Studies of Statoblasts of the Bryozoan, Cristatella mucedo
Biological Sciences, Lethbridge, Alberta, Canada
| Abstract | Introduction | Methods | Results | Discussion
| Acknowledgement | References |
The phylum Bryozoa, also called Ectoprocta, is found in unpolluted and unsilted waters. In general, Ectoprocta are
marine organisms, but Cristatella mucedo is one of four freshwater species found in Alberta. Most bryozoans are
hermaphroditic. Growth and proliferation of a colony is simply by asexual budding or statoblast formation. Statoblasts are
asexual internal buds, whose functional cells are enclosed tightly by two sclerotized valves to endure unfavourable environmental
conditions. Very few studies have been done on C. mucedo, although these organisms are potential environmental indicators,
and also contain useful biochemical genetic markers. This study concentrates on the C. mucedo statoblast in an attempt to
observe the internal and surface structures of the organism by scanning electron microscopy (SEM). Samples were collected at
the University of Lethbridge irrigation pond, and then processed and examined with SEM. Special attention is given to the unique
features of the capsules, suture zone, spicules, and internal structures of C. mucedo.
The phylum Ectoprocta
is found in unpolluted waters, especially ponds, shallows of lakes, and slow
streams. Generally, Ectoprocta are marine organisms. However, of the 20 freshwater
species, four of them are found in Alberta. Freshwater bryozoans live in colonies.
Each colony may consist of thousands of complete individuals called zooids which
are enclosed in a zooecium, a hard gelatinous case. Most bryozoans are hermaphroditic
(1). Growth and proliferation of a colony occurs through simply asexual budding
or by statoblast production. Statoblasts are asexual internal buds, whose functional
cells are enclosed tightly by two sclerotized valves to endure unfavourable
environmental conditions. They are produced within the zooid following sexual
reproduction. Statoblasts are classified into three categories according to
their morphology: (1) spinoblast: statoblasts with spines; (2) floatoblasts:
statoblasts that are filled with gas and float; and (3) sessoblasts: statoblasts
that are sessile, fixed or cemented to the zooecium wall (2). Very few studies
have been done on bryozoans, especially Cristatella mucedo, although
they are potential environmental indicators (3), and contain useful biochemical
genetic markers (4). This SEM study concentrates on the statoblast of Cristatella
mucedo, the most complicated spinoblast, in an attempt to observe the internal
and external structures of the organism.
Cristatella mucedo samples were
collected from the University
of Lethbridge irrigation pond with an aquarium net, and then processed by
standard SEM procedures as outlined below:
2. dorsal-ventral dissected
3. radially dissected
sodium cacodylate buffer (0.1 M, pH 7.2)
15 sec. sonication in phosphate buffer saline (PBS) plus 0.5 % Extran
in ethanol series
CRITICAL POINT DRYING
aluminum stub with double-sticky tape
30 nm gold
||Fig. 1. Side view of the spinoblast of Cristatella
The capsule (C) has numerous tiny spines. Note the hooks (H) arise
from the capsular periphery of the statoblast suture zone and the
honeycombed annulus (A) along the periphery of the statoblast.
||Fig. 2. Ventral view of the spinoblast of C. mucedo.
The ventral capsule (VC) also has a raised reticulated surface and
numerous tiny spines (S). These spines are outgrowths from the hardened
raised walls of the capsular reticulations. The hooks (H) also arise
from the capsular periphery of the statoblast suture zone.
||Fig. 3. Inner surface of dorsal capsule. 130X
The inner surface of dorsal capsule of the statoblast is fairly smooth.
||Fig. 4. The interior structure of dorsal capsule. 200X
The fibrous connective tissue (F) underlines the capsule. Note The
aggregated egg mass (E).
||Fig. 5. The inside structure of dorsal capsule. 600X
The outer surface (O) is reticulated and has numerous spines that
project from it. The interior of the capsule is occupied by fibrous
connective tissue (F). The annulus cells (A) are honeycombed to enable
||Fig. 6. A closer view of the interior of ventral capsule.
Note the groove between the honeycombed annulus (A) and the fibrous
connective tissue (F) inside the capsule. This groove is the juncture
of the two valves. The annulus cells (A) are porous.
||Fig. 7. The terminal hooks of the statoblast. 450X
The surface of these hooks is relatively smooth. Note the ends the
bifurcated hook (B).
||Fig. 8. Transverse section of a capsular wall. 1500X
This cross section is taken from the dorsal valve. The chitinous
valve is approximately 15m thick.
||Fig. 9. The symbiosis between diatoms and Cristatella
Two diatoms (D) attached to the outer surface of the capsule. Note
the surface texture of the groove (G) and the capsular wall.
The statoblast of Cristatella mucedo consists of
two hardened, chitinous valves that protect the egg cells inside of it. Both
dorsal and ventral capsules of the statoblast have a raised, reticulated surface
and small spines that grow out of it. Enlarged terminal hooks arise from the
capsular periphery of the statoblast suture zone, and are also evident on
both capsules. These hooks may help the dispersal of the statoblasts. The
honey-combed annulus cells provide buoyancy to the organism. Large amounts
of connective tissues underlie the internal surface of the ventral valve,
whereas the internal surface of the dorsal valve is relatively smooth.
I am happy to thank Mr. Doug Bray for helping me with specimen
preparation, and Dr. Margaret Lewis and Dr. Paul Lewis for discussions and
suggestions concerning the organism. I am also indebted to University of Lethbridge
for the use of the SEM facilities.
1. Clifford, H.F. 1991. Bryozoans Pp. 55-59 in
Aquatic invertebrates of Alberta (H.F. Clifford ed.). The University of Alberta
2. Hyman H.L. 1959. The Lophophorate coelomates -
phylum Ectoprocta Pp. 275-501 in The invertebrates Vol. V: smaller
coelomate groups (H.L. Hyman ed.). McGraw-Hill, New York.
3. Pennak, R.W. 1989 Bryozoa Pp. 269-289 in
Freshwater invertebrates of the United States (R.W. Pennak ed.). A
Wiley-Interscience Publication, New York.
4. Okamura, B., C.S. Jones, and L.R. Noble. 1993.
Randomly amplified polymorphic DNA analysis of clonal population
structure and geographic variation in a freshwater bryozoan.
Proc. R. Soc. London. Biol. Sci.. 253:147-154