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Arachnoides placenta (Linnaeus, 1758)
Cake Sand Dollar (Arachnoides placenta)

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Cake sand dollar on Changi Beach (Image by Hannah Yeo, 2016)

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Sand dollars are echinoderms, this means that they are related to sea cucumbers, sea stars and sea urchins. As a matter of fact, sand dollars are flattened sea urchins. Typically, we think of sea urchins (regular) as looking something like this.

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White salmacis urchin (Salmacis sphaeroides) on Changi Beach (Image by Hannah Yeo, 2016)

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The body changes from a round (spherical shape) to a flat (disc shape) skeleton like this.

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Flat disc like body of the cake sand dollar (Image by Ria Tan, 2009)

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Spines of regular sea urchins are usually elongated, but the spines in sand dollars are tiny and short, which helps them move or burrow into the sand. This can be seen in the video embedded below.

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Short spines and burrowing movement of a sand dollar 

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A distinctive trail is left behind in the sand at low tides and the sand dollar can be found just beneath the surface at the ends of these trails. Look out for these trails the next time you visit the intertidal zone!

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Trails left in the sediment by the cake sand dollars at low tide (Image taken by Hannah Yeo, 2016)

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The jaw apparatus of sand dollars (called the Aristotle’s lantern) is compressed and modified into a crushing mill.

The Aristotle’s lantern in regular urchins are usually thicker and looks like this.

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Aristotle’s lantern in regular urchins

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“What (and How) do Sea Urchins Eat? Sea Urchin Feeding Roundup!”, by Chris M, The Echinoblog (9 Jul 2013). URL:http://echinoblog.blogspot.sg/2013/07/what-and-how-do-sea-urchins-eat-sea.html (accessed on 17 Nov 2017).

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Here is a picture showing the Aristotle’s lantern in sand dollars.

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Aristotle’s lantern in a sand dollar

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“My star of sand dollar doves”, by Keri Fabin, Fabin Bros. Farms (10 May 2011). URL:http://kerimehome.blogspot.sg/2011/10/my-star-of-sand-dollar-doves.html (accessed on 29 Oct 2017).

(Permission pending)


Can you spot the five doves? Take a closer look at the photo.

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Five kissing doves also known as the Star of Bethlehem

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“My star of sand dollar doves”, by Keri Fabin, Fabin Bros. Farms (10 May 2011). URL:http://kerimehome.blogspot.sg/2011/10/my-star-of-sand-dollar-doves.html (accessed on 29 Oct 2017).

(Permission pending)

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. At the heart of the skeleton is the Star of Bethlehem, which actually is the complete Aristotle’s lantern. The five doves that are thought to spread peace are the broken fragments of the lantern. 

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Five doves are broken fragments of the Aristotle’s lantern

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“5 "Doves" escaping from inside a Sand Dollar”, by photoholic1. Flickr, 31 Aug 2009. URL:https://www.flickr.com/photos/lenbo/3920415711 (accessed on 29 Oct 2017).


Just Google “the legend of the sand dollars” and you can find crafts or old post cards like this one. 

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Old post card on the legend of the sand dollar

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“Sand Dollars ARE Sea Urchins. Please make a note of it!”, by Chris M, The Echinoblog (13 Mar 2012). URL:http://echinoblog.blogspot.sg/2012/03/sand-dollars-are-sea-urchins-please.html (accessed on 17 Nov 2017).

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, the cake sand dollar is widely distributed throughout the Indo-West Pacific region. 

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Global distribution of the cake sand dollar (Retrieved from the Global Biodiversity Information Facility

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 “Arachnoides placenta (Linnaeus, 1758)”, by Global Biodiversity Information Facility (GBIF). GBIF Backbone Taxonomy. Checklist Dataset. URL:https://www.gbif.org/species/4875329 (accessed on 25 Oct 2017).

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Distribution of the cake sand dollar

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A map displaying the localities in which the cake sand dollar has been recorded. (Map from Global Administrative Areas

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“Global administrative areas (GADM)”, by R. Hijmans (30 Aug 2009). URL:http://gadm.org/ (accessed on 30 Nov 2017).

. Annotations by Hannah)

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Aung, W. (1975). Observations on the reproductive biology of the tropical sand dollar Arachnoides placenta (L.) (Echinodermata: Echinoidea). M.Sc. Thesis Department of Marine Biology, James Cook University, Townsville, Queensland Australia. 141 pp.

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An example of tropical intertidal sandy habitat with high abundance of cake sand dollars. Those small "bumps" on the sand are actually all cake sand dollars! (Image by Hannah Yeo, 2016)

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“Sand Dollars Order Clypeasteroida”, by Ria Tan, Wild Singapore (11 Apr 2009). URL:http://www.wildsingapore.com/wildfacts/echinodermata/echinoidea/sandollar/sandollar.htm (accessed on 11 Nov 2017).

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Aboral (left) and oral (right) view of A. placenta on Changi Beach, Singapore (Image by Hannah Yeo, 2016)


Although colorful when alive, dead sand dollars are equally just as interesting! They may have lost their colour and spines, but every feature can be clearly seen, as labelled in the diagram below. The aboral and oral terms are common when describing echinoderms and refer to the surface opposite the mouth and the side with the mouth respectively

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Labelled aboral (left) and oral (right) view of A. placenta (Image taken from WoRMS, and was edited to include labels)

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Arachnoides placenta (Linnaeus, 1758)”, by World Register of Marine Species (WORMS) (9 Mar 2014). URL:http://www.marinespecies.org/aphia.php?p=taxdetails&id=513113 (accessed on 25 Oct 2017).

(This work is licensed under a Creative Commons)

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Species namePhotoDistinguishing features
The Cake Sand Dollar 
(Arachnoides placenta

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(Image by Hannah Yeo, 2016)

  1. Circular with no slots in the body.
  2. Star/flower shaped pattern is not obvious

Keyhole sand dollar

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“Keyhole Sand Dollar Echinodiscus sp. Family Astriclypeidae”, by Ria Tan, Wild Singapore (13 Apr 2009). URL:http://www.wildsingapore.com/wildfacts/echinodermata/echinoidea/sandollar/echinodiscus.htm (accessed on 25 Oct 2017).


(Echinodiscus sp.)

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(Image by Ria Tan, 2008)

  1. Less circular
  2. Have slots in the body
  3. Larger in size (8-10cm)
  4. Star/flower shaped pattern is obvious

Thick-edged sand dollar

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Jacksonaster Lambert in Lambert & Thiery, 1914, p. 313”, by The Echinoid Directory, Natural History Museum, London (21 Apr 1997). URL:http://www.nhm.ac.uk/our-science/data/echinoid-directory/taxa/taxon.jsp?id=791 (accessed on 12 Oct 2017).

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 “Thick-edged sand dollar Jacksonaster depressum Family Laganidae”, by Ria Tan, Wild Singapore (8 May 2015). URL:http://www.wildsingapore.com/wildfacts/echinodermata/echinoidea/sandollar/depressum.htm (accessed on 11 Nov 2017).


(Jacksonaster depressum)

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(Image by Ria Tan, 2008)

  1. Oval shaped instead of circular
  2. Thicker test
  3. Longer spines
  4. Star/flower shaped pattern is obvious

Pink sand dollar

Footnote Macro

Jacksonaster Lambert in Lambert & Thiery, 1914, p. 313”, by The Echinoid Directory, Natural History Museum, London (21 Apr 1997). URL:http://www.nhm.ac.uk/our-science/data/echinoid-directory/taxa/taxon.jsp?id=791 (accessed on 12 Oct 2017).

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“Sand Dollars Order Clypeasteroida”, by Ria Tan, Wild Singapore (11 Apr 2009). URL:http://www.wildsingapore.com/wildfacts/echinodermata/echinoidea/sandollar/sandollar.htm (accessed on 11 Nov 2017).


(Peronella lesueuri)

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(Image by Ria Tan, 2011)

  1. Oval shaped instead of circular
  2. Bright pink colour
  3. Star/flower shaped pattern is obvious

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. Sometimes juveniles, very small ones (0.6-0.8cm), can be found buried under the sand near groups of larger individuals.

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Very small cake sand dollar found amongst larger ones at East Coast Beach (Image by Hannah Yeo, 2016)


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Small to large cake sand dollars on East Coast Beach (Image by Hannah Yeo, 2016)

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. The picture below shows a very nice gradual colour progression as the species get bigger. However, little is known/studied on why there this change in colour from juveniles to adults.

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Size and colour progression of A. placenta found in the study by Haycock (2004) [7]

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“Internal buttressing”, by The Echinoid Directory, Natural History Museum, London (8 May 2015). URL:http://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/dollars/buttressing.html (accessed on 12 Oct 2017).

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Series of buttresses in the internal test of sand dollars

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“Internal buttressing”, by The Echinoid Directory, Natural History Museum, London (8 May 2015). URL:http://www.nhm.ac.uk/our-science/data/echinoid-directory/morphology/dollars/buttressing.html (accessed on 12 Oct 2017).

(Permission pending)

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“Keyhole Sand Dollar Echinodiscus sp. Family Astriclypeidae”, by Ria Tan, Wild Singapore (13 Apr 2009). URL:http://www.wildsingapore.com/wildfacts/echinodermata/echinoidea/sandollar/echinodiscus.htm (accessed on 25 Oct 2017).

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, the larvae goes through several stages of development. These stages can be seen in the video below (Note: the video shows the life cycle of a sea biscuit (Clypeaster subdepressus) larvae, which are closely related to the cake sand dollars).

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The life of a cycle of a sea biscuit 

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. Over its lifetime, the cake sand dollar will gradually grow in size. A diagram of the life cycle of a typical sea urchin is shown below. Remember how sand dollars are related to sea urchins? [link to urchins to sand dollars]

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The life cycle of a typical sea urchin

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 “Reproduction and life history”, by The Echinoid Directory, Natural History Museum, London (17 Oct 2015). URL:http://www.nhm.ac.uk/our-science/data/echinoid-directory/intro/reproduction.html (accessed on 8 Oct 2017).

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Crossland, M. R., Collins, J. D. & Alford, R. A., 1993. Host selection and distribution of Hypermastus placentae(Eulimidae), an ectoparasitic gastropod on the sand dollar Arachnoides placenta (Echinoidea). Australian Journal of Marine and Freshwater Research 44(1):835-844.

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Eulimid snail on A. placenta (Image by Ria Tan, 2011)

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“Cake sand dollar Arachnoides placenta Family Clypeasteridae”, by Ria Tan, Wild Singapore (6 Mar 2010). URL:http://www.wildsingapore.com/wildfacts/echinodermata/echinoidea/sandollar/arachnoides.htm (accessed on 11 Nov 2017).

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Print left behind by birds around a flipped sand dollar (Image by Ria Tan, 2017)



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Grey bonnet snail feeding on cake sand dollar (Image by Ria Tan, 2011)



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Haddon's carpet anemone eating a cake sand dollar (Image by Ria Tan, 2004)

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Sometimes, the washed up skeleton of dead sand dollars are used to make beautiful art, souvenirs and jewelry such as depicted in the following photo. 

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A pair of sand dollar earring by

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 “Sand Dollar Earrings, White Sand Dollar Jewelry Beach Jewelry, Nautical Earrings Nautical Jewelry Hawaiian Jewelry Hawaii Jewelry Ocean O22”, by Kari, DRaeDesigns, featured on Etsy.com (1 Jan 2017). URL:https://www.etsy.com/sg-en/listing/218559609/sand-dollar-earrings-white-sand-dollar?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_qu (accessed on 3 Sep 2017).

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It is interesting to note that previous classification placed the species in the genus Echinarachnius and Scutella (indicated by dotted arrows) which are not closely related to Arachnoides (indicated by bold arrow). However, the relationships for Echinarachnius and Scutella have small bootstrap support (less than 20), possibly due to lack of data.

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Parsimony phylogenetic analysis in TNT and PAUP. Bootstrap values are based on 100,000 fast heuristic searches, excluding any value below 20. Diagram from

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Kroh, A. & Smith, A. B., 2010. The phylogeny and classification of post-Palaeozoic echinoids. Journal of Systematic Paleontology 8, 2010: Issue 2.

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Comparison of phylogenetic trees generated from morphology and molecular data gave rise to differences in the relationships between Clypeasterines, Scutellines and Cassiduloids. This mismatch could be due to the small taxon selection used in molecular analysis and/or because sequences available online are usually represented by one specimen from just one region within its full species distribution. As such, much more work on the phylogenetic tree of Clypeasteroida needs to be done to resolve the disagreements in relationship with other orders.

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Figure 4. from Kroh and Smith (2010) showing a simplified phylogenetic tree based on morphology and molecular sequence data

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Kroh, A. & Smith, A. B., 2010. The phylogeny and classification of post-Palaeozoic echinoids. Journal of Systematic Paleontology 8, 2010: Issue 2.

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Smith, A. B., Pisani, D., Mackenzie-Dodds, J. A., Stockley, B., Webster, B. L. & Littlewood, D. T. J. 2006. Testing the molecular clock: molecular and paleontological estimates of divergence times in the Echinoidea (Echinodermata). Molecular Biology and Evolution, 23, 1832–1851

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