Parasitic platyhelminthes, particularly the flukes, have the most complex life cycles of any animal, involving two or more hosts and an elaborate sequence of developmental stages that allows each egg laid by the adult to spawn huge numbers of infective juveniles via asexual reproduction. The photo and video below illustrate a part of this developmental sequence in a fluke that uses periwinkle’s as its intermediate host.
The photo shows part of the periwinkle’s gut dissected to reveal an infestation of pale, ovoid structures. These are known as sporocysts and they’re much less than 1 mm long. Every one of these sporocysts developed from a single miracidium, a ciliated organism that hatched from an adult fluke’s egg. The video reveals the macabre secret contained within these sporocysts.
The cercariae released from the bulging sporocysts are another infective stage in the parasite’s life cycle and it is their task to penetrate the body of the definitive host (unknown in this case) where they develop into the adult fluke. For a fluke, the example above is a relatively simple life cycle. In other species there are yet more body forms and serial repetition of some stages to produce colossal numbers of cercariae from a single egg. What is the significance of this singular reproductive strategy? Well, in short, huge numbers of offspring produced asexually offset the huge odds stacked against any one juvenile completing its development. You have to love the incredible complexities of such animals that are often disregarded as ‘simple’.