Ocelli in a Cnidaria polyp: The ultrastructure of the pigment spots in Stylocoronella riedli (Scyphozoa, Stauromedusae)

Abstract

Within the Cnidaria, the occurrence of ocelli at the polyp stage is only known in the species of Stylocoronella (Scyphozoa, Stauromedusae). The light-sensitive organs of S. riedli are ultrastructurally investigated. In this interstitial-living species, each of the up to 24 ocelli is composed of between seven and nine monociliary sensory cells and between one and four pigment cells. A striking feature of the photoreceptive cilia is their peculiar axonemal pattern. This is expressed (a) by the presence of a third central microtubule at a certain point and (b) by the balloon-like swelling of the distal portion of the cilium, with clearly scattered microtubules in this area. Although the polyps of S. riedli show no distinct reaction to light stimuli, the ultrastructural results corroborate the hypothesis that these organs are light-sensitive organs. The possible function of the pigment granules is discussed.

Full text

Zoomorphology (1995) 115:221-227 9 Springer-Verlag 1995
Michael J.E Blumer 9 Luitfried v. Salvini-Plawen
Reinhard Kikinger. Thomas Btichinger
Ocelli in a Cnidaria polyp: the ultrastructure of the pigment spots
in Stylocoroneila riedli
(Scyphozoa, Stauromedusae)
Accepted: 1 September 1995
Abstract Within the Cnidaria, the occurrence of ocelli
at the polyp stage is only known in the species of S~lo-
cotvnella (Scyphozoa, Stauromedusae). The light-sensi-
tive organs of S. riedli are ultrastructurally investigated.
In this interstitial-living species, each of the up to 24
ocelli is composed of between seven and nine monocili-
ary sensory cells and between one and four pigment
cells. A striking feature of the photoreceptive cilia is
their peculiar axonemal pattern. This is expressed (a) by
the presence of a third central microtubule at a certain
point and (b) by the balloon-like swelling of the distal
portion of the cilium, with clearly scattered microtubules
in this area. Although the polyps of S. riedli show no dis-
tinct reaction to light stimuli, the ultrastructural results
corroborate the hypothesis that these organs are light-
sensitive organs. The possible function of the pigment
granules is discussed.
Abbreviations bb basal body 9 c cilium 9 co collar 9
csv crescent-shaped vesicle 9 cv clear vesicle 9 dcv
dense-core vesicles 9 k kinetosome 9 m mitochondrion.
mvb multivesicular body - n nucleus 9 oc ocellus 9
pc piment cell- pg pigment granule - sc sensory celt.
sr striated rootlet 9 v vesicle
A. Introduction
The first species of the genus Stylocoronella, S. riedli,
was found in the Adriatic Sea and was described as the
polyp of a Scyphozoa species (Salvini-Plawen 1966). A
second species, S. variabiIis, which was found in the
Plymouth area (England) was described and a summary
of other localities where species of the genus were found
was included (Salvini-Plawen 1987) which was subse-
quently extended to include Galicia (Spain) (Besteiro
M.J.E Blumer (~) 9 L. Salvini-Plawen 9 R. Kikinger
T. Btichinger
Institut fiir Zoologic der Universitfit Wien,
Abteilung fiir S),stematische Zoologic, Althanstrasse 14,
A- 1090 Wien, Osterreich
and Urgorri 1988). The life cycle of both known repre-
sentatives of Stylocoronella classified them among the
Lucernariidae (Stauromedusae) (Kikinger and Salvini-
Plawen 1995). Two characters of both known species are
exceptional. Firstly, the polyp stages (up to 800 gm in S.
variabilis) inhabit the interstitia of marine coarse sand
and, thus, represent the only known mesopsammic
Scyphozoa. Secondly, these polyps possess dark pigment
spots at the oral side of the calyx. Based on histological
investigations, these pigment spots were interpreted as
being ocelli (Salvini-Plawen 1966) which, in a somewhat
altered arrangement, are retained throughout the medusa
stage (Kikinger and Salvini-Plawen 1995).
Photoreceptive sense organs in Cnidaria are well
known within the medusa stage of most Anthomedusae
(Hydrozoa) and of several Scyphozoa (see Salvini-Pla-
wen and Mayr 1977, for a summary). The ocelli de-
scribed in some Stauromedusae also clearly represent or-
gans of the medusa stage (Clark 1878; Berrill 1962).
These sense organs have been the subject of several ul-
trastructural studies (for Hydrozoa see Eakin and West-
fall 1962; Yamasu and Yoshida 1973; Bouillon and Niel-
sen 1974; Singla 1974; Yamamoto and Yoshida 1980;
Weber 1981; for Scyphozoa and Cubozoa see Singla and
Weber 1982; Yamasu and Yoshida 1973, 1976; Bouillon
and Nielsen 1974). The situation in Stylocoronella spe-
cies, however, is unique among Scyphozoa and Cnidaria
in general in regularly possessing what are presumed to
be ocelli at the polyp stage. The object of the present
study is to clarify whether these organs in S. riedli pop
yps are true ocelli as proposed by Salvini-Plawen (1966).
B. Materials and methods
Specimens of
Stylocoronelta riedli
Salvini-Plawen, 1966, were
collected from coarse sand in the northern Adriatic Sea at Punta
Croce near Rovinj, Istria (Croatia). The sand was dredged at
depths of 6-8 m in March, April, July and October 1989 and in
January, March, July and August 1990 (see Kikinger and Salvini-
Plawen 1995). All animals represented the typical interstitial stage
having a calyx diameter of 350-600 gm with up to 24 terminally

222
Fig. 1 Polyp of
Stylocoronella riedli
(400-650 ~tm) with the char-
acteristic pigment spots (=ocelli) each at the inner bases of the ter-
minally bent tentacles
bent ("hooked") tentacles and a corresponding number of up to 24
basitentacutar ocelli (Fig. 1).
The specimens were anaesthetized prior to fixation in magne-
sium chloride made isotonic with sea-water. The polyps were
fixed in 2.5% gtutaraldehyde (+sodium cacodylate buffer pH 7.2,
0.05 M, 10% saccharose) at 4 ~ C for two hours and rinsed in sodi-
um cacodylate buffer. Subsequently, they were postfixed in 1% os-
mium tetroxide (+sodium cacodylate buffer pH 7.2, 0.05 M, 10%
saccharose) at 4 ~ C for one hour and rinsed again in sodium caco-
dylate buffer. Finally, the specimens were dehydrated with ethanol
in graded steps and embedded in Spurr's epoxy resin. Three com-
plete series of ultrathin sections (80 nm) from the ocelli were
made with a diamond knife on a Reichert Ultracut. The sections
were mounted on dioxane formvar-coated copper grids, stained
with aqueous m'anyt acetate and lead citrate using a Reichert U1-
trostainer and examined with a Zeiss EM902. The computer pro-
gramme Pc-3d-sis was used tbr the three-dimensional reconstruc-
tion.
C. Results
I. General observations
StyIocoronella riedli
polyps have an interstitial life-style.
In cultured specimens, reaction to light stimuli was not
obvious. The investigated structures of the interstitial
polyp stage of
S. riedli are
visible as dark spots at the
base of each tentacle (Salvini-Plawen 1966; Kikinger
and Salvini-Plawen 1995) (Fig. 1). They are composed
of seven to nine sensory cells and one to four pigment
cells (Fig. 3). The pigment cells are in front of and occa-
sionally lateral to the sensory cells (Fig. 2a). The sensory
cells lie next to the tentacular mesogloea. A conspicuous
feature of both sensory and pigment cells is their large
size. Sections show that the longitudinal axis of sensory
cells measures up to 16 btm and that of pigment cells up
to 20 ~tm.
Fig. 2a Schematic three-dimensional reconstruction of the ocellus
S. riedli
showing three sensory cells and three pigment cells, b
Schematic drawing of a monociliary sensory ceil
II. Sensory cells
The sensory cells are not situated at one level, but are ar-
ranged at various distances below the surface. Each of
the generally flask-shaped, monocitiary cells contains a
distal, pear-shaped lumen (Figs. 2b, 3, 7). This lumen is
surrounded by the collar, whose distal surface is differ-
entiated into short microvilli (Figs. 2b, 7). The pear-
shaped lumen is filled with an electron-translucent ma-
trix and contains a single cilium (Figs. 7, 12). About
one-half of this cilium lies outside the lumen; this outer
part of the cilium is ensheathed by a canal formed by a
neighbouring pigment celI (Fig. 2b). The structure of this

223
Fig. 3 Cross-section through the ocellus
region is not uniform. In one illustrated ocellus, the out-
ermost end of two ciliary canals fuse into one canal,
which now bears two cilia (Fig. 4). In another ocellus,
the tips of six sensory cilia are ensheathed in one canal
(Fig. 5). In a third ocellus, no fusion of ciliary canals is
visible.
The cilium of each sensory cell consists of an axo-
neme, a basal body with a basal plate and a rootlet struc-
ture. Most of the peripheral microtubules appear singly
along the whole axoneme; the remaining peripheral mi-
crotubules appear to be arranged as doublets. All micro-
tubules lack dynein arms. (Figs. 10-12). Above the basal
plate, the axoneme shows a 9x1+2 pattern over a short
distance. Cross-sections through the more distal parts of
the sensory cilium reveal an increasing breakdown of the
axonemal pattern. Shortly above the basal plate, a third
central microtubule becomes visible and cilium cross-
sections reveal a 9x 1+3 arrangement. Terminally, the cil-
ium shows a balloon-like swelling and the entire axone-
real pattern becomes disorganized, with the shaft consist-
ing of clearly scattered microtubules (Figs. 9-13). It
should be emphasized that all sensory cilia show the ul-
trastructural features described above. Cilia examined
from other regions of the animal show the typical 9x2+2
axonemal pattern. No balloon-like swellings resembling
those in the sensory cilia were found here. Thus, a fixa-
tion artefact can be excluded.
In the proximal part of the cilium, the basal body re-
veals the typical cartwheel structure in cross-sections
(Fig. 14). A second kinetosome is arranged perpendicu-
larly to the basal body. This basal body gives rise to the
striated rootlet (Fig. 6).
The middle and distal portions of the sensory cells are
much narrower than the basal part. Both portions contain
numerous mitochondria, with a matrix including elec-
tron-dense grains, multivesicular bodies, smooth endo-
plasmic reticulum and small vesicles. The majority of
these vesicles are dense cored. Less numerous are elec-
tron-translucent, or clear, vesicles and crescent-shaped
vesicles. These vesicle types are distributed throughout
the cytoplasm of the sensory cells, but are concentrated
distally (Figs. 6-8).

224

Proximally, the sensory celt bears the nucleus. It is
regularly shaped and its matrix is electron-translucent. A
huge vesicle adjoins the nucleus (Fig. 7). The receptor
cells narrow abruptly at their bases but, although many
sections were examined, no axons were found. The cells
are connected with the neighbouring pigment cells by
poorly developed septate junctions.
III. Pigment cells
The pigment cells ensheath the photoreceptive cilia and
are round in shape. A striking feature of these cells are
the remarkably structured pigment granules. The pig-
ment granules are irregularly shaped and are enveloped
by a membrane. An electron-translucent area is visible
between this membrane and the electron-dense portion.
Serial sections through the pigment granules and higher
magnifications show the electron-dense portion to be
composed of tightly stacked electron-dense membranes.
The dark pigment visible in living animals is associated
with these membranes. Apart from the electron-dense
membranes, small vesicles are also enveloped by the
membrane of the pigment granules (Figs. 15, 16).
In addition to the pigment granules, the pigment cells
contain a few small vesicles, multivesicular bodies and
several mitochondria.
D. Discussion
The present results corroborate the hypothesis of Salvini-
Plawen (1966) that the pigment spots at the base of the
tentacles of S. riedli are tight-sensitive organs. Here, it is
demonstrated that the photoreceptive cells contain cres-
cent-shaped vesicles and dense-cored vesicles. Both
types of organelles are described in the tentacular senso-
ry cells of the polyp of Aurelia aurita Linne, 1785 (Cni-
daria, Scyphozoa) (see Chia et al. 1984). Additionally,
the monociliary sensory cells of S. riedli show all the
features, such as shape and ultrastructural organelles,
which are characteristic for photoreceptive cells (see Sal-
vini-Plawen and Mayr 1977; Eakin and Hermans 1988).
The most conspicuous feature of the light-sensitive cells
is the intraciliary structure demonstrating that these cilia
are immobile (see Golz and Thurm 1993). It is well
known from light-sensitive organs in other groups that
Fig. 4 Cross-section through a ciliary canal containing two senso-
ry cilia
Fig. 5
Cross-section through a ciliary canal containing six senso-
ry cilia
Fig. 6 Longitudinal section through the distal portion of a sensory
cell
Fig. 7 Longitudinal section through a sensory cell; note the distal-
ly pear-shaped lumen bearing a single cilium (asterisk)
Fig. 8 Longitudinal section through a sensory celt; note the nu-
merous dense-cored vesicles
225
photoreceptive cilia lose their central microtubules and
that the axonemal pattern becomes disorganized more
distally (Eaking and Hermans 1988). However, a similar
mode of disorganization in microtubule arrangement - as
described particularly for the distal, swollen parts of the
ciliary sheaths of S. riedli - has only been described in
the non-homologous ocelli of the cubomedusan Tamoya
bursaria Maas, 1903 (=Z gargantua Haeckel, 1880 ?)
(see Yamasu and Yoshida 1976). The presence of a third
central microtubule at a certain point and the single ap-
pearance of most of the peripheral microtubules differs
from all known types of photoreceptive cilia within the
Eumetazoa; it is assumed to be an autapomorphic char-
acter ~br S. riedli. According to Eakin (1982), Salvini-
Plawen and Mayr (1977) and Blumer (1994), the distal
balloon-like swelling of the sensory cilia increases the
light-absorbing sin-face, which is a typical feature for this
type of modified organelle.
Another remarkable feature of the ocellus is the
unique ultrastructure of the pigment granules in the pig-
ment cells. These irregularly shaped granules consist of
stacks of electron-dense membranes. Other investiga-
tions on cnidarian ocetli demonstrate that the pigment
granules are round in shape, membrane-bound and con-
tain melanin (see Yamasu and Yoshida 1976; Yamamoto
and Yoshida 1980; Weber 1981; Singla and Weber
1982). Membranous elements resembling those of S.
riedIi have never been found and this extraordinary type
of pigment granule appears to be unknown within the
Eumetazoa. Fahrenbach (1963) described enormous
stacks and whorls of endoplasmic reticulum in the retin-
ula cells of the nauplius eye; he concluded that the endo-
plasmic reticulum, in its condensed form, acts as a light-
concentrating organelle. In the eyes of Fartulum orcutti
(Dall, 1885) (Gastropoda), a light-gathering and light-
concentrating function was suggested for the ciliary
whorls positioned in front of the photoreceptive micro-
villi (Howard and Martin 1984). The dark pigment in the
ocelli of living S. riedli polyps indicates a light-absorb-
ing function; in addition, however, the present results as
well as the studies by Fahrenbach (1963) and Howard
and Martin (1984) indicate that the membrane stacks in
the pigment granules may also act as light-refracting or-
ganeltes.
Figs.
9-12 Cross-sections through a photoreceptive cilium at dif-
ferent levels; note the breakdown of the axonemal pattern. Arrow-
head in Fig. 11 demonstrates the appearance of a third central rni-
crotubule at a certain point. Asterisk in Fig. 12 indicates the elec-
tron-translucent matrix in the pear-shaped lumen
Fig. 13 Longitudinal section of a sensory cilium with its balloon-
like swelling (asterisk)
Fig. 14 Cross-section through a basal body of a sensory cilium
showing the typical cartwheel structure
Fig. 15 Section through a pigment granule; note the stacks of
electron-dense membranes
Fig. 16 Section through a pigment cell; note the small vesicles
and the densely arrmaged membranous stacks of the pigment gran-
ules

226

In
S. riedti,
the ocelli of the polyp stage are known to
undergo change and rearrangement during transforma-
tion into the medusa stage (Kikinger and Salvini-Plawen
1995). Currently, we have no information on whether the
ultrastructure of the sensory cells and the pigment cells
is also altered.
Acknowledgements The present study is a joint collaboration in
the framework of the research programms P 8377-Bio and P 9658-
Bit supported by the Fonds zur Ftrderung der wissenscbaftlichen
Forschung, Osterreich, We would like to thank Dr. M. Stacho-
witsch for correcting the English and Dr. S. Neulinger and H. Gril-
litsch for drawing the illustrations.
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