Epidermis - Lined by longitudinal muscle fibers. Epitheliomuscular cells: they are not true muscles cells but muscle fibers located at the base of epithelial cells. They are responsible for contraction and movement of the animal. Interstitial cells: also referred to as I cells, they are multipotent stem cells which can differentiate into other cells. Sensory cells: They include receptor cells and motor neurons and play a role in gathering and processing information about the surroundings. Nematocytes: One of the three types of cnidocytes that occur in the phylum Cnidaria. Cnidocytes are one of the defining characteristics of cnidarians. Within the nematocytes are thick walled nematocysts which are responsible for the stinging pain. Nematocysts typically occur in high density on the tentacles (prey catching, defense) and inside the acrorhagi (offense).

Gastrodermis - Surrounded by circular muscle fibers. Enzymatic gland cells: They secrete digestive enzymes to aid in digestion. Flagellated nutritive muscular cells: Similar to epitheliomuscular cells, but also ingest small particles of food that have been broken down by the digestive enzymes. Germ cells: originate in the endoderm and grow and differentiate into the gastrodermis.

Mesoglea - Longitudinal muscle fibers line the mesoglea, also provide anchorage for the thin cylindrical bases of epitheliomuscular cells. Gelatinous nature is due to the abundant collagen fibers that make up the mesoglea. Functions as gelatinous support of the body.

The definitive defining characteristic of the phylum Cnidaria are cells called cnidocytes that produce the capsular organelles, cnidae. Cnidae are important morphological characteristics within the taxonomy of sea anemones for they define the higher taxa (Schmidt, 1974) and identify species within a difficult group (Manuel, 1988). There are three types of cnidae, namely spirocysts, ptychocysts and nematocysts with the latter occurring in all groups of cnidarians.  As such, this study aimed to focus on discussing the functional morphology of nematocytes. A layer of collagen is used to thicken and stiffen the body wall of the nematocyst. Within the nematocyst itself, the coiled tubule is enclosed in a fluid containing a high amount of ions, amino acids and proteins. As such, many of the ions form complexes with molecules of the venom reducing the osmotic pressure. When the nematocysts are stimulated, ions dissociate from the complex to create a high intracapsular osmotic pressure. Water moves from low osmotic pressure to high osmotic pressure, consequently, this result in water from the cytoplasm rushing into the capusle, forcing the tubule to be ejected forcefully and rapidly. After discharging the nematocyst, the nematocyte degenerates and is replaced by a new one thus, there is a high energetic cost associated with every tubule fired. As a result, cnidarians seem to have evolved mechanisms involved with the controlling of nematocyst discharge. At least two modes of stimulation (mechanical and chemical), are required before the firing of nematocysts. Such adaptations have allowed the cnidarians to decrease incidences of unintentional and insignificant discharges. The nematocytes are primarily adapted for prey capture, defense and offense against neighboring individuals. Therefore, it could be expected that nematocytes would be present in dense concentrations on the tentacles, septal filaments and acrorhagi.  

Materials and methods

A. tenebrosa were collected from the rocky sub tidal areas of Heron Island, removed using a spatula and fixed in 4% neutral buffered formalin to prevent autolysis and distortion of tissue structure. Fixed specimens were then sent off for sectioning. Prior to staining, the slides were placed in three beakers of xylene for three minutes each followed by three beakers of absolute ethanol for three minutes each, then 90% ethanol, 70% ethanol, 50% ethanol and distilled water for two minutes each. The sections were stained with haematoxylin and eosin before analysis under a light microscope.

Results

Within each nematocyte is a thick-walled nematocyst, a capsular organelle enclosing a thin, eversible invagination of the capsule wall which contains spines or barbs on the surface. The opening for the tubule is covered by three apical flaps. Nematocysts were found on the tentacles (Figure 1), acrorhagi (Figure 2) and septal filaments (Figure 3) of A. tenebrosa and were not present elsewhere. Based on gross morphology, the nematocysts on the tentacles and acrorhagi appeared to be of the same type. Nematocysts of the septal filaments were too immature to be compared. On both the tentacles and acrorhagi, nematocysts occurred on the most exterior margin of the structures in dense concentrations.

Discussion

Consistent with my predictions, nematocysts were found in abundance on the tentacles, acrorhagi and septal filaments, demonstrating their roles in prey capture for food, defense against predators and aggression against competitors. Their discharge is most likely facilitated by their position within these structures. Their anatomical locations and their use in particular situations are most probably the best indicators of their function (Chapman & Hall). For instance, the nematocysts present on the tentacles have a role in nutrition because they are employed in the capture and envenomation of prey. When the prey is caught in the tentacles, the nematocysts penetrate the prey and inject toxins resulting in the paralysis and the subsequent capture of the prey. The nematocysts on both the acrorhagi and septal filaments are responsible for the offense and defense against non- clonal conspecifics and predators respectively. Aggression is observed when tentacular contact between two genetically dissimilar A. tenebrosa is initiated. The acrorhagi becomes inflated and discharge nematocysts when they are in contact with their opponent, causing necrotic lesions on the epidermis of their opponent. Only nematocysts present on the acrorhagi are involved in this act of aggression. Septal filaments are usually expelled through the mouth or from the sides of the body wall at the approach of food or predators, thus nematocysts on the septal filaments can be inferred to be involved in both prey capture and defense. Within the three varieties of cnidae, it is only the nematocysts that contain toxins. Toxins purified from A. tenebrosa included tenebrosin- A, tenebrosin- B and tenebrosin- C, all of which are cytolytic toxins capable of immobilising or killing small prey (Macek, 1992).

Although A. tenebrosa is capable of locomotion, it is mostly sessile and movement, if any, is generally restricted to a very small range. Nematocysts are utilised for nutrition, defense and offense, all of which plays a part in fitness which is imperative for greater survivability. Further empirical studies about the evolutionary history of nematocysts in anemones might provide valuable insight into some of the evolutionary changes that shaped the phylum Cnidaria.