Question

A generalized life cycle of the liver fluke Clonorchis sinensis is shown below. Label the fluke's stages. Identify the intermediate host(s). Identify the definitive host(s). To what phylum and class does this animal belong?

Short answer

Intermediate hosts: snails and fish. Definitive host: humans/mammals. Phylum: Platyhelminthes; Class: Trematoda.

Step-by-step solution

Identify the Fluke's Life Stages

The life cycle of Clonorchis sinensis involves several key stages. First, the eggs are released in the feces of a host. These eggs are then ingested by snails, which are the first intermediate host. Inside the snail, they develop into miracidia, followed by rediae, and then into cercariae. Cercariae leave the snail and penetrate fish, the second intermediate host, where they encyst as metacercariae. Humans or mammals consume infected fish, and the metacercariae mature into adult flukes in the bile ducts of the definitive host.

Identify the Intermediate Hosts

In the life cycle of Clonorchis sinensis, the intermediate hosts are the snail and the fish. The snails are the first intermediate hosts that ingest the eggs, and the fish are the second intermediate hosts in which the cercariae form cysts as metacercariae.

Identify the Definitive Host

The definitive hosts of Clonorchis sinensis are humans and other mammals. These hosts consume infected fish and provide an environment for the flukes to mature and reproduce in the bile ducts.

Determine the Phylum and Class

Clonorchis sinensis belongs to the phylum Platyhelminthes, which comprises flatworms. It falls under the class Trematoda, commonly known as flukes, which are parasitic flatworms.

Key concepts

Intermediate Host

In the parasitic life cycle of Clonorchis sinensis, intermediate hosts play a crucial role. These organisms temporarily harbor the parasite at various developmental stages, but they do not reach sexual maturity here. In this cycle, both snails and fish serve as intermediate hosts.

Initially, snails become the first intermediate host by ingesting fluke eggs present in the environment. Within the snail, eggs develop into miracidia, then progress into rediae, and finally transform into free-swimming cercariae.

These cercariae exit the snail and search for another host. Fish become the second intermediate host as cercariae attach to them and encyst as metacercariae. This transformation is vital for the fluke's continuation to its next life stage.

Definitive Host

A definitive host is one where a parasite reaches adulthood and reproduces sexually. In the life cycle of Clonorchis sinensis, this role is filled primarily by humans and some mammals.

When humans or other mammals consume raw or inadequately cooked fish containing metacercariae, the parasites proceed to mature in the bile ducts of the liver. Here, they can reproduce and complete the life cycle.

The environment within a definitive host is highly conducive for the parasite. Consequently, this allows it to subsist and propagate effectively, often causing disease in the host. Identification of the definitive host is crucial in understanding and controlling the spread of this parasite.

Platyhelminthes

Clonorchis sinensis belongs to the phylum Platyhelminthes, widely known as flatworms. These organisms are characterized by their flattened body structure, bilateral symmetry, and lack of body segmentation.

Flatworms are primarily divided into free-living and parasitic species. While some reside freely in aquatic environments, others, such as Clonorchis sinensis, lead entirely parasitic lives.

The phylum Platyhelminthes includes other classes alongside Trematoda, such as Cestoda (tapeworms) and Turbellaria. These organisms display diverse lifestyles and adaptations within their environments.

Trematoda

Trematoda, also known as flukes, is the class to which Clonorchis sinensis belongs. This class is characterized primarily by their parasitic lifestyles, usually infecting the tissue of various animals.

Flukes have complex life cycles involving multiple hosts. Most typically have suckers or hooks used for attachment in their host's tissue, which aids in nutrient absorption and survival.

The variety within Trematoda is vast, with species infecting a range of definitive hosts including humans, birds, and fish. Understanding Trematoda is essential in tackling diseases caused by these parasites, as they pose significant health concerns globally.

Parasitic Life Cycle

A parasitic life cycle refers to the series of developmental stages a parasite undergoes, often involving more than one host. Clonorchis sinensis, like many other parasitic organisms, relies on a complex cycle involving both intermediate and definitive hosts.

This cycle begins with egg release into the environment from a definitive host. Intermediate hosts allow for critical developmental transformations, such as from miracidia to rediae. Eventually, the parasite reaches a final host, where it can mature and reproduce.

Understanding the parasitic life cycle is pivotal for the control and prevention of parasitic infections. Effective public health measures can break this cycle by eliminating or reducing exposure to the parasite at various stages.