Factsheet 2. Shells of Rocky Shorelines
How do seashells form?
Predatory marine snails
Seashells are the exoskeletons (exo – Greek, ‘outside’)
of invertebrate animals (animals that have no
backbone) and are mostly made up of a chemical
compound called calcium carbonate. Different types of
animals make different types of shells. This is a very
general guide to some common seashells, but there is
great variation and range in shapes and sizes. For a
more detailed identification guide Seashells of NSW,
accessible at www.seashellsofnsw.org.au, is an
excellent and comprehensive resource.
All marine snails have spiral shells - but if the shell has a
small groove at the mouth it is probably a carnivorous
whelk, triton or auger shell. The groove is for the animal to
extend its siphon into the water. A siphon is an organ that
detects prey by picking up chemical signals in the water.
Chitons, limpets and cowries.
If it is rounded and
concave, it could be a
chiton, limpet, cowrie or
siphon shell. The texture
of these shells can range
from smooth to strongly
corrugated.
Bivalves
If it is fan shaped, or looks like a pair of wings, it could
be a scallop, clam, pipi, oyster, or mussel. These types
of animals are called bivalves. When inhabited, these
shells are joined at the base by a type of hinged valve.
This auger shell (right)
displays the characteristic
groove at the tip of the shell
opening.
Herbivorous marine snails
Herbivorous marine snails
also have spiral shells, but
can be distinguished from
those of carnivorous species
by the lack of the groove at
the shell opening. Shells such
as these belong to as
periwinkles, topshells, turbans
and nerites, and can range
from flat and rounded to
elongated, even slightly
triangular.
Please note: Illustrations are not to scale.
WetlandCare Australia: Supporting the community to protect and restore Australian wetlands since 1991
What type of animals make seashells?
Whelks
All seashells are made by a group of organisms called
molluscs. Molluscs have a specialized type of toothed
tongue called a radula, which they use to scrape algae of
rocks, or to cut up their prey if they are carnivorous. The
‘teeth’ on the radula of the chiton actually contain a metal
coating, making them hard enough to grind through rock.
Seashells that are spiral in shape are all made by
gastropods (marine snails).
The main types of whelks found on rocky shorelines
are mulberry whelks (smallish, about 3cm long), cart rut
shells (about 8 cm long) and spenglers rock whelk (up
to 15cm long). Sphengler’s rock whelks are known to
prey on cunjevoi.
The Mulberry Whelk, Morula marginalba
The mulberry whelk has a distinctive cone shape, with
rounded nodules that give it the appearance of a
mulberry. They are found across all tidal levels and are
able to prey on limpets and barnacles by applying acid
from a gland in their foot, which creates a hole in the
shell. They then use their radula to saw up the body of
their prey so they can eat it. This process can take up
to four days.
Did you know?
The word gastropod comes from the
Greek gastro – stomach, and pod – foot.
Here are two types of shellfish that are commonly found in
intertidal areas along the NSW coast.
Zebra Top Shell, Austrocochlea porcata
These shells have a distinctive striped pattern. This occurs
because the algae that they graze on produce a substance
that causes the animal to secrete a darkened band as a by
product of the digestive process. Because the algae
produce this substance on a seasonal basis the shell then
becomes patterned with a dark and light stripe.
Mulberry whelks are able to drill a hole through the shell of
their prey by applying acid from a gland in their foot.
Other factsheets in this series are:
The zebra top shell is very common in tidal areas. The striped
pattern on their shell is a result of a chemical produced by the
algae they eat, and can vary considerably in width.
1. Rocky Shoreline Ecology
3. Birds of Rocky Shorelines
4. Rock Pool Creatures
5. Marine Plants of Rocky Shorelines
6. Caring for Rocky Shorelines
For further information on the Rocky
Shores Protection and Education
project contact:
Adam Gosling
WetlandCare Australia
PO Box 114 Ballina NSW 2478
T: (02) 6681 6169
E: [email protected]
W: www.wetlandcare.com.au
Prepared by S. Haigh, WetlandCare Australia
(Version 10/09) All photos Adam Gosling except
for auger shell: wikipedia and mulberry whelk:
S. Haigh
WetlandCare Australia: Supporting the community to protect and restore Australian wetlands since 1991