and Bonnet snails
Where seen? These large snails are rarely seen on our shores.
These slow-moving carnivores are found half buried on large sand flats. Such areas are rare in Singapore as most shallow sandy areas have already been reclaimed.
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
On top of a Cake sand dollar.
Cyrene Reef, Aug 11
Cyrene Reef, Aug 11
Thick strong foot.
Cyrene Reef, Aug 11
|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.
On our shores, the Grey
bonnet snail has been seen on top of a Cake
sand dollar, and when the sand dollar is examined, a hole is seen in the sand dollar skeleton. This suggests the snail bored the hole.
Half buried next to a Cake sand dollar.
East Coast Park, Jul 15
Changi, May 08
|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 is listed as Endangered
in the Red List of threatened animals of Singapore.
|Some Bonnet snails on Singapore shores
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
Cassidae on The Gladys Archerd Shell Collection at Washington
State University Tri-Cities Natural History Museum website: brief
fact sheet with photos.
Cassidae on The
Seashells of New South Wales by Des Beechey Research Associate,
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.
- Tan Heok Hui
and Tan Siong Kiat. 30 November 2016. Grey bonnet preying on sand dollar at Changi. Singapore Biodiversity Records 2016: 166-167.
- 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.
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.