Geography 316:  Biogeography    
Update 01/19/2005

Thank you for visiting our site. This web pages was written by a student in Geography 316: Biogeography and edited by the instructor, Barbara Holzman, PhD.  All photos and maps are posted with specific copyright permission for the express use of education on these web pages. The students have tried to be as accurate as possible with the information provided and sources and references are cited at the end of each page.
 

The Biogeography of the

  Western Sand Dollar       (Dendraster excentricus)
 

by Karen Vitulano,
student in Geography 316
 


Kingdom: Animalia
    Phylum: Echinodermata
        Class: Echinoidea
            Order: Clypeasteroida
                Family: Dendrasteridae
                    Genus: Dendraster
                        Species:
Dendraster excentricus 

 

 

Description of Species:  

The Sand Dollar Dendraster excentricus is a marine invertebrate that lives in the sandy bottoms of sheltered bays and open coastal areas.  They form dense beds in the low intertidal and subtidal zones of sheltered bays, and in the subtidal zone just beyond the break zone of coastal areas.

The body of Dendraster excentricus consists of a rigid test (hard external covering) covered with movable spines.  It commonly reaches a size of 75mm in diameter although this is highly location-specific.  Although the common conception of the Sand Dollar is the smooth white empty test that washes ashore on North American west coast beaches, the living animal is colored from a pale gray-lavender to a dark purplish black, its aboral (top) spine tips sometimes much lighter.  It has a water-vascular system – a system of water-filled canals derived from the internal cavity or coelom that connect with tube feet.  The tube feet are arranged in five paired rows that extend from the mouth on underside to the top (aboral) pole.  The tube feet are protruded and withdrawn from the ambulacra – the five radial areas on the undersurface of the animal, and serve a locomotion, feeding, and possibly a respiratory function.  The five ambulacral rows alternate with five interambulacral areas, where calcareous plates occur in patterns and are bound into the rigid test.


A flower-like pattern is seen on the aboral side of the body composed of pore pairs where specialized tube feet perform gas exchange.  At the center is the madreporite - a perforated platelike structure that forms the intake for their water-vascular system, and adjacent to this on the interambulacra, the genital pores.  Radiating out from the genital pores are the five flower petals, which represents the ambulacral radii.  The flower pattern in this species is off-center, thus the meaning of the name excentricus.
 

Natural History:  

Feeding

Dendraster excentricus is considered a suspension feeder and feeds on suspended organic particulate matter in the water currents.  Various specialized spines, tube feet, mucus-secreting glands and the pedicellariae – small pincher-like organs with moveable jaws, are involved in feeding (Jangoux and Lawrence 1983).  The spines on the aboral side are club-shaped and covered with cilia.  When small organic particles and organisms flow onto this surface, little eddies are created and particles become trapped by mucus secreted on the spines.  They are directed by these club-shaped spines and cilia to the margins and around to the oral side to the food grooves, where they move in mucus streams that lead to the mouth in the center (Morris 1980).       

Larger suspended particles may be held by sucker-tipped podia and transported to the food grooves.  For living prey such as crustacean larvae and small copepods, the spines on the oral side converge and enclose it into a little tent-like cone, where it is passed to a food groove by the jaws of the pedicellariae (Morris 1980).  Sand is ingested with the food and it is thought that this is deliberately done by juveniles to act as a “weight belt” for stability in the sand. 
    

The Western Sand Dollar is able to feed in this manner because of its eccentricity, which allows it to bury itself “standing up” obliquely in the sand.  This is their feeding position, parallel to the surge current with the anterior end buried (Lawrence 1987).  By positioning themselves closely together, they may exploit their hydrodynamic shape and influence the current flow past their bodies.  It is likely that this is the reason they form densely packed beds of several hundred individuals per square meter.  In light currents, they may stand perpendicular, lay flat, or even bury themselves (Jangoux and Lawrence 1983). 

Dendraster excentricus at Monterey Bay Aquarium, Monterey, California

Reproduction

Reproduction is sexual and the Western Sand Dollar reaches sexual maturity between 1 and 4 years of age (Smith, 1984; Morris 1980).  The swelling of the gonads in May marks the beginning of the major spawning period, which lasts through July or August.  The female Dendraster discharges the eggs already ripe (meiosis already occurred) through her gonopores and are fertilized externally by the male, who protrudes his genital papilla from his body wall to increase the distance that the sperm can reach the eggs.  Within 2-4 days, four-armed pluteus larvae develop and become a part of the floating zooplankton.  Adult Sand Dollars will eat their own larvae but not their eggs, due to a protective jelly coating.  Individuals probably spawn multiple times.  Females can produce over 350,000 eggs per year (Morris 1980).  Brooding, or the association of the juvenile with the female parent, is not common in echinoderms because it would decrease reproductive productivity. 

 The larvae may travel quite a distance with the currents and do not necessarily stay near the parent bed group.  After the larvae develop, they pick up a chemical cue that is produced by the adult Dendraster excentricus (Highsmith 1982).  They then settle within or adjacent to an existing sand dollar bed.  After they settle, they undergo metamorphosis and begin to grow their adult sand dollar form.  This whole process has taken 62-162 days when replicated in the laboratory (Morris 1980).  Studies within the intertidal environment have shown that if the larvae do not settle in an existing bed, they are more likely to be eliminated by predators (Highsmith 1982).    

Life and Death

While the dense beds that adults form offer some protection from predators, predation remains a principal cause of death.  Fish are the most important predators of echinoids in general, and Dendraster must look out for the California sheephead, the starry flounder, and the large pink sea star in particular.  When sea stars walk through a sand dollar bed, a meter-wide clearance has been observed around it where the sand dollars have disappeared below the sand (Morris 1980).  Crabs also attack sand dollars by using their claws to crack open the test at the margin.  Some 5% of one population of Dendraster sand dollars had abnormally configured tests, thought to be signs of failed crab attacks (Smith 1984).  Sea gulls will eat Dendraster when washed ashore or exposed at low tide. 

Large storms can cause mass mortality of sand dollars in shallow-water, smothering them in deep sand or washing them onto the beach in large numbers where they are pulverized.  High temperatures and desiccation also cause mass mortality if low tide coincides with a hot midday and the animals are exposed to air for just 2 to 3 hours.  But strangely, old age is thought to be the principal cause of death of Dendraster excentricus (Smith 1984).  Their life span can range from 1 to 15 years. (Smith 1984). 


Evolution
 

All members of the Phylum Echinodermata possess radial symmetry – the symmetrical arrangement of constituents, especially of radiating parts, about a central point.  It is thought that this feature evolved from an ancestor that was sessile or attached.  Since a sessile way of life is an evolutionary dead end, the body changes that subsequently occurred for a free-moving lifestyle were on top of the significant modifications that already occurred for sessile life (Lawrence 1987).  All extant classes except for one are free-living and have been throughout their fossil record.  Their pentamerous (having 5 similar parts) body form has remained, but the significance of this is unknown.   

Members of the Class Echinoidia first appeared in the Ordovician period (the second period of the Paleozoic era).  Rigid tests were evolved and they underwent a great adaptive radiation in post-Paleozoic times.  There was a marked reduction in the Permian and Triassic periods of the Mesozoic Era after which it resumed its diversification, which has continued until the present day. Fossils of the inner skeletons of these echinoderms are common in both Mesozoic and Cenozoic sediments.  

Dendraster excentricus shares the class Echinoidea with the Sea Urchins.  Echnoidea are divided into two groups – Regularia and Irregularia on the basis of body form, with the Regular Echinoids being spherical with radial symmetry.  The irregular echinoids lost their pentameral symmetry and are bilaterally symmetrical instead.  Dendraster is considered an irregular echinoid. 
 

The cladogram at the right shows the earliest echinoids to be the lepidocentrids, which includes the ancestors of several later lineages.  Several offshoots occurred throughout the Palaeozoic but by the end, echinoids were on the decline.  Only one genus is believed to have survived the Permo-Triassic life crisis – the Miocidaris, which saved the group from extinction.  All post-Palaeozoic echinoids evolved from them, and a great adaptive radiation occurred.  Two different types of echinoids – the cidaroids and euechinoids emerged.  From the euechinoids, the remaining forms emerged including the irregular echinoids in the early Jurassic, who diversified rapidly and now represent 47% of all living species of echinoids (Smith 1984).    

Whereas the food  of the regular echinoids is primarily macroscopic, the development of irregular echinoids that feed on particulate matter was a major evolutionary event that allowed them to exploit habitats and food sources unavailable to regular echinoids (Lawrence 1987). 
 

  Inferred phylogeny of major echinoid groups  Source: Smith 1984

The clypeasteroids are the most recent group of echinoids to have evolved and represent the Order of Dendraster excentricus
 

The Order Clypeasteroida originated from the Upper Palaeocene of West Africa and rapidly diversified to become world-wide by the Oligocene, with all extant families evolved by the Miocene (Smith 1984).  The earliest disk-shaped animal appears in the southern U.S. (Gulf/Caribbean region) by the late Eocene (Superfamily Scutellidea) and diversified and spread out in three independent lines.  This southern US group is thought to be a sister group to the Echinarachniidae, which appeared in the Oligocene and spread up the west coast of America. 

  Evolution of sand dollars in time and space. Ornamentation indicates the present day distribution of the various groups of sand dollars. Stars and circles on the map mark approximate centers of origin for lineages as indicated; arrows show the suggested migration pathways.                             
 

Source: Smith 1984

   
       

The Genus Dendraster originated in the northwestern Pacific in the Pliocene and may have evolved from the Echinarachniidae.  Another theory, however, points to the tooth structure of Dendraster, suggesting a derivation from mellitids instead (Smith 1984).

Smith (1984) gives a complete evolutionary history of the clypeasteroids.  Rapid expansion led to geographical separation, and a certain amount of convergence occurred among different lineages.  The convergent features include the evolution of the discoid shape and branched food grooves, and the evolution of lunules and notches, increasing the test periphery.  All these features help improve feeding.  Smith notes that this parallel evolution in different lineages was possible because of a change in feeding strategy by their ancestor that resulted in the ability to feed on finer sediments.  This proved so successful that different groups evolved independently to feed solely on these particles, leading to the important morphological adaptations mentioned above.  This was likely the reason for the dramatic radiation of clypeasteroids.  
 

 

Distribution

Dendraster excentricus are common marine invertebrates that occur off the northeastern shores of the Pacific Ocean.  They reside in the low intertidal and subtidal zones of sheltered bays, and in open coastal areas to depths of 40 to 90 meters (Morris 1980, Mooi 1997).  They occur in localized aggregations in large numbers wherever there is a suitable sandy or sandy-mud substrate, preferring fine siliceous sands (Mooi 1997).  Range is from southern Alaska continuously to the south to Baja California, Mexico.  Studies show Juneau, Alaska is considered the northern limit, and Cabo San Lucas, Mexico is likely the southern limit (Mooi 1997).  Out of the three extant species of Dendraster, the range for Dendraster excentricus is the largest and includes the range of the other two: D. vizcainoensis and D. terminalis, both of which are much more limited (northern Baja only, and southern California to northern Baja, respectively).

This distribution originates from one of six important centers of endemism or faunal provinces that have been identified for echinoids towards the end of the Cretaceous period– in this case the Gulf and Caribbean center (Smith 1984).               

 


Other interesting facts:

 
 Christians have found symbols in the form and appearance of another species of Sand Dollar.  They call it the “Holy Ghost Shell” because:

"…the markings on the shell symbolize the Birth, Crucifixion and Resurrection of Christ. On top of the shell, an outline of the Easter Lily is clearly seen. At the center of the Lily a five pointed Star representing the Guiding Star of Bethlehem that led the Wise Men. The five narrow openings are representative of the four nail holes and the spear wound made in the body of Christ during the Crucifixion.

Reversing the shell you will easily recognize the outline of the Poinsettia, the Christmas flower, and also the Bell. When broken, inside the shell are five little birds called the Doves of Peace. Some say they are the Angels that sang to the Shepherds the First Christmas Morning."

 -  THE LEGEND OF THE SAND DOLLAR, from http://www.tropicabelize.com/Crab%20Collection/Crabs%20Photos/Legend%20of%20the%20Sand%20Dollar.htm

 

 

 

 

Bibliography
 

Allaby, Alisa and Michael Allaby. 1999.  A Dictionary of Earth Sciences.  Oxford: Oxford University Press.  Available: http://www.xrefer.com/entry.jsp?xrefid=613915

Chia, Fu-Shiang.  1969.  “Some observations on the locomotion and feeding of the San Dollar, Dendraster Excentricus.”  Journal of Experimental Marine Biology and Ecology. 3(2): 162-170.

Emlet, R.B.  1986.  “Larval production, dispersal and growth in a fjord: a case study on larvae of the Sand Dollar Dendraster Excentricus.”  Marine Ecology Progress Series. 31(3): 245-254

Highsmith, Raymond C.  1982.  “Induced settlement and metamorphosis of Sand Dollar (Dendraster excentricus) larvae in predator-free sites: adult sand dollar beds.”  Ecology.  63(21): 329-337

Houghton Mifflin Company. 1992.  The American Heritage Dictionary of the English Language, Third Edition.  Electronic version licensed from Lernout & Hauspie Speech Products N.V. and utilized as Microsoft Bookshelf 2000 CD-ROM.

Jangoux, Michel and John M. Lawrence.  1983.  Echinoderm Studies.  Rodderdam: A.A. Balkema.

Lawrence, John.  1987.  A Functional Biology of Echinoderms.  London & Sydney: Croom Helm.

Lowndes, Michael (webmaster).  2002.  Natural History Museum of the UK website.  London.  Available: http://www.nhm.ac.uk/

Mooi, Rich.  1997.  “Sand Dollars of the Genus Dendraster (Echinoidea: Clypeasteroida): Phylogenic Systematics, Heterochrony, and Distribution of Extant Species.”  Bulletin of Marine Science, 61(2): 343-375.

Morris, Robert.H., Donald Abbot and Eugene Haderlie.  1980.  Intertidal Invertebrates in California.  Stanford:  Stanford University Press.

Smith, Andrew.  1984.  Echinoid Palaeobiology.  London: George Allen & Unwin. 

Smith, R., and James T. Carlton.  1975.  Light’s Manual:  Intertidal Invertebrates of the Central California Coast.  Berkeley: University of California Press.

Wagner, Carol D.  1974.  “Fossil and Recent Sand Dollar Echinoids of Alaska.”  Journal of Paleontology.  48(1):105-123

 

send comments to bholzman@sfsu.edu
 

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