Heavy metals (e.g. mercury, arsenic, copper, selenium, lead, and cadmium) are a natural component of the Earth’s crust, hence why natural processes like erosion and weathering can transport them from land into our oceans. However, an increase in industrial activity over the past few decades means that we are now the largest contributor to heavy metals at sea! The United States National Mercury Deposition Network reveals the Gulf Coast region, particularly Florida, has consistently high atmospheric deposition of inorganic mercury compared to the rest of the United States due to high annual rainfall. In fact, Florida has mercury impairment state-wide, with the Florida Department of Health (DOH) and Florida Department of Environmental Protection identifying over 400 waterbodies that are impaired due to mercury found in fish tissue. Most of them are freshwater lakes and rivers but also some coastal/marine areas, like Charlotte Harbor located along southwest Florida’s coast, a recognized mercury hotspot.
Like other marine animals, sharks are exposed to heavy metals through a variety of pathways in the environment. So how is this affecting them – and is it impacting them since birth? That is what scientists Nicole A. Reistad and Sarah B. Norris from Florida Gulf Coast University were keen to find out. “Once I gained an understanding of toxicant transference via placental connection, I wanted to investigate how much mercury is transferred during gestation compared to dietary exposure once [young are] born. While much is known about mercury effects in young humans, there is still a knowledge gap about how much mercury affects young sharks,” said Reistad, co-author of this latest study. Co-author Norris agreed: “For me, it was primarily because everyone knows about mercury in seafood and there are so many guidelines about what not to eat because of mercury. But we know so little about how mercury impacts the species in which it bioaccumulates, particularly the young. We have few to no baselines for the young of most of these species so I was intrigued to take something everyone is aware of and dig deeper.”
Reistad and Norris decided to look at maternal offloading and diet-based mercury exposure for neonatal and juvenile blacktip sharks (Carcharhinus limbatus). The species is widespread in warm-temperate, subtropical, and tropical waters worldwide and is seen in beaches, bays, estuaries, over coral reefs, and off river mouths. Blacktips are a viviparous species, meaning mother and baby shark have a placental connection that allows the transfer of nutrients and toxins to reach the embryo for up to a year. It has been previously found that their embryos have higher mercury concentrations within their tissues compared to embryos of similar viviparous shark species with shorter gestation periods, like Atlantic sharpnose (Rhizoprionodon terranovae) and bonnethead sharks (Sphyrna tiburo).
The team set out to survey the mercury distribution in various tissues of neonate and juvenile blacktip sharks in Charlotte Harbour as the estuary is known to be used as a nursery. The small sharks were captured using gillnets and fishing rods, then placed in a small pool to collect muscle and blood samples. “Blood, muscle, liver, and kidney tissues were all analyzed for total mercury concentrations using a direct mercury analyzer and reference materials. We also inspected erythrocytes (red blood cells) in blood smears and tissue cross-sections for abnormalities that may be indicative of high mercury exposure,” explained Reistad.
They found that neonates had the highest total mercury concentrations in their kidneys and muscles, followed by their liver and blood. Juveniles, on the other hand, has the highest concentration in their liver, followed by their muscles, kidneys, and blood. Reistad and Norris concluded that the distribution of mercury among tissues and liver-to-muscle ratios indicated that mercury originated primarily from maternal offloading in neonates, whereas juveniles continued to accumulate this heavy through dietary exposure post-parturition. “Since the percentage of transfer seems to be highly variable in different shark species based on several factors (e.g. geographical location, size, diet, and reproductive strategies), our results contribute to baseline knowledge of mercury exposure for future studies of both similar and contrasting species,” said Reistad. “These comparisons would be particularly beneficial for regional populations, as the blacktips in this study are from a known mercury hotspot. Comparing this population to species from regions that are not known as mercury hotspots provides additional baseline data and corroborates the present study’s use as a general risk assessment.”
The results reveal significantly different mercury concentrations between the two developmental stages. What it means for sharks in the long term… we just don’t know.
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