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Tharyx illustration and names

Petersen, Mary Elizabeth MEPetersen at zmuc.ku.dk
Mon Mar 9 16:06:00 EST 1998


Monday, 9 March 1998

Dear Tom [and others?],

I hate to spoil a popular misconception, but it is unrealistic to believe
that cirratulids - and members of most other families - can be identified
by counting segments.  Yes, there are some where "the numbers game" works,
e.g., most Ampharetidae (but even here there seem to be some that flunked
math and have more than the decreed number of segments, and I do not
consider them new species because of this), and lots more families where
it does not, and to persist in attributing to these groups characters they
do not possess does not make any of us any wiser.

In cirratulids, the size of an expanded "thoracic" region usually varies
with the size of the worm and perhaps also with state of sexual maturity -
immature, approaching sexual maturity, fully mature, etc.  Sometimes the
variation is small, sometimes larger.  Take any species you like and look
at enough specimens and you will see that smaller worms have one
appearance, larger ones usually a quite different one.  Whether more than
one species is involved should preferably not be decided from figures
alone, as even if these are extremely accurate, there may be important
information that is not shown.

If I contributed to the misconception that some cirratulids can count with
100% accuracy by my comment that the abrupt change figured by Hartman 1960
was real, I apologize.  There seem to be 1-3 segments that are
transitional, but by comparison with many species where the transition is
so gradual that it is hard to decide where it starts and stops, here it
must be considered abrupt.

Regarding heads and tails, I have yet to see anyone mistake a cirratulid
tail for an anterior end; apart from what should be obvious differences
between a prostomium + peristomium and a pygidium, there are rarely any
branchiae and never any tentacles on the posterior end, and (on the
anterior end) even when these are lost, the scars or tentaculophores
persist so that the site from which they arise is visible.  In
Aphelochaeta, there are also marked differences in chaetal length in these
two regions, with chaetae of anterior segments long, those of posterior
ones very short.  I know of no cirratulid posterior ends that could
successfully imitate an anterior end.

One other factor should be considered with Hartman's descriptions.  She
often described a huge number of worms from many different areas in a
relatively short time.  One can discuss whether it would have been
preferable to have worked up less material and done it in more detail, but
it seems like her approach to survey material, despite errors, has
nonetheless probably been the most useful one in that we have more quickly
become aware of the immense diversity present in the many areas in which
she worked. It is simply not possible to get through so much material in
so short a time without shortcuts, and the presence of several clearly
distinct species in some of the larger samples of a species suggests that
she did not [have time to] examine every specimen, but perhaps
spot-checked samples sorted to species by nonspecialists. As long as one
is aware of this, it's possible to keep certain reservations in mind when
reviewing her material, but to start dissecting descriptions without
having seen the material is not very useful.  Anyone who has seen the
material knows there are errors, but also that there is a lot more that is
correct.

Best regards,

Mary
 --------
Mary E. Petersen
Zoological Museum, University of Copenhagen
mepetersen at zmuc.ku.dk


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