What is the Difference Between Monogenea and Digenea

The key difference between monogenea and digenea is that monogenea reproduce sexually while digenea reproduce both sexually and asexually.

Flatworms or Platyhelminthes are a phylum of unsegmented, simple bilaterian, soft-bodied invertebrates. Similar to other multicellular animals, they possess three embryonic layers: endoderm, ectoderm, and mesoderm. Some flatworms are free-living and non-destructive, while many are parasitic to humans and animals. Monogenea and digenea are two types of flatworms. Monogenea are commonly found in aquatic habitats where the majority are present as ectoparasites of fish and other organisms. Digenea is the largest group of endoparasites.

CONTENTS

1. Overview and Key Difference
2. What are Monogenea 
3. What are Digenea
4. Similarities – Monogenea and Digenea
5. Monogenea vs Digenea in Tabular Form
6. Summary – Monogenea vs Digenea

What are Monogenea?

Monogenea are ectoparasitic flatworms that commonly reside on the skin, gills, or fins of fish. They have a direct lifecycle; hence, they do not require an intermediate host. Monogenea are either oviparous or viviparous. Oviparous monogenea release their eggs directly into the water. Viviparous monogenea release larvae, which immediately attach to another host. Monogenean adults possess both male and female reproductive structures.

Monogenea are less than 2 cm in length. However, several species infecting marine fish are larger. They have the ability to elongate and shorten as they move. They lack circulatory, respiratory, and skeletal systems. Monogenea have no or weakly developed oral suckers and have no true body cavity. Similar to ectoparasites, they have well-developed attachment structures. Anterior structures are known as prohaptor, while posterior structures are known as opsithaptor. They contain hooks, clamps, and anchors for attachment. Freshwater fish infected by monogenea become lethargic and gradually end up swimming toward the surface of the water. They also rub the sides or bottom of their skin where the parasite resides, and the gills appear swollen. Infected fish also show loss of scales and produce a pinkish fluid. Saltwater fish infected by monogenea show irritated swollen skins and gills. Severe infections result in erratic swimming behaviour.

What are Digenea?

Digenea are parasitic flatworms with a syncytial tegument with two suckers, one ventral and one oral. Adult digenea commonly reside within the digestive tract and occur throughout the organ system of vertebrates. Digeneans possess an unsegmented, vermiform body plan. They have solid parenchyma with no body cavity. The two suckers present are the anterior oral sucker, which surrounds the mouth, and the ventral sucker on the ventral surface. The ventral sucker is also referred to as the acetabulum. The majority of digenea are hermaphrodites, where both male and female reproductive structures are present in one organism. This is a favourable adaptation to allow the continuous life cycle where only one individual infects the final host successfully.

Generally, testes are present as male organs, and a single ovary with an oviduct is present as the female organs. Most digenea possess terminal and subterminal mouths and a muscular pharynx, which provide the force for the ingestion of food in their digestive system. Their nervous system contains paired ganglia at the anterior end of the body, and they serve as the brain. Alteration of sexual and asexual generations is an important feature, and this involves the presence of several discrete generations in a single life cycle.

What are the Similarities Between Monogenea and Digenea?

• Monogenea and digenea are flatworms (Platyhelminthes).
• Moreover, they are invertebrates.
• They have no body cavity.
• They are parasitic organisms.
• Both organisms are soft-bodied.
• Monogenea and digenea are hermaphrodites, so both male and female reproductive structures are present in one organism.

What is the Difference Between Monogenea and Digenea?

Monogenea reproduce sexually, while digenea reproduce both sexually and asexually. Thus, this is the key difference between monogenean and digenea. Besides, monogenea are ectoparasites, while digenea are endoparasites. Furthermore, monogenea consist of a series of hooks for attachment, whereas digenea consist of two suckers (oral and ventral) for attachment.

The below infographic presents the differences between monogenean and digenea in tabular form for side-by-side comparison.

Summary – Monogenea vs Digenea

Flatworms are unsegmented, simple, bilateral, soft-bodied invertebrates. Monogenea and digenea are two types of flatworms. Monogenea reproduce sexually, while digenea reproduce both sexually and asexually. Furthermore, monogenea are ectoparasitic flatworms, which commonly reside on the skin, gills, or fins of fish. Digenea are parasitic flatworms with a syncytial tegument with two suckers, one ventral and one oral. So, this summarizes the difference between monogenea and digenea.

Reference:

1. “Digenea.” An Overview | ScienceDirect Topics.
2. “Monogenean Parasites of Fish.” The Fish Site.

Image Courtesy:

1. “Pseudorhabdosynochus maaensis” By Jean-Lou Justine – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Parasite210036-fig3 – Creptotrema (Digenea, Allocreadiidae)” Авторство: Lidiane Franceschini, Aline Aguiar, Aline Cristina Zago, Priscilla de Oliveira Fadel Yamada, Mariana Bertholdi Ebert and Reinaldo José da Silva. (2021). “Three new species of Creptotrema (Trematoda, Allocreadiidae) with an amended diagnosis of the genus and reassignment of Auriculostoma (Allocreadiidae), based on morphological and molecular evidence”. Parasite 28: 69. DOI:10.1051/parasite/2021065 (CC BY 4.0) via Commons Wikimedia