HELGOLANDER MEERESUNTERSUCHUNGEN
Helgol~inder Meeresunters. 36, 231-241 (1983)
M o r p h o l o g y and fine structure of two marine tubificids
(Oligochaeta), closely related to the gutless
P h a l l o d r i l u s spp.
O. G i e r e
Zoologisches Institut und Zoologisches Museum, Universit8t Hamburg;
Martin-Luther-King-Platz 3, D-2000 Hamburg 13, Federal Republic of Germany
ABSTRACT: The morphology a n d ultrastructure of the two interstitial m a r i n e tubificids Phallodrilus prostatus a n d Aktedrilus monospermathecus (Oligochaeta), closely related to the gutless
PhaHodrilus spp., is described. Since p e r t i n e n t histological descriptions of 'regular' m a r i n e
tubificids were found to b e non-existent, this study provides a structural basis allowing comparisons
both with limnetic species a n d the recently described m a r i n e anenteric forms. C o m p a r e d to the few
freshwater tubificids investigated histologically, major differences are the substitution of the dorsal
blood vessel b y a large sinus, the close contact b e t w e e n n e p h r i d i a l tissue a n d the subintestinal
vessel, a characteristic 'wavy' a r r a n g e m e n t of cuticular collagen fibres, a n d the formation of
b r a n c h e d epicuticular projections. The structural divergences from the gutless relatives refer
mainly to the body wall, the vascular system a n d its endothelial structure. The various peculiarities
outlined here indicate that generalizations of the situation as found in the few histological studies
a r e as yet problematical.
INTRODUCTION
I n a s p e c i a l r e s e a r c h p r o j e c t , t h e g u t l e s s o l i g o c h a e t e Phallodrilus leukodermatus
f r o m B e r m u d a is t h e s u b j e c t of t h o r o u g h e x a m i n a t i o n ( G i e r e , 1979, 1981; G i e r e e t al.,
1982, 1983). T h i s s p e c i e s h a s , i n c o n t r a s t to t h e a n e n t e r i c P o g o n o p h o r a , c l o s e r e l a t i v e s
which possess a regular digestive and excretory system. However, direct comparisons
h a v e b e e n i m p o s s i b l e so f a r d u e to t h e l a c k of i n f o r m a t i o n o n t h e s t r u c t u r e of t h e s e
'normal' forms. Both in classical monographs on oligochaete morphology (Stephenson,
1930) a n d r e c e n t c o m p i l a t i o n s of t h e i r u l t r a s t r u c t u r e ( J a m i e s o n , 1981), m a r i n e t u b i f i c i d s
h a v e b e e n d i s r e g a r d e d . H e n c e , t h e first a i m of t h i s s t u d y w a s to e s t a b l i s h a b a s i s for
comparison between 'regular' and extremely modified forms by examining two common
s p e c i e s , t a x o n o m i c a l l y a n d e c o l o g i c a l l y c l o s e to t h e g u t l e s s w o r m s , t h e i n t e r s t i t i a l
c o n g e n e r Phallodrilus prostatus a n d a r e p r e s e n t a t i v e f r o m a r e l a t e d g e n u s , Aktedrilus
monosperma£hecus.
However, since morphological and histological knowledge w a s found to be based,
also in freshwater tubificids, on only very few species (mainly Tubifex tubifex and
l_Jmnodrilus spp., see 'Discussion'), it w a s felt that this w o r k should not restrict itselfto
selected ultrastructural details, but should cover the gross-morphological organization
© Biologische Anstalt H e l g o l a n d
232
O. Giere
b y use of light- a n d low p o w e r - e l e c t r o n microscope, a n d scrutinize some more r e l e v a n t
organs (vascular system, n e r v e cord, e p i d e r m i s / c u t i c l e complex) at the s u b c e l l u l a r level.
MATERIAL AND METHODS
Phallodrilusprostatus (Kn611ner, 1935) was collected i n October 1982 at the b e a c h i n
Strande (Kiel-Fjord, Baltic Sea; salinity ca 15 %o). T h e worms were d e c a n t e d from
m e d i u m to coarse s a n d slightly above the water l i n e i n ca 20 cm d e p t h just above
g r o u n d w a t e r level. Aktedrilus monospermathecus (KnSllner, 1935) was collected i n
October 1982 from the s a n d y u p p e r shore at Keitum (Isle of Sylt, North Sea) 2 m from
h i g h w a t e r l i n e at a d e p t h of ca 30-40 cm (interstitial salinity 22 %0, m e d i u m sand).
O n e day after s a m p l i n g , a d u l t worms without d e v e l o p e d g e n i t a l organs were fixed
a n d p r e p a r e d for microscopy: fixation for 2 h i n 5 % g l u t a r a l d e h y d e buffered i n 0.1 m
N a - C a c o d y l a t e with 7 % sucrose, CaC12 a n d MgC12 a d d e d (according to Pearse, 1972);
2 % OsO a for 1 h, acetone, Spurr's resin (all procedures at 4 °C). Ultrathin sections
(Reichert O M U2) were s t a i n e d with u r a n y l a c e t a t e (5 rain) a n d leadcitrate (1 min) for
electron microscopy (Zeiss E M 9S-2); 1 ~m-sections were s t a i n e d with t o l u i d i n e - b l u e /
p y r o n i n e for light microscopy (Zeiss-Axiomat). This study is c o n f i n e d to cross sections
t h r o u g h the postclitellar, c e n t r a l parts of the worm's body. It omits c o m p l i c a t i n g structures such as setae with their special m u s c u l a ~ . r e a n d the g e n i t a l organs.
RESULTS
Phallodrilus prostatus
Light-microscopic survey
The d o m i n a t i n g structure is the v o l u m i n o u s i n t e s t i n e with a well d e v e l o p e d l u m e n
with l o n g cilia e x t e n d i n g into it from the i n n e r surface of the wall cells (Fig. la). The
enteric wall is s u r r o u n d e d b y a large sinus complex of irregular width. E v e n the large
'dorsal blood vessel' lacks a n e n d o t h e l i a l l i n i n g , i.e. it has to b e r e g a r d e d as a w i d e sinus
(Figs la, ld). The v e n t r a l b l o o d vessel is relatively small and, if not filled with blood,
sometimes hardly d i s c e r n i b l e u n d e r the light microscope. T h r o u g h peristaltic movem e n t s of the body it is often f o u n d dislocated from its m e d i a n s u b i n t e s t i n a l location
above the v e n t r a l n e r v e cord into a position lateral to the n e u r a l tissue. A s u b n e u r a l
l o n g i t u d i n a l vessel is absent. A p a i r e d a r r a n g e m e n t of n e r v e trunks i n the v e n t r a l cord
could not b e detected histologically.
The chloragogenic tissue forms a thick layer on the dorsal a n d latero-dorsal parts of
the gut e n c l o s i n g the large b l o o d plexus, w h i l e it covers the gut i n a t h i n layer a l o n g the
lateral a n d ventral parts of the i n t e s t i n a l tube. The coelomic cavity is w i d e i n the dorsolateral parts of the body; it n a r r o w s latero-ventrally w h e r e its l u m e n is filled w i t h large
n e p h r i d i a , the n e r v e cord a n d often the o b l i q u e setal m u s c u l a t u r e . T h e c o n s i d e r a b l e size
of each excretory o r g a n is i n d i c a t e d b y the fact that n e p h r i d i a l tissue, mostly d i v i d e d in
several l o b e d portions is to b e e n c o u n t e r e d i n almost every section (Fig. lb). The outer
m e s o d e r m a l wall of the coelome, a thin, b u t clearly d i s c e r n i b l e p e r i t o n e a l l i n i n g ,
M o r p h o l o g y a n d fine structure of m a r i n e tubificids
233
b e l o n g s f u n c t i o n a l l y to the r e l a t i v e l y t e n u o u s p a r i e t a l wall. Here, w e l l d e v e l o p e d
l o n g i t u d i n a l m u s c l e s a r e e n c l o s e d b y a frail r i n g of circular m u s c u l a t u r e . T h e e p i d e r m i s
w i t h c o n s p i c u o u s n u c l e i secretes a c l e a r l y v i s i b l e cuticle.
Transmission-electron-microscopic
observations
T h e active m e t a b o l i c function of t h e i n t e s t i n a l cells b e c o m e s e v i d e n t in the l i g h t of
t h e i r v a s t o r g a n e l l e content (Pig. lc): t h e c o m p a c t cytoplasm, h i g h l y e n r i c h e d w i t h
g l y c o g e n rosettes, is c r a m m e d w i t h m i t o c h o n d r i a of r a t h e r i r r e g u l a r cristae structure.
T h e d e n s e e n d o p l a s m a f i c r e t i c u l u m is l i n e d w i t h ribosomes. G o l g i b o d i e s a n d l a r g e
n u c l e i a r e r e g u l a r l y v i s i b l e w h i l e t h e r e a r e o n l y few v e s i c u l a r inclusions. T h e cells
e x t e n d into t h e i n n e r i n t e s t i n a l l u m e n t h r o u g h a d e n s e l a y e r of c i l i a (9 + 2 pattern); t h e
o u t e r cell surface is in d o s e contact w i t h t h e c i r c u m i n t e s t i n a l b l o o d plexus. T h e
i n c o n s p i c u o u s i n t e s t i n a l m u s c u l a t u r e consists of s m a l l portions of l o n g i t u d i n a l fibres a n d
a n a r r o w i n n e r r i n g of circular fibres, a n d s e p a r a t e s the i n t e s t i n a l cells a n d b l o o d sinus
from t h e m e s o d e r m a l chloragocytes.
W i t h o u t g o i n g into the d e t a i l s of d e v e l o p m e n t a n d n o m e n c l a t u r e of c h l o r a g o g e n i c
tissue (see J a m i e s o n , 1981), d e s c r i p t i o n will b e c o n f i n e d h e r e to one cell category, the
l a r g e a n d i r r e g u l a r l y s h a p e d c h l o r a g o c y t e s , c e n t r e s of o l i g o c h a e t e m e t a b o l i s m (Pig. ld).
T h e y c a r r y rich a m o u n t s of g l y c o g e n rosettes. T h e i r c y t o p l a s m a p p e a r s flimsy d u e to t h e
m a n y m e m b r a n e - b o u n d l i p i d (?) d r o p l e t s of v a r y i n g e l e c t r o n d e n s i t y f i l l e d w i t h a n
a m o r p h o u s mass. F u r t h e r c h a r a c t e r i s t i c o r g a n e l l e s are t h e n u m e r o u s e l e c t r o n d e n s e
' c h l o r a g o s o m e s ' and, e s p e c i a l l y in e r g a s t o p l a s m a t i c s t a g e s (Jamieson, 1981), t h e s t a c k e d
s y s t e m of p l a s m a t i c r e t i c u l a p a c k e d w i t h ribosomes, the h u g e n u c l e u s a n d the c o m m o n
m i t o c h o n d r i a w i t h t h e i r d e n s e m a t r i x a n d small, i r r e g u l a r cristae. Also the thin
p e r i t o n e a l cells at the outer e d g e of t h e c o e l o m i c c a v i t y c o n t a i n n u m e r o u s mitochondria.
W h i l e a d o r s a l contractile b l o o d v e s s e l is s u b s t i t u t e d b y a m e r e b l o o d sinus (Pigs l a ,
d), t h e w a l l of the s u b i n t e s t i n a l v e s s e l is s u p p h e d w i t h m u s c u l a t u r e c o n s i s t i n g of a b o u t
six b u n d l e s of i n n e r l o n g i t u d i n a l m y o f i b r i l s a n d v e r y fine o u t e r circular s t r a n d s (Pig. 2a).
In t h e f i n e - g r a n u l a r b l o o d p l a s m a n o h e m o c y t e s or b l o o d e l e o c y t e s (Jamieson, 1981)
have been encountered.
T h e v e n t r a l b l o o d v e s s e l is a l w a y s closely a s s o c i a t e d w i t h the n e p h r i d i a l tissue of
the v a s c u l a r w a l l s (Pig. 2b). In this distal, v e n t r o - l a l e r a l portion of the n e p h r i d i u m , the
tissue is c h a r a c t e r i z e d b y e x t e n s i v e f o l d i n g of c e l l u l a r l a m e l l a e (Pig. 2b). Cross sections
t h r o u g h t h e s e s l i m folds r e v e a l c i r c u l a r to oval structures. Since o n l y s m a l l p a r t s of the
m u c h t w i s t e d n e p h r i d i a l ' c a n a l h a v e b e e n cut, it a p p e a r s slitlike or t u b e l i k e . E s p e c i a l l y
in t h e p r o x i m a l p o r t i o n of t h e n e p h r i d i u m , t h e d e n s e p l a s m a s h o w s m i t o c h o n d r i a , m a n y
m i n u t e t u b u l a r structures, a n e n d o p l a s m a t i c r e t i c u l u m w i t h r i b o s o m e s a n d s o m e elong a t e nuclei.
T h e p e r i n e u r a l s h e a t h a r o u n d t h e v e n t r a l n e r v e cord (Pig. 2c) is o n l y d o r s a l l y w e l l
d e v e l o p e d . It consists m a i n l y of l o n g i t u d i n a l m u s c u l a t u r e , b u t l a c k s b l o o d vessels. T h e
t u b u l e s t r a v e r s i n g t h e m u s c l e fibres a r e d i r e c t e d at right a n g l e s t o w a r d s the n e r v e cord.
N e u r o g t i a l cells w i t h t h e i r c o n s p i c u o u s n u c l e i c a n b e e n c o u n t e r e d at the v e n t r a l e d g e of
t h e n e r v e cord, b u t n e v e r form a m a r k e d g l i a l ' l a y e r ' . T h e n e u r o p i l e w i t h its characteristic m e s h - l i k e tissue of n e u r o n a l p r o c e s s e s contains only few m i t o c h o n d r i a a n d is of a
v e r y uniform structure t h r o u g h t h e a b s e n c e of g i a n t fibres. In the p a r i e t a l m u s c u l a t u r e
(Pig. 2d), t h e p r e d o m i n a n t l a y e r is the i n n e r 'ribbon' of l o n g i t u d i n a l m y o f i b r e s (one fibre
234
O. Giere
deep) hanging
peripheral
densely
on a thin basement
sarcoplasma,
packed
with
showing
membrane
many
mitochondria.
and enclosed
glycogen
Small
in a fine sarcolemma.
rosettes, contains
triangular
fibres
are
the nucleus
irregularly
The
and is
spaced
F i g s 1-2: Phatlodrilus prostatus. Fig. l a : L i g h t m i c r o s c o p i c foto; F i g s l b - 2 f : T E M fotos; (la) D o r s o
- l a t e r a l h a l f - s e c t i o n . (lb) S e c t i o n t h r o u g h l a t e r a l p a r t s of b o d y . (lc) I n t e s t i n a l cell. (ld) C h l o r a g o c y t e s a n d d o r s a l b l o o d s i n u s . (2a) V e n t r a l b l o o d v e s s e l . (2b) F o l d e d l a m e l l a e i n d i s t a l n e p h r i d i a l
t i s s u e . (2c) V e n t r a l n e r v e cord w i t h c o v e r of m u s c u l a t u r e . (2d) B o d y w a l l . (2e) E p i d e r m i s cell. (2f)
C u t i c l e t r a v e r s e d b y microvillL A b b r e v i a t i o n s : b s = b l o o d s i n u s , by----blood v e s s e l , b w = b o d y
wall, ci = cilia of gut, c h = c h l o r a g o c y t e s , c m u ----c i r c u l a r p a r i e t a l m u s c l e s , c o e = c o e l o m i c cavity,
col = c o l l a g e n fibres, cs = c h l o r a g o s o m e s , c u = cuticle, dr ----d r o p l e t s , e p = e p i d e r m i s , e p i d e r m a l
cell, e p r = e p i c u t i c u l a r p r o j e c t i o n s , er = e n d o p l a s m a f i c r e t i c u l u m , g o ----G o l g i v e s i c l e s , il = i n t e s t i n a l l u m e n , i w = i n t e s t i n a l walt, t m u = l o n g i t u d i n a l p a r i e t a l m u s c l e s , m i = m i t o c h o n d r i u m ,
m s = m y o s a r c w i t h m i t o c h o n d r i a , m n = m u s c u l a t u r e , m y = microvilli, n e = n e r v e cord, n e g = n e u r o g l i a (nucleus), n e p = n e u r o p f l e , n m u = m u s c u l a t u r e a r o u n d n e r v e cord, n p = n e p h r i d i u m ,
nephridial tissue, nu = nucleus, vs = vesicle
~D
~D
C~
o
©
236
O. G i e r e
Figs 3a-f: Aktedrilus monosperma~ecus. TEM fotos, for abbreviations see l e g e n d s to Figs 1-2. (3a)
Body wall. (3b) Mucous (?) droplets in epidermal cell. (3c) Intestinal cell with dense endoplasmatic
reticulum. (3d) Nephridial wall folded i n blood vessel lumen. (3e) Chloragocyte. (3f) Excretion (?) of
membrane-bound 'bubble'
Morphology a n d fine structure of m a r i n e tubificids
237
b e t w e e n the l a r g e r n o r m a l l o n g i t u d i n a l myofibres. Both types are traversed b y sarcoplasmatic r e t i c u l u m t u b u l e s e x t e n d i n g from the p e r i p h e r a l p l a s m a l e m m a at o b l i q u e to
right a n g l e s a n d s u b d i v i d i n g the thick m y o f i l a m e n t s i n r e g u l a r c o m p a r t m e n t s of three to
four p a r a l l e l f i l a m e n t a l rows. T h i n filaments s u r r o u n d i n g the thick ones (Jamieson,
1981) could n o t b e observed. In the outer single b a n d of circular muscle fibres, separated
from the l o n g i t u d i n a l ones b y the b a s e m e n t m e m b r a n e , sarcotubules are absent. Here,
the sarcoplasma with its m i t o c h o n d r i a extends e x t e r n a l l y towards the epidermis, from
which it is set off only b y a t h i n m e m b r a n e .
The typical epidermis cell ("supporting cell", Jamieson, 1981) is more or less
cuboidal, h i g h e r t h a n w i d e (Fig. 2d), with a large n u c l e u s a n d cytoplasma rich in
g l y c o g e n rosettes. T h e r e is a d e n s e e n d o p l a s m a t i c r e t i c u l u m a n d n u m e r o u s Golgi (Fig.
2e). M i t o c h o n d r i a are frequent, w h i l e excretory activity, j u d g i n g from the occurrence of
droplets a n d m u c u s ceils, seems m o d e r a t e i n this species (see below).
The e x t e r n a l surface of the e p i d e r m a l cells is h i g h l y digitated forming irregular
c o l u m n a r supports w h i c h b r a n c h or simply taper into long microvilli. The cuticle,
traversed b y these microvilli, consists of a thick matrix (550 nm) with a n irregular p a t t e r n
of t h i n c o l l a g e n fibres (Fig. 2f) a n d a n epicuticle (140 nm) with a s o m e w h a t c o n d e n s e d
fibrillar structure, not set off s h a r p l y from the b a s a l layer. Very n u m e r o u s e p i c u t i c u l a r
projections (175 n m long, 36 n m wide, 65-70 n m centre-to-centre distance) e x t e n d from
the surface. T h e y s t a n d on fine pedestals a n d carry at their tips long, b r u s h - l i k e
branches. Some sections d e p i c t e d m u c u s floccules c o m i n g off the tips of the e p i c u t i c u l a r
projections.
Aktedrilus monospermathecus
Light-microscopic survey
Ecologically similar to P. prostatus, this species also i n h a b i t s the interstitial r e g i o n of
the h i g h - w a t e r zone of s a n d y beaches. The worm is t a x o n o m i c a l l y very closely r e l a t e d to
the other species s t u d i e d here a n d for a time it was i n c l u d e d i n the g e n u s Phallodrilus
(Brinkhurst & Jamieson, 1971).
Sections t h r o u g h A. monospermathecus show a parietal m u s c u l a t u r e with s o m e w h a t
thicker l o n g i t u d i n a l fibres. S u b s t a n t i a l differences at the light-microscopic level could
not b e detected.
Transmission-electron-microscopic observations
Here, the p r o m i n e n t l o n g i t u d i n a l muscle fibres i n relation to the worm's d i a m e t e r
are conspicuously d e p i c t e d {Pig. 3a), b u t i n structural details they do not seem to differ
from P. prostatus. However, the i n t e g u m e n t is d i s t i n g u i s h e d b y a n a u g m e n t e d excretory
activity: vesicles occur regularly; m e m b r a n e - b o u n d spheres arise from the epicuticular
projections (Pig. 3f) a n d cells filled with droplets are m i x e d with the s u p p o r t i n g
e p i d e r m a l cells (Fig. 3b). T h e s e e x h i b i t at their surface m a n y digitiform b r a n c h e s w h i c h
m e r g e into l o n g microvilli. The e p i c u t i c u l a r projections are of the same shape a n d size as
i n Po prostatus. The fibre system i n the cuticular matrix is also similarly w a v y a n d
irregular.
Other organs, with i d e n t i c a l or slightly d i v e r g i n g structure:
- In the cells of the i n t e s t i n a l wall, the e n d o p l a s m a t i c r e t i c u l u m l i n e d with ribosomes
(Fig. 3c) is the d o m i n a t i n g o r g a n e l l e b e s i d e the large n u c l e u s ; b u t this extreme
d e v e l o p m e n t m a y b e due to l i m i t e d activity phases only.
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O. Giere
The v e n t r a l n e r v e cord seems less clearly covered b y d i s c o n t i n u o u s l o n g i t u d i n a l
m u s c l e s on the dorsal side. Ventrally, a massive l a y e r of glia cells with m a r k e d n u c l e i
u n d e r l i e s the central n e u r o p i l e which, again, lacks g i a n t fibres.
- The v e n t r a l b l o o d vessel, always i n close contact with the a d j a c e n t n e p h r i d i a l tissue,
is often found laterally dislocated. A c o r r e s p o n d i n g dorsal vessel does not exist. As i n
P. prostatus it is s u b s t i t u t e d b y a large sinus w h i c h m e r g e s into the i n t e s t i n a l plexus.
Location a n d structure of c h l o r a g o g e n i c cells is similar to P. prostatus: especially
a r o u n d the dorsal sinus, they form a n irregular layer of large, fluffy cells, v a r y i n g i n
shape, with large n u c l e u s , m a n y organelles, g l y c o g e n rosettes a n d droplets, i n d i c a t i n g
their active m e t a b o l i s m (Fig. 3e).
-
-
DISCUSSION
In the first part, the b o d y structure of Phallodrilus prostatus a n d A k t e d r i l u s m o n o spermathecus, as e v i d e n t from cross sections, will b e c o m p a r e d w i t h that of other
oligochaetes, i n particular tubificids; the second part will discuss the results i n r e l a t i o n
to the gutless Phallodrilus spp.
In v i e w of the scarce a n d scattered specific a n a t o m i c a l - h i s t o l o g i c a l literature on
tubificids, the g e n e r a l reviews of S t e p h e n s o n (1930), Brinkhurst & J a m i e s o n (1971), a n d
J a m i e s o n (1981} i n a d d i t i o n to some original papers were u s e d here as a source of
information. However, on s c r u t i n i z i n g literature for details of the different o r g a n systems, the m e a g e r extent of i n f o r m a t i o n on tubificids b e c o m e s apparent, particularly with
reference to ultrastructure as s u m m a r i z e d b y J a m i e s o n (1981). Here, the a l i m e n t a r y
c a n a l a n d the excretory system of T u b i f i c i d a e are only v a g u e l y m e n t i o n e d , the description of n e r v e cord, m u s c u l a t u r e a n d i n t e g u m e n t is mostly restricted to T u b i f e x tubifex
and, to some extent, to Limnodrilus spp. Hence, so far, m a n y literature 'results' on
Tubificidae are, i n fact, extrapolations from facts o b t a i n e d from other oligochaete
groups. The ultrastructure of the i n c r e a s i n g n u m b e r of species i n the m a r i n e s u b f a m i l y
Phallodrilinae is c o m p l e t e l y u n k n o w n except for that of two gutless forms.
While the chloragocytes a n d m u s c u l a t u r e of the two 'regular' m a r i n e tubificids
studied here do not a p p a r e n t l y differ m u c h i n ultrastructure from some other tubificids
with gut or e v e n from other families (chloragocytes of T. tubifex: Lindner, 1965; Fischer
& Horvgth, 1978, 1979; other families: Jamieson, 1981; m u s c u l a t u r e of T. tubifex a n d
Limnodrilus udekemianus: Lanzavecchia, 1971), major o r g a n systems are d i v e r g e n t i n
the two e x a m i n e d species c o m p a r e d with literature reports.
S t r u c t u r e of t h e n e r v e c o r d
W h i l e for T. tubifex {Dixon, 1915), L. hoffmeisteri (Satija & Garg, 1976) and, finally,
for microdriles as a w h o l e (Cook, i n Brinkhurst & J a m i e s o n , 1971) g i a n t dorsal fibres
w e r e described a n d the d o u b l e structure of the d e a r l y e n s h e a t h e d n e r v e t r u n k s reported
to b e of g e n e r a l occurrence, there is no sign of these structures i n the worms s t u d i e d
here. A "fibrous capsule" c o v e r i n g the n e r v e cord is missing.
Dorsal blood vessel
This vessel, d e s i g n e d b y Dixon (1915) with a n o w n v a s c u l a r w a l l for T. t u b i f e x a n d
later r e p e a t e d l y s t u d i e d in T u b i f i c i d a e (see literature i n Brinkhurst & J a m i e s o n , 1971), is
M o r p h o l o g y a n d fine structure of m a r i n e tubificids
239
l a c k i n g i n t h e two p h a l l o d r i l i n e m a r i n e s p e c i e s in question. Here, the c o r r e s p o n d i n g
structure c l e a r l y h a s o p e n c o n n e c t i o n s to t h e i n t e s t i n a l s i n u s - c o m p l e x and, in contrast to
t h e v e n t r a l vessel, l a c k s e n d o t h e l i a l l i n i n g (Fig l a , ld). Since Dixon (1915) does not
d i f f e r e n t i a t e b e t w e e n ' v e s s e l ' a n d 'sinus', t h e " s u p r a - i n t e s t i n a l v e s s e l " , d e s c r i b e d b y h e r
as g i v i n g rise to the " i n t e s t i n a l v e s s e l s " m a y in fact r e p r e s e n t a l a r g e dorsal b l o o d sinus.
The intimate connection between the ventral blood vessel and the excretory system
in P. prostatus and A. m o n o s p e r m a t h e c u s is d o c u m e n t e d b y folds in t h e v a s c u l a r wall,
w i t h n e p h r i d i a l tissue p r o j e c t i n g into t h e v e s s e l l u m e n or e v e n b y "branches' p r o t u b e r a t i n g into it (Fig. lf). A s i m i l a r l y close a l l i a n c e w i t h t h e b r a n c h e s w a s d e s c r i b e d b y N a s s e
(1882, i n Dixon, 1915) for T u b i f i c i d a e , b u t l a t e r r e j e c t e d b y V e j d o v s k y (1884) a n d D i x o n
(1915). H o w e v e r , close p h y s i o l o g i c a l c o n n e c t i o n b e t w e e n b l o o d a n d n e p h r i d i a is stress e d e v e n in r e c e n t c o n t r i b u t i o n s (see J a m i e s o n , 1981).
G r a s z y n s k i (1963) found in L u m b r i c u s terrestris a p l a s m a t i c structure in the n e p h r i d i a l w a l l s h o w i n g characteristic l a m e l l a e a n d circular sections w i t h m i t o c h o n d r i a . His
figures r e s e m b l e v e r y closely the m i c r o g r a p h s g i v e n in this s t u d y (Fig. 2b), so that they,
with s o m e p r o b a b i l i t y , r e p r e s e n t sections t h r o u g h t h e p l a s m a t i c w a l l of the distal
n e p h r i d i a l c a n a l ( " S t h b c h e n k a n a r ' in G r a s z y n s k i , 1963; p e r h a p s "porci6n C" in Pefia,
1978).
Cuticle
T h e c o n s p i c u o u s p a t t e r n of r e g u l a r l y crossing c o l l a g e n fibres, t y p i c a l for some
l u m b r i c i d s a n d e n c h y t r a e i d s (Richards, 1977), is not p r e s e n t in the m a r i n e tubificids
i n s p e c t e d here. T h e i r i r r e g u l a r w a v y fibres, r u n n i n g m a i n l y in a circular direction, b e a r
m o r e r e s e m b l a n c e w i t h the cuticle in the n a i d i d Dero obtusata (Krall, 1968). In this
p a t t e r n t h e y contrast to t h e g u t l e s s Phallodrilus s p e c i e s (Giere, 1981; Richards, F l e m i n g
& J a m i e s o n , 1982) w h i c h s h o w a n o r t h o g o n a l c o l l a g e n grid. This difference w i t h i n
f a m i l i e s a n d e v e n w i t h i n o n e g e n u s of interstitially l i v i n g worms r e n d e r s g e n e r a l i z a t i o n
or r e l a t i o n of t h e s e c u t i c u l a r structures to life h a b i t s p r o b l e m a t i c a l (Richards, 1977;
W e s t h e i d e & Rieger, 1978).
T h e s h a p e a n d size, too, of t h e e p i c u t i c u l a r p r o j e c t i o n s s e e m to v a r y c o n s i d e r a b l y
(Jamieson, 1981). Both T u b i f e x t u b i f e x (Fischer & Horv~th, 1977) a n d t h e gutless
r e l a t i v e s h a v e shorter a n d s i m p l e e p i c u t i c u l a r p r o j e c t i o n s w h i c h l a c k a n y b r u s h - l i k e tips
(Giere, 1981; R i c h a r d s et al., 1982). This l e a v e s r o o m for s p e ~ a t i o n on t h e function of
t h e s e structures, w h e t h e r as m e a n s for t h e t r a n s e p i d e r m a l u p t a k e of n u t r i e n t s or, as
s u g g e s t e d b y Richards et al., (1982), for m e r e protection.
This l e a d s to a further c o m p a r i s o n w i t h the a n a t o m y of t h e two gutless Phatlodrilus
species, w i t h t h e restriction t h a t in P. albidus (Richards et a l . , 1982), o n l y t h e d i s t a l
e p i d e r m i s a n d cuticle h a v e b e e n d e s c r i b e d . In P. I e u k o d e r m a t u s (Giere, 1981), t h e most
s t r i k i n g difference, t h e c o m p l e t e a b s e n c e of gut a n d n e p h r i d i a , h a s a p p a r e n t l y m a r k e d
c o n s e q u e n c e s for t h e struchlre of t h e v a s c u l a r s y s t e m a n d c h l o r a g o c y t e s : w h i l e i n the
' n o r m a l ' w o r m s the v e s s e l s a r e v i a t h e c h l o r a g o g e n i c tissue in loose c o n n e c t i o n w i t h the
i n t e s t i n e a n d n e r v e cord, in t h e a n e n t e r i c form t h e two l a r g e l o n g i t u d i n a l vessels,
c o n n e c t e d b y a m u s c u l a r strand, t r a v e r s e more or less freely the ' e m p t y ' c o e l o m i c cavity.
H e n c e , a n y h o m o l o g y for a v e n t r a l or dorsal v e s s e l is h e r e p r o b l e m a t i c a l . T h e e n d o t h e lial ceils, too, h a v e a t t a i n e d a different a p p e a r a n c e in the gutless coelome: t h e y s e e m
' b l o w n up' and, in s o m e sections, fill the w h o l e cavity almost r e s e m b l i n g a fluffy
240
O. G i e r e
m e s e n c h y m a (Fig. 1 in Giere, 1981). T h e i r rich content of e n d o p l a s m a t i c reticulum, l i n e d
b y ribosomes, m i t o c h o n d r i a , e l e c t r o n - d e n s e grana, h o w e v e r , f u n c t i o n a l l y r e s e m b l e s
n o r m a l chloragocytes (see above). Transformation of t h e i n t e g u m e n t t h r o u g h the incorp o r a t i o n of b a c t e r i a (Giere e t al., 1983) h a s m o d i f i e d t h e e p i d e r m i s / c u t i c l e c o m p l e x in
b o t h gutless s p e c i e s c o m p a r e d to t h e r e g u l a r e p i d e r m i s as d e s c r i b e d above. L a c k i n g
'cavities' at the e p i d e r m i s - cuticle interface for s t o r a g e of b a c t e r i a a n d m u l t i - v e s i c u l a r
b o d i e s , t h e d e r m a l layer, here, is less t h i c k t h a n the c o r r e s p o n d i n g ' m a n t l e ' of P.
l e u k o d e r m a t u s w h i c h attains a l m o s t half t h e d i a m e t e r of the cross s e c t i o n (Fig. 1 in
Giere, 1981). H o w e v e r , e x c r e t i o n of m e m b r a n e - b o u n d ' b u b b l e s ' , a p p a r e n t l y m e d i a t e d
b y t h e e p i c u t i c u l a r p r o j e c t i o n s (Fig, 6 in G i e r e et al., 1983), occurs b o t h in P. leukoderm a t u s a n d the s p e c i e s d e s c r i b e d h e r e (Fig. 3f).
Musculature
T h e l o n g i t u d i n a l m u s c l e fibres of b o t h gutless s p e c i e s differ in the e n l a r g e m e n t of
the sarcoplasmatic, n o n c o n t r a c t i l e p o r t i o n of the cell w h i c h ' h a n g s ' into the c o e l o m i c
cavity, a n d w h i c h i n s o m e sections (see Fig. 1 in Giere, 1981) is as l a r g e as or l a r g e r t h a n
t h e fibrous p a r t (see also J a m i e s o n , 1981). S m a l l t r i a n g u l a r m u s c l e cells a r e e m b e d d e d in
the n o r m a l l o n g i t u d i n a l m u s c u l a t u r e . T h e y are a p p a r e n t l y a c o m m o n f e a t u r e in
tubificids as shown for T u b i f e x tubifex, P. prostatus a n d A. monospermathecus, b u t occur
in P. l e u k o d e r m a t u s in i n c r e a s e d n u m b e r a n d in p a r t i c u l a r l y r e g u l a r a r r a n g e m e n t . In
this s p e c i e s t h e u l t r a s t r u c t u r e of t h e l o n g i t u d i n a l m u s c l e s differs from that of t h e s p e c i e s
e x a m i n e d above, e a c h (thick) myofibril b e i n g e n c i r c l e d b y thin fibrils.
Nerve cord
In contrast to t h e two 'normal" p h a l l o d r i l i n e w o r m s i n s p e c t e d in this study, t h e n e r v e
cord of P. l e u k o d e r m a t u s h a r b o u r s two or t h r e e l a r g e r d o r s a l fibres. H o w e v e r , since their
d i a m e t e r is m u c h less distinct t h a n that of the p r o m i n e n t ' g i a n t fibres' in Lumbricus
terrestris (Jamieson, 1981) or T u b i f e x tubifex (Dixon, 1915), it c a n n o t b e s t a t e d at this
s t a g e t h a t t h e y r e a l l y a r e t h e i r e q u i v a l e n t . O n l y in s o m e sections t h r o u g h P. leukoderm a t u s is a slight b i p a r t i t i o n in two n e u r a l t r u n k s visible.
CONCLUSIONS
T h e m a r i n e P h a l l o d r i l i n a e , s t u d i e d h e r e for the first time, e x h i b i t m a j o r m o r p h o l o g i cal a n d histological differences from the k n o w n l i m n i c T u b i t i c i n a e . This refers m a i n l y to
the h i s t o l o g y of the n e r v e cord, a r r a n g e m e n t of the v a s c u l a r s y s t e m a n d u l t r a s t r u c t u r e of
the cuticle. C o m p a r e d to t h e ' r e g u l a r ' P h a l l o d r i l i n a e , d e g e n e r a t i o n of t h e g u t a n d
n e p h r i d i a in the r e c e n t l y d i s c o v e r e d a n e n t e r i c r e l a t i v e s h a s d e e p l y c h a n g e d t h e b o d y
organization: not o n l y has the d e r m a l w a l l s p e c i a l i z e d b y t h e u p t a k e of s y m b i o t i c
bacteria, b u t also t h e structure of t h e l o n g i t u d i n a l m u s c u l a t u r e a n d the e n d o t h e l i a l tissue
w h i c h a c c o m p a n i e s the l a r g e b l o o d v e s s e l s in the ' e m p t y ' c o e l o m i c c a v i t y h a v e u n d e r g o n e modifications.
F u r t h e r structural i n v e s t i g a t i o n s on other gutless o l i g o c h a e t e s p e c i e s (Giere, in
p r e p a r a t i o n ) will a s c e r t a i n w h e t h e r t h e s e 'derivations' are of a m o r e g e n e r a l n a t u r e or
M o r p h o l o g y a n d f i n e s t r u c t u r e of m a r i n e t u b i f i c i d s
241
d i f f e r i n e a c h a n e n t e r i c s p e c i e s . H o w e v e r , it h a s a l r e a d y b e c o m e d e a r t h a t a r e l i a b l e
h i s t o l o g i c a l a n d c y t o l o g i c a l a s s e s s m e n t of t h e a b e r r a n t g u t l e s s s i t u a t i o n is o n l y f e a s i b l e
i n t h e l i g h t of t h e ' n o r m a l ' o r g a n i z a t i o n a s s h o w n i n r e l a t e d w o r m s f r o m t h e m a r i n e b i o t a .
T h e r e f o r e , g e n e r a l i z a t i o n s f r o m t h e f e w s c a t t e r e d s t u d i e s o n f r e s h w a t e r t u b i f i c i d s or
even other families are misleading and less than helpful.
Acknowledgement. My thanks to Dr. O. Pfannkuche for his cooperation.
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