SEM Studies of Statoblasts of the Bryozoan, Cristatella mucedo
Department of Biological Sciences, Lethbridge, Alberta, Canada

| Abstract | Introduction | Methods | Results | Discussion | Acknowledgement | References |

ABSTRACT

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.

INTRODUCTION

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.

METHODS

 

SPECIMEN COLLECTION
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:

SPECIMEN DISSECTION
1. undissected
2. dorsal-ventral dissected
3. radially dissected

FIXATION

sodium cacodylate buffer (0.1 M, pH 7.2)

RINSING
15 sec. sonication in phosphate buffer saline (PBS) plus 0.5 % Extran

DEHYDRATION
in ethanol series

CRITICAL POINT DRYING

SPECIMEN MOUNTING
aluminum stub with double-sticky tape

SPUTTER COATING
30 nm gold

SEM VIEWING
Hitachi S-500

RESULTS

Fig. 1. Side view of the spinoblast of Cristatella mucedo. 140X

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. 300X

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 buoyancy.

Fig. 6. A closer view of the interior of ventral capsule. 1400X

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 mucedo. 2000X

Two diatoms (D) attached to the outer surface of the capsule. Note the surface texture of the groove (G) and the capsular wall.

DISCUSSION AND CONCLUSION

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.

ACKNOWLEDGMENTS

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.

REFERENCES

1. Clifford, H.F. 1991. Bryozoans Pp. 55-59 in Aquatic invertebrates of Alberta (H.F. Clifford ed.). The University of Alberta

Press, Edmonton.

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

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