shelled snails text index | photo index
Phylum Mollusca > Class Gastropoda
Helmet and Bonnet snails
Family Cassidae
updated Oct 2016

Where seen? These large snails are rarely seen on our shores. These snail live in sandy areas and are slow-moving carnivores.

Features: The shell is usually helmet-shaped with a large body whorl and tiny spire. All have a small, elongated operculum made of a horny material. In Phalium, it is fan-shaped.

What do they eat? They feed almost exclusively on echinoderms, sea urchins or sea stars, mainly at night and often while both predator and prey are buried in the sand. Snails with heavy helmet-shaped shells eat sea urchins. The smaller Phalium feed on sand dollars.

The Gladys Archerd Shell Collection describes their feeding technique: To feed on sea urchins, the helmet snail creeps up slowly, raises its heavy shell quite high, then abruptly drops the shell in such a way that the urchin is completely engulfed. Since urchin spines contain a poison, the helmet snail releases a paralytic enzyme from its salivary gland, then it secretes sulfuric acid sufficiently strong to dissolve the sea urchin shell in about 10 minutes before consuming its meal. Wow!

According to Poutiers, the snail first squirts neurotoxic saliva over its prey to paralyse the spines. The snail is initially protected from these spines by the thick skin of the foot. Then, the snail pushes its snout through the unprotected anus, or through a hole rasped by radula in the test of the victim which may also be crushed under the weight of the snail.

While the Seashells of New South Wales website says that most helmet shells live buried in the sand by day, coming out at night to feed on echinoderms, especially sea urchins, which they can locate by smell from at least 30cm away. They immobilise the urchin, crawl onto it with the thick skin of the foot providing protection from the spines, and make a hole in the urchin shell by an acidic secretion and by rasping with the radula, and suck out the soft parts.

Baby helmets: The snails have separate genders and practice internal fertilisation. The shell of the female is often larger. Eggs laid in large masses of numerous, small horny capsules, forming irregular or cylindrical, tower-like structures. Each capsule contains several hundred eggs, most of which often serve as food for the developing embryos. The eggs hatch into planktonic larvae or crawling juveniles, depending upon the species.

Human uses: These snails are commonly collected for food and their large decorative shells are popular in the shell trade. Some of the larger species are used as raw material for lime or for cameo-carving.

Status and threats: The Grey bonnet snail (Phalium glaucum) is listed as Endangered in the Red List of threatened animals of Singapore.

Snorkelling from under the sand.
Cyrene Reef, Aug 11


On top of a Cake sand dollar.
Cyrene Reef, Aug 11



The 'burn' marks on the test of this
recently dead sea urchin might have
been made by a Helmet snail!

Changi, May 08

Family Cassidae recorded for Singapore
from Tan Siong Kiat and Henrietta P. M. Woo, 2010 Preliminary Checklist of The Molluscs of Singapore.
in red are those listed among the threatened animals of Singapore from Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore. Nature Society (Singapore). 285 pp.

  Family Cassidae
  Phalium decussatum
Phalium glaucum
(Grey bonnet snail) (EN: Endangered)

Semicassis bisulcata (Japanese bonnet snail)

Links
  • Family Cassidae on The Gladys Archerd Shell Collection at Washington State University Tri-Cities Natural History Museum website: brief fact sheet with photos.
  • Family Cassidae on The Seashells of New South Wales by Des Beechey Research Associate, Australian Museum.
  • Family Cassidae in the Gastropods section by J.M. Poutiers in the FAO Species Identification Guide for Fishery Purposes: The Living Marine Resources of the Western Central Pacific Volume 1: Seaweeds, corals, bivalves and gastropods on the Food and Agriculture Organization of the United Nations (FAO) website.

References

  • Tan Siong Kiat and Henrietta P. M. Woo, 2010 Preliminary Checklist of The Molluscs of Singapore (pdf), Raffles Museum of Biodiversity Research, National University of Singapore.
  • Tan, K. S. & L. M. Chou, 2000. A Guide to the Common Seashells of Singapore. Singapore Science Centre. 160 pp.
  • Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore. National Council on the Environment. 163pp.
  • Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore. Nature Society (Singapore). 285 pp.
  • Abbott, R. Tucker, 1991. Seashells of South East Asia. Graham Brash, Singapore. 145 pp.
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