Flatworms again

I checked on the flatworms a few days ago and found that:

1) I had a few less flatworms than I did before (specifically, the Dendrocoelum lacteum were missing)
2) I had 5 small brown balls attached to the upper sides of the tank, near but below the water line.

My first thought is that the Dendrocoelum had encysted somehow, as perhaps my tank was not to their liking, but after delving into the subject, I'm afraid it looks like Dendrocoelum has become the perfect meal for my Dugesia, supporting a round of egg-laying by that species.  The eggs themselves are inside the cocoons, but i expect in a few weeks I'll have numerous baby Dugesia circling my tank.

It came from the compost heap!

One of the more familiar sights of summer is the hoverfly, that yellow bee-like insect that darts from flower to flower, or hovers in mid-air, surveying its surroundings.

But how many people have ever seen their larva?

Below is a rat-tailed maggot... larva of one of the larger groups of hoverflies, the Eristalinae.  This fellow had established himself in our compost bin and was quite happy to turn our waste veggies into hoverfly material until I tipped him onto the compost heap out back.

That long bit sticking out the back (the 'rat tail') is a breathing tube allowing the maggot to live in waterlogged sewage, in temporary ponds, or in any environment with lots of water and lots of organic material.  I'm guessing this is a sign I need to empty the compost more frequently, though on the flip side, I can think of no better fly to be chowing down on our leftovers.

Incidentally, the fly pictured at top, Episyrphus balteatus, doesn't produce rat-tailed maggots...  its larva are one of the few active, predatory fly larva and look something like small green slugs.  They feed on aphids and can be found, with some careful searching, on garden plants that have been aphid infested.

Flatworm pictures

Some pictures of the flatworms.  The animals were transferred to a small plastic container and photos were taken with a macro lens.  It was a bit harder than I thought it would be to get decent pics, but these are probably the best I could hope for with the equipment I have.

Incidentally, its not really clear what I should feed them.  In some of the old literature with Dugesia tigrina, the authors use small cubes of liver.  In the 60's, that probably wasn't hard to find, but few supermarkets stock liver these days.  Liverwurst is not a substitute, apparently, but I will try some chicken meat soon.  I've also tried dried bloodworms, which some of them have gone for, but its not entirely clear if they are feeding or just clinging on to them.  The problem is that these are predators (Dendrocoelum in particular) and only occasionally scavengers, so I imagine simply wiggling a dead bloodworm in front of them isn't going to provoke much of a response.

Dugesia lugubris

Dendrocoelum lacteum

(Note the eyes - Dugesia has large eyes that look 'cross-eyed' - the white bit is many retinal cells, the dark patch a few pigment cells. Dendrocoelum has just two dark dots composed of two retinal cells and a pigment cell.)

Flatworms III

Some pictures of my flatworms: Dendrocoelum lacteum (left) and Dugesia lugubris (right). If the picture looks a bit odd, its because it was taken through the plastic of the tank with a macro lens, and what your seeing is the undersides. The lighter patch in the middle of each worm is the pharynx, which is popped out when the animal wants to suck up some food. Flatworms don't have a through digestive system, so its also where the waste leaves when digesting is done.


Despite their similarity, these species are not close relatives. Dendrocoelum is in the family Dendrocoelidae, a group of active hunters with very simple eyes and an 'adhesive organ' (sucker) at the front for holding their prey, while Dugesia is in the family Dugesiidae, a more opportunistic predator that lacks a sucker but has a much more complex eye. These species used to be all lumped together in a larger group, the Paludicola, defined as all triclad planarians (triclads are flatworms whose intestine divides into three parts) that live in fresh water. They were believed to have descended from terrestrial flatworms, who were themselves descendents of marine flatworms.


About 10 years ago, a series of molecular studies completely rewrote the book on flatworm taxonomy and gave us a very different view of their evolution. The freshwater triclad flatworms are now believed to have evolved from marine flatworms (which when you think about it, makes more sense than if they had come from terrestrial species), and at some point around 100 million years ago, the ancestor of the Dugesiids and all terrestrial flatworms split off from the ancestor of Dendrocoelum. In the process they gained a rather unique mutation - a second version of their ribosomal RNA genes. All animals have multiple copies of their ribosomal RNA genes, but because all these copies are next to each other on the DNA strand, the mechanisms of DNA copying ensure that they are all identical copies. At some point, the ancestor to the Dugesiids and land flatworms underwent a mutation that moved some copies of their ribosomal RNA genes to a different location in their chromosomes, where they could evolve in a different direction. Its not really clear what these second ribosomal RNA genes do, but they are expressed and they can be found in all of the descendents, so they must be doing something important, but different from the original ribosomal RNA. My guess is that they may only be expressed in certain tissues or at certain times of development, but I don’t think anyone has really looked into it.

Flatworms II

Part of my fascination with these humble flatworms comes from their ability to regenerate - as a youth, I remember coming across drawings of two-headed flatworms, multiheaded flatworms, and full-sized flatworms with a 'miniature' flatworm emerging from their sides. Not all flatworms display this amazing ability to regenerate, of course - depending on which version of flatworm taxonomy you accept, regeneration is thought to be an ancestral characteristic that has been reduced or lost multiple times in the evolution of this group. Some flatworms can't regenerate at all, others can regenerate most of their body, as long as the brain is intact, but for most flatworms regeneration of amputated parts tends to be limited to an area from their brain to their mouth (located around the middle of their body). They do a bit better at regenerating cuts, which leads to two-headed flatworms if you make a cut along their midline.

The flatworms that do regenerate have a cells scattered through their body called neoblasts that are thought to be totipotent - the flatworm equivalent of stem cells. When a flatworm is damaged, these cells migrate to the wound site and begin dividing and differentiating to make new tissues. What is interesting about this process is why not all flatworms regenerate, and why some are not as good at it as others even though they have neoblast cells... if you think about it, these cells are a two-edged sword. On the one hand, they allow the animal to quickly repair damage and replace lost body parts (and considering how soft and delicate these animal are, I'm sure damage is quite common), on the other hand, these cells have to be kept under tight regulation or they run the risk of replicating out of control - essentially becoming cancer cells. Even a regeneration that is 'mostly' correct can be fatal - two headed flatworms appear to die after a month or so. I suspect that for some lineages of flatworms, the risks of uncontrolled regeneration or cancer have outweighed the benefits of fast healing and replacing amputated parts, and over time the ability to regenerate has either been lost outright or various mechanisms to regulate and restrict these cells have evolved.

Of course there are other factors that may help maintain the ability to regenerate. A number of flatworm species can reproduce by splitting in two, and while there are differences between fission and regeneration, neoblast cells are involved in both processes. Interestingly, in some of these asexual species, if they do become sexual and develop ovaries, they lose much of their ability to regenerate. There are also similarities between early growth and regeneration, suggesting that regeneration could be a retention in the adults of growth mechanisms in the juvenile. It would be interesting to look at some of the poor- and non-regenerating species and see if their juveniles show greater regenerative powers than the adults.

As for my own flatworms, I'm a bit undecided as to whether I will be cutting them up to create two-headed monsters of the water-tank... for one, cutting a tiny flatworm is probably a lot harder than it sounds if all you have is an old microscope, a hand-lamp, and a kitchen knife (ah for the days when I had access to a full laboratory!). For another, where people have bothered to study the fate of these multi-headed worms, their lifespans have been much shorter than normal, and for now I just want to see if I can keep them alive in a small tank. These aren't the 'classic' lab flatworms with the arrow-shaped heads (Dugesia tigrina), so I don't have as much to go on about culturing these species.

Flatworms

Been catching flatworms in the river near where I live... I was able to catch several black ones, two grey ones, and a white one.  The black ones are almost certainly Dugesia polychroa... the white one possibly Phagocota vitta.  I'll have to wait a bit until they settle down before I can take a closer look at the white and grey ones.  Flatworms have always fascinated me, ever since reading about them in an old copy of 'Animals without backbones'.  The problem of course was finding them... the streams near where I lived (as a child) were muddy and not really suitable for them.  My identifications are from Ball and Reynoldson's "British Planarians", which may be a bit dated.  Hopefully I'll figure out a way to get some pictures of them posted.

Beautiful blue

This blue jellyfish was kind enough to do a somersault, giving me a good view of both its top and underside.

My god, its full of stars!

Asterias rubens to be exact. These starfish were numerous at the dive site, mostly on rocks but some were on the shell-gravel. If you look carefully at both photos, you can see a second smaller starfish nearby in each of them.

This individual below is eating a clam - the clam isn't obvious, but the body of the starfish is positioned over it, and the stomach has been inserted into the clam shell.

Diving at St.Abbs

Common sea urchins in abundance at St. Abbs, Scotland.



Below, you can see the tube feet wafting in the current.