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"A Nature Observer′s Scrapbook"

Leeches and aquatic worms.

Individual embryo Immature specimen at rest Mature specimen in motion. When submerged,
 the front of the body would be raised. Embryos on underside of body
visible through translucent body. Hemiclepsis marginata extended Hemiclepsis marginata carrying
embryos on underside of body. Hemiclepsis marginata carrying young

no common name

Hemiclepsis marginata

It's amazing what turns up when you get around to doing a bit of routine pond maintenance. The upper image is of a group of Hemiclepsis marginata leeches that were found on the side of the pond pump when it was hauled from the water. Moving the mouse over the image will identify the individuals. There is a very well established large colony of these leeches in my small garden pond which has a constant flow of re-circulated water from a waterfall.

This species is comparatively small compared to the large medicinal leech (about 90mm). The overall length of a mature H. marginata can be up to about 43mm fully extended (second image). At rest,12mm to 20mm is about average, and when disturbed it can roll into an 8mm ball like a pill bug, which allows it to roll freely down a slope or be washed along under the influence of water currents.

The various leech species have developed their own unique method of ovulation. Some round bodied species carry their eggs in a girdle round the body in earthworm fashion and then slough the girdle off prior to hatching. This flat bodied species passes its eggs from the genital pore into a sac-like cocoon which is held against the underside of the body for about 10 days. One adult carrying a large number of green eggs (third image) was observed to remain virtually imobile in a very protective manner while other non egg carrying adults were quite active.

When the embryos have developed their sucker pads, they hatch from the egg but remain attached to the parent (fourth image) for a further 14 days before dropping off.

With a sucker pad at each end of the body, their mode of locomotion is to maintain hold with the large posterior sucker, extend the body forward, often swaying from side to side, until a suitable new hold point is latched onto with the anterior sucker, and then the body is drawn forward in a pronounced arch towards the front sucker - very similar to 'looper' caterpillar action.

Hemiclepsis marginata head detail

The key to identification of leeches covers several features, including the number and placement of the eyes. As can be seen in the head closeup, this species has four eyes closely aligned in two pairs (fifth image).

The variation in coloration of the images illustrates a chameleon like ability to adapt to different lighting conditions. Found in shade, the true natural colour is a greyish green. But in bright sunlight it appears very yellow/brown.

H. marginata is a blood sucking parasite of fish, tadpoles and water molluscs. Considering the large leech population, I have only occasionally seen an odd one attached to the Golden Orfe which co-habit the pond but have never seen one on a tadpole. This may not be too surprising because one meal can last a leech a considerable time. Some species need only feed once or twice a year.

Lifespan will be largely dependent upon habitat and water temperature. But even in a friendly environment it is not thought likely that many will survive more than two years.

30.05.2004Resident all year long in garden pond.
10.03.2005One seen attached to fish for three days by rear sucker, but not feeding all the time.
15.05.2005During pond maintenance, several found to be carrying embryos.
22.05.2006One found to be externally carrying a large number of green eggs.
26.06.2007One found carrying a brood of small leeches.
12.01.200840mm specimen found on pond pump.

Dina lineata ? Dina lineata ?

no common name

Dina lineata,
... or Erpobdella occulata,
....... or Erpobdella testacea ?

Confusion reigns. It seems that from shape, size and coloration the specimen shown upper right, might be Dina lineata, Erpobdella occulata or E. testacea. It would take microscopic examination of its 20 plus body segments with 3 to 6 annulii per segment to come up with a definitive species.

Under a lens, a specimen that I found in 2004 had 8 eyes arranged in two forward curving arcs of four. The four eyes in the forward arc were all equi-spaced, whereas the second shallower arc had two pairs with a central gap. All of which appeared to point to a classic Erpobdella testacea arrangement. However, the specimen examined in 2006 had 6 eyes as seen in the lower image. And that is consistent with Dina lineata which can have a variable number of eyes! So, presuming that there is only the one species in the pond, the odds seem to favour Dina lineata - maybe!

They are round bodied, worm like species with a dark 'liver red' coloration ( the lighting of the lower image was enhanced to accentuate the eyes ). They move by extending and contracting the body but, with a much less pronounced looping action than the previous species, H. marginata. The posterior sucker is very effective and, once it has taken hold, it can be very difficult to dislodge.

Whatever the species, the garden pond hosts a large population of them and they can be found at any time throughout the year.

Whereas most leech species are recognised as blood suckers, this group are of a later evolutionary period and are truly carnivorous, swallowing their prey whole (tiny worms, insect larvae, etc.). At full stretch they can measure up to 30mm and are capable of free swimming with a very effective rapid undulating action.

19.11.2004 Resident all year long in garden pond.
22.05.2006 Resident all year long in garden pond.
12.01.2008 Two 40mm specimens found attached to pond pump, one with 6 eyes, one with 8 eyes!

Piscicola geometra

no common name

Piscicola geometra

Whereas the two previous species are both common in the garden pond, I only saw this species on two occasions; once on the side of a fish and the last time was when this image was taken.

It may well be that the small garden pond was not sufficiently well oxygenated to sustain this species. It favours a faster flowing environment (which begs the question of how it arrived in the pond; probably as an embryo on the leg of a coot or moorhen).
Or, since the Piscicola spp. are more closely related to marine leeches, perhaps they are less well adapted to the changes in water temperature which could occur in a small pond.

One of the characteristic features of this species are the pulsatile vesicles (knobbles!) which lie down the sides of the body. These pulsating organs are associated with gill respiration and are adapted to maximise the oxygen intake needed by this parasitic species to actively swim to its host. Other less active species are able to absorb sufficient oxygen by a rythmic undulation of the body.

My preoccupation with leech eyes was thwarted here because I could not find any on the head area. There should be four in two pairs. In leech terminology an 'eye' comprises a group of simple light sensing ocelli. And I subsequently found that Piscicola geometra has as many as 12 to 14 'single' ocelli - in a ring around its posterior sucker pad!
I never thought to examine that end.

05.03.2004 Short term resident of garden pond.

Flatworm, Dugesia polychroa Flatworm, Dugesia polychroa


Dugesia polychroa

Initially, I thought these might be D. lugubris but, I am reliably informed that solely on the photographic evidence, they are more likely to be Dugesia polychroa.

Although it may look superficially leech-like, this is one of the freshwater worms, of which there are three different groups. There is the annelid group (with round segmented bodies), the round unsegmented bodied group and the flat bodied group.

This flatworm shows no body undulation as it moves. It slithers quite slowly (3mm per sec. max), slug like, so one has to very patiently look for them. The two close set eyes are barely discernible without the aid of a lens. As seen alongside, there are two colour forms. The paler version was found in a waterfall pan by the garden pond and the darker form was found in a field drainage ditch a few yards away. They favour slow moving water and there appeared to be well established colonies in both sites.

They are carnivores, feeding on small shrimps and other small lifeforms and they are also scavengers of animal scraps and the recently dead. When deprived of food they have the ability to shrink dramatically in size before succumbing to terminal starvation. At 12mm and 17mm respectively, these specimens were not full size. They can reach 20mm in length.

30.05.2004 First seen but not identified in pond waterfall.
23.03.2005 Several pale specimens seen in the shallow pan of the pond waterfall.
13.11.2005 Several of the dark form found in an adjacent field drainage ditch.
26.06.2007 Pale and dark forms found in same locations as in 2005.

Horsehair worm

Horsehair worm

Gordius species

Yes, it may look like a question mark but, take my word for it, this was a very lively thin aquatic worm. Currently, in 2011, there are thought to be six different UK species in the genus Gordius but the job of finding differentiating features in something less than 1mm wide requires detailed microscopic examination.

This specimen, at 8 inches, 200mm, long was on the small side as they can grow up to 4 times that length. I did see one contorted mass of wriggling thread in the garden pond some time ago that must have been much longer than 200mm but I did not manage to investigate that. This one was found in a field side drainage ditch.

The adult worm has a completely aquatic vegetarian diet and produces an enormous number of eggs. The minute free swimming larvae rely upon being ingested by specific insect and arthropod species drinking from the infested water. Known hosts include crickets, grasshoppers, beetles, cockroaches, spiders and woodlice.

After two or three months development, the young adult worms break out of the host's body - but only when the host is in contact with water! It may be that by devouring the host's body juices, thirst drives the insect back to water where it dies. Or, it may just be pure chance that an infected host comes into contact with water. If that is the case then that could well justify the adult producing so many eggs to raise the odds of completion of a very uncertain life cycle.

23.06.2006 Found in a field drainage ditch.
28.06.2006 3 found swimming together in the field drainage ditch.
18.05.2007 6 found, 25mm to 150mm, in same location as 2006.
04.06.2008 Several found in same location as 2006 and 2007.
13.06.2011 Two found in same location, the outfall of an underground drainage pipe. Now beginning to wonder at the co-incidence of always finding them at precisely the same place.

Square-tail worm

Square-tail worm

Eiseniella tetraedra

It looks like an earthworm, it is an earthworm - but one that has adapted to an aquatic lifestyle.

It is omnivorous, devouring tiny creatures and decaying plant material that accumulate in the silt at the bottom and sides of slow moving water courses. I have a large community of them living in the sediment trap of the pond filtration system. They appear to thrive in the black sludge that gets trapped there - but, they are not 'sludge worms', they are quite a different animal.

The specimen shown here is not fully mature, as it lacks the mid body girdle or egg sac called the clitellum. But, forward of where the clitellum should be, the body cross-section is cylindrical as in earth worms. The rear part of the body tends to be squared off, rather more rectangular in cross-section. And the most obvious feature is that the rear tip of the body terminates in a markedly 'squared off' fashion.

They normally grow to a length of about 50 - 60mm.

23.06.2006 Large colony permanently resident in pond filtration tank.

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