat version 2011 (Di Rienzo et al. 2011). Results During both years, we collected a total of 234 stomatopods, belonging to two Squilla species. Of these, 45% were S. chydaea and 55% were S. empusa (Table 1). In 2012, S. chydaea was present at 9 of the 26 sites, and in 2013, at 9 of the 34 sites. For this species, maximum densities were observed in 2012 during both day (8.4 ind ha−1) and night (6.8 ind ha−1) at two sites located offshore off Terminos Lagoon (19°32´N, 91°44´W, 41.5 m depth, and 19°33´N, 91°54´W, 55 m depth) (Fig.