The Morphological Study of Cnidocysts
Under Light Microscope and Scanning Electron Microscope
Sau Yu Grace Wong, BSc Zoology, The School of Animal & Microbial Science, Reading University, England
Erasmus Exchange Programme Project Supervisor: Carina Östman, Animal Development and Genetics, Uppsala University, Uppsala, Sweden
Period of study: between the September – Augusti 2001
Abstract: The morphology of the nematocysts or the cnidocysts of three Classes of cnidarians were studied in light microscopy (LM) and scanning electron microscopy (SEM), namely, Carophyllia smithii, Metridium senile, Protanthea simplex and Sagartiogeton viduatus of the Class Anthozoa, Hydractinia echinata of the Class Hydrozoa and the moon jellyfish Aurelia aurita and Cyanea capillata of the Class Scyphozoa.
Some of the nematocysts identified in the Scyphozoa were similar to those of the hydrozoans, namely the homotrichous small a-isorhiza haplonemes and the heterotrichous microbasic eurytele heteronemes. The nematocysts identified were however closely similar in the two scyphozoans. Most of the anthozoan cnidae identified in this study were confined to themselves; the microbasic b-mastigophore heteronemes, the heterotrichous microbasic and mesobasic p-mastigophore heteronemes, the heterotrichous microbasic p-amastigophore heteronemes and the spirocysts. Many of the anthozoan cnidae were larger in size and more elongated in shape compared to those of the hydrozoan and scyphozoan cnidae, which were sub-spherical and ovate in shape. The size ranges and the length to width ratio of identified anthozoan cnidae were greater than the ones of examined hydrozoans and scyphozoans.
The anthozoans studied, except for P. simplex, possessed acontia where the largest and the most potent cnidocysts were kept. The acontia of the stone coral C. smithii differed from the acontia in the other sea anemones in their size and purpose. They were too short to extend outside the body and they remained in the gastrovascular cavity. The purpose of these acontia are still in question.
The stone coral C. smithii possessed the most categories and size classes of cnidocysts; four size classes of microbasic b-mastigophores, four size classes of microbasic p-mastigophores, two types of isorhizas and two sizes of spirocysts. Furthermore, it possessed the largest nematocysts, namely the microbasic p-mastigophores in which the capsule reached up to 100 m in length and 14 m in width. Cnidae heavily armed with course spines were suppose to be penetrators, whereas tubules armed with weaker and smaller spines functioned to entangle prey. Although the isorhizas found in the mesenteric filament of C. smithii were heavily armed, they functioned in a similar way as to the spirocysts, which were used to entangle preys.
The presence, function and structure of the “dart-like” structure, previously found in the sea anemones M. senile and S. viduatus, had been much debated. The images produced in this study strengthened its presence.
The birhopaloids were thought to possess a terminal opening and were categorised as Stomocidae. It was found that the birhopaloid of the scyphozoan medusae A. aurita possessed a closed terminal tip. The terminal closing of the birhopaloids together with the “dart-like structure” indicate the need of modifying the nomenclature system.