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CU Boulder researchers use a unique, noninvasive method to determine the environmental factors contributing to several symptoms among tropical fish

For many researchers in biology and other natural sciences, dissecting specimens may not be desirable, though it is often necessary. This is because dissection means killing the animal a researcher is trying to study—a big issue, especially if the species is experiencing population decline.

Over time, such concerns have led scientists to develop a number of non-invasive techniques, including video transects. This is a type of video recording used in marine biology, in which divers film along a line of fixed length and depth to record images for computer-assisted analysis, obtain permanent data that can be reassessed later and survey wider areas in shorter amounts of time.

 

CU Boulder scientist Pieter Johnson and his research colleagues use a unique, noninvasive method to determine the environmental factors contributing to several symptoms among tropical fish.

A by Pieter Johnson, a �鶹��Ƶ professor of distinction in the Department of Ecology and Evolutionary Biology, and lead author Cheyenna de Wit of the University of Amsterdam, demonstrates the benefits of recording rather than dissecting specimens.

In their paper on black spot syndrome in ocean surgeonfish, the researchers use video transects to measure the severity of the disease among thousands of fish and identify the environmental factors contributing to its distribution.

What is black spot syndrome?

Black spot syndrome is a collection of several symptoms, the most prominent being the dermal lesions or spots for which the condition is named, according to Johnson. In many species, Johnson says, these lesions are black, “but in some species they’ll show up as white.” They form on the skin, scales and fins of fish.

The spots appear when the free-swimming, larval form of trematodes—commonly known as flukes, a type of parasitic flatworm—penetrate the skin of the fish and form cysts inside them. The distinctive coloration occurs when fish surround the cyst with melanin in response to the invasion, similar to the formation of pearls in oysters.

Relatively little is known about the genus of trematode that causes black spot syndrome, Scaphanocephalus. “Prior to us detecting it in 2017,” Johnson says, “it had never been reported from Caribbean fish. So, it was wholly undescribed from that area.” Much remains unknown about this trematode, including the type of snail that Scaphanocephalus infects before moving on to fish.

However, trematode infection is clearly very common in certain regions: In Johnson’s study, 70% of observed fish showed signs of infection, while demonstrated both how high the parasite loads are in that region, and how many different fish species seem to be affected, according to Johnson.

As to the consequences of infection for fish, there is some evidence, Johnson says, that infected fish may graze less and have more trouble maintaining buoyancy. Researchers also hypothesize that they are more conspicuous to predators.

“One in particular, of course, is osprey, which are visual, fish-specialized predators that are looking for fish through the water,” Johnson says. “When these infected fish tend to flash or turn sideways, and you can see those black spots, it probably makes it a lot easier for the bird to detect them.”

If this hypothesis is true, black spot syndrome could bolster the numbers of the trematodes that cause it, as Johnson says osprey are their definitive host. That means these trematodes must enter the body of an osprey to reproduce. The transmission of the parasites is trophic, so they are passed along when infected fish are eaten.

Noninvasive methods

While black spot syndrome can have negative effects on infected fish, the most important consequences could be for reef ecosystems. According to Johnson, black spot syndrome has been increasingly prevalent in important herbivorous grazing fish in the Caribbean, such as surgeonfish and parrotfish.

“In tropical coral reef ecosystems,” Johnson explains, “surgeonfish and parrotfish, and other herbivores play a key role by grazing on algae.” Since infected fish are evidenced to graze less, and since they may be more likely to be eaten by osprey, the population of algae in the affected area can increase.

“Algae and coral are in a dynamic balance,” Johnson says, and if there is enough algal growth, “it can start to overwhelm and kill corals. So, in these areas, we try to keep those populations of surgeonfish and parrotfish as viable as possible, so that they can continue to regulate and graze down the algae.”

In fact, some studies have even said that , with particular emphasis on parrotfish because the prior primary grazer in the Caribbean, spiny sea urchins, were killed off by disease in the 1980s. Also, trematode infection isn’t the only thing threatening surgeonfish and parrotfish populations, as they are popular catches for fisheries.

Because the fish being studied are ecologically important, it is particularly important to avoid interfering with their populations. Ordinarily, this is difficult, since dissection is the surest way to confirm a trematode infection—the parasites being clearly visible inside the fish’s bodies. In this case, though, the black spots characteristic of black spot syndrome allowed for a different approach: the video transect method.

To record as many surgeonfish as possible, and therefore provide an accurate estimate of how many fish were infected, SCUBA divers filmed at 35 sites along the coast of Curaçao, an island in the southern Caribbean. They recorded two and five meters below water for either 10 minutes or until 20 adult surgeonfish had been filmed.

 

An ocean surgeonfish with black spot syndrome. (Photo: Cheyenna de Wit)

Environmental factors

Besides determining that 70% of surgeonfish showed visible signs of black spot syndrome, Johnson and de Witt correlated different environmental factors with the severity of the syndrome, which they based on the average number of spots per fish.

One of the most significant effects the researchers observed arose from longitude—that is, the position of fish from east to west along the leeward (downwind) shore. Both the prevalence and intensity of black spot syndrome was lower toward the east and higher toward the west.

Johnson hypothesizes that this effect is caused by urban and industrial development, as the east end of Curaçao, where a portion of the research took place, is privately owned and less developed. The researchers observed the same association between development and infection intensity in Bonaire, the neighboring island.

The first component of the effect was wave intensity, which was negatively associated with infection intensity because the larval form of trematode that infects fish can’t swim well enough to overcome opposing tides. Wave energy is usually greatest at the eastern end of Curaçao, so this will have contributed to the lower intensity of infection at the east end.

The other components were positively associated with infection intensity. Nitrogen concentration increases with sewage and domestic runoff, which can contain nutrients and other pollutants. Nutrients can increase the population of trematode hosts, and pollutants can weaken the immune systems of fish that trematodes infect.

While fishing pressure can be either positively or negatively correlated with parasite abundance, Johnson says, this depends on the species involved. In the case of Scaphanocephalus, fishing pressure could increase abundance if it removed predatory fish from the environment, resulting in an increased snail population.

Since most of the factors composing the difference between the east and west ends come from human action, it is possible that the severity of black spot syndrome could be significantly reduced if the handling of runoff and/or fishing behavior were changed.

A unique methodology

One noteworthy part of the way Johnson and de Witt’s study was conducted is that, with the videos collected, the researchers had observers record the number of lesions on each fish. This is unique, as prior studies have simply noted whether lesions were present, leaving the severity of infection uncertain.

Moreover, methods like the one used in this study may help to solve the challenges that come with observing ocean life. “There's a lot of ocean out there and not a tremendous number of people to study it,” Johnson explains, “so I think approaches like this could be applied in other areas where we're detecting blackspot syndrome.” Photos are an especially useful way to study the ocean because they are easy for anyone to take thanks to digital technology, he adds. For this reason, community science platforms like can be used to aggregate a large amount of data.

“When people are on vacation, or they’re diving, or they’re swimming,” Johnson says, “they upload all of their observations and fish photos, and we’ve been using that to scan across large sections of the Caribbean and lots of different fish species; and now some of the undergrads in the lab are also extending that to look into parts of the Indo-Pacific and other regions of the world where Scaphanocephalus occurs.

“So, I think those kinds of approaches, video transects and these community science-uploaded images, together start to give a much bigger picture of patterns of infection over large geographic areas.”

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