Monday, October 22, 2012

Helmet Shell & Bonnet Snails (Phylum Mollusca: Family Cassidae) of Singapore

The cassids, commonly called helmet shells or bonnet snails, have helmet-shaped shells with short spires and inflated body whorl. The aperture is long and narrow. The outer lip is thickened and recurved outwards to form a helmet-like rim. The operculum is thin and corneous. They are usually found on sandy substrates and are good burrowers. They usually feed at night, hunting for echinoderms such as sea stars or sea urchins that dwell in the sand. Some species first secrete a paralysing saliva to prevent the sea urchin prey from releasing its venom, then secrete an acid to dissolve the shell so that they can feed on the prey.

Helmet shells are commonly collected for food. Their large pretty shells are also collected for the shell trade. Some of the larger species are used as raw material for lime.

Here are the helmet shells that I have seen on our shores so far (thanks to Siong Kiat who gave tips on how to identify them!). You may want to take a look at my diagram on the parts of a snail's shell if you are not familiar with the names of the parts, so as to better understand the terms used below.

Japanese Bonnet (Semicassis bisulcata)
The Japanese Bonnet (Semicassis bisulcata) is sometimes seen on our sandy shores. The body whorl is marked with spiral rows of brown squarish patterns on a pale brown or whitish background colour. It can grow to about 8cm long.

Japanese Bonnet (Semicassis bisulcata)
Looking at the underside, The operculum is corneous and thin. The siphon is long and brown in colour.

Grey Bonnet (Phalium glaucum)
I have seen the Grey Bonnet (Phalium glaucum) a few times on our sandy shores, but unfortunately did not take any photos then. the above photo was taken while I was diving in Sulawesi. The body whorl is grey in colour, and it has a few spines on its outer lip. It feeds on sand dollars, and can grow to about 9cm.

  • Abbott, R. T., 1991. Seashells of Southeast Asia. Graham Brash, Singapore. 145 pp.
  • Carpenter, K. E. & V. H. Niem (eds), 1998-2001. FAO species identification guide for fishery purposes. The living marine resources of the Western Central Pacific. Volumes 1 to 6. FAO, Rome. pp. 1-4218.
  • Gladys Archerd Shell Collection at Washington State University Tri-Cities Natural History Museum. 2008. Retrieved October 22, 2012, from
  • Oliver, A. P. H., 2012. Philip's guide to seashells of the world. Philip's, London. 320 pp.
  • Tan, S. K. & H. P. M. Woo, 2010. A preliminary checklist of the molluscs of Singapore. Raffles Museum of Biodiversity Research, National University of Singapore, Singapore. 78 pp. Uploaded 02 June 2010.
  • World Register of Marine Species. 2012.  Retrieved Oct 3, 2012, from

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