Aquatic Parasite Observatory

Megalodiscus spp.

    • Species Name: Megalodiscus spp.
    • Synonyms: None
    • Taxonomy: Animalia, Platyhelminthes, Trematoda, Echinostomida, Paramphistomidae, Megalodiscus spp.
    • Life Cycle: “Eggs are deposited into water from adults in the intestine of frogs and tadpoles which hatch into miracidia. Young ram’s horn snails become infected and the parasite develops into mother sporocysts. After three generations of rediae, cercariae are produced which emerge from the snail and find a frog host. The cercariae attach and encyst on the skin of the frog host and become metacercariae. The frogs typically eat the sloughed skin containing the metacercariae. Inside the last frog host, the infection develops into the adult stage and eggs are produced” (Olsen, 1974).
    • Description:
      1. Specimens vary in length from 1.2 to 6. One of medium size measured 3.62 in length, 1.23 greatest breadth of body, and 1.49 breadth of acetabulum. The body narrows from the acetabulum forwards, first slowly but at the anterior third more quickly, to the mouth-sucker. Sections of the anterior end are circular but farther back elliptical. The mouth-sucker opens forwards, the large posterior sucker backwards and downwards. The cuticle is thick and smooth. The mouth opens back into right and left pharyngeal-pockets, between and below which it also opens into a rather long tube resembling an oesophagus. This bears posteriorly a pharynx-like swelling, beyond which are broad right and left caeca extending almost to the posterior sucker. There are two testes in the anterior half of the body, situated ventrally, a little distance apart, the anterior slightly to the right, the posterior slightly to the left of the median plane. From each springs a short vas deferens, the two uniting into a vesicula seminalis that anteriorly coils on itself and passes into a short ductus with prostate glands and a structure resembling a thin-walled penis-sac. A short genital-sinus opens ventrally, just under or behind the forking of the intestine. The ovary is smaller than the testes, is dorsally situated, in the median plane, a little way behind the second testis. A Laurer's canal runs upwards and slightly forwards to the surface. A shell-gland lies behind the ovary and the broad uterus passes first back and then forwards, making several transverse folds on its way to the genital pore. The vitellaria extend from the level of the posterior testis to near the ends of the coeca. The anterior follicles are below but the posterior ones are above the coeca. From the centre of each half, where it is passing round the inner side of the coecum, springs the transverse duct. Between ovary and acetabulum is the bladder of the excretory system, opening upwards in the median dorsal line. From it, below, spring right and left lateral ducts, which pass below the ends of the coeca and fold up and down along the outside of the latter on their way forwards to the region of the pharyngeal-pockets. Two large lymph-vessels lie along the inner, lower sides of the coeca and posteriorly approach to each side of the bladder but remain distinct from it.
    • Sources: Olsen, O.W. 1974. Animal parasites, their life cycles and ecology. University Park Press, London, Baltimore & Tokyo, 3rd Revd Edn., p. 278-279.
    • APO Parasite Records:

Parasite Images:

Distribution Map:

  • University of Colorado Boulder