CHAPTER II
MORPHOLOGY
2.1 General appearance
From the veliger stage the bilateral symmetry of the abalone body turns into a spiral shape. Its body is divided into three parts: head, foot and saccate intestine.
2.1.1 Head
The head is located on the anterior part of the body and is bilaterally symmetrical. It comprises a mouth, appendages and sensory organs. Compared to other shellfish, abalones have a developed and complex head. It has a pair of tentacles and two eyes at the tip of eye stalks which originate at the bottom of the tentacles.
2.1.2 Foot
The foot of the abalone is a creeping organ of muscular tissue which lies on the ventral part of the body. The well developed foot has a broad “sole” which allows the animal to strongly adhere to rocks or other hard substrates. The epipodes that occupy the lower part of foot form a broad-plate shape structure, while the interior surface has ten pairs of brown stripes on a grey background. Branch-shaped tentacles are typically found at the end of the epipodes.
2.1.3 Intestinal sac
The intestinal sac is located on the dorsal side of abalone and has several internal organs. On the outside, the mantle runs from the back to the ventral margin. The abalone shell is formed by the secretion released from the epidermal cells placed at the front margin and tip of the mantle.
2.1.4 Shell
Abalones belong to one of the most primitive gastropods and its round, elliptical or ear-shaped shell has a row of respiratory pore located along the left margin. As the animal grows, older pores are successively filled in and closed. The number of open pores varies among different abalone species. Shell exterior and interior color varies with species, but it is an unreliable form of identification since the color may not be clearly visible or the original description may not be accurate.
The shell comprises three layers. The cuticle, which is the outer layer, is an organic matter purely formed of concholin. It is very thin. The hard shell or middle layer (horny layer) is composed mainly of calcium carbonate crystals embedded in concholin whose chemical components are similar to the one in cuticle layer. The innermost layer called pearl layer has chemical components similar to the horny layer.
The pearl layer is made of secretions occurring over the surface of the entire mantle. The cuticle and horny layers are secreted by the mantle margin and the growth line appears on the edge of the shell. During the winter and spawning season, or under adverse environment conditions, no growth occurs, thus showing the growth stages and the physical condition of the abalone.
2.2 Internal anatomy
2.2.1 Calyx lobe and radula
In the pharynx cavity there is a pair of calyx lobes and a radula which are located at the anterior part of digestive gland. These structures are important in terms of abalone taxonomy. The calyx lobe is made up of concholin and serves as the feeding organ. The long radula moves back and forth, chopping down the food with its small teeth (visible under a microscope) which is then further broken and absorbed during its passage through the esophagus, crop, stomach, spiral caecum, and intestine (Fig. 1).
2.2.2 Digestive system
The digestive system of abalone is placed on the left side of the adductor muscle located in the middle of the body. The digestive organ consists of mouth, gullet, stomach, intestine rectum and anus (Fig. 2).
Due to the spiral shape of the body, its digestive organs are curved, and the mouth and anus lay near to one side. The mouth is gibbous and ovate in shape and its walls composed of thick muscular tissue.
In the mouth are located the calyx lobe, radula and salivary glands. The gullet of the abalone is elongate and narrow. The abalone stomach is saccate and V-shaped and placed next to the gullet. The vermiform appendix and liver are attached to the stomach.
The liver is placed at the right side of the adductor muscle, protruding in the shape of an ox horn. The intestine and rectum start from the back side to the front margin of its body going round the left side of adductor muscle and then it curves back to the front side again. The intestine length is 3.27 times that of the shell. The digestive system of the abalone is long and complex, as it is with many herbivores.
Figure 1.
Figure 1. External features of an abalone without its shell (1-anus; 2-tentacle; 3-liver; 4-gill; 5-epipodium; 6-eye; 7–10-mantle; 11–12-mantle tactus; 13-heart; 14-foot; 15–16-adductor muscle; 17-spiral intestine).
Figure 2.
Figure 2. Digestive organs of an abalone (1-anus; 2-intestine; 3-liver; 4-gullet; 5–7-salivary gland; 8-radula; 9-mouth; 10-stomach).
2.2.3 The respiratory and circulation systems
The gills, lying immediately below the shell pores, function as the respiratory organ of the abalone. There is a pair of gills at the centre of the respiratory chamber, the left one being bigger than the right one. The left and back sides of the respiratory chamber are closed so as to allow the water into the gills from the upper and right sides of its head. The sea water undergoes gas exchange in the gills and runs out through the respiratory pores on the shell along with other excretions from the anus and ostium. The circulatory system of abalone is characterized by its patency. The heart in the pericardial cavity consists of one ventricle and two atria. The blood is colourless and contains amoebocytes. The blood from the heart runs to the blood sinus placed between organs through the circulation system. The blood in the sinus runs to gill artery and then to the gill itself. Oxygenated blood then runs to auricle through the gill vein. Abalones have a pair of gill arteries and a pair of gill veins.
2.2.4 Nerve system
The abalone nerve system is poorly developed. There are four pairs of brain ganglion located around the mouth: foot nerve chain, intestine ganglion, side nerve chain and the nerve conjunction which are connected lengthwise and crosswise (Fig. 3).
Figure 3.
Figure 3. Nervous system of an abalone (1-brain-linked nerve; 2-brain ganglion; 3-nerve beside the brain; 4-nerve under the brain; 5-side pedal ganglion; 6-pedal nerve; 7-ventral ganglion; 8-pedal nerve chain under gullet; 9-intestine ganglion over the gullet; 10-gill ganglion).