A few examples are given of variability and taxonomic difficulties within the Desmoscolecida as well at species level as at genus level. The diagnostic characters generally used within two genera: Desmoscolex Claparède, 1863 and Tricoma Cobb, 1894 and the status of the genus Quadricoma Filipjev, 1922 are discussed

Decraemer, W.

Open Marine Archive

TAXO NO M IC PROBLEMS 
W ITH IN  THE DESMOSCOLECIDA (NEM ATODA)
by
W i l f b i d a  DECRAEMER  
Instituut voor Dierkunde, Rijksuniversiteit Gent,
Ledeganckstraat 35, B 9000 Gent, Belgium
A B S T R A C T
A  few examples are given of variability and taxonomic difficulties within the Des­
moscolecida as well at species level as at genus level. The diagnostic characters generally 
used within two genera : Desmoscolex C l a f a r è d e , 1863 and Tricoma C o b b , 1894 and 
the status of the genus Quadricoma F i l i p j e v , 1922 are discussed.
IN T R O D U C T IO N
The taxonomical problems in the Desmoscolecida on species level vary according 
to the genus. For several genera the value of the characters generally used within 
the order e.g. the number of main rings, the setal pattern, differs. Since little is 
known about variability it is difficult to judge the diagnostic value of many cha­
racters used. This paper deals with a few examples of variability and taxonomic 
difficulties : (1) at species level within two genera : Desmoscolex C l a r a r è d e , 1863 
and Tricoma C o b b , 1894 and (2) at genus level with the acceptance or not of the 
genus Quadricoma F i l i p j e v , 1922.
I. s o m e  t a x o n o m i c  d i f f i c u l t i e s  a t  s p e c i e s  l e v e l
1. Genus Desmoscolex C l a p a r è d e , 1863
A first important character is the number of main rings. Within the genus 
Desmoscolex two types of main rings can be found : (1) the desmoscolecoid type 
(S t e i n e r , 1916, p. 324 ; D e c r a e m e r , 1976, p. 125) and (2) the tricomoid type 
(S t e i n e r , 1916, p. 324 ; D e c r a e m e r , 1977b, p. 6) (Fig. 1A, IB).
Considering the number of main rings in all species of Desmoscolex known, it 
was found that the majority of species (i.e. 52 out of 69) possesses 17 main rings 
of the typical desmoscolecoid type. Consequently a first major distinction is made 
between these and the other species possessing a larger number of main rings. 
Within the latter group the number of main rings is still used for distinguishing 
further between 8 species having 18 main rings of desmoscolecoid type (in two 
species one animal formed an exception w'ith 19 rings) and the remaining species 
with 22-44 main rings. Among the latter group the species with the largest number 
of main rings, i.e. from about 29 rings on have a tricomoid type of main ring ; thus 
in the group containing the species with the largest number of rings the type of 
main ring is used as an additional diagnostic character.
30 )J m 
!------1------1----- 1
30 p m
50 p m
D
30 pm  
H I---------'---------1---------1
Only two examples of sexual dimorphism in the number of main rings are 
known : D. aquaedulcis S t a m m e r , 1935 with 18 rings in males, 22 rings in females 
and D. velifer T i m m , 1970 with 43-44 rings in males, 31-38 rings in females.
Some variability in the number of main rings occurs in the species with a 
number of rings higher than 25 (e.g. in D. vinealis W e i s c h e r , 1962 : 11$$ with 
25-29 rings). Among the 17-ring species no variability in the number of rings was 
ever noticed except for D. geraerti D e c r a e m e r , 1974 were a male specimen was 
found with only 16 rings ; it was however considered as aberrant and having lost 
a ring between its 8th and 9th main ring as could be deduced from the setal pattern.
A second important diagnostic character in Desmoscolex is the setal pattern. 
Departures from the typical scheme for 17-ring species ( T i m m , 1970) are charac­
teristic for a given species. In a few species e.g. D. dimorphus D e c r a e m e r , 1975,
D. sieverti F r e u d e n h a m m e r , 1975 sexual dimorphism occurs in the setal pattern. 
As far as I know, apart from sexual dimorphism, only one example D. falcatus 
L o r e n z e n , 1969 was found showing an intraspecific variability in the setal pattern 
among specimens from different localities (cf. D e c r a e m e r , 1975, p. 248).
Third, the head shape, the shape and length of the amphids (cf. Fig. 1C-1G) 
are usually of taxonomic value e.g. the possession of a rostrum (Fig. 1C) and the 
elongation of the amphids as in D. asetosus D e c r a e m e r , 1975. However several 
species may posses a similar head structure as e.g. in D. geraerti, D. nymphianus 
D e c r a e m e r , 1974, D. brevisetosus D e c r a e m e r , 1974.
Fourth, the presence or absence of cephalic setae, their length, shape (with 
or without a membrane) and the position of the cephalic setae on the head are of 
great diagnostic value (Fig. 1C-1H).
In males the copulatory apparatus, mainly the length and shape of the spicules 
may be of diagnostic value ; the gubeinaculum can rarely be used since it often is 
lacking or if present it consists of a thin trough-like structure parallel to the spicules 
in about all the species known so far.
Additional characters such as the shape and length of the end rings and spin­
neret, the structure of the cuticidar rings (with or without spines or pores) are 
often given some diagnostic value.
Fig. 1. —  Genus Desmoscolex
A : D. asetosus (<J) : detail of dorsal body wall at the level of main rings 10 and 11 : 
desmoseolecoid type (cf D e c r a e m e r , 1976)
B : D. labiosus ($ : nr. 4454 Landbouwhogeschool, Wageningen) : surface view of part 
of body wall at the level of main rings 9-12 : tricomoid type (cf D e c r a e m e r , 1976)
C : D. asetosus : surface view of head, lateral side (<J) (cf D e c r a e m e r , 1975)
D  : D. leptus : surface view of head, lateral side ($) (cf D e c r a e m e r , 1975)
E : D. longisetosus : surface view of head, lateral side (holotype <J) (cf D e c r a e m e r , 
1975)
F : D. spinosus : surface view of head, lateral side ($) (cf D e c r a e m e r , 1976)
G : D. falcatus : surface view of head, lateral side (holotype <J) (cf D e c r a e m e r , 1974) 
H : D. velifer : surface view of head, dorsal side ((J) (cf T i m m , 1970, fig. 41).
Considering the diagnostic value of the number of main rings we can conclude 
that when that number is low, it is constant and when it is higher, then its diag­
nostic value is more limited and a certain range of variability should be taken into 
account. As appears from the data mentioned above, most characters used to 
characterize species and to distinguish them from one another are surface ones, 
which give only a partial picture of the structure and may be more subjected to 
variation than internal features. Detailed morphological studies of the anatomy of 
many species of Desmoscolex ( D e c r a e m e r , 1976) showed a large similarity between 
them. Consequently at the present time the internal organs can rarely be of any 
use for distinguishing species. In future further detailed studies of the female repro­
ductive system and of the anterior sensorial organs would presumably be of greater 
help in solving taxonomic problems within this group.
2. Genus Tricoma C obb , 1894
In this genus the taxonomic problems are even larger than in other genera 
of the Desmoscolecida. This is due to (1) the large number of species, (2) the many 
inaccurate and superficial descriptions and figures lacking data on internal organs, 
anterior sensorial structures and labial region, (3) descriptions based on a single 
specimen even when orientated in a dorso-ventral position, (4) the emphasis put 
on the number of main rings without taking into account the occurrence of a possible 
variation. Two species, T. dimorpha D e c r a e m e r , 1977 and T. similis C o b b , 1912, 
illustrate the large variabilities that can occur in characters used as diagnostic 
features (such as the number of main rings, body length and genital setae). Both 
species are easily distinguished from all other species of the genus.
a) Tricoma dimorpha (Figs. 10, 11 in D e c r a e m e r , 1977b)
In a first study of the type material of T. dimorpha from Yonge Reef (Australia) 
a tentative distinction was made between two different forms based on a difference 
in the total number of main body rings and in the number of tail rings : (1) a 
« typical » form with 61-65 main rings and 12-13 tail rings in males ; 62-67 main 
rings and 12-13 tail rings in females and (2) an « aberrant » form with only male 
specimens having 52-54 main rings and 10-11 tail rings.
No differences were found between both forms in habitus, setal pattern, copu- 
latory apparatus, internal organs and detailed morphology of the head region.
Later on, other specimens of T. dimorpha became available from Gannet Cay 
(Australia). Among them two male specimens were found possessing a number of 
main rings lying in between the range recorded for the « typical » and the « aberrant » 
form i.e. with 58 and 60 rings. The number of tail rings was respectively 11 and 12.
Taking into account the specimens from Gannet Cay, T. dimorpha can no 
longer be split up on the basis of the number of main rings since the larger the 
number of specimens available the greater the chance to cover completely the 
difference in number of main rings observed in the type material.
Apart from a variability in the number of main rings in T. dimorpha there 
was also a relative large variation in body length. One male and one female paratype 
were clearly longer than the other specimens : 600 ii.ni (<J) against 305-370 jim 
(other males) ; 650 [xm (?) against 335-410 fxm (other females).
b )  Tricoma similis ( c f .  Figs. in C o b b , 1912 ; Figs. 30-35 in D e c r a e m e r , 1977b)
In T. similis the following variations were found in diagnostic features : (1) the 
number of main rings varies between 80 and 85 in males and between 77 and 84 in 
females, (2) in males the number and position of the genital setae (mostly 2 setae 
situated on rings 55 and 57) varies between 1 and 3 setae situated between rings 52 
and 62 and (3) in females the position of the vulva varies from ring 43-46 or inter- 
zones 43-44, 44-45.
Apart from the number of main rings, following characters are used as diag­
nostic features : the setal pattern, the head- and amphid-shape, the length and 
shape of spicules and gubernaculum. More recently ( D e c r a e m e r , 1977b) an attempt 
was made to study the internal organs, the labial region and anterior sensorial 
organs more in detail so that they could also be used in solving taxonomic problems.
Considering the setal pattern, it was found that the number of subdorsal setae 
is mostly constant within the same species and differs only slightly among species 
with 9, 11 and 13 pairs as most frequent numbers. The subventral setae vary in 
number within the species and among different species.
Conclusion
From the large intraspecific variation observed in the number of main rings 
in T. dimorpha we can conclude that this character generally accepted as the most 
or one of the most important diagnostic features in the genus Tricoma, is of more 
limited value. The number of main rings may be helpful for the identification of 
species, but should not be overweighted especially since for the majority of the 
species nothing is known about its variability.
The diagnostic value of the setal pattern in the genus Tricoma is more limited 
compared with that in Desmoscolex (see above).
I I .  T A X O N O M IC  D IF F IC U L T IE S  A T  T H E  G E N U S  L E V E L
Some taxonomic problems of the Desmoscolecida exist at the generic level 
e.g. the status of the genus Quadricoma F i l i p j e v , 1922.
The original diagnosis of F i l i p j e v  (1922) is of little use, but on the basis of 
characters of the type species Quadricoma loricata F i l i p j e v , 1922 (cf. also D e c r a e ­
m e r , 1978) the genus can be clearly distinguished from other genera.
In contradiction with L o r e n z e n  (1969) who synonymized without comment 
Quadricoma with Tricoma while introducing a new subfamily Tricominae, I am 
inclined to follow T i m m  (1970) and F r e u d e n h a m m e r  (1975) who considered Qua­
dricoma as a separate genus.
However, the differences between both genera : Quadricoma and Tricoma are 
relatively small. The main difference is found in the shape of the main rings i.e. all 
main rings quadricomoid and with a clear ring of inversion in Quadricoma (see 
Timm , 1970, p. 38 ; D ecraem er , 1978) while in Tricoma the main rings are mainly 
tricomoid except for those in the extreme anterior body region and tail which may 
be quadricomoid in some species ; an inversion rings is always lacking (Fig. 2A, 
2B, 2D).
Among the type material of Q. noffsingerae D e c r a e m e r , 1977 a male and a 
female specimen were found possessing (as opposed to the other specimens) only 
quadricomoid rings in the anterior part of the body and in the tail region ; the rings
A :
B :
C : 
D : 
E :
F :
f ----------------------- 1------------------------ 1-----------------------1 I---------- .---------- 1-------- h
D F
Fig. 2. —  Genera Quadricoma and Tricoma.
Q. crassicomoides : detail of dorsal body wall a level of main rings 29 and 3 3  (cf 
D e c r a e m e r , 1978)
Q. crassicomoides : surface view of part of body wall at level of main rings 29 and 3 3  
(of D e c r a e m e r , 1978)
Q. noffsingerae (?) : surface view of tail region (cf D e c r a e m e r , 1 9 7 7 c )
T. similis ($) : detail of dorsal body wall (cf D e c r a e m e r , 1977b)
T. dimorpha : surface view of the ventral side of the posterior body region of a 
female (cf D e c r a e m e r , 1977b)
T. dimorpha : posterior body region of a male (cf D e c r a e m e r , 1977b).
situated in between are tricomoid, also due to the absence of foreign material on 
the interzones. Consequently no ring of inversion was observed in both specimens. 
If only a specimen was found showing the aberrance in the structure of the main 
rings the definition of the genus based only on the shape on the rings would be 
uncertain.
However, apart from the structure of the main rings additional characters may 
be used in distinguishing both genera i.e. (1) the shape of the endring and spinneret. 
In Quadricoma the spinneret consists of a fine, elongated, naked structure, offset 
from the broader covered anterior part of the end ring ; in Tricoma the naked spin­
neret is short, not offset from the covered anterior part of the end ring which gra­
dually tapers posteriorly. (2) The shape of the head : the conspicuous triangular 
head shape in lateral view typical for Tricoma, is not present in Quadricoma.
Conclusion
Since the genus Quadricoma can be clearly distinguished from Tricoma mainly 
on the base of the structure of the main rings and since until now' as far as I know 
only one exception (Q. noffsingerae) exists I tentatively recognize Quadricoma as 
a separate genus for practical reasons and until more detailed studies of both genera 
will allow well founded conclusions.
A C K N O W L E D G E M E N T
I am very grateful to Professor Dr. A. Coomans for critical reading of the 
manuscript. I wish to thank Professor Dr. L. De Coninck and Dr. Geraert for the 
fruitful discussions.
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Taxonomic problems within the Desmoscolecida (Nematoda)

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