Feeding & growth
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Research study 1
 

photograph of a sand dollar in feeding posture in the sanddrawings of side views of sand dollars to show their postures at high and low tidesA unique behaviour of sand dollars Dendraster excentricus is their propensity to sit in the sand with their anterior ends buried.  This exposes the feeding surface to the water currents. Sand dollars have their mouth and anus on the same surface, called the oral surface.  The opposite surface is called the aboral surface and is also involved in feeding, but to a lesser extent. 

Scattered over the oral surface are spines, pedicellariae, and photograph of a sand dollar Dendraster excentricus with details of feeding structurestube feet, and all take part in food capture.  Of several modes of feeding, the simplest is the catching of edible particles by sticky mucus on the tube feet. This material is collected in the food grooves and moved by cilia to the mouth.  Mucus for transport is secreted in the floors of the food tracts (on the oral surface) and ambulacral grooves (mostly on the aboral surface).  Buccal tube feet and oral spines assist in directing the mucous streams into the mouth.  Later, the undigested residues are expelled from the anus.  Foods are small crustaceans, phytoplankton (diatoms), algal bits, and bacterial coatings on sand grains.  It is not known whether buried individuals can feed.  Chia 1969 J Exp Mar Biol Ecol 3: 162.

NOTE  Lit. “away” “mouth”

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Research study 2
 

drawing of the spines, tube feet, and food groove showing a food particle being transportedMost interesting of the feeding modes of sand dollars is capture of live prey by spines, tube feet, and pedicellariae.  A study on Dendraster excentricus in the Los Angeles, California area indicates that about 80% of the food comprises small crustaceans. In this feeding mode, a prey is sensed, and the spines move to form trapping cones.  One or more pedicellariae then capture the prey from within the spine cone and crush it.  On its release from the pedicellariae, spines bat at the prey, and tube feet push it towards a food tract.  On its way to and in the groove more pedicellariae may bite the prey in a kind of “pre-mastication” process. 

A second feeding mode is used for non-motile food such as sand grains and algal bits.  These are grasped by tube feet and pushed into the food tracts (see drawing on Right). 

A third mode is a type of suspension-feeding where small organic particles are enveloped by mucus in the food tracts and pushed along by tube feet or by cilia (described in Research Study 2 above).  Rejection of unwanted particles usually occurs at the Y-junctions of the food tracts.  On arrival at the mouth, spines deflect outwards, and the food is moved into the mouth by tube feet and then crushed by the Aristotle’s lantern.  For live prey, the capture and transfer processes require about 15min, with 15min more required for mastication by the Aristotle’s lantern. Timko 1976 Biol Bull 151: 247; drawing of sand-dollar feeding adapted from Ruppert & Barnes 1994 Invertebrate Zoology 6th Ed Saunders College Publ NY

SUMMER DIET OF Dendraster excentricus IN THE LOS ANGELES AREA:

Phytoplankton 54%
(dinoflagellates 42%, diatoms, protists 12%)
Crustaceans 10%
Algae 6%
Sand grains 14%
Detritus 16%

  photograph showing a close view of the food tracts leading to the mouth of a sand dollar Dendraster excentricus
Food tracts leading to the mouth in D. excentricus. The grooves are full of sand being moved to the mouth. The Y-junctions are are points of rejection 2X
photograph of details of mouth, spines, and food grooves of a sand dollar Dendraster excentricus
Close view of mouth (2 o'clock location) in D. excentricus. The grooves are lined by spines, tube feet (withdrawn), and pedicellariae (small whitish bumps) 7X
photograph of a food groove of a sand dollar Dendraster excentricus in close view
View of a food groove at edge of test showing spines and tube feet. The tube feet can reach out some distance to snag edible items 10X
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Research study 3
 

graph showing test length with age of sand dollars Dendraster excentricusBy sanding and polishing the test of a sand dollar, it is easy to estimate years of growth by counting the number of annual lines seen on each ossicle.  Ossicles are hexagonally shaped.

From the graph on the Left an individual Dendraster excentricus of 8cm test-length is estimated to be 7-9yr of age. Birkeland & Chia 1971 J Exp Mar Biol Ecol 6: 265.

NOTE  each ossicle is a single crystal of calcium carbonate.  The crystals are hexagonal in shape and are separated from one another by a thin organic layer.  The layer secretes the calcareous material on both sides, adding new material to adjacent crystals seasonally in the form of rings (as on a tree)

NOTE  the authors compare growth in 2 populations in Puget Sound, which explains the 2 lines visible in the graph. Of more interest here, though, is the generalised growth curve for both populations shown in blue, allowing an averaged estimate of age to be made from a measurement of test length

 

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Research study 4
 

graph showing relationship between density and posture in sand dollars Dendraster excentricushistogram showing effect of organic content of sediments and density of individuals on proportion of sand dollars Dendraster excentricus exhibiting deposit-feeding modeSand dollars Dendraster excentricus are able to feed on either deposited or suspended organic particles, but what regulates the proportion of time spent in each mode?  This is investigated by researchers at the Scripps Institution of Oceanography, La Jolla, California, who conclude that sand-dollar density and organic content of the sediments are the main factors regulating the proportions that feed on deposits versus photograph of dense bed of sand dollars Dendraster excentricussuspended matter. When densities are high, fewer sand dollars are deposit-feeding (see graph on Right). Conversely, when densities are low, a greater proportion of individuals are deposit-feeding. The proportion can be regulated by changing the organic content of the sediments.  The researchers do this by establishing mesh-enclosed plots in a bed and mixing in known quantities of organic-rich sediments collected near to an adjacent marsh. Sand-dollar densities are adjusted to 3 levels in the plots.  Note in the accompanying histogram (one of several sets of data presented by the authors) that the proportion of deposit-feeding animals depends both on density and organic content, with high-density/unenriched plots having significantly fewer deposit-feeding individuals. Additionally, analyses of carbon stable-isotope ratios (delta13C)  of sand-dollar tissues and different food types suggest that suspended organic matter, including particles of drift macroalgae, represent the predominant food.  Fodrie et al. 2007 J Exp Mar Biol Ecol 340: 169.

NOTE  beds studies by the researchers are located in northern Baja California, Mexico

NOTE  densities are 80, 160, and 400 individuals . m-2 (a dense natural bed may have 600 individuals . m-2 or more

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