MODULE 3: ADVANCED IDENTIFICATION OF
FLATWORMS (TRICLADIDA) AND LEECHES
(HIRUDINEA) - BY RICHARD CHADD
Introduction to this module
If you are reading this, it is likely
that you will have undertaken
and passed the 'Basic Level' of
certification in species - work
(Modules 1 & 2) - so well done!
It's now time to delve a little
deeper into the subject of
freshwater invertebrate species
identification and get to grips
with the trickier stuff. The
remaining parts of the course
(Modules 3 to 11) stand alone
as individual courses, based on
major taxonomic groups. They
are broadly arranged in order of difficulty, with a certain adjustment for
'usefulness' in tackling the group. So this is the easiest one and, when
passed, will give you the Advanced Level for identification of freshwater
flatworms and leeches.
Inevitably, because freshwater Hirudinea and Tricladida comprise two
reasonably small and distinctive groups of organisms, much of the advice
on identifying them was covered in Module 2, so there will be a certain
amount of reiteration and this Module is mercifully short. It will, however,
cover the species for which close examination of fine features or dissection
is necessary.
As with the first two Modules, there are mandatory exercises which will be
marked by your tutor. You can submit them by printing out this workbook
and filling in the appropriate spaces by hand (but please add your name to
the front of the workbook), or you can use the template appended to the
introductory part and send completed exercises by email. There are also
occasional self-assessment exercises, which are not mandatory and do not
contribute to your certification, but may be helpful in understanding a
principle or technique.
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SECTION 1: FLATWORMS (TRICLADIDA)
For this section, you'll need a copy of the FBA guide to Freshwater
Tridclads: Scientific Publication no. 58 (Reynoldson & Young, 2000).
Having completed Module 2, you will be aware that the freshwater triclad
fauna in Britain and Ireland consists of a relatively small number of species
(12 in Britain and 11 in Ireland), so the guidance given in the basic Module
covers much of what you need to know.
To recap, the features you need to use to separate virtually all of them are:
1.
Eyes (number, arrangement/position and separation);
2.
General colour (and distribution of pigment, if any);
3.
Shape (including possession of 'tentacles').
As mentioned in Exercise 1 of Module 2, using these features for
identification is much easier with live material. Dead flatworms either
degrade very rapidly and turn to mush, or, if fixed/preserved, crinkle-up like
a cornflake, making identification a bit problematic. It is possible,
however, to identify preserved material using the key on pages 48 & 49
of Reynoldson and Young (2000).
In essence, the key to preserved material uses the features above in a
substantially simplified way and with a few additional useful bits of
'jizz' (this term was outlined on page 4 of Module 1). For example, the
dendrocoelids Dendrocoelum lacteum (Müller) and Bdellocephala punctata
(Pallas) go 'frilly' on the front end and 'crinkly' on the lateral edges,
respectively, when pickled, unlike planariids (see Figs. 1 & 2).
Fig. 1: Preserved Dendrocoelum
lacteum (Müller) - frilly.
Fig. 2: Preserved Dugesia lugubris
(Schmidt), or Dugesia polychroa
(Schmidt) - smooth.
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SECTION 1: CONTINUED
As you can see, Dendrocoelum, apart from being unpigmented and
whitish - a characteristic shared with Phagocata vitta (Dugés) - has a 'nose'
when preserved. The corners of the anterior edge fold-in towards the
middle, forming a cleft, and the section between the eyes pushes upwards.
In planariids, the anterior edge remains smoothly rounded in preserved
specimens. So, having identified live specimens in Module 2, it's now time
to try identifying preserved ones.
Preserved or otherwise, some of the species-groups of triclads are
impossible to separate reliably without dissection and examination of the
penial papilla. Pigmentation in Polycelis nigra (Müller) and P. tenuis (Ijima)
cannot be relied upon for diagnosis. In the closely related pair of Dugesia
lugubris (Schmidt) and D. polychroa (Schmidt), pigmentation and shape are
sufficiently different to separate the species when alive, but preserved
specimens lose this distinction and, again, dissection becomes necessary.
The process was described on page 31 of Module 1 and on page 30 and 31
of Reynoldson and Young (2000). In the latter, the authors use pressure
applied to a cover slip to thin-out the obstructing gunge so as to be able to
view the penis, which works well. You may prefer to tease it away with a
mounted needle. Either way, it's worth practising with a number of
specimens and see how you get on, which takes us to the next Exercise.
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EXERCISE 1: IDENTIFY A PRESERVED TRICLAD
This mandatory exercise is worth 5 points.
Find a preserved triclad of any family (Planariidae, Dugesiidae or
Denrocoelidae). In the space below, either attach a photograph (or more
than one, if necessary) or make a sketch of your specimen. The sketch
needn't be of the whole animal, just the diagnostic features, if you wish.
Write down which species you think it is (or species group in the case of
P.nigra/tenuis and D.lugubris/polychroa). Also write down why you believe it
to be this species (your 'diagnosis'), with arrows pointing to the relevant bits
if you feel it necessary to do so.
Complete exercise 1 in the space below or use the template appended to
the introductory leaflet.
Mark =
/ 10
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EXERCISE 2: DISSECT A TRICLAD
This mandatory exercise is worth 10 points.
Obtain a pile1 of flatworms of the Polycelis nigra/P.tenuis species-pair
(alternatively you can use Dugesia lugubris/D.polychroa, but the diagnostic
differences in the penis are more subtle in the latter). You'll find the process a
little easier if they are alive - the un-fixed body tissues come away more
easily - but use preserved specimens if you're squeamish (the upside is that
they keep still and the bits you're looking for are firmer).
Dissect a specimen to reveal the penis and, in the space on the next page,
attach a photograph or make a sketch of the appropriate member. With
reference to pages 46 and 47 (or 40 and 41) of Reynoldson and Young
(2000), write down which species you think it is.
1
You'll find that you start-off pretty ham-fistedly - this is the reason that it's worth
having a number of specimens on which to practice your skills. Having pounded the
table in frustration, persevere, wind your neck back in, and it will all become more
straightforward as you go on.
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EXERCISE 2: CONTINUED
Complete exercise 2 in the space below or use the template appended to
the introductory leaflet.
Mark =
/ 10
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SECTION 1: CONTINUED
You may derive some benefit from identifying flatworm cocoons (the
products of sexual reproduction), should you come across them. You can
find them attached to the underside of stones, on floating plant matter, or in
the leaf-axils of submergent vegetation. They start off as tiny orange or
yellow blobs, usually around 1 to 2 mm in size (but can be as large as 4
mm), which rapidly darken to deep brown. Although broadly similar, subtle
differences in shape and attachment can allow some degree of diagnosis.
The three species of Dugesia all produce spherical cocoons with a stalk
attaching them to the substratum, like this:
Fig. 3: Cocoon of Dugesia sp.
The alien species Dugesia tigrina (Girard) does not, however, reproduce
sexually anywhere other than America, so any cocoons that you find like
this in Britain and Ireland will be D.lugubris (Schmidt) or D.polychroa
(Schmidt).
The cocoons of Dendrocoelidae - Dendrocoelum lacteum (Müller) and
Bdellocephala punctata (Pallas) - are also spherical, but have no stalk and
are only just attached to the substratum. They also tend to be a bit larger
(up to 4 mm for Bdellocephala).
Only the planariid species produce oval cocoons, particularly noticeable in
Polycelis nigra/tenuis, which attach them strongly to the substratum, as
does Planaria torva (Müller). Most populations of the lotic species
(Phagocata vitta (Dugés), Crenobia alpina (Dana) and Polycelis felina
(Dalyell)) reproduce asexually, so you won't tend to find cocoons in small
streams with an obvious flow.
• Round, stalked cocoons = Dugesia spp.
• Larger, round, unstalked cocoons = Dendrocoelidae
• Unstalked, well stuck-down oval cocoons = Planariidae
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SECTION 1: CONTINUED
For a full account, see Reynoldson and Young (2000) page 50 & 51.
Finally, it would be worth mentioning the invasive alien species of flatworms
currently resident in the UK which are not aquatic. This is because they are
regarded as a threat to important British invertebrates, such as Earthworms,
so are worth looking-out for on the underside of stones in damp places
(though terrestrial, they can't stand desiccation).
The New Zealand flatworm Artioposthia triangulata has a dark, purplishbrown dorsal side, with a pale buff edge. The ventral side is also pale buff.
They are pointed at both ends. A mature flatworm at rest is about 10 mm
wide and 60 mm long. When extended, it can be up to 300 mm long, and
proportionally narrower. Its cocoons are said to look like blackcurrants.
There is a useful picture of A.triangulata at:
http://www.introduced-species.co.uk/Species/worms/NZ%20flatworm.htm
The Australian flatworm Australoplana sanguinea var. alba (Dendy) is a
similar shape, but tends to be a bit smaller and is bright orange-red, see:
http://www.introduced-species.co.uk/Species/worms/Australian
%20flatworm.htm
Another recent immigrant is Phagocata woodworthi Hyman, a native from
North America which, thus far, has only been found in Loch Ness. It
probably arrived on equipment brought over by Monster hunters (I'm not
joking!). It has a multiple-branched pharynx (as seen through the upper
surface) which makes it a very efficient feeder. This, together with its ability
to breed sexually (unlike Crenobia and P.vitta, for example) makes it a
potential threat to native species.
• Watch for the invasive alien species, even if they inhabit only the
riparian zone and are not strictly aquatic. Their presence is
important.
So that's it for the flatworms. We'll now move on to another so called 'lower
order' of freshwater invertebrates, of the Phylum: Annelida (segmented
worms) - the Class: Hirudinea or leeches.
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SECTION 2: LEECHES (HIRUDINEA)
For this section, you'll need a copy of the FBA guide to leeches (Elliott
& Mann, 1979).
As with the flatworms, much of the British freshwater leech fauna (8 of the
16 known species) was covered in Module 2, using the following series of
features:
1.
Eyes (number and arrangement);
2.
Colour;
3.
Degree of 'squishiness' (relating to the relative numbers of
muscle fibres in the body of the beast);
4.
The presence of surface features, such as a dorsal scute in
Helobdella.
I don't propose to recap any further. The purpose of this Module is to cover
the eight species of leech which were not covered previously, by virtue of
being slightly more fiendish to put a name to (3 from F: Glossiphoniidae and
all of F: Erpobdellidae).
Starting with the remaining Glossiphoniidae, then:
The rare species Haementeria costata (Müller), which, like the equally rare
Medicinal Leech (Hirudo) is capable of sucking blood, has only two eyes - a
feature it shares with the very common Helobdella stagnalis (Linn.). Unlike
Helobdella, it doesn't have a dorsal scute, but does possess papillae on the
dorsal surface and has its mouth on the anterior rim of the front sucker. It's
also rather big (20 - 70 mm long) - Helobdella doesn't get above 10 mm.
So, if you see a big, 2-eyed, papillate leech, especially in the extreme south
of England, it is probably Haementeria. Be very careful not to confuse it
with Helobdella, though, and always get your specimen confirmed by
an expert (such as a County Recorder or national expert at the FBA).
This is a truly unusual species in the British Isles. Incidentally, the
species has recently been placed into Genus: Placobdella.
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SECTION 2: CONTINUED
There are four leeches with 3 pairs of eyes2. Those of G: Glossiphonia are
easy to identify and very common and are of the 'firm consistency'. The
other two are 'squishy' and rather less common, though records seem to
be increasing in number for both.
The first, Boreobdella verrucata (Müller) has similarities to Glossiphonia
complanata (Linn.) in possessing very obvious papillae. It has a very
different 'look', however, with irregular pigment spots down the sides of the
dorsal surface, especially at the level of the genital pores (see the rather
sad, curled-up & preserved specimens in Fig. 4, below). The absolute
clincher, however, is in the arrangement of the papillae: in Boreobdella,
each set of three segments begins with one segment which has no papillae,
the second has up to six (two in the midline and four either side) and the
third has a pair of papillae. In G.complanata, only the second segment in
every set of three has papillae on it - so it is only 33% papillate as opposed
to 66%.
B
A
Fig. 4: Preserved Boreobdella verrucata (Müller), showing papillae (A) and
pigmentation (B). The arrangement of papillae and intensity of pigment is even
clearer in live leeches.
2
Batracobdella paludosa (Carena) frequently has these fused into 4, 2 or 1 - see
later.
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SECTION 2: CONTINUED
Batracobdella paludosa (Carena) is a little trickier, in that, although
obviously squashy, you identify it by an absence of the diagnostic features
present in Boreobdella - always a slightly dubious course to take!
B. paludosa does possess papillae on every third segment, which interrupt
a pair of rather faint brown lines running down the middle of the back of the
leech, but they are rather small and insignificant (see Fig. 5). Essentially, it
looks like a rather pale Glossiphonia complanata, with a squishy
consistency and rather pathetic papillae. The eyes are often fused into four,
two, or even a single pigment spot (see Elliott and Mann, 1979, Figs. 25 to
29). It is, fundamentally, a rather dull and featureless species with squiffy
eyes!
A
B
Fig. 5: A preserved specimen of Batracobdella paludosa (Carena). The brown
pigment line (A) has faded to near invisibility in preservation, but is pretty faint and
sometimes absent in life anyway. One of the papillae is marked B.
Incidentally, both Batracobdella paludosa (Carena) and Boreobdella
verrucata (Müller) have recently been placed into Genus: Glossiphonia.
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SECTION 2: CONTINUED
Finally, we come to the task of separating species of F: Erpobdellidae. In
all erpodellids, it is wise to confirm the species using the degree of
separation of the genital pores (like all annelids, leeches are
hermaphrodites), of which more in a moment.
Colour on the upper surface will, however, often give you a good startingpoint, as Erpobdella testacea (Savigny), Dina lineata (Müller) and Trocheta
subviridis Dutrochet are frequently uniformly reddish brown or grey-green,
especially in immature specimens (the latter two may have two to four faint
lines running down them when more mature). The very common Erpobdella
octoculata (Linn.) is nearly always flecked with black and yellow pigment,
however, and the rather rare Trocheta bykowskii Gedroyć has dirty white
papillae on a black background.
The segmentation (or 'annulation') is also diagnostic, both species in
Genus: Erpobdella having a uniform segmentation, whereas Dina has every
fifth segment broader than the rest with a shallow furrow across it, and both
Trocheta have a pattern of three broad segments followed by five narrow
ones. Make sure that you follow the recommendations on gentle snuffage
of leeches outlined on page 30 of Module 1, using gradual alcoholic
narcosis. You won't have a whelk's chance in a supernova of sorting-out
the pattern of segmentation if your specimen is covered in slime, which will
tend to happen if you take a live leech and boil it or frazzle it in neat alcohol.
In a preserved sample, the slime will gradually slough-off, so identification
is facilitated after a time.
So, to separate the erpobdellids:
• Look at the colour on the dorsal surface;
• Look at the pattern of segmentation (or 'annulation');
• Confirm the identification by looking at the separation of the genital
pores.
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SECTION 2: CONTINUED
What's the best way of looking at an erpobdellid's rude bits? It is possible
to do this with a live leech, by letting it flatten-out on a clean (preferably
glass) petri dish, then looking through the glass at the underside of the
leech, but you may have difficulties with the specimen moving about and/or
swathing the view in slime. A less frustrating option is to take a dead
specimen and pin it out, ventral surface uppermost, with a pin through
each sucker, as shown in Fig 6.
Fig. 6: Erpobdella octoculata (Linn.) pinned to a corkboard through the anterior and
posterior suckers - the anterior end is to the right of the picture. The arrow indicates
the position of the genital pores.
The pins are quite large - you may wish to use finer entomological pins, which will
destroy rather less of your specimen.
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SECTION 2: CONTINUED
Next, examine your leech (in the area shown by the arrow in Fig. 6) under a
microscope. You should get a view like those in Figs. 7 and 8, below:
♂
♀
Fig. 7: Erpobdella octoculata (Linn.) male and female genital pores. If you count the
segments between them, the pores are separated by 3.5, which is diagnostic of
Erpobdella spp.
♂
♀
Fig. 8: Trocheta subviridis Dutrochet male and female genital pores. In this case, the
pores are separated by 6 segments, which is diagnostic of this species alone.
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SECTION 2: CONTINUED
The male pore, towards the front end, is always big and easy to see. The
female pore, on the other hand, is often little more than a dent in the
'groove' between two segments, especially in specimens preserved for a
while. You may have to search about with a pair of fine forceps or a
mounted needle to find it, but be very careful not to punch a hole of your
own while you're at it - you may misidentify your beast!
As you may have surmised, examining the genitals in erpobdellid leeches is
not always essential - it is perfectly possible to identify them by colour
alone or with a combination of colour and patterns of annulation.
Separation of the pores is extremely useful, however, when doubt creepsin, especially if you think you have something unusual.
A final, minor point. In the more unusual species (i.e. those other than
Erpobdella octoculata), the eyes are frequently rather small, although the
pattern and position of them is the same in all erpobdellids. Along with the
colour, this is a useful 'alarm bell' which rings to tell you that you might
have something out of the ordinary.
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EXERCISE 3: IDENTIFY AN ERPOBDELLID LEECH
This mandatory exercise is worth 10 points.
Find an erpobdellid. In the space below, attach pictures (preferably
photographs, but sketches of diagnostic features will do) to show (1) the
whole dorsal surface and (2) the ventral surface in the area of the genital
pores. Identify the species and write down why you think it is what you
believe it to be, discussing colour, annulation, pore-separation and
anything else you think is pertinent.
Complete exercise 3 in the space below or use the template appended
to the introductory leaflet.
Mark =
/ 10
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SECTION 2: CONTINUED
Hooray - you've finished! Your final mark is written in the box below. Now,
why don't you try out the next Module (4) & get to grips with
Ephemeroptera and Plecoptera?
/ 30 =
%: Pass
Fail
Module 3: Version1, July 2007 (Richard Chadd)