Characteristic Features of Trematodes

Introduction

Trematodes are parasites that live in various organisms’ bodies including human beings. They can be divided into more than 20,000 species some of which are associated with parasitic infections in humans and animals. This tutorial offers a snapshot of what trematodes are, how is their body organs, and how they reproduce and grow. Examples of trematodes and some infections they cause have also been included.

What are Trematodes?

Trematodes are a class of parasitic leafworms. They are also known as flukes and they are linked mostly to aquatic fauna such as mollusks and fish. Trematodes also infect vertebrates and hence they are a subject of medical and veterinary interest.

There are more than 20,000 species in the class of trematoda some of which infect human beings and animals. These organisms can be found in various habitats across the globe. Trematodes depend on one or several hosts in their lifecycle and this depends on the type of species of trematodes.

Some examples of trematodes include the following −

• S. japonicum

• F. hepatica

• C. sinensis

• S. haematobium

• P. heterotremus

• P. westermani

Classification of Trematodes

• Animalia Kingdom − Being part of this kingdom, trematodes use oxygen, and consume organic materials. They are capable of movement and can grow through reproduction.

• Platyhelminthes Phyllum − The phylum of trematodes is Platyhelminthes (flatworms) and hence they show bilateral symmetry. They are of monogenean and digenean orders and fall in the classification of obligate parasites.

Morphology

Different trematode species may have different features but there are some common features that are found in almost all types of flukes. For example, the flukes have dorsoventrally flattened (unsegmented) bodies which are bilaterally symmetrical. The bodies of flukes are leaf-shaped.

There are some characteristic features that can be associated with trematodes.

• While most of them are small, some species can grow up to 10 cm in length. Most of the flukes are however only a few millimeters in length.

• Flukes have physiologically and morphologically complex body covers (teguments). However, there is no body cavity. Therefore, the bodies of flukes are bound in parenchyma. The somatic muscles of the parenchyma attach to the body cover.

• The flukes however have a well-developed alimentary canal that is branched and has an esophagus and muscular pharynx. The intestine is also well-developed but it does not open through the anus. Instead, the two branches of two sides of the body run into a common pore or excretory vesicle that is located at the posterior end of the organism.

• The alimentary canal of flukes is covered by a single layer of epithelial cells.

• Flukes may have a secondary or a tertiary branch of the intestine depending on the species.

• Apart from the body cavity, flukes also do not have a respiratory system.

• It is notable that flukes need to attach to the host body for survival. For this purpose, there are two specialized suckers in the body of each adult fluke. One of these is located in the mouth area while the other is located in the middle part of the body. These suckers help flukes remain attached to their hosts.

• There is a powerful muscular pharynx in the mouth of the flukes that help them suck food from the hosts. This is found in the moth part of both larvae and adult flukes. This powerful sucker helps them to suck blood which is their food from the hosts.

  This, however, harms the hosts and that is why flukes are harmful to human beings and animals. The posterior and ventral suckers of a trematode act as powerful pumps to suck the blood from the body of the hosts. The sucked blood is the source of nutrition for the flukes. Out of the two suckers, the ventral sucker is only for attachment purposes while the sucker at the mouth is both for attachment and collecting food.

• As flukes do not have an anus to excrete the waste products, the wastes are excreted through the mouth opening. The wastes are carried from excretory vesicles to the mouth for this reason. Thus, the mouth opening of the trematodes serves two purposes - it collects food from the hosts as well as dispels excretory products.

• In terms of reproduction, most of the flukes are found to be hermaphroditic. So, they have both a male and female reproductive organs. Here, the male reproductive system consists of: Two testes, a cirrus (acts as the penile part), and accessory glands. The female reproductive system consists of: an ovary, Ootype, oviduct, seminal receptacle, yolk glands.

  While most flukes are hermaphroditic, they use self-fertilization as a process of reproduction. However, flukes can reproduce through cross-fertilization too where one fluke fertilizes another. Blood flukes are not hermaphroditic in nature. So, they do not possess two types of reproductive organs.

• The larval stage of flukes goes through many stages from larvae to adulthood and these stages may take place in different hosts which include aquatic organisms to vertebrates.

• Fluke eggs are hard and smooth which is a characteristic of the organism. They are usually yellow-brown in color. The eggs may have spines and they are of the size of 30 to 175 microns depending on the species of the fluke.

• Flukes have a primitive nervous system that consists of two ganglia on the two sides of the esophagus. There is a nerve ring around the esophagus that connects the two parts. This is the only simple nervous system of flukes and apart from this, there is no other additional sensory organs of flukes. Flatworms differ from flukes in this sense as they have additional sensory organs.

• Apart from sexual reproduction, flukes can also reproduce sexually. For example, liver flukes can reproduce both sexually and asexually at given stages of their lifecycle. However, they need to depend on various hosts to complete these processes at different stages of their lifetime.

Some of the most common infections caused by trematodes include −

• Clonorchiasis

• Fascioliasis

• Opisthorchiasis

• Paragonimiasis.

Conclusion

Flukes are an important organism to study because they infect us and cause various types of illnesses. They are unique in nature and their existence and reproduction are subject to medical and veterinary interest as they can infect animals and human beings. Studying the life of flukes that can reproduce both sexually and asexually is thus important to stay uninfected from these parasites.

FAQs

Qns 1. What are trematodes? Why are they of medical interest?

Ans. Trematodes are a class of parasitic leafworms. They are also known as flukes and they are linked mostly to aquatic fauna such as mollusks and fish. However, trematodes also infect vertebrates and hence they are a subject of medical and veterinary interest.

Qns 2. What are the male and female reproductive organs of flukes made up of?

Ans. The male reproductive system of flukes consists of −

• Two testes

• A cirrus (acts as the penile part)

• Accessory glands

The female reproductive system consists of −

• An ovary

• Ootype

• Oviduct

• Seminal receptacle

• Yolk glands

Qns 3. How many species of flukes are there? In how many genres and families are they further divided?

Ans. There are more than 20,000 species of flukes that can be further divided into 2,200 genres and 256 families.