MONOCYSTIS – The Earthworm Parasite

Monocystis is an endoparasite in the seminal vesicle and coelom of earthworms. The feeding stage is called trophozoite that develops within the sperm morula, which is a group of developing sperms in the seminal vesicles of earthworm. There is only one host, the earthworm.


Young trophozoites are rounded and oval, about 5 micron long but full grown trophozoites are elongated, spindle-shaped, about 500 micron long and 40 micron broad. Body of trophozoites is covered with pellicle that contains longitudinal contractile fibres called myonemes, which help in metabolic locomotion. Endoplasm contains paraglycogen globules and volutin granules. Nucleus is single and placed anteriorly in the upper half of body. It contains haploid chromosomes. The anterior end bears a pair of elongated roptries, whose secretion helps the trophozoite in penetrating through the host tissues. There are also conoids and micronemes in the anterior end, whose function is not well known. Nutrition is saprozoic but also feeds on sperm heads.


Monocystis Is a monogenetic parasite which means its life cycle is completed in a single host that is earthworm. The motile infective stage is a sporozoite which is haploid and enters the sperm morula of earthworm to feed and grow. This spindle shaped stage is called trophozoite, which eventually differentiates into a gamont or gamete producing cell or gametocyte. The process is called gamontogamy.

Two gametocytes or gamonts become rounded and make a non-sexual association which is called Syzygy. The two gamonts then become enclosed in a common envelope called the gametocyst or gamontocyst. This process is called encystation.

The gametocytes then undergo multiple fission called gametogony to produce large number of gametes. The cell membrane between two parent gametocytes break and the gametes of the two cells mix together and fuse in pairs. This fusion of gametes is called syngamy, which forms zygote or sporont.

The diploid zygote secretes around it a tough covering called the sporocyst and the resultant structure is now called spore. Sometimes it is named as pseudonavicella because of its boat-shaped structure. The zygotic nucleus undergoes meiosis to restore the haploid chromosome number and then mitosis produces eight cells, which differentiate into spindle- shaped sporozoites.

The sporocyst remains viable in the environment for a long time. The sprozoites are liberated from the spore only after it is ingested by an earthworm. They penetrate the intestinal lining to reach seminal vesicles where they transform into trophozoites.

Exact mode of transmission from one host to another is not known. The sporocysts are liberated into soil by the death of earthworms or when earthworm is eaten by birds and sporocysts are excreted through the faecal matter of birds. The sporocysts are also believed to be transferred from one host to another during copulation.

Although all earthworms are infected with monocystis, their fertility is not greatly impaired by the action of parasite. The trophozoites invade and grow inside sperm morulae, inhibiting the development of spermatogonia and spermatids but this does not significantly reduce the number of sperms produced.