Summary
Physical Description
Ecology
Life History & Behaviour
Anatomy & Physiology
Evolution & Systematics
Biogeographic Distribution
Conservation & Threats
References & Links | Anatomy & Physiology
External Anatomy
External anatomical features of a bleached test of D. savignyi.
The urchin body is essentially divided into the oral and aboral hemispheres, with organs arranged radially around the central pole. The oral surface faces the substratum and contains the mouth, while the aboral surface houses the anus and faces away from the substratum. Radially, the body is divided into 10 sections, alternating between ambulacral and interambulacral areas, each consisting of two skeletal plates. Ambulacral areas bear paired holes for tube feet, while interambulacral areas do not. The mouth is essentially a large aperture in the centre of the oral surface of the test, surrounded by a flexible membrane. This membrane bears buccal podia (short tube feet) and branching gills. On the opposite site, the anus (the large open pore in the centre of the test) is surrounded by a circular membrane called the periproct. Bordering the anus is 5 plates, 4 of which are genital plates, housing gonopores which release gametes, and 1 of which is a the madreporite. The madreporite is the site where water enters the urchins body. The spines are joined to the test via a ball and socket joint, where the ‘ball’ is called a tubercle. Most urchins have primary and secondary spines, which are long and short respectively. Spines of D. savignyi are cylindrical and hollow, and taper to a sharp point. The outer surface is circled in tiny barbs that point towards the spine tip. Pedicellariae are located across the test and peristome, and are tridentate, slim and needle-shaped in D. savigny (Lessio & Pearse, 1996).
Image showing the periproct and two spine types on D. savignyi found on Heron Island Reef
Internal anatomy
Internal anatomy of a regular urchin.
Adapted from Ruppert et al, 2004.
Urchins have a specialized jaw structure to enable them to feed successfully on algae. The urchins jaw apparatus, named Aristotle’s Lantern, is composed of 5 pyramidal calcareous plates arranged around the pharynx, each containing a sharp tooth at its tip that protrudes externally. Specialized muscles invert and retract the lantern through the mouth and move the lantern side to side, while other muscles open and close the teeth. These movements enable the Echinoid to scrape and tear at the substratum.
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Teeth of D. savignyi captured on Heron Island Reef, 2012
The pharynx joins to the esophagus, which leads to the stomach. The stomach secretes enzymes and is the site of extracellular and intracellular digestion, endocytosis and nutrient storage. A cecum is present where the esophagus meets the stomach, which may serve as the site of micro-flora culture for digestion of cellulose (Lewis, 1964, Lawrence, 1975), The stomach joins to the intestines, which are arranged in a zigzagging fashion by mesenteries. The rectum follows, which forms feces for expulsion out the anus.
Gas exchange is achieved through 5 pairs of peristomal gills on the oral surface, where gases diffuse directly from the water column into the coelomic fluid. The tube feet also act as gills that supply the water vascular system with water and gases. Coelomocytes within the system carry particulate waste to the gills, tube feet and axial organ (heart-kidney complex) for disposal or storage.
The water vascular system is supplied by the madeporite, which is essentially a slender canal that descends from the centre of the aboral surface to the ring canal that encircles and eosophagus. Radial canals extend from the ring canal and run along the inside of the test, and provide a medium for gas, food and waste transportation. The water vascular system is also responsible for locomotion via tube feet, which are simply small outpockets of the water vascular system that are powered by hydraulic pressure.
The nervous system of urchins consists of a nerve ring that encircles the pharynx inside the lantern, with radial extensions that run along the inside of the test. These radial nerves link to sensory cells in the epithelium of the test, spines, pedicellariae and tube feet. Sea urchins have statocysts located in stalked, spherical bodies called spheridia, found on ambulacral areas.
(Information summarised from Ruppert et al, 2004)
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