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Abstract

The green sea turtle (Chelonia mydas) is facing the deadly issue of Spirorchiidae spp. parasites. There remain many unknowns about the life cycle of the parasite; not very much is understood beyond a gastropod intermediate host. Research into parasitic life cycles is important because it can positively impact conservation awareness, as seen in the example of Echinococcus granulosis and marsupials. In order to begin a conservation effort for the turtles and to control the spread of Spirorchiidae spp., a fuller life cycle must be better understood through various measures. Thus, I suggest we investigate Scyllaea pelagica as a possible gastropod intermediate host. This is due to how recently there has been a massive influx of Sargassum seaweed, which contains S. pelagica’s food source and habitat. The theory is that the turtles may ingest the seaweed with the gastropod and become infected with Spirorchiidae spp. through trophic transmission. However, before any control measures can be taken against the S. pelagica, it has to be determined if it is an intermediate host for Spirorchiidae spp. There are two methods that could help with this. One is to collect many S. pelagica and wait to see if they produce the proper sporocysts. Another is to run a DNA assay on the S. pelagica. If evidence of Spirorchiidae spp. is found in C. mydas then it is a step in making a complete life cycle. The completion of the life cycle with all the intermediate hosts present will allow for conservation of the C. mydas.

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Spirorchiidae spp. are platyhelminth trematode digenean blood flukes that have the ability to cause extreme pathologies in many species of sea turtle (Snyder 2004). One species of turtle that has been particularly affected by this parasite are the Chelonia mydas, or the green sea turtles. The Spirorchiidae spp. cause issues surrounding tissue inflammation and mortality in the turtles (Cribb et al. 2019). The infected turtles will show signs of weight loss, anorexia, and a swirling behavior due to neurological damage (Cribb et al. 2019). Spirorchiidae spp. eggs are also found in many organs and other tissues in the turtle body and were especially prevalent in the serosa of the intestine (Cribb et al. 2019). These turtles experienced cardiac hemorrhagic lesions and granulomatous reactions to the eggs (Santoro et al. 2017). When dissected, the researchers found that the Spirorchiidae spp. flukes attached themselves to the chambers of the heart and the great vessels in the cardiac region (Santoro et al. 2017). Because of the flukes’ presence, the lumens of many cardiac vessels were destroyed beyond repair because of the host inflammatory response to the parasite (Santoro et al. 2017). This pathology is one of the leading causes of ‘stranding’ green turtles across the world (Santoro et al. 2017). There were also large amounts of endothelial necrosis found in the foci of the lower intestine’s mucosal lining (Santoro et al. 2017). Alongside the necrosis were the eggshells of the parasite, encased by a granuloma response (Santoro et al. 2017). The infection of Spirorchiidae spp.will often be indicted as the main cause of death in the turtle (Chapman et al. 2019).

While incomplete, researchers are making progress in determining the life cycle for Spirorchiidae spp. Researchers recently determined that an intermediate host for the parasite is a gastropod, and a nonconfirmed pulmonate snail (Pinto et al. 2015). It is also true that cercariae that resembles Spirorchiidae spp. were also found in two marine polychaetes, which raises them as another possible intermediate host (Buron et al. 2018). That said, there is very little information on the life cycle of the parasite, which is integral to understanding how to quell the spread of the parasite.

Understanding the life cycle is imperative to forming necessary functional conservation plans, which are needed as the C. mydas population has declined by as much as over 60% throughout many years and is considered endangered (Troeng et al. 2005). The Spirorchiidae spp. have been determined to be a key cause in sea turtles’ mortality globally and is thus playing a role in the decline (Stacy et al. 2010). There are some regions of the world with sea turtles where the prevalence of the parasite is near 100% (Chapman et al. 2019). Specifically, there was a study done by Santoro et al. (2017) which shows tropical green sea turtles experience a prevalence rate of 95% with 39 of the 40 turtles sampled as testing positive for Spirorchiidae spp. Almost 1200 Spirorchiidae spp. were found in those 39 turtles (Santoro et al. 2017) which gives a mean intensity of approximately 31 parasites per host. In order to move forward in being able to save the turtles from the parasite this life cycle beyond a general grouping of gastropods must be understood.

The gastropod Scyllaea pelagica may be connected to the life cycle of the Spirorchiidae spp. S. pelagica lives in warm water, and eats hydroids that live on Sargassum seaweed, which it also uses as an environment to live on (Vaughn n.d.). Huge amounts of this weed have accumulated on the coastlines where the Chelonia mydas live (Maurer et al. 2021). The build-up of floating Sargassum is covering over 600,000 m² of the Mexican Caribbean, a habitat where the C. mydas live(Vazquez-Delfin et al. 2021). This covering can weigh up to 10,000 tons across the Caribbean Island shores (Marx et al. 2021). Not only is the seaweed a general ecological issue, but it also interacts with the C. mydas species; non-juvenile green sea turtles’ main diet is composed of seagrass (Stubbs et al. 2019; Arthur et al. 2008). In the reefs, floating Sargassum seaweed was the main vegetation consumed, with almost 60% of green turtles ingesting it (Machovsky et al. 2020). Through trophic transmission it is possible that the turtle will consume the Sargassum, and thus become the next host (Moore 2010). That said, the C. mydas will hypothetically come into some form of contact with the S. pelagica, given that the gastropod could harbor Spirorchiidae spp. parasites. However, this is not enough to label the S. pelagica an intermediate host. There are many different tests and techniques available which will help construct a full life cycle.

There are a few ways to determine if an organism is an intermediate host. One way is drawn from an experiment which determined polychaetes as a possible intermediate host for Spirorchiidae spp. parasites. Buron et al. (2018) began by collecting the species over four years. They waited for sporocysts to emerge from the hemocoel of the polychaetes (Buron et al. 2018). While some of the sporocysts found released cercariae on their own, some were intervened with to induce the release of cercariae (Buron et al. 2018). This was shown to be a useful method for this research project. Therefore, for this study of determining if S. pelagica is an intermediate host these methods can be applied. However, since it is not known if S. pelagica is an intermediate host, the method will have to be tweaked slightly. After collecting, the researchers will have to either wait for sporocysts to potentially emerge from the gastropod or use the mechanical intervention previously mentioned to induce potential sporocyst activation. If the sporocysts emerge, then it is a likely intermediate host.

Another method to determine an intermediate host is a DNA assay. DNA assays are commonly seen when trying to uncover and determine microorganisms in food, but in general have the overarching ability to detect many different types of organisms using genome sequencing (Kovac et al. 2022). A large benefit to a DNA assay is that it is time efficient. For example, a study done on Aeromonas hydrophila used a DNA assay and found results within thirty minutes (Elsheshtawy et al. 2019). The speed of this is very quick is in comparison to the testing option which waited for sporocysts.

In continuation, an example of a DNA assay being beneficial was when researchers tried to understand the presence of Fasciola hepatica in its intermediate host (Kaplan et al. 1997). While this study used the DNA probe to figure out how F. hepatica transmits throughout the seasons, the goal was to test if there was evidence of the parasite in the intermediate host (Kaplan et al. 1997). The researchers engineered an assay with 100% specificity which is easily able to detect one miracidium (Kaplan et al. 1997). The assay’s specificity means that it was designed to not cross hybridize with the DNA of other very similar trematodes (Kaplan et al. 1997). Since Spirorchiidae spp. is also a trematode, perhaps the assay could be tweaked to recognize this specific trematode parasite instead. The high level of specificity proves the DNA assay to be a reliable and viable method for Spirorchiidae spp. detection. If there is evidence of the parasite in the gastropod, it can be concluded that it is a good candidate for further testing of it as an intermediate host.

Another example of DNA assay working to determine in intermediate host was done on porpoises with a lungworm. Scientists studying harbor porpoises (Phocoena phocoena) and harbor seals (Phoca vitulina) were investigating the lungworm Metastronglyoida (Lehnert et al. 2010). Metastronglyoida is a nematode that lives in the water where the P. phocoena and P. vituline live. Similarly to Spirorchiidae spp., much of its life cycle is very unknown (Lehnert et al. 2010). Therefore, the researchers took regions of ribosomal DNA, ITS-2, from all of the lungworms which live in the habitat and can live in the two animals (Lehnert et al. 2010). From there, the ribosomal DNA from the lungworms was compared by sequencing the DNA (Lehnert et al. 2010). This was done using by utilizing polymerase chain reaction (PCR) methods (Campbell et al. 1995). By using the ITS-2 nucleotide sequences, the researchers ran the DNA through multiple wild fish to try to get a match for an intermediate host for the larval stage of the lungworm (Lehnert et al. 2010). When a fish was a match, it was dissected, and larvae were found inside and their hypothesis of fish as an intermediate host was confirmed (Lehnert et al. 2010). This method could be applied to the Spirorchiidae spp. and S. pelagica by using the right nucleotide sequence from the Spirorchiidae spp. and running it up against S. pelagica. To think broader, this clearly previously effective method could be used more generally to find other intermediate hosts that are not gastropods by screening as many organisms in the Caribbean waters as possible. Through these three examples of DNA assay working for different reasons, the assay is proven to be an effective tool in the process of determining the Spirorchiidae spp. life cycle.

By developing a complete life cycle for Spirorchiidae spp., necessary conservation of C. mydas can be facilitated. There are many examples of conservation projects that have benefitted from knowing a complete life cycle. For instance, a study done in Australia discusses the profound impact of discovering a sylvatic life cycle for Echinococcus granulosis (Thompson et al. 2010). E. granulosis has been established as having a domestic life cycle since its introduction to Australia in the 1700s (Thompson et al. 2010). However, since understanding the more recent sylvatic life cycle a series of ecological struggles can be predicted by this development (Thompson et al. 2010). For example, the sylvatic life cycle is able to thwart a multitude of predatory-prey interactions as well as the survival of a host (Thompson et al. 2010). In the event of a sylvatic life cycle, an E. granulosis infection in a wild kangaroo will attack its lungs (Thompson et al. 2010). The infection within the lungs of the kangaroo makes it easier for its predator, the dingo, to kill (Thompson et al. 2010). Australia’s marsupials are under low population threat as is, and the introduction of the sylvatic E. granulosis life cycle could be incredibly harmful for the population (Thompson et al. 2010). Discovering the kangaroo as an intermediate host allowed scientists to understand the potential ecological shifts that the parasite could cause. Through this example it is clear how facilitating research on completing a life cycle can positively impact conservation efforts.  

In conclusion, Chelonia mydas is an endangered species dying due to Spirorchiidae spp. It is imperative to understand the life cycle of the parasites to begin a control effort. Therefore, there are at least two tests shown to work to determine other intermediate hosts, which can also be applied more broadly for other organisms like Spirorchiidae spp. While S. pelagica was the example for an intermediate host, these techniques could be applied to another researched potential intermediate host. These research methods include waiting for sporocyst release and DNA assays. The research to complete the life cycle is important, and this is seen in how understanding a new life cycle for E. granulosis alerted wildlife conservationists to the perils marsupials may face in the wake of the parasite. Therefore, it is evident that a step in the conservation efforts for C. mydas lies in understanding the full life cycle of Spirorchiidae spp. parasites.

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