Trematodes!

Recently we collected some Arenicola egg cases in preparation for our unit on annelid worms. These egg cases were covered in tiny snails. We assumed the snails were eating the eggs, so we pulled them off of the cases and put them in a separate tupperware. The next day when we checked the tupperware we found tiny animals swimming around in it along with the snails that were crawling around on the sides. When we put these animals under the microscope, no one recognized them, so we consulted with one of the station parasitologists, Chelsea Wood, who had this to say:

"It's a trematode cercaria!  I sometimes get them in water from the seawater system, but I've never seen one with such luxuriant setae on its tail - good find!"

Trematodes are mostly parasites on snails. They belong to the phylum of worms Platyhelminthes, and often have complex life cycles. The cercaria we found swimming in the water are larvae looking for second hosts that they can infect until they develop into sexually mature adults. The adults will produce eggs and sperm, which will develop into miracidia larvae and look for more snail hosts.

Circaria larvae have long tails with bristles on them. Chelsea sent us this description from Sukhdeo and Sukhdeo (2004):

"Swimming cercariae have tails of various lengths that may be forked, single, or bestudded with setae and other appendages (Kearns 1998). Cercaria setifera and Opectonia bacillarus have tufted tails that greatly enhance their speed; the rapid undulations of the tails sweep the tufts of setae backwards to push against the water to generate speeds of up to 13 m/h (Koie 1975; Bartoli 1984). These tufted tails often accumulate debris and these species have an interesting behaviour where they stop swimming, reach around with their body to clasp the tail, and then they draw the tail through their body to clean it."

Here are some pictures of the snails that came in on our egg cases:

And here are some videos of the ghostly and beautiful cercaria larvae:

http://youtu.be/_PmkshAyvVU

http://youtu.be/kPx2rc8XB7Q