The area of the apical system is naked, as are the interambulacral areas on the upper surface of the test (Hendler et al. 1995). Four prominent plates cover the periproct. Pore pairs are arranged in a vertical series, and the interambulacral tubercles are equal in size. While most urchins possess both long primary and short secondary spines, A. punctulata possesses only the long type (Ruppert & Barnes 1994).
The purple-spined sea urchin, Arbacia punctulata, is named for its long slender spines, which are often purple to brown in color. However, spines of some individuals can also vary in color from reddish or reddish gray to almost black (Harvey 1956). The spines are generally the same color as the test, and the muscle bases are whitish with several brown to purple spots (Hendler et al. 1995). Tube feet are olive in color and inconspicuous aborally. Stalks of the aboral pedicellariae share the same color as the muscle bases. The oral side of the test is light brown to light purple, with a white peristome area, and silvery white circumoral feet with white terminal disks. The oral primaries are lighter than the aboral primaries, and sometimes appear with white and pink bands.
HABITAT & REGIONAL OCCURENCE:
A. punculata occurs in beds of the turtle grass, Thalassia testudinum, under coral rubble, and on rock, sandy or shelly bottoms (Hendler et al. 1995). Most individuals reside in shallow waters less than 50 m deep, although A. punctulata has been found down to 225 m (Serafy 1979).
The range of A. punctulata extends from Maine to Cuba, the Yucatán Peninsula, Florida to Texas in the Gulf of Mexico, Panama to French Guiana and north to Barbados (Serafy 1979, Kaplan 1988).
INDIAN RIVER LAGOON DISTRIBUTION:
The distribution of A. punctulata in the IRL is undocumented. However, they tend to be concentrated around rock jetties and other hard structures near inlets (Ruppert & Fox 1988; LH Sweat, personal observation).
A. punctulata reaches a total diameter, including spines, of about 100 mm. The test alone reaches a diameter of about 50 mm (Hendler et al. 1995).
The sex ratio of A. punctulata has been reported to be 1.03 males to every female (Shapiro 1935). Detailed abundance data remain unreported for the IRL, but both solitary individuals and aggregated urchins can be found in the lagoon.
REPRODUCTION & EMBRYOLOGY:
Shapiro (1935) reported an incidence of hermaphrodism in 1 of every 2,350 A. punctulata. The reproductive season of this urchin is long in the warm waters of Florida, but discrete spawns are more evident in spring and summer near the northern part of its range (Harvey 1956, Serafy 1979). A. pulchella is a popular species for biological research, especially embryological and larval studies.
The purple-spined sea urchin is primarily an herbivore, although it has been known to consume sponges, coral polyps, sand dollars, and even other A. punctulata (Harvey 1956, Fell et al. 1984). Foraging on algae and observed chemical attraction to preferred algal species increases at night (Hay et al. 1986) when the risk of predation is probably lower.
Of the many algal species available, studies have documented preferred grazing by A. punctulata on Gracilaria tikvahiae and Sargassum filipendula. However, food preference studies are not necessarily comprehensive, and may exclude species that would be consumed under certain circumstances. For example, Renaud et al. (1990) showed that the A. punctulata preferred dried Padina gymnospora over G. tikvahiae, suggesting that desiccation decreased the chemical defenses of the characteristically unpalatable P. gymnospora.
MOVEMENT & BEHAVIOR:
Locomotion in A. punctulata is achieved primarily via the spines, and not the tube feet (Agassiz 1873). When a shadow is cast on the urchin, as might occur in the case of an approaching predator, the protective spines are pointed in the direction of the object (Ruppert & Fox 1988).
Commensal relationships have been described between A. pulchella and up to 145 other species, especially copepods (Bell & McClintock 1982).
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Bell SS & JB McClintock. 1982. Invertebrates associated with echinoderms from the west coast of Florida with special reference to herpactacoid copepods. In: Lawrence JM (Ed.). 165-171. Echinoderms: Proceedings of the International Conference. Tampa Bay, 14-17 September 1981. Balkema, Rotterdam.
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Ruppert EE & RD Barnes. 1994. Invertebrate zoology, 6th edition. Saunders College Publishing. Orlando, FL. USA. 1056 pp.
Ruppert, EE & RS Fox. 1988. Seashore animals of the Southeast: A guide to common shallow-water invertebrates of the southeastern Atlantic coast. University of SC Press. Columbia, SC. USA. 429 pp.
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Shapiro H. 1935. A case of functional hermaphrodism in the sea urchin, Arbacia punctulata, and an estimate of the sex ratio. Amer. Nat. 69: 286-288.