Orientation, aggregation, & locomotion
  black dot
Research study 1
 

photograph of a sand dollar Dendraster excentricus in feeding posture
drawing of a sand dollar Dendraster excentricus in feeding postureA unique behaviour of sand dollars Dendraster excentricus is their propensity to sit in the sand with their anterior ends buried.  This exposes the feeding surfaces to the water currents. Most of the food collection is done by the oral surface (see drawing on Right). Any particles caught up in the aboral surface layer of mucus are moved over the edge of the test and onto the oral surface. Food particles such as phytoplankton and organic matter are caught by sticky mucus on the tubefeet and passed along special food grooves to the mouth. O'Neill 1978 Oecologia 34:157.

The aboral surface of a sand dollar Dendraster
excentricus
has a dense pavement of short spines
and only small potential for food-gathering

  black dot
Research study 2
 

drawing of a juvenile sand dollar Dendraster excentricus showing heavy grains of iron oxide in the gut diverticula being used for stability in currentsThe higher surface area/volume relationships in juvenile sand dollars Dendraster excentricus as compared with adults increases the chance of currents disturbing their correct orientation in the sand.  A juvenile counters this by preferentially eating heavy sand grains and storing them in special diverticula of the gut.  These diverticula are distributed more towards the anterior end and form a kind of “weight-belt” to ensure that the front of the animal is buried in its correct angle of orientation.  Later in life the adults lose the sand grains.  Chia 1973 Science 181:73.

NOTE  lit. “turn aside”, referring to the fact that such small pouches lie to the side of a main chamber.  A human appendix is a diverticulum off the main gut tube

  black dot
Research study 3
  drawing of a side view of a sand dollar to show how its shape creates potential for lift in currentsInterestingly, the shape of the test of a sand dollar in cross section is like an airfoil and, indeed, water currents flowing across the test actually produce lift.  In higher current velocities, frictional drag forces become dominant, and the sand dollars are more likely to be lying flat on the sand surface.  At extreme current velocities, individuals may even bury themselves. Despite the obvious airfoil shape of a sand dollar’s test, however, the author considers that lift is not involved in attaining an oblique or vertical orientation. This apparently is accomplished by the spines crawling against sand grains.  Nakamura 1994 J Exper Mar Biol Ecol 178: 275.
  black dot
Research study 4
 

photograph of a bed of sand dollars Dendraster excentricus in Barkley Sound, British ColumbiaWhether a population of sand dollars Dendraster excentricus is in an inclined orientation or not depends upon habitat.  A survey along southern-California and Baja-California coasts shows that in sheltered bay areas with little water movement, sand dollars move about frequently and lie flat on the bottom where they feed on deposited material.  In tide channels on the outer coast with moderate water low, individuals tend to be stationary and in an inclined position, and in this manner they feed primarily on suspended material.  In wave-exposed outer-coast areas, adults are usually buried and presumably feed on deposited organic material.  Merrill & Hobson 1970 Amer Midl Nat 83: 595.

 

Bed of Dendraster excentricus in a wave-exposed
sandy location in Barkley Sound, British Columbia

 
  black dot
 
photograph of a sand dollar bed Dendraster excentricus in a commercial aquarium taken from a video

CLICK HERE to see a video of a sand-dollar bed in a commercial aquarium. The main difference between this view and the one above in Research Study 4 is that the sand dollars are standing straighter in this one.

NOTE the video replays automatically

  black dot
Research study 5
 

Sand dollars typically form large aggregations.  This may result from preferential settlement of larvae near to adults or from later movement of juveniles and adults.  That aggregation is important to sand dollars is shown in the following experiment done by researchers in Puget Sound, Washington.  Fifty adult sand dollars Dendraster excentricus are distributed randomly within a 2 x 2m-square fenced arena and left to move around (a simulation of the experiment is shown below). After 45d the 30 remaining individuals have aggregated within a 5 x 7cm area within the arena.  Birkeland & Chia 1971 J Exp Mar Biol Ecol 6: 265. 

NOTE  the authors of the study record densities in parts of their study area in excess of 600 individuals . m-2

NOTE  this behaviour is common in marine invertebrates.  Settlement close to established adults is a type of “bet-hedging”, for the newly metamorphosed juveniles are able start out in life in an area, as evidenced by the presence of the adults, where conditions are suitable for survival

  photograph of a sand arena for studies of aggregation behaviour of sand dollars Dendraster excentricus courtesy Birkeland & Chia 1971 J Exp Mar Biol Ecol 6: 265
At the start of the experiment the arena is cleared of sand dollars and other larger organisms, including a few predatory sea stars. NOT SHOWN TO SCALE
photograph of a sand arena for studies of aggregation behaviour of sand dollars Dendraster excentricus courtesy Birkeland & Chia 1971 J Exp Mar Biol Ecol 6: 265
Fifty sand dollars are added to the arena and left for several weeks. Not shown is a screen fence to minimise invasion of predatory sunflower stars
photograph of a sand arena at the end of a study on aggregation behaviour of sand dollars Dendraster excentricus courtesy Birkeland & Chia 1971 J Exp Mar Biol Ecol 6: 265
After 45d the remaining 30 sand dollars are aggregated within a small area within the enclosure. Most deaths result from sea stars that squeeze past the fencing
  black dot
Research study 6
  photo-schematic showing possible deflection of currents within a bed of sand dollars creating more favourable feeding conditionsWhy do sand dollars aggregate?  One idea relates to enhanced feeding by reduction of current flow within the group.  When individuals are densely packed, the entire mass acts to deflect water currents up and over the group.  The current increases in speed as it rises over a localised aggregation and slows close to the substratum surface. Small spin-off turbulence eddies make it easier for individuals to capture food particles.  Nakamura 1994 J Exper Mar Biol Ecol 178: 275.
  black dot
Research study 7
 

Another theory about why sand-dollars aggregate, also related to feeding, stems from the observation that in an aggregation of sand dollars more drawing showing how lift is generated by a sand dollar test in currentsindividuals are found in an oral:aboral orientation (62%) than in oral:oral (25%) or aboral:aboral (13%) orientations.  The theory goes as follows:

drawing showing effective feeding range of tube feet of a sand dollar if no lift were generated by the testBecause of its cambered airfoil shape, a sand dollar generates lift as it stands vertically in the current (imagine you are looking straight down at the sand dollar from above).





If there were no lift, the current would flow straight past and food particles would be caught by drawing showing effective feeding range of tube feet of a sand dollar if lift is generated by the testtubefeet and pedicellariae only out to the distance that they could reach, say, 4mm or so.







With lift, however, the food-bearing stream of water deflects inwards to follow the slight curvature of the oral surface, thus bringing more particles in reach of the food-capturing organs. Now the effective capture distance is much greater, say, 8mm.


drawing showing how the effective feeding range of tube feet of adjacent sand dollars is enhance when lift is generated by the tests


In an aggregation, even of 2 individuals, the streamlined curvature of water flow induced over the aboral surfaces of neighbours will thus enable more efficient feeding. This may explain why sand dollars are more often packed in oral:aboral/aboral:oral orientations.

photograph of the aboral surface of a sand dollar Dendraster excentricus

The aboral spines on Dendraster excentricus are short (about 1mm in length) and have clublike ends which meet to form a tight, pavement-like surface (photo on Left: note the longer oral spines just visible at the edge of the test).  This presents little frictional resistance to the water flowing past.  On the oral surface, however, the spines are longer (about 4mm in length), and are moved about to trap food.  As water flows past the oral spines in a current, turbulence is created, thus further enhancing feeding. O’Neill 1978 Oecologia 34: 157.

schematic showing expected frequencies of oral/aboral pairings of sand dollars if their side-to-side pairing were a result of random sorting
NOTE
if orientation of paired individuals were random then we would expect aboral:oral and oral:aboral pairings 50% of the time, and the other pairings each 25% of the times, as shown in the schematic on the Right. The author's results, however, show a significant deviation from the predicted 50% value of oral:aboral and aboral:oral pairings

  black dot
Research study 8
 

drawing showing optimum spacing between 2 sand dollars for maximum feeding effectivenessBy measuring the capture of micro-beads by sand dollars Dendraster excentricus in the oral:aboral orientation, the optimal between-animal distance is found to be 1cm.  At greater separation distances, the efficiency of bead capture goes down.  In the author's view the mutualistic use of water-flow patterns by sand dollars in this way uniquely explains their propensity to aggregate. O’Neill 1978 Oecologia 34: 157.

  black dot
Research study 9
 


A study on movements of sand dollars Dendraster excentricus at Alki Point, Washington shows that locomotory modes differ between young and adult stages.  Densities in the study site can be as many as 600 individuals . m-2.  The animals bury themselves at low tide and emerge at high tide, positioning themselves with anterior ends mostly downwards for feeding. While juveniles locomote using suckered tube feet, the adults move using their large oral spines.  Burial to a depth of 10cm takes about 15min for an adult (juveniles take 5min), while righting requires 2h or more.  An adult moves at 2cm . min-1.  If an obstacle is encountered while moving the animal rotates around it.  Chia 1969 J Exp Mar Biol Ecol 3: 162.

 

Sand dollars Dendraster excentricus buried at low tide at Crescent Beach, British Columbia. Such
individuals are highly susceptible to being crushed by careless and/or unknowing beach walkers. A few decades
ago, this particular beach was famous for it dense sand-dollar beds, but now only a few individuals exist

  black dot
Research study 10
 

chart showing acoustic backscatter from sand-dollar beds off Humboldt Bay, CaliforniaA research consortium based largely at Humboldt State University, California discover that side-scan sonar devices can be used to detect beds of sand-dollars Dendraster excentricus.  At first, the map showing distribution of sand dollars Dendraster excentricus near Humboldt Bay, CAresearchers were uncertain as to the origin of enhanced acoustic backscatter signals (see chart on Right) that they were receiving during routine scanning just seaward of the entrance to Humboldt Bay in northern California but, later, with the use of bottom-grab samplers they confirmed the presence of large beds of adult sand dollars Dendraster excentricus at around 20m depth.  Sizes range from 6-8cm diameter, with abundances up to 300 . m-2 (see map on Left).  The authors suggest that use of enhanced acoustic-backscatter methodology could be handy for studying the distribution and abundance of sand dollars.  Fenstermacher et al. 2001 Mar Georesources Geotechnology 19: 135.

  black dot
  RETURN TO TOP