Keyhole Urchin

Keyhole Urchin

Mellita quinquiesperforata

Figure 1. Ventral and dorsal sides of Mellita quinquiesperforata

Kingdom: Animalia

Phylum: Echinodermata

Class: Echinoidea

Order: Clypeasteroida

Family: Mellitidae

Genus: Mellita

Species: M. quinquiesperforata

Introduction

Mellita quinquiesperforata, or the keyhole urchin, is a member of the mellitidae family under the echinoderm phylum. The keyhole urchin is commonly found on the warm southeastern coast and northern gulf coast of North America, along with the northern coast of South America (Weihe & Gray 1968). According to Salsman and Tolbert in Limnology and Oceanography in 1969, the keyhole urchin was found to be the most populous macroorganism along the shallows of the northern Gulf of Mexico. Keyhole urchins are more commonly found in shallow water; the population is most dense at 7 meters, and disappears completely at 15 meters (Salsman & Tolbert 1969). Keyhole urchins prefer a clean, sandy substrate and are able to handle wide seasonal temperature fluctuations (Weihe & Gray 1968). While keyhole urchins commonly only burrow under a thin layer of sediment, they have been found to burrow up to 8-10 centimeters deep during storms for protection (Weihe & Gray 1968). They are round and flat, with five holes called lunules, and their mouth and anus are on the ventral side of their body. They have a web of food tracts branching outwards from their mouths (Ghiold 1979). Keyhole urchins range in size from 60mm-110mm (Weihe & Gray 1968). Keyhole urchins are most commonly known as sand dollars, and are often collected after dying and being washed up on beaches.

Figure 2. Mellita quinquiesperforata burrowing and feeding along the ocean floor

Feeding Behaviors and Locomotion

Keyhole urchins have very interesting feeding behaviors. At night, keyhole urchins feed as they burrow through the sediment on the ocean floor; when the sediment is swept over the dorsal side of the keyhole urchin, small sediment particles are collected and digested (Findlay & White 1983). Small particles of sediment are passed through the lunules to the food tracts on the ventral side, while large particles slide right off the posterior dorsal side (Ghiold 1979). Once passed through the lunules, small particles of sediment are moved to the mouth through the food tracts by cilia (Ghiold 1979). Gut analysis of keyhole urchins revealed they eat diatoms, foraminifera, dinoflagellates, and other organic material (Findlay & White 1983). They cover about one centimeter of ocean floor every ten minutes (Salsman & Tolbert 1969). Keyhole urchins are very effective eaters, taking up to 50% of foraminifera in the sediment in a single pass (Findlay & White 1983). They move horizontally across the ocean floor, using spines along their ventral surface as tools for locomotion (Salsman & Tolbert 1969). The ambulatory spines move in continuous waves along the ventral side (Bell & Frey 1969). The locomotive spines wave anteriorly to posteriorly, and push the animal forward into the sand (Ghiold 1979). Around the posterior edge of the keyhole urchin, numerous podia (tube feet) can be found. These podia take up grains of sediment that have previously been passed over and move them to the food tracts (Findlay & White 1983).

Figure 3. Dorsal view of Mellita quinquiesperforata with labeled lunules and petaloids. Ghiold 1979.

Lunules and Petaloids

As can be assumed by the name Mellita quinquiesperforata, this species of keyhole urchins has five lunules. Lunules are the oval shaped holes seen above in figure 3. Keyhole urchins have two lateral lunules, two posterior lunules, and one anal lunule. The lunules are a very important and interesting part of keyhole urchin anatomy. They act as food gathering structures (Alexander & Ghiold 1980), and they are essential for efficient locomotion (Telford 1983). Located within the lunules, there are numerous podia, or tube feet, that can take up larger grains of sediment that have previously been passed over (Findlay & White 1983).

In terms of locomotion, when the key urchin moves horizontally across the ocean floor, water passes through the lunules, creating lift, making it easier for the urchin to continue moving (Telford 1983). As the size of the lunules increase, more water can pass through, and the critical velocity of the keyhole urchin also increases (Telford 1983). Another interesting feature of lunules in keyhole urchins is that the lunules grow at a faster rate than the rest of the body as a whole (Alexander & Ghiold 1980).

Another structure seen on the dorsal side of the keyhole urchin are the five petaloids seen above in figure 3. Petaloids are gill like structures that are used for gas exchange (Ghiold 1979).

Reproduction

Keyhole urchins are generally reproductively active in the spring and summer months. Before reproduction can occur, keyhole urchins store nutrients in their gonads during the fall and winter months to prepare (Tavares & Borzone 2006). After nutrients are built up in the gonads, the gametes begin to grow. In female keyhole urchins, it takes approximately three months for gametes to grow into mature eggs; while it takes one month for gametes to grow into mature sperm in male keyhole urchins (Lane & Lawrence 1979). The highest number of gonad indices are found in the early spring months, and spawning occurs in the later spring months (Lane & Lawrence 1979). After spawning occurs, the remaining oocytes are reabsorbed by the keyhole urchin (Lane & Lawrence 1979). Gonadal indices of larger keyhole urchins were greater than gonadal indices of smaller keyhole urchins, and the smaller keyhole urchins spawn later than their larger counterparts (Lane & Lawrence 1979). A possible simple explanation of this is that larger and more mature keyhole urchins are more efficient at storing nutrients, and therefore can devote more energy to gamete production (Lane & Lawrence 1979).

Alexander, D. E., & Ghiold, J. (1980). The Functional Significance Of The Lunules In The Sand Dollar,mellita Quinquiesperforata. The Biological Bulletin, 159(3), 561-570. doi:10.2307/1540822

Bell, B., & Frey, R. (1969). Observations on Ecology and the Feeding and Burrowing Mechanisms of Mellita quinquiesperforata (Leske). Journal of Paleontology, 43(2), 553-560. Retrieved from http://www.jstor.org/stable/1302333

Findlay, R. H., & White, D. C. (1983). The effects of feeding by the sand dollar Mellita quinquiesperforata (Leske) on the benthic microbial community. Journal of Experimental Marine Biology and Ecology, 72(1), 25-41. doi:10.1016/0022-0981(83)90017-5

Ghiold, J. (1979). Spine Morphology and Its Significance in Feeding and Burrowing in the Sand Dollar, Mellita Quinquiesperforata (Echinodermata: Echinoidea). Bulletin of Marine Science, 29(4), 481-490. Retrieved November 28, 2017, from http://www.ingentaconnect.com/content/umrsmas/bullmar/1979/00000029/00000004/art00004#expand/collapse

Lane, J., & Lawrence, J. (1979). Gonadal growth and gametogenesis in the sand dollar Mellita quinquiesperforata (Leske, 1778). Journal of Experimental Marine Biology and Ecology,38(3), 271-285. doi:10.1016/0022-0981(79)90072-8

Salsman, G. G., & Tolbert, W. H. (1965). Observations on the sand dollar,Mellita quinquiesperforata. Limnology and Oceanography, 10(1), 152-155. doi:10.4319/lo.1965.10.1.0152

Tavares, Y. A., & Borzone, C. A. (2006). Reproductive cycle of Mellita quinquiesperforata (Leske) (Echinodermata, Echinoidea) in two contrasting beach environments. Revista Brasileira de Zoologia, 23(2), 573-580. doi:10.1590/s0101-81752006000200033

Telford, M. (1983). An experimental analysis of lunule function in the sand dollar Mellita quinquiesperforata. Marine Biology, 76(2), 125-134. doi:10.1007/bf00392729

Weihe, S. C., & Gray, I. E. (1968). Observations on the Biology of the Sand Dollar Mellita quinquiesperforata (Leske). Journal of the Elisha Mitchell Scientific Society, 84(2), 315-327. Retrieved November 28, 2017, from http://dc.lib.unc.edu/cdm/singleitem/collection/jncas/id/2630/rec/8