"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.


  1. Does this trematode castrate the snail host? How does the species weather that kind of predation level?

  2. Yes, just about all trematode castrate their snail host. There are two ways the hosts have evolve to mitigate the effects of these castrating parasites.

    Firstly, there is coevolution, so the host and parasite population are in an constant evolutionary race; in every generation, if there happen to be rare individuals that are immune from the parasites, then those individuals that escape parasitism will come to dominate the next generation - vice versa for the parasite to evolve to infect these new host.

    Secondly, there is host maturation time - it appears that the snails do not become infected until they reach above a certain size threshold, and after that, they almost always become infected (and castrated), thus snails that can reach sexually maturity at an earlier age has an advantage because they will be able to have at least *some* offspring before they eventually become castrated - in fact, that's what Fredensborg (the main author for 2 of the above cited papers) found in his research - snails which are found in areas where parasites are common reach sexual maturity at an earlier age than snails in areas where parasites are rare. This is a pattern which has been found not just for this particular host-parasite system, but also in other host-parasite relationships where the parasite (either through killing or castrating the host) drastically reduces the host's reproductive fitness.