Nick Baker takes a look at the fascinating life of a green banded broodsac, which turns snails into disco zombies
What is a green-banded broodsac?
The green-banded broodsac – what kind of a name is that? Normally we name an animal after the appearance or behaviours of the adult, but in the case of the fluke Leucochloridium paradoxum, unless you want to go poking around the guts of snails or the bums of birds, its broodsac – a membranous sac containing larvae – really is the most visible part of this parasitic flatworm’s life.
And when I say visible, it’s no exaggeration. If you’re lucky, you might come across the unassuming amber snail (Succinea putris) putting on an uncharacteristic and extrovert performance.
Green-banded broodsac lifecycle
With its eyestalks pulsating in high-contrast patterns of green, black and red, you are witnessing the phenomenon that gives this fluke its name. These are the fluke’s broodsacs and they are part of a complicated life-cycle that involves a bird, a snail and a fluke.
It all starts with a snail stumbling upon a dollop of nutritious bird poo containing the eggs of the fluke: this is species jump developing in the snail’s eyestalks.
With up to 250 of these larvae in each broodsac, each covered in a protective mucous coating, room is tight and the eyestalks swell and distort; the snail’s skin gets stretched so thin the parasites can be seen within.
The snail can no longer retract its eyestalks, and it is now that the light show begins. If the amber snail was an introvert before, it has now come out as a mollusc fit for Mardi Gras.
The effect is mesmerising, as the coloured bands of the boldly patterned broodsac throb at a frequency of up to 80 pulses per minute up and down inside the snail’s distended eyestalks. The palpitating broodsacs have a single purpose: to draw the eye of a predatory bird. Although it has not been witnessed in the wild, studies in captivity show that birds find these eyestalks irresistible, perhaps because they mimic the motion of a delicious caterpillar.
This is a textbook example of ‘aggressive mimicry’ – when an animal impersonates another species with the purpose of gaining an advantage. Once inside the bird, the larvae pass through its gut and metamorphose into the adult form, called a distome. The distome attaches itself to the wall of the bird’s cloaca with a pair of suction discs. Here, the cling-on waits for others to join it so it can mate and lay eggs to be distributed in the bird’s droppings, completing the circuit when devoured by an unsuspecting snail.
Main image: green-banded broodsac, parasitic flatworm in the Succinea snail’s tentacles. © Getty Images