Sharks are apex predators in virtually all marine environments and impact ecosystem structure and function through trophic cascades [1,2]. However, shark populations are experiencing global declines as a result of over-fishing, largely driven to support the burgeoning shark fin trade [3,4,5]. A minimum of 26 to 73 million sharks per year, representing a combined weight of 1.7 million tons are killed in both target and bycatch fisheries to support the high demand for fins in Asian markets [6]. High exploitation rates continue to increase annually driven by the rising demand for highly prized fins used to make shark fin soup, an Asian delicacy and one of the world’s most expensive fishery products [7]. Shark fins consist of cartilage with fibrous protein collagens that add texture and consistency to the soup. The larger the fin and higher fin needle content (collagen fibers), the more expensive the soup. Sharks accumulate mercury and other heavy metals [8] that pose health risks to consumers of shark products, including shark fin soup.

The neurotoxin BMAA is produced by diverse species of free-living cyanobacteria found in terrestrial and aquatic environments [9] and cyanobacterial symbionts [10]. BMAA has been linked to the development of neurodegenerative brain diseases, such as Alzheimer’s disease and Amyotrophic Lateral Sclerosis (ALS) [11,12]. Cyanobacteria are found in lakes, rivers, estuaries, and marine waters with bloom growth increased due to nutrient loading from agricultural and industrial runoff, farm animal wastes, sewage, groundwater inflow and atmospheric deposition [13]. The occurrence of BMAA has been reported in isolated cyanobacteria from waters in the Baltic Sea [14], China [15], Holland [16], South Africa [17], British Island [18], and Peru [19] as well as in laboratory cultures of free-living marine cyanobacteria [20].

BMAA has been measured in high concentration in marine fish and invertebrates collected from South Florida coastal waters [21] and the Baltic Sea [14]. Given the ubiquity of cyanobacteria in marine ecosystems, BMAA could bioaccumulate up the marine food web to sharks, potentially posing health risks to consumers of shark products.

Given the increasing exploitation of sharks and the potential health hazard associated with bioaccumulation of BMAA in marine food webs, we conducted a study to determine if BMAA could be detected in shark fins. Specifically, we sampled fins and select organs from seven common shark species found in South Florida waters (USA) for analysis and detection of BMAA using multiple analytical techniques.

The fins of seven shark species collected in South Florida coastal waters (Table 1) were analyzed by high performance liquid chromatography with fluorescence detection (HPLC-FD). BMAA was detected in a total acid hydrolysate using HPLC-FD and validated by triple quadrupole liquid chromatography tandem mass spectrometry (LC/MS/MS). Precolumn derivatization of the amino acids in the sample was performed using the fluorescent tag 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC). AQC universally tags amino acids at primary and secondary nitrogens producing complex molecules that do not degrade during high pressure separation [22].

The AQC-derivatized BMAA standard elutes closest to methionine (Met). Figure 1A illustrates the HPLC-FD separation of the standard amino acids, BMAA and its isomers N-2(amino)ethylglycine (AEG) and 2,4-diaminosuccinic acid (2,4-DAB). The relative retention time for BMAA (30.89 min) was clearly separated from AEG (29.67 min) and 2,4-DAB (32.91 min).

These results demonstrate that BMAA did not coelute with any of the natural or diamino acids contained in the shark matrix. A representative HPLC-FD chromatogram of a great hammerhead shark fin sample shown in Figure 1B illustrates the BMAA peak. BMAA in the shark sample shown in Figure 1 was confirmed using triple quadrupole LC/MS/MS (Figure 2). The mass spectrometric verification of the BMAA peak confirms HPLC detection of BMAA in the shark sample [9,16,17,23]. The product ions with masses of m/z 171, 289, and 119 were detected in the third quadrupole for both the sample and the BMAA standard and the ratio of the three fragmentation product ions were within normal variation as described previously [18].

We detected and quantified BMAA in the fins of all shark species with concentrations ranging from 144 to 1836 ng/mg wet weight (Table 2). BMAA was not detected in six out of the total number (n = 29) of individual fin clip specimens assayed. The results demonstrate high concentrations of BMAA in shark fins collected in areas with or without active cyanobacteria blooms. We observed considerable variability within the same shark species having a similar body length and taken from the same collection sites. For example, the bonnethead shark had BMAA concentrations that ranged from 320 to 1836 ng/mg over a range of only 76 to 79 cm. Of the 7 members of the elasmobranch family surveyed, both the nurse shark and the blacktip shark had fin clip samples where BMAA was not detected (Table 2). Interestingly, the two samples taken from nurse sharks sampled in Florida Bay were positive for BMAA while only one of the five sampled from Biscayne Bay had a quantifiable peak (Table 2). There was no apparent correlation of BMAA concentration with the size of the shark or lifespan at sampling.

We measured BMAA using HPLC-FD in the organs and muscles of great hammerhead sharks killed as a result of recreational fishing activities. As shown in Table 3, BMAA was detected in kidney, liver, and muscle but was not measured in the heart tissue for this species. The highest levels were observed in the kidney, suggesting that uptake and excretion of BMAA along with other natural amino acids occurs in this organ. Although the heart sample had no detectable BMAA, further studies are needed to rule out possible accumulation of BMAA in contractile cardiac tissue.

Cyanobacterial blooms in South Florida coastal waters occurred in the 1980s and have persisted ever since [21]. Most cyanobacteria are known to produce the neurotoxin BMAA that has been linked to development of the neurodegenerative brain diseases [10,11,24]. Brand et al. [21] recently reported that BMAA was detected in several species of crustaceans and fish from the same South Florida coastal waters surveyed in the present study. These marine species are part of the diet of some groups of sharks. Since sharks are at the highest trophic level, they may bioaccumulate BMAA from active exposure to cyanobacterial bloom sites. All seven shark species analyzed in this study had BMAA detected in high amounts in their fins. Interestingly, high concentrations of BMAA were detected in the fins of some sharks collected in areas that had no active cyanobacteria blooms. Sharks are highly migratory, making it likely that they pass in and out of areas where cyanoblooms may have occurred over time [21,25]. While planktonic cyanobacteria are abundant, benthic and cyanobacteria epiphytic on seagrass and macroalgal blades are also present, providing a source of BMAA from the lowest trophic levels to higher animals within the same marine ecosystem.

The bonnethead shark that had the highest levels of BMAA in this study are known to primarily feed on members of the benthic zone, including blue crabs and pink shrimps which reportedly have very high concentrations of BMAA (mean concentration of 2505 µg/g and 2080 µg/g, respectively [21]). Sharks as long-lived apex predators may concentrate protein-associated BMAA over time in certain tissues. This pattern of bioaccumulation is what has been observed for mercury and other heavy metal toxins in sharks across the lifespan [8]. The range of BMAA concentrations measured in the different sharks surveyed most likely reflect their ecological niches, different foraging patterns, and their size and age differences.

BMAA was measured in select organ tissues including the kidney, liver, and muscle of the great hammerhead shark (Sphyrna mokarran). The tissue uptake of BMAA has been previously reported in the brain and muscle of bottom-dwelling fishes in the Baltic Sea [14], muscle and tissues from fish and crustaceans in South Florida coastal waters [21], and in brain, muscle, skin, intestine, kidney and fur in flying foxes from Guam [23]. Taken together, these studies suggest that BMAA may be misincorporated into proteins where it bioaccumulates with repeat exposures.

Shark fins consist of cartilage with fibrous protein collagens. Shark fin cartilage powder or capsules are marketed as dietary supplements and claimed to combat and/or prevent a variety of illnesses. However, the benefits of this supplement have not been significantly proven, nor has shark cartilage been reviewed by the US Food and Drug Administration (FDA). Recently Field et al. [26] hypothesized that collagen abnormality in the skin of sporadic ALS patients may be caused by the misincorporation of BMAA leading to misfolding of the collagen proteins. In keeping with this hypothesis, the highest levels of BMAA found in the Guam flying fox were detected in skin tissue known to contain collagen as a major component [23].

The elevated level of BMAA in shark fins provides additional support that marine cyanobacteria may represent a route for human exposure to BMAA. Further studies are needed to confirm this finding and to demonstrate that widespread BMAA detections in sharks may occur outside of South Florida coastal waters. The recent finding that BMAA co-occurs with other cyanotoxins in contaminated water supplies raises the possibility that low-level human exposure to BMAA exists in many parts of the world [17]. The possible link between BMAA and gene/environment interactions in progressive neurodegenerative diseases [9] warrants concern for exposure to BMAA in human diets. In Asia, shark fin soup is considered a delicacy, which drives a high consumer demand for this product. Our report suggests that human consumption of shark fins may pose a health risk for BMAA exposure especially if it occurs with mercury or other toxins.

BMAA can be transferred from cyanobacteria in the lower trophic levels (teleosts and crustaceans) to marine apex predators. Sharks are among the most threatened marine vertebrates [28] due in part to the high demand of their fins for dietary and medicinal purposes. The consumption of shark products that contain the cyanotoxin BMAA could increase risk for development of neurodegenerative diseases, including Alzheimer’s disease and ALS [11,24]. The worldwide prevalence of Alzheimer’s disease is estimated to quadruple in 2050 by which time 1 in 85 persons worldwide will be living with the disease [29]. Until more is known about the possible link of BMAA to Alzheimer’s disease and other neurodegenerative diseases, it may be prudent to limit exposure of BMAA in the human diet. Our report suggests that consumption of shark fins increases the risk for human exposure to BMAA, a neurotoxic amino acid that accumulates in biological tissues.