This Spider Lassos Her Prey from the heavens above! (Monteverde Cloud Forest)
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Echinodermata have no definite circulatory system, Simple nervous system, respiratory system, or excretory system. They survive using a unique water vascular system that is involved in respiration, movement, and food gathering. The mouth is located on the underside of the body and has been termed ÒAristotleÕs lanternÓ. The organ consists of a five symmetrical parts including teeth and a fleshy tongue-like structure. The digestive track is also made up of five parts that come together at the anus on the top of the test body.
The skeletal structure of the Diadema antillarum is made of radial symmetrical plates that join together to form a round or flattened disk shape. The test is the outer shell of the body and is composed of calcium carbonate and is covered in a thin epithelium layer. The test is rigid to protect the inner body but is also very fragile and is rarely found whole along the beach or seabed after death. The size of this species varies based on habitat and population density but typically the height measures up to 50% if the width. Diadema antillarum reach up to an average of 10 cm in diameter as adults.
The typical test color of Diadema range from dark purple to black and some can have detectable blue spots set in row along the top of the test. These blue spots are light sensors covered with the dark pigment of the epithelium that end up producing a bluish pigment. Diadema are very sensitive to both light and dark, which can sometimes reflect in the shade of the test. Darker species are found in brighter areas with clear water, and in darker area such as crevices or deep water, lighter urchins inhabit. Juveniles have black bodies with either white spines or black spines banded with white. A mature Diadema will most always loss any white pigment and have long black spines.
The spines are one of the most identifiable characteristics of the Diadema antillarum species. Spines are a characteristic shared in all Echinodermata, and are essential for locomotion, protection, and trapping food. They have a ball-in-socket movement and are set in tubercles. They are long, slender, and hallow making them brittle and susceptible to break off if they pierce an object. The spines of Diadema antillarum can reach up to 16 inches long and are typically three to four times as long as the diameter of the test. There are also secondary spines that are shorter but just as flexible and play more of a role in food collection than locomotion. Five double rows of spines make up the largest tubercles and give the test a symmetrical banded appearance. The spines are especially useful protecting the very active Diadema antillarum. Although the spines are hallow, they contain glandular tissue that produces a toxin. Along with mild toxin, small barbs directed toward the tip cover the spine to further prevent predators from reaching the test.
Tube feet are another characteristic that is associated with all of the species in the Echinodermata phylum. Sea urchins use their tube feet to capture food, for respiration, move about, and to hold on to their base. The five radial plates are perforated for the tube feet. They are aligned in ten rows that are equally proportioned from the top of the test down to the underside. UrchinsÕ tube feet are like our finger tips in that they are extremely sensitive. They are capable for extreme extension that aid in locomotion and the suckers, or terminal disk at the end of the feet allow the urchins to hold on to coral reef or rock beds. The tube feet on the upper side of the test are used more for respirations rather than locomotion or stability.
As discussed before, Diadema had highly sensitive light receptors on the surface of the test that detect changes in both darkness and light. During the day they typically find shelter under reef structures or other types of crevices. Diadema can also be seen exposed during the day where predators are not as much of a treat. In the evening, the urchins become more active as they seek lighter areas due to adaptation to light. As night falls, the dark protective pigment in the test withdraws allowing the nerves to be exposed. This results in the urchins to become more sensitive to light thus influencing them to seek dark areas as the light exposure increase before sunrise.
Diadema manage to inhabit almost all marine environments such as sea grass beds, mangroves, coral reefs, and sand or rock. In relation to other species of sea urchin, Diadema antillarum canÕt tolerate heavy wave action so they tend to be found in shallower waters around 1 to 10 meters deep, but can also be found depths of 400 meters. During the day they can be found in clumps as much as 20 per square meter. At night they move out of their protective covering to graze on the wide variety of plant material found along reefs and coral rock.
Sea urchins foraging behavior is fundamental in maintain the health of the coral reefs that they inhabit. As they feed on plant material along coral rock, the clear area left behind provides a suitable environment for coral larvae. They also tend to avoid grazing in areas that they had fed on the previous night in order to maximize food intake while managing algae growth. The digestive material that urchins secret provides supplemental nitrogen to the nutrient-limited algal turfs. The mass mortality of the black sea urchin in 1983 proved just how influential the species is to its habitat. A 97% loss of adult Diadema antillarum lead reproduction levels to be low thus leading to a slow recovery in population.
Spawning seasons typically begins late in the winter and continues into early summer. The process involves a simultaneous release egg and sperm into the surround water. For this reason most of the sea urchins in the area will spawn all at the same time. It is not uncommon to see mass spawning to travel up current and eventually travel through the whole populated area.
The process begins when a chemical substance is release in the water by either the male or female that produces a scent that signals every sea urchin in the area to release its sperm or egg. This can be identified by a slender stream of yellowish egg or white sperm being release from the five reproductive glands on the top of test called gonopores. The females are capable of releasing several million eggs in the water and mass spawning ensures an increase likelihood that they eggs will be fertilized. Once the sperm reaches the egg the embryo enters in the first stage of development called a bastula which is essentially a hallow ball of cells. The embryo then develops two layers, or gastrula. The gastrula then develops into a platonic larval stage called an echinopluteus where four to six pairs or arms begin to grow. This process last around four to six weeks until the larvae settle into the bottom where the process of metamorphoses begins.
The development of larvae into to a juvenile urchin occurs as quickly as one hour but only become recognizable after one to two months when the test diameter reaches about one centimeter. At this stage in development they are at high risk of being eaten by vast array of fish predators. The juvenile urchins may be seen at night when they come out to feed; otherwise they find protection under rock and crevices. In the first months after metamorphose, Diadema have the most rapid period of growth. Once a juvenile Diadema reached around 6cm in diameter growth slows down tremendously. After about three to four years the Diadema antillarum should be at its full size. Life expectancy of Diadema antillarum is estimated to be four to five years but has yet to be proven.
The significant influence that Diadema antillarum sea urchin has on the marine environment is undeniable. As a relatively simple organism in anatomical structure, they play a significant role in the health coral reefs. The risk of their extinction a few decades ago proved that their existence is essential to the well being of all the marine life in their habitat.
Works Cited
Ogden, J. C. & Carpenter, R. C. (August 1987). Long-Spined Black Sea Urchin. Species Profiles: Life Histories and Environmental Requirements of Coastal Fishes and Invertebrates (South Florida)
Lawrence, J. M. (Ed.). (2001). Ediblie Sea Urchins: Biology and Ecology Vol. 32. Tampa, Florida: Elsevier.
Hendler, G., Miller, J. E., Pawson, D. L. & Kier, P. M. (1995). Echinoderms of Florida and the CaribbeanSea Stars, Sea Urchins, and Allies Washington: Smithsonian Institution.
Martin, R. E. (Ed.). (2000). Other Aquatic Life of Economic Significance: Sea Urchins. Marine & Freshwater Products Handbook (pp. 265-272). Lancaster, Pennsilvania: Technomic Publishing Company, Inc.
Farland, K. C. Long-spined Sea Urchin (Diadema Antillium) Marine Invertebrates of Bermuda. Retrieved May 24, 2008, from http://www.thecephalopodpage.org/MarineInvertebrateZoology/Diademaantilla rum.html
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