Parasitic Helminths (Trematoda, Nematoda, Cestoda) as Bioindicators of Heavy Metal Pollution in Marine Ecosystems: A Comparative Study on Saurida undosquamis

Imagine the ocean as a giant, bustling city where everything is connected. In this city, there are invisible troublemakers called heavy metals—substances like lead and cadmium that can sneak into the water from factories or runoff and make the whole ecosystem sick. Scientists have been trying to catch these troublemakers for a long time. Usually, they check the water itself or dig up the mud, but these places are like a busy highway; the pollution levels change fast, making it hard to get a clear picture. They also check the fish, but fish are like smart bodyguards; they have complex systems to push out toxins and keep their own bodies clean, which can hide the true amount of pollution around them.

Enter the "parasitic detectives." These are tiny worms that live inside fish. Unlike the fish, these worms don't have the same fancy bodyguards. They are like sponges that soak up everything in their immediate neighborhood, including the heavy metals. Because they are so good at holding onto these pollutants, scientists think they might be the perfect messengers to tell us exactly how dirty the water really is. This study dives into the world of these tiny worms to see if they can help us solve the mystery of ocean pollution.

---

The Case of the Bristly Fish and the Metal-Sucking Worms

In the waters off the coast of Misurata, Libya, there lives a fish called the brushtooth lizardfish (Saurida undosquamis). It's a popular catch for local fishermen, but it also has a very crowded house inside its belly. A team of researchers from Misurata University and other local institutions decided to investigate the tenants living inside these fish to see what they could tell us about the water's health.

The researchers collected 88 of these fish during both the hot summer months and the cold winter months of 2022. When they opened the fish up, they found something amazing: 100% of the fish were infected with parasitic worms. Every single fish had them! The worms fell into three main groups: Trematoda (flukes), Nematoda (roundworms), and Cestoda (tapeworms).

The team then took these worms to the lab and tested them for four specific heavy metals: Copper (Cu), Zinc (Zn), Lead (Pb), and Cadmium (Cd). They used a machine called an Atomic Absorption Spectrophotometer (AAS) to measure exactly how much metal was inside the worms, reporting the results in micrograms per gram of wet weight (µg/g w.w.).

The Worms Have Different Personalities

The study found that while all the worms were living in the same fish, they acted very differently when it came to soaking up metals.

The Metal Sponges: The Trematodes and Cestodes were like heavy-duty sponges. They were particularly good at soaking up the toxic, dangerous metals like Lead and Cadmium. In fact, for these two groups, Cadmium made up a huge chunk of their total metal load—about 19% of everything they held onto. This suggests they have a special way of grabbing onto these toxic metals and keeping them safe inside their bodies.

The Regulators: The Nematodes, on the other hand, were more like careful accountants. They preferred to hold onto essential metals like Copper and Zinc, which living things actually need to survive. They kept the levels of toxic Lead and Cadmium much lower than the other two groups. The researchers think this is because Nematodes have a tougher outer skin (a cuticle) and better systems for cleaning themselves out, which stops the bad metals from getting in as easily.

The Seasons Play a Role

The time of year also changed the story. Generally, the worms had higher concentrations of metals during the cold season compared to the hot season. This was especially true for the Cestodes (tapeworms). For example, the amount of Copper in Cestodes went from 0.143 µg/g in the hot season to 0.157 µg/g in the cold season. The Lead and Cadmium levels also jumped up during the colder months.

Why the cold season? The researchers suggest a few ideas. Maybe the worms slow down their metabolism in the cold, meaning they don't excrete (poop out) the metals as fast, so they build up more. Or, perhaps the fish eat different things in the winter, or runoff from the land brings more metals into the water when it's wet and cold. The Nematodes, however, stayed very steady, showing almost no change between the seasons, which proves their "body armor" is very effective.

Why This Matters

The big takeaway from this paper is that these parasitic worms are excellent "bioindicators." That's a fancy way of saying they are nature's pollution detectors. Because they soak up the toxic metals so well and are found in 100% of the fish, they give scientists a clear, time-integrated picture of how polluted the environment is.

Unlike checking the water, which changes every minute, or checking the fish, which tries to hide the pollution, these worms hold onto the truth. The study suggests that Trematoda and Cestoda are the most sensitive detectors for toxic metals like Lead and Cadmium. This is the first time this specific type of fish and its parasites have been used to map out heavy metal pollution along the Libyan coast, providing a new, reliable tool for keeping an eye on the health of the Mediterranean Sea.