INTRODUCTION
Marine macroalgae, as primary producers in coastal environments that accumulate metals, can be a source of metals for herbivores. Macroalgal species have different affinities for different metals related to their structural composition as green, brown and red algae, which may reflect competition among metals for binding or uptaking sites in the macroalgae, resulting in different bioaccumulation patterns (Sawidis et al., Reference Sawidis, Brown, Zachariadis and Sratis2001). Potential bioaccumulation of metals is related mainly to their concentration in the environment and their chemical presentation, which can be influenced by water effluents generated by human activities and environmental factors, such as upwelling.
An important source of metals in aquatic ecosystems is mineral deposits, including phosphorite (Mann & Ritchie, Reference Mann and Ritchie1995). One of the largest phosphorite deposits in the world is located in the south-western region of the Gulf of California, about 50 km north-west of the city of La Paz, Baja California Sur (Riley & Chester, Reference Riley and Chester1971). In phosphate rocks, common impurities are cadmium (Cd), lead (Pb), copper (Cu) and zinc (Zn) (Voulgaropoulos et al., Reference Voulgaropoulos, Paneli, Papaefstathiou and Stavroulias1991; Sabiha-Javied et al., Reference Mehmood, Chaudhry, Tufail and Irfan2009). Although, phosphate rocks are not a common source of iron (Fe), they are important as a measure of re-suspension of sediments from the continental shelf (Elrod et al., Reference Elrod, Berelson, Coale and Johnson2004). Among other factors, Cu, Zn and Fe can alter Cd and Pb bioavailability (Lönnerdal, Reference Lönnerdal2000). All of these metals accumulate in marine biota in nearby coastal areas (Ray, Reference Ray1984).
Sargassum species are the most conspicuous component of marine flora, forming extensive beds covering from a few square metres to several hectares throughout the Gulf of California (Hernández-Carmona et al., Reference Hernández-Carmona, Casas-Valdéz, Fajardo-León, Sánchez-Rodríguez and Rodríguez-Montesinos1990; Casas-Valdéz et al., Reference Casas-Valdéz, Sánchez and Hernández1993), where numerous species of molluscs, crustaceans and fish inhabit this ecosystem (Foster et al., Reference Foster, McConnico, Lundsten, Wadsworth, Kimball, Brooks, Medina-López, Riosmena-Rodríguez, Hernández-Carmona, Vásquez-Elizondo, Johnson and Steller2007). The sea slug Elysia diomedea (Bergh, 1894) (Gastropoda, Sacoglossa) is one of the common invertebrate species in Sargassum beds in the Gulf of California (Bertsch, Reference Bertsch, Danemann and Ezcurra2008), including Bahía de la Paz (Pacheco-Ruíz et al., Reference Pacheco-Ruíz, Zertuche-González, Espinoza-Ávalos, Riosmena-Rodríguez, Galindo-Bect, Gálvez-Télles, Meling-López, Orduña-Rojas, Danemann and Ezcurra2008). Elysiid sea slugs are suctorial feeders with distinct uniseriate radula. They are specialized herbivores, feeding on marine plants; the majority of species feed on siphonaceous green algae, such as the genera Caulerpa, Codium or Halimeda, among others (Bradley, Reference Bradley1984; Jensen, Reference Jensen1997; Gavagnin et al., Reference Gavagnin, Mollo, Montanara, Ortega and Cimino2000; Curtis et al., Reference Curtis, Massey, Scwartz, Maugel and Pierce2005; Pierce et al., Reference Pierce, Curtis, Massey, Bass, Karl and Finney2006; Trowbridge et al., Reference Trowbridge, Hirano, Hirano, Sudo, Shimadu, Watanabe, Yorifuji, Maeda, Anetai and Kumagai2010, Reference Trowbridge, Hirano and Hirano2011). Elysia diomedea feeds on marine algae (Bertsch & Smith, Reference Bertsch and Smith1973; Trowbridge, Reference Trowbridge2002; Hermosillo et al., Reference Hermosillo, Behrens and Ríos-Jara2006; Bertsch, Reference Bertsch, Danemann and Ezcurra2008), perhaps Caulerpa or Codium (Trench et al., Reference Trench, Greene and Bystrom1969, Reference Trench, Trench and Muscatine1972).
The aim of our study was to compare the concentrations of Cd, Pb, Cu, Zn and Fe between primary producers and a herbivore, the sea slug E. diomedea, in marine ecosystems dominated by Sargassum species and located close to one of the largest exploited phosphorite deposits. The information generated contributes to the knowledge about potential mobility and metal bioaccumulation at lower trophic levels in marine ecosystems.
MATERIALS AND METHODS
Field surveys were conducted at three sites in the south-western region of the Gulf of California: Las Animas (24°31′43.8″N 110°44′1.50″W), San Juan de la Costa (24°21′58.2″N 110°40′50.7″W) and El Sauzoso (24°18′38.6″N 110°38′28.8″W; Figure 1), which are characterized by rocky reefs where beds of Sargassum spp. predominate. At San Juan de la Costa there is a phosphorite deposit that has been exploited for nearly 30 years (Servicio Geológico Mexicano, 2008). Las Animas is 14.1 km to the north and El Sauzoso is 10.3 km to the south of San Juan de la Costa. There are no indications of human influences at either Las Animas or El Sauzoso (Méndez et al., Reference Méndez, Palacios, Acosta, Monsalvo-Spencer and Álvarez-Castañeda2006).
Collection sites in the south-western region of the Gulf of California at Baja California Sur, Mexico. ★, collection sites; 
, mine at San Juan de la Costa.
At each site, 30 healthy fronds of three macroalgal species Codium simulans Setchell & Gardner (Chlorophyta), Sargassum sinicola Setchell & Gardner (Ochrophyta: Phaeophyceae) and Gracilaria pachydermatica Setchell & Gardner (Rhodophyta), and 18 adults of the sea slug Elysia diomedea (20.3 ± 7.1 mm) were randomly collected along Sargassum beds at a depth of 0.5–2 m in March 2011. Of the three macroalgae, C. simulans was considered the main food item of the sea slug (Trench et al., Reference Trench, Greene and Bystrom1969, Reference Trench, Trench and Muscatine1972). The other two represent up to 70% of macroalgal biomass in coastal communities dominated by Sargassum spp. in the region. The taxonomic identification was performed with keys for macroalgae (Setchell & Gardner, Reference Setchell and Gardner1924; Joly, Reference Joly1967; Abbott & Hollenberg, Reference Abbott and Hollenberg1976; Guiry & Guiry, Reference Guiry and Guiry2013) and gastropods (Abbott, Reference Abbott1974; Hermosillo et al., Reference Hermosillo, Behrens and Ríos-Jara2006).
Macroalgae were cleaned by hand to remove epiphytes. Concentrations of Cd, Pb, Cu, Zn and Fe were measured on three pooled samples (six specimens) of each species of macroalgae and of the sea slug at each site. The samples of macroalgae and sea slugs were dried in an oven at 75°C for 24–48 h and then ground in a grinder/mixer fitted with a steel vial and ball pestle to make a homogeneous mixture. Approximately, 0.5 g dry weight of each sample of macroalgae and E. diomedea were subjected to acid digestion with a 3:1 ratio of nitric acid and hydrogen peroxide (analytical grade; Mallinckrodt J.T. Baker, USA) in a microwave oven (Mars 5X, CEM; Matthews, USA). Levels of Cd, Cu, Fe, Pb and Zn in the digested samples were quantified by atomic absorption spectrophotometry (Avanta, GBC Scientific Equipment, Australia) with an air–acetylene flame (Matusiewicz, Reference Matusiewicz, Mester and Sturgeon2003). Reference standards TORT-2, DORM-2, and ALGAE (National Research Council Canada, Institute for Marine Biosciences, Certified Reference Materials Programme, Halifax, NS, Canada) were used to validate the accuracy of the analytical method. Recovery percentages were above 95%. Detection limits (in µg g−1) for Cd, Pb, Cu, Zn and Fe were 0.017, 0.07, 0.02, 0.02 and 0.07, respectively.
Element concentration datasets were not normally distributed (Shapiro–Wilk test). To assess differences among metal concentrations among sites, nonparametric tests (Kruskal–Wallis test followed by Mann–Whitney U-test) were performed. Separately for each site, the species was considered as an independent variable and the element concentration as the dependent variable. Spearman rank correlation analyses between elements for each species and among species were also conducted (Zar, Reference Zar2010). Analyses were carried out with the STATISTICA 8 software (Statsoft, 2007). For elements that were present in concentrations below the detection limit, the half value of the respective detection limit was used for statistical analyses (Farnham et al., Reference Farnham, Singh, Stetzenbach and Johannesson2002). All results with a P < 0.05 were considered significant.
RESULTS
Macroalgae
Concentrations of Cd, Pb, Cu, Zn and Fe in the macroalgae were significantly different at the three sites (Table 1). Average Cd concentrations in Codium simulans and Sargassum sinicola were also significantly different among sites. For C. simulans, the average Cd concentrations in San Juan de la Costa and El Sauzoso were higher than the average in Las Animas (0.7 ± 0.3 µg g−1). For S. sinicola, specimens from San Juan de la Costa had an average Cd concentration (9.6 ± 0.8 µg g−1), 1.8 times higher than that from Las Animas and 1.4 times than that from El Sauzoso. For the red seaweed Gracilaria pachydermatica, no significant differences were found among collections sites, with mean values ranging from 5.1 to 6.0 µg g−1. Cadmium concentrations in G. pachydermatica were significantly correlated (r s = 0.72) with Fe concentration.
Concentration of cadmium (Cd), lead (Pb), copper (Cu), zinc (Zn) and iron (Fe) (µg g−1 dry weight) in three macroalgal species and a sea slug collected at three coastal sites in the south-western region of the Gulf of California. Average value ±standard deviation; superscript letters denote significant differences by Kruskal–Wallis test followed by Mann–Whitney U-test (P < 0.05) among sites in concentrations of the elements in a species.
LA, Las Animas; SJ, San Juan de la Costa; ES, El Sauzoso.
In C. simulans, detectable concentrations of Pb were found in specimens from Las Animas and San Juan de la Costa but not in specimens from El Sauzoso; average Pb concentration in Las Animas was 13 times higher than the average in San Juan de la Costa (0.3 ± 0.5 µg g−1). Lead in the brown alga S. sinicola was only detected in specimens from San Juan de la Costa (2.8 ± 1.9 µg g−1); it was below the detection limit in G. pachydermatica specimens from San Juan de la Costa, while levels in Las Animas (2.4 ± 0.7 µg g−1) and El Sauzoso (2.2 ± 1.3 µg g−1) were not significantly different. The Pb levels in S. sinicola were correlated with Cd (r s = 0.82) and Zn (r s = 0.76) concentrations; furthermore, Zn levels were correlated with Cu concentrations (r s = 0.72).
Copper levels in the macroalgae C. simulans and S. sinicola showed significant differences among sites. The highest concentration of Cu in C. simulans specimens from Las Animas (3.2 ± 0.8 µg g−1) was 1.9 times higher than the levels recorded in specimens from the other locations. For S. sinicola, mean Cu concentrations recorded in Las Animas and San Juan de la Costa were 2–3 times higher than those recorded in specimens from El Sauzoso (1.5 ± 0.4). Concentrations of Cu were significantly correlated (r s = 0.72) with Zn concentration in S. sinicola. For the red seaweed G. pachydermatica no significant differences were found, with mean values ranging from 1.6 to 5.1 µg g−1.
Zinc concentrations in the three macroalgal species were significantly different among sites; the highest levels were found in San Juan de la Costa. Zinc concentrations in C. simulans were correlated with concentrations in S. sinicola (r s = 0.83). The average concentration in G. pachydermatica (18.7 ± 1.7 µg g−1) was 1.7 times higher than the levels found in specimens from Las Animas and two times those at El Sauzoso.
Iron concentration in macroalgae was significantly different among sites only in G. pachydermatica. Levels recorded in specimens from San Juan de la Costa (197.0 ± 58.9 µg g−1) were three times higher than those recorded in Las Animas and two times those at El Sauzoso.
Cadmium concentrations in the three macroalgae showed the same pattern (C. simulans < G. pachydermatica < S. sinicola) in all the three sites, as they were for Zn concentrations (G. pachydermatica < C. simulans ≈ S. sinicola) and Fe levels (G. pachydermatica ≈ S. sinicola < C. simulans).
Sea slug
Concentrations of Cd, Pb, Cu, Zn and Fe in the sea slug were significantly different between the three sites (Table 1). Detectable concentrations of Cd were found in organisms from the three sites with those from San Juan de la Costa (17.7 ±4.2 µg g−1) showing significantly higher values than those from Las Animas and El Sauzoso. Detectable levels of Pb were found only in El Sauzoso (3.7 ± 6.3 µg g−1). No detectable levels of Cu were found in Las Animas. In addition, Cu concentrations were positively correlated (r s = 0.73) with Cd values. Zinc concentrations showed significant differences between Las Animas and El Sauzoso but not with San Juan de la Costa (29.7 ± 4.5 µg g−1). The average Fe concentration recorded in El Sauzoso (117.5 ± 82.6 µg g−1) was 3.2 and 2.5 times higher than the average concentration recorded in Las Animas and San Juan de la Costa, respectively. Overall, concentrations of the five elements showed an ascending pattern: Pb ≈ Cu < Cd < Zn < Fe across the three sites.
Sea slug vs macroalgae
The average Cd concentration in Elysia diomedea ranged from 3.4 to 7.4 times higher than the levels found in C. simulans from the three sampled sites, and it was from 1.8 to 3.0 times higher than the levels recorded in S. sinicola and G. pachydermatica, respectively, from San Juan de la Costa. The average Cd concentration in E. diomedea was lower than or similar to that found in S. sinicola and G. pachydermatica from Las Animas and El Sauzoso. The average Pb, Cu and Fe concentrations in E. diomedea were lower than or similar to those found in the three species of macroalgae, except for Pb in relation to G. pachydermatica from El Sauzoso, where the average Pb concentration in the sea slug was 1.7 times higher than the levels found in specimens of G. pachydermatica. Zinc concentrations in E. diomedea were between 1.3 times higher than those recorded in specimens of S. sinicola from San Juan de la Costa and 3.3 times higher than those recorded in specimens of G. pachydermatica from El Sauzoso.
DISCUSSION
Macroalgae
Metal concentrations in the three macroalga species (Table 1) were all within the ranges previously reported for these and other macroalgae in the Gulf of California and along the west coast of the Baja California Peninsula, Cd: 0.03–4.60 µg g−1, Pb: 0.1–30 µg g−1, Cu: 0.5–82 µg g−1, Zn: 2–96 µg g−1 and Fe: 140–2898 µg g−1) (Huerta-Díaz et al., Reference Huerta-Díaz, De Leon-Chavira, Lares, Chee-Barragan and Siqueiros-Valencia2007; Jara-Marini et al., Reference Jara-Marini, Soto-Jiménez and Páez-Osuna2009; Rodríguez-Figueroa et al., Reference Rodríguez-Figueroa, Shumilin and Sánchez-Rodríguez2009; Riosmena-Rodríguez et al., Reference Riosmena-Rodríguez, Talavera-Sáenz, Acosta-Vargas and Gardner2010; Patrón-Prado et al., Reference Patrón-Prado, Casas-Valdéz, Serviere-Zaragoza, Zenteno-Savín, Lluch-Cota and Méndez-Rodríguez2011).
Cadmium levels in Codium simulans were similar to findings in C. amplivesiculatum Setchell & Gardner and C. cuneatum Setchell & Gardner at sites with no or little influence of human activity but subject to natural influences, such as upwelling (Huerta-Díaz et al., Reference Huerta-Díaz, De Leon-Chavira, Lares, Chee-Barragan and Siqueiros-Valencia2007). Moderate upwelling has been reported in Bahía de la Paz (Jiménez-Illescas et al., Reference Jiménez-Illescas, Obeso-Nieblas, Salas-de León, Urbán and Ramírez1997). In Sargassum sinicola, Cd concentrations are similar to those reported in previous studies, 11 µg g−1, at the same area of the Bahía de La Paz (Patrón-Prado et al., Reference Patrón-Prado, Casas-Valdéz, Serviere-Zaragoza, Zenteno-Savín, Lluch-Cota and Méndez-Rodríguez2011). However, higher mean levels of Cd were found in Gracilaria pachydermatica in relation to the range previously reported in this region for the red macroalgae G. crispata Setchell & Gardner, G. textorii Setchell & Gardner, and G. vermiculophylla (Ohmi) Papenfuss (0.6–4.6 µg g−1) at sites with rare human activity but subject to upwelling (Talavera-Saenz et al., Reference Talavera-Saenz, Gardner, Riosmena-Rodríguez and Acosta2007; Riosmena-Rodríguez et al., Reference Riosmena-Rodríguez, Talavera-Sáenz, Acosta-Vargas and Gardner2010).
Levels of Pb in C. simulans (3.9 µg g−1), S. sinicola (2.8 µg g−1), and G. pachydermatica (2.4 µg g−1) are lower than the concentration range found in sites with low anthropogenic influence compared with levels in algae from industrial areas, in which concentrations are reported to be 29.7 ± 3.3 µg g−1 in the green algae Caulerpa serrulata (Forsskål) J. Agardh, 25.8 ± 2.9 µg g−1 in the brown algae Sargassum dentifolium (Turner) C. Agardh, and 19.8 ± 1.8 µg g−1 in the red algae Hypnea comuta (Lamoroux) C. Agardh (Abdallah et al., Reference Abdallah, Abdallah and Beltagy2005). This element has been considered to have little potential to be bioaccumulated or biomagnified in lower trophic levels (Dietz et al., Reference Dietz, Riget, Cleemann, Aarkrog, Johansen and Hansen2000; Ruelas-Inzunza & Páez-Osuna, Reference Ruelas-Inzunza and Páez-Osuna2008), which is consistent with our results.
Copper, Zn and Fe levels in the species studied were lower than the concentrations recorded in the brown macroalgae Padina durvillaei Bory Saint-Vincent from sites with copper mining and smelting activities (Cu: 53 ± 38 µg g−1, Zn: 63 ± 43 µg g−1 and Fe: 2243 ± 2325 µg g−1) (Rodríguez-Figueroa et al., Reference Rodríguez-Figueroa, Shumilin and Sánchez-Rodríguez2009). However, Zn and Fe levels in C. simulans and S. sinicola tend to be higher than concentrations in species of the same genus from undisturbed sites with influence of upwelling, for example in S. sinicola (Zn: 9.3 µg g−1 and Fe: 186 µg g−1) (Huerta-Díaz et al., Reference Huerta-Díaz, De Leon-Chavira, Lares, Chee-Barragan and Siqueiros-Valencia2007) and C. cuneatum (Zn: 7.3 µg g−1 and Fe: 284 μg g−1) (Riosmena-Rodríguez et al., Reference Riosmena-Rodríguez, Talavera-Sáenz, Acosta-Vargas and Gardner2010).
Sea slug
The sea slug Elysia diomedea contained Cd concentrations similar to concentrations reported for the chocolate clam Megapitaria squalida (Sowerby, 1835) collected at the same study sites (Méndez-Rodríguez et al., Reference Méndez, Palacios, Acosta, Monsalvo-Spencer and Álvarez-Castañeda2006). Low Pb levels (<0.07 µg g−1) were found in the sea slug, but concentrations of 0.3 ± 0.2 µg g−1 were reported in the chocolate clam M. squalida at Las Animas, 4.8 ± 0.5 µg g−1 at San Juan de la Costa, and 7.8 ± 1.9 µg g−1 at El Sauzoso (Méndez-Rodríguez et al., Reference Méndez, Palacios, Acosta, Monsalvo-Spencer and Álvarez-Castañeda2006). Such differences suggest that the ability of the sea slug (E. diomedea) to accumulate Cd from macroalgae is similar to that of the chocolate clam, but its ability to accumulate Pb is lower than that of molluscs in this region.
In our study, the levels of Cu, Zn and Fe found in E. diomedea (<0.02–1.6 µg g−1, 24.7–30.1 µg g−1 and 36.4–70.0 µg g−1, respectively) were lower than the amounts detected by Méndez et al. (Reference Méndez, Palacios, Acosta, Monsalvo-Spencer and Álvarez-Castañeda2006) in the chocolate clam M. squalida at the same sites (Cu: 5–8 µg g−1, Zn: 55–63 µg g−1 and Fe: 276–385 µg g−1). The differences between E. diomedea and this filter-feeder bivalve seem to reflect differences not only in feeding habits between the two species, but also differential regulation of essential metals, depending on the specific metabolic functions they perform in each species. The lower Cu levels in E. diomedea (maximum 1.6 ± 1.2 µg g−1), compared to those reported for other molluscs, such as the sea snail Bembicium auratum (Quoy & Gaimard, 1834) (88 ± 15 µg g−1), as well as for Fe (70.0 ± 10.3 µg g−1) (Barwick & Maher, Reference Barwick and Maher2003), may be associated with differences in respiratory pigments. Elysia diomedea seems to have haemoglobin (a Fe-based respiratory protein) as does the California sea hare Aplysia (Linnaeus, 1758) (Barnes, Reference Barnes1986), rather than hemocyanin (a Cu-based respiratory protein), as is the case for B. auratum (Barwick & Maher, Reference Barwick and Maher2003).
Organisms from the same site incorporate Cd and Pb based on differences in diet, as would be the case of planktivorous bivalve molluscs and herbivorous gastropods (Barwick & Maher, Reference Barwick and Maher2003; Jara-Marini et al., Reference Jara-Marini, Soto-Jiménez and Páez-Osuna2009). For example, in a seagrass ecosystem, Barwick & Maher (Reference Barwick and Maher2003) found the Sydney cockle Anadara trapezia (Deshayes, 1840) has lower Cd concentrations (0.030 ± 0.002 µg g−1) but higher Pb concentrations (1.1 ± 0.1 µg g−1) than B. auratum (0.5 ± 0.1 µg g−1 and 14 ± 1 µg g−1, respectively). The authors state that these metals are accumulated by the animals from their food—in the bivalve by eating plankton and in the gastropod by eating the green macroalga Ulva sp. (referred to as Enteromorpha sp.). Those observations are consistent with our findings for E. diomedea, indicating that the fraction of metals that is accumulated depends not only on their concentration in food but also on their chemical presentation. Macroalgae contain proteins and various carbohydrates with which different metal ions can react, in turn, modifying their bioavailability. Polysaccharides are the main component in most algae, and vary between species, favouring some elements to be more easily accumulated than others, as well as being more or less bioavailable (Hu et al., Reference Hu, Tang and Wu1996; Chan et al., Reference Chan, Wang and Ni2003). Most brown algae contain alginates and fucoidin; red algae mainly contain carrageenans and agar, and green algae contain ulvan (Vera et al., Reference Vera, Castro, Gonzalez and Moenne2011). Polysaccharides in brown algae contain carboxyl groups, whereas those in green and red algae mostly contain sulphated groups. The balance between the different functional groups in each algae group will favour metals with similar properties to be absorbed. In our study, C. simulans had the highest Fe concentration, which is consistent with the results of Robledo & Freile (Reference Robledo and Freile1997) for green algae, suggesting this species has more affinity for Fe than other metals.
The highest concentrations of Cd recorded in E. diomedea, as well as the highest levels of Cd and Zn in the three macroalgae species, were found at San Juan de la Costa where, unlike Las Animas and El Sauzoso, discharges occur in an environment enriched with phosphorite-associated elements, such as Cd and Zn (Méndez-Rodríguez et al., Reference Méndez, Palacios, Acosta, Monsalvo-Spencer and Álvarez-Castañeda2006). Absorption of these metals by macroalgae is increased by the addition of ammonia and nitrate (Lee & Wang, Reference Lee and Wang2001; Evans & Edwards, Reference Evans and Edwards2011). Nitrogen enrichment stimulates the synthesis of amino groups in macroalgae, in addition to indirectly increasing the synthesis of carboxyl and carbonyl groups, as photosynthesis is promoted (Lee & Wang, Reference Lee and Wang2001). These functional groups facilitate binding of metals, such as Cd and Zn, promoting their accumulation in macroalgae. In addition, nitrogen enrichment facilitates growth in macroalgae, which increases the demand for essential elements, such as Zn. In the absence of selective Cd transport, its accumulation in different macroalga species will follow the same path as Zn (Chan et al., Reference Chan, Wang and Ni2003). The difference being that Zn is used for various metabolic functions, while Cd is not. Mechanisms used for element homeostasis can also be used for detoxification. Bound to various molecules, such as metallothioneins (in animal cells) or phytochelatins (in plant cells), metals such as Cd will be gradually eliminated from the body although the process might take years in some species (Hu et al., Reference Hu, Tang and Wu1996). For most organisms, Cd and Pb are not essential elements; Cu, Zn, and Fe are. These three metals are metabolically regulated (Rainbow, Reference Rainbow2002). For example, planktivorous bivalves incorporate over twice as much Zn as do herbivores feeding on macroalgae, indicating that organisms from different groups have different mechanisms for accumulating this metal (Barwick & Maher, Reference Barwick and Maher2003; Ruelas-Inzunza & Páez-Osuna, Reference Ruelas-Inzunza and Páez-Osuna2008).
Sea slug vs macroalgae
The ratio of metal concentrations between primary producers and their consumers is reported to be 0.7 for Pb, and up to 31 for Cd (Barwick & Maher, Reference Barwick and Maher2003; Ruelas-Inzunza & Páez-Osuna, Reference Ruelas-Inzunza and Páez-Osuna2008). The concentrations of Cd and Pb found in the three macroalgae species and its potential consumer, E. diomedea, in this study follow these patterns. The concentrations of Cd and Pb found in E. diomedea could be related to its potential feeding on C. simulans, as the genus Elysia (Risso, 1818) includes specialist herbivores (Trowbridge, Reference Trowbridge1991; Jensen, Reference Jensen1997; Gavagnin et al., Reference Gavagnin, Mollo, Montanara, Ortega and Cimino2000; Curtis et al., Reference Curtis, Massey, Scwartz, Maugel and Pierce2005; Pierce et al., Reference Pierce, Curtis, Massey, Bass, Karl and Finney2006; Trowbridge et al., Reference Trowbridge, Hirano, Hirano, Sudo, Shimadu, Watanabe, Yorifuji, Maeda, Anetai and Kumagai2010, Reference Trowbridge, Hirano and Hirano2011), which look for specific siphonaceous green alga species to suck their cellular contents after piercing the cell walls (Jensen, Reference Jensen1997, Reference Jensen2009; Hermosillo et al., Reference Hermosillo, Behrens and Ríos-Jara2006; Trowbridge et al., Reference Trowbridge, Hirano, Hirano, Sudo, Shimadu, Watanabe, Yorifuji, Maeda, Anetai and Kumagai2010; Giménez-Casalduero et al., Reference Giménez-Casalduero, Muniain, González-Wangüemert and Garrote-Moreno2011). In our study, specimens of E. diomedea sitting on C. simulans thalli were commonly observed during field surveys (Figure 2) and green algae chloroplasts in digestive diverticulae of sea slugs sampled were observed. Some Elysia species seek specific algae and sit on them to feed and lay eggs (Hermosillo et al., Reference Hermosillo, Behrens and Ríos-Jara2006; Giménez-Casalduero et al., Reference Giménez-Casalduero, Muniain, González-Wangüemert and Garrote-Moreno2011). The presence of chloroplasts from Codium sp., Sargassum sp., and an unidentified red alga species in the digestive diverticulae of Elysia cf. furvacauda (Burn, 1958) is evidence of consumption of several macroalgae species and, therefore, a wide variation in the diet of one of these gastropods (Bradley, Reference Bradley1984). These observations suggest that S. sinicola could constitute an alternative food source for E. diomedea, based on the correlations between Cd concentrations in the two species (r s = 0.86) and its abundance in this environment. On the other hand, the lack of correlation with the concentration of metals in G. pachydermatica suggests that this red alga is not part of the gastropod's diet. However, further studies of E. diomedea feeding habits at different sites in the region are needed because feeding patterns might vary across its distribution range, as observed in other species of the same genus (Trowbridge, Reference Trowbridge1991; Jensen, Reference Jensen1997; Giménez-Casalduero et al., Reference Giménez-Casalduero, Muniain, González-Wangüemert and Garrote-Moreno2011).
Specimen of sea slug Elysia diomedea on thallus of Codium in Bahía de La Paz, Baja California Sur, Mexico.
Like other herbivores that feed on macroalgae (Barwick & Maher, Reference Barwick and Maher2003; Ruelas-Inzunza & Páez-Osuna, Reference Ruelas-Inzunza and Páez-Osuna2008), concentrations of Zn, Cu and Fe in E. diomedea might be from feeding on some of the macroalgae, such as in the Sargassum beds from El Sauzoso. At this site, the highest concentrations of Zn found in E. diomedea (30.1 ± 4.4 µg g−1) compared to those recorded in C. simulans (16.6 ± 1.1 µg g−1) and in S. sinicola (15.4 ± 0.4 µg g−1), as well as the ratio between the concentrations observed in the slug and the macroalgae (1.8–3.3), are consistent with the proportions observed in other herbivores and their food (Barwick & Maher, Reference Barwick and Maher2003; Ruelas-Inzunza & Páez-Osuna, Reference Ruelas-Inzunza and Páez-Osuna2008). For example, Barwick & Maher (Reference Barwick and Maher2003) reported that concentrations of Zn in the green alga Ulva sp. (53 µg g−1) were lower than those recorded in its consumer, the gastropod B. auratum (87 µg g−1), having a ratio of 1.6 between them. The low ratio for Cu concentrations (maximum values of 0.9) between the three macroalgae and E. diomedea in this study contrast with the highest ratio (27) reported between Ulva sp. and B. auratum (Barwick & Maher, Reference Barwick and Maher2003). There are no previous reports for Fe concentrations that could allow a comparison of the proportions found in E. diomedea and its potential food sources. Studying the dynamics of metals at different trophic levels is of potential interest because their concentrations can increase from lower to higher trophic levels, including species consumed by humans.
In general, Cd and Zn concentrations in E. diomedea were higher than those recorded in specimens of C. simulans, G. pachydermatica, and S. sinicola. In contrast, Pb, Cu, and Fe concentrations in the sea slug were lower than or similar to those in macroalgae from all sampled sites. Studying the accumulation of metals in marine organisms at different trophic levels provides important information about their potential mobility and accumulation. In some cases, incorporated metals may eventually reach high concentrations with potential increases between successive trophic levels and potentially affect economically-important species, sometimes affecting humans directly.
ACKNOWLEDGEMENTS
We thank Alejandra Mazariegos Villarreal, Alejandra Piñón Gimate, Baudilio Acosta Vargas, Griselda Peña Armenta, and Horacio Bervera León. Ira Fogel and Diana Dorantes of CIBNOR provided valuable editorial services.
FINANCIAL SUPPORT
This work was supported by the CIBNOR (PC 0.5), and Consejo Nacional de Ciencia y Tecnología of Mexico (CONACYT CB2012 179327). P.H.A. is a recipient of a doctoral fellowship (CONACYT 48345).
