"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift
April 8, 2010
April 8 - Maritrema novaezealandensis
The parasite Maritrema novaezealandensis is commonly found on the coast of South Island, New Zealand. It is a trematode with a typical three host life-cycle, using a snail as a first host intermediate host where clonal multiplication occurs, a crustacean as a second intermediate host where it form a cyst-like waiting stage, and gulls as the definitive host where it matures into a hermaphroditic adults and sexually reproduces.
This trematode parasite uses the New Zealand mudsnail (Zeacumantus subcarinatus) to asexually multiply, and in certain areas, more than 60% of the snails found are infected with this parasite. The parasite takes over the most of the innard of the snail, filling it up with clones of itself and diverting resources from its reproductive organs, thus castrating it. The snail is essentially a zombie under the control of the parasite. Maritrema then use the snail as a kind of "parasite factory" producing free-living swimming larval stages (also clones) call cercariae (pictured) which are released into the environment to infect the next host in the cycle which are crustaceans such small crabs and amphipods (tiny beachhopper-type animals). The cercariae penerate weak spots in the cuticle using a specialised structure known as a stylet which functions rather like a cross between a scalpel and a saw. The little cercariae (which are less than 0.15 mm long) use the stylet to cut their way into the crustacean. Once inside, they grow over the course of a few weeks and develop into a cyst. There, they wait to be eaten by a sea gull to complete their life-cycle.
The cue for the infected snails to release the free-swimming cercariae stage is an increase in temperature, and when that happens, hundreds of cercariae swarm out of an infected snail. The hotter it gets, the more parasites are released. In the summer during low tide, pools or puddles gathered on the mudflat can get quite warm in the glare of the afternoon sun. Trapped within those tide pools are various crustaceans and many, many infected snails. Triggered by the heat, each infected snail releases hundreds of cercariae into the water, turning the water into a swarming "cercariae soup" and rendering any crustaceans in the area into hapless parasite pin-cushions. For tiny crustaceans like amphipods, the experience of being penetrated simultaneously by multiple cercariae can be quite traumatic (imagine being stabbed multiple times by scalpels) and the experience can often be lethal. While killing the intermediate host before it can be passed on to the next host is not good for the parasite either, it demonstrates one of the ways that parasites can regulate the population of its host.
Some relevant papers on this parasite are:
Fredensborg, B.L., K.N. Mouritsen, and R. Poulin. 2004. Intensity-dependent mortality of Paracalliope novizealandiae (Amphipoda: Crustacea) infected by a trematode: experimental infections and field observations. Journal of Experimental Marine Biology and Ecology 311: 253-265.
Fredensborg, B.L., K.N. Mouritsen, and R. Poulin. 2005. Impact of trematodes on host survival and population density in the intertidal gastropod Zeacumantus subcarinatus. Marine Ecology Progress Series 290: 109-117.
Keeney, D.B., J.M. Waters, and R. Poulin. 2007. Clonal diversity of the marine trematode Maritrema novaezealandensis within intermediate hosts: the molecular ecology of parasite life cycles. Molecular Ecology 16: 431-439.
Martorelli, S.R., B.L. Fredensborg, K.N. Mouritsen, and R. Poulin. 2004. Description and proposed life cycle of Maritrema novaezealandensis n.sp. (Microphallidae) parasitic in red-billed gulls Larus novaehollandiae scopulinus from Otago Harbor, South Island, New Zealand. Journal of Parasitology 90: 272-277.
Contributed by Tommy Leung.