Here is a video from Robert Sandler, an undergraduate in Casey Dunn’s Biology0410 Invertebrate Zoology class. In this video Robert introduces gyrodactylids, a group of flatworms that live attached to fish.
Few places capture the biologist’s imagination quite like New Zealand. A fragment of the former supercontinent Gondwana, subject to countless (and continuing) mountain-building events and glaciations, Aotearoa cradles a host of singular creatures such as the Tuatara, the giant Weta, the tiny relict Rifleman, and numerous endemic soil arthropods, hiding out in the leaf litter of the country’s own flavor of the Antarctic flora. It’s also, happily, home to two (known) species of the flatworm Prorhynchus, members of an easily-overlooked group restricted to freshwaters and humid leaf litter.
Prorhynchus in this country were first discovered almost by accident, when sampling wells on the Canterbury plain. They apparently prefer cold, stable groundwater habitats, where they reign as the apex predators of these simple ecosystems. Throw a net down in the right places, and there’s a decent chance that you’ll pull up, besides a few terrified stygian scuds, a horned Prorhynchusputealis (=”well-dweller”) – blind, wraith-white, retreating, slick with mucous, and more than anything else, enormous. Most Prorhynchus are tiny animals, some species only 0.5 mm long as adults. In New Zealand, Prorhynchus attains lengths of 7 cm or more (the creature in the video below was almost 10 cm outstretched). But for all their imposing heft, they are fragile creatures, being well-adapted to their subterranean life, and when exposed to even pale sunlight, or a slight rise of temperature, will dissolve with alarming rapidity into a formless ectoplasm. It’s easy to imagine that we don’t know very much about the biology of these animals.
Wells provide only haphazard glimpses into these ecosystems, so if they were our only means of access, we’d have to count ourselves ignorant indeed. But New Zealand is a land with a hydrology as unusual as its biology, particularly in the alps, which are incised by dozens of sprawling braided-river valleys (which can make for interesting travel experiences). Follow one of the narrow, waterweed-choked channels that feed the rivers, and, in half-an-hour’s walking, you’ll come to the head – a deep, steel-blue pool, lined by moss, unbroken graywacke cobbles at its base, with, in places, fountains of dancing sand animated by waters cold enough to numb the hand in seconds – a spring.
Natural interfaces between surface waters and the rivers beneath, springs can provide a portal into a world we rarely see. At the source, especially when the flow of water is great, groundwater fauna can sometimes become accidentally trapped in springs. They’re usually not common – but spend a few hours in the icy water pushing aside cobbles and turning over rocks, and amidst the stoneflies and snails, you’ll soon notice the sickly-seeming but vigorous sideswimmers, the threadlike, pulsating, pale earthworms, and, fleeting downward between the smoothened shingles, the ghostly Prorhynchus of New Zealand, struggling to move under its own weight.
The sense that the self exists somewhere close to the brain or heart is an intuitive one for humans. It also seems to apply to most of the animals we regularly encounter, even when they can regrow parts of their body. When a crayfish gets into a tight spot and loses one of its claws, the part of the crayfish with the head will regrow the lost claw, but the claw won’t regrow a body and head.
For many animals, though, there is no such essential center of the organism. When a flatworm gets its tail cut off, both the tail and head will fill in the missing parts and make two whole flatworms that are clones of each other. Its body is arranged such that there isn’t a single part of the animal that can be identified as the core.
Here is a bit of footage taken by Stephanie Spielman, an undergraduate in Casey Dunn and Gary Wessel’s seminar on the evolution of multicellularity at Brown University. The clip features the flatworm Dugesia tigrina swimming around the Dunn lab. It is released under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 license. The crawfish video is from Day at the River (1928), a video from DeVry School Films, Inc., which is under public domain.