Download Miranda shoreline shell study

Miranda shoreline
shell study
The Firth of Thames Food Basket – Kete kai moana
The mudflats and sandflats stretching far out into the Firth of Thames are
the larder or food pantry for tens of thousands of birds.
It is so bountiful that – after breeding in the South Island of New Zealand
– wrybills and South Island pied oystercatchers fly all the way to this and
other northern harbours each autumn to feed through the winter. When
they fly south again in spring their place at the Firth of Thames dinner
table is taken by many thousands of migrant waders from the northern
hemisphere.
These are mostly godwits, knots, turnstones and red-necked stints which
must escape from their breeding grounds in Siberia and Alaska when the
tundra and wetlands begin to freeze over as northern winter approaches.
It is remarkable that they come all the way to New Zealand – about
12,000 km – but they are rewarded with a banquet that will help
condition them well for their next breeding season.
Most of their food lives buried below the surface but, with their probing
beaks, the waders can find plenty to eat. It is difficult for us to get across
the muddiest stretches to discover exactly what the birds are eating.
However studying the banks of shells washed up along the top of the
shore shows the range of molluscs that make up a good portion of some
birds’ food.
Mudwhelk
Stalk-eyed mudcrab
Other animals are very important foods too but they are either
fragmented or completely digested, leaving few recognisable remnants.
Many worms and crustacea (especially small crabs, shrimps, sealice
and sandhoppers) are also a major part of the waders’ diets.
Flea mussel
Making a collection of the different shells along the top of the shore
(washed up by wave action) is a useful exercise to demonstrate the
variety and relative abundance of the marine wildlife that inhabit these
extensive flats.
Teaching possibilities
Three simple class exercises can be completed along the top of the
shore in a short time to complement the studies of the waders and
other sea birds.
●
Biodiversity – a collection of different shell species
abundance – from quadrat counting of the most
common species
● Relative
●
Same shells but varied sizes – different food for different birds
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Miranda shells
Pre-visit background: some facts about shells
Molluscs are soft-bodied animals with shells. A few, like the octopus and
many slugs, do not have shells but are evolved from shelled ancestors.
Molluscs may have one shell (or valve), two shells (or valves) or eight shells.
Mudcrab
Hornshell
One shell: univalves (snails) – includes whelks, winkles, paua, limpets,
topshells, cats eyes, and etc.
Two shells: bivalves – includes clams (like pipi, cockle and tuatua)
oysters, mussels and scallops
Eight shells: chitons (could be called octavalves!)
Mollusc lifestyles
Chitons (pronounced ‘kite – ons’) are from ancient mollusc stock.
All are grazers that feed by scraping algal films from rocks and other
hard surfaces (including the shells of other molluscs).
Bivalves are nearly all filter feeders. Their food is mainly microscopic
plant plankton (phytoplankton) strained from the seawater. A few
bivalves, like nutshells and wedge shells, are sediment sorting deposit
feeders that are able to separate microscopic organisms from sand
and mud grains.
Univalves are adapted to feed in various ways on different foods. Many
are grazers of algal films and some browse on seaweeds. Many others
are predators or scavengers with adaptations for catching and opening
or penetrating other marine life. Some snails are specialist deposit
feeders that can extract microscopic life from muddy or sandy substrates.
A few snails are filter feeders.
The mollusc shell
Structure
Shells are made of chalk (calcium carbonate) that is laid down by the
mantle. (The mantle is the outer flap or cloak of flesh that encloses the
animal.)
Mollusc shells also have an outer horny skin (the periostracum) secreted
by the mantle edge. This may be obvious, giving a smooth, shiny skin to
the shell (eg. green-lipped mussel) or so thin that it is difficult to see
(eg. cockle). The periostracum often gives shells their characteristic
colours and patterns.
Shell form
The shells of each different species have a characteristic shape and
thickness and fairly standard ratios of length, width and breadth. The
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shape and size ratios remain constant throughout their lives. Particular
species of shellfish may be recognised by:
●
shape (round, long, pointed, fat, thin)
●
distinctive bumps, knobs, shoulders, folds or spouts
Topshell
● characteristic
surface sculpture of ridges, grooves, flanges that may
run across or around the shell
Mudsnail
●
particular colours or splashes and bands of colour
● the
position, shape and clarity of muscle and mantle attachment scars
inside a shell
●
the shape and complexity of the interlocking ‘hinge teeth’ in bivalves
Shell function
Shells give protection from:
● Predation:
The shell is an armoured box to stop casual predators
getting access to the soft flesh inside. Some animals have developed
equipment (chisels, pincers, rasping drills) or techniques (acid saliva
to dissolve shell chalk) to penetrate shell armour. However, the shell is
generally a safe box for molluscs to hide in.
● Physical
damage: Shellfish dislodged by waves may be thrown against
rocks or tumbled across abrasive sand. Wave-tossed stones and other
debris could also damage their soft flesh if they were not protected by
their tough shell armour.
● Drying
out: Most shells hold a reservoir of water to keep the animal
moist while the tide is out. Bivalves that live at or near the surface have
shells that close perfectly all around the gape to retain the reservoir of
water.
Deep burrowing bivalves may not have shells that can close perfectly.
This becomes less important the deeper the shellfish burrow because
drying out, disturbance by waves and predation are all reduced by the
animals being deeply buried in the substrate.
The shell serves as a rigid skeleton against which muscles are attached
and can contract.
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Harbour earshell
Teaching exercises
a) Biodiversity – shell search
At the beach divide a class into groups of four students and one teacher
or parent helper to supervise and encourage.
Equipment for each group:
●A
black/dark plastic sheet about 0.5m x 1m to lay shells on
●4
stones or 2 pieces of driftwood to hold down sheet
● Plastic
collecting bag (to hold a post-visit study set of shells for the class)
Method:
● Allocate
a small strip of beach to each group to search for different
species of shells
● Space
the groups about 5m apart: far enough to avoid interference
between groups but close enough for all the class to be within sight and
hearing range of the teacher
●
The class groups are given 15 minutes to search their strips
● Engender
a low level of competition between groups to stimulate
industry and careful observation
(Refer students to the shell shape and surface sculpture sheet, page 101,
before coming to Miranda and again before this exercise begins)
● Place
collected shells in separate species piles on the plastic-collecting
sheet
● Teacher/helper
checks that different species are properly separated and
that students are searching carefully. They may be missing very small
shells – under 6mm. Some species of molluscs are fully grown
at about 6mm or less
NB: Most students will initially overlook the very small shells. Remind
students that to find very small shells they will need to get down on hands
and knees with their eyes close to the ground
b) Relative abundance
Counting and recording the numbers of the most common species
within a quadrat.
Method:
group randomly casts a quadrat frame of standard size (eg. 0.1m2)
on to the shore at the base of the shell bank (along the line of mean
high tide)
● Each
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● Gather
● Count
all the shells from the surface and sort into distinct species
the numbers of each species and enter the results in a table
● Repeat
the procedure twice and enter the results in the table
● Sum
the numbers for each species and divide by the number of
samples ( 3 ). This gives the average and smoothes out any
irregularities of rogue samples
● Compare
the results between groups. (They should all be similar)
● Work
out averages for the whole class. Work out the ratios of different
species, eg. if pipi = 12 and cockle = 96, then ratio of cockle to pipi is
8 : 1. Such results give a rough indication of the relative abundance
NB: The results are only approximate because the shell ratios in the
driftline may not be the same as the ratios of live animals out on the
flats. Some shells are more easily carried in by waves and others may
live deeper in the sand and are therefore less easily uncovered.
c) Same shells but varied sizes:
different foods for different birds
Mixed flocks of waders frequently have completely different diets so they
do not compete for the same food. The contrasting shapes of their beaks
are often a sure sign of their food preferences.
Knots are regularly seen with godwits. The knots dine mainly on small
shellfish (such as nutshells and baby cockles) while the godwits are
believed to feast on worms and small crustaceans.
Sometimes food size determines the feeding method. Knots and
oystercatchers both eat shellfish, often the same species. However knots
eat only small specimens and eat them whole, while oystercatchers eat
clams, eg. tuatua over 60mm long. They are experts at opening them so
they only swallow the flesh.
Exercise
Have the class make a collection of cockle shells from very small (about
5mm) to large (up to 50mm). This will make the point that birds have
distinct diets and eat a range of food within just one species.
A breeding cockle may release more than 100,000 eggs, sometimes
several hundred thousand eggs. Only a fraction will become fertilised
and eventually settle as baby cockles (spat) but that fraction may still
amount to tens of thousands of little cockles that provide food a plenty
for thousands of knots. If only a dozen survive to breed some years later
the original spawning will still have been a success.
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Godwit
Knot
Univalves and bivalves
Preparing a shell collection for use as an identification aid
It is helpful for students, when sorting shells, to have access to a labelled
set that includes all of the species commonly found at the beach visited.
This can be prepared by a teacher or by keen and careful students working
under supervision.
Species of shells commonly found at Miranda are listed in the sheet headed:
Miranda univalves – Miranda bivalves
This sheet is designed to be photocopied and cut up to make labels for
a shell set display.
● Mount
the shells on stiff card cut accurately to the size of an A4 sheet.
Card cut from an ordinary brown cardboard box (from dairy or supermarket)
is ideal
● Most
shells are light coloured or bleached white and so will show up well
against the plain brown background of packaging cardboard
● Select
the best shells from those collected and arrange on the A4 cards
before gluing each shell in place with PVA or similar woodworking glue
● Apply
glue fairly generously to the parts of each shell which touch the board
● Leave
sufficient space between shells of different species for the
identification labels
Tips for shell selection and arrangement
●
Use separate cards for univalves and bivalves
● With
univalve shells (snails and limpets) mount two of each species, one
with the aperture up and another with the aperture down
● With
bivalve shells mount one to show the outside and another to show
the inside
● With
common shells, like cockles, it is useful to mount a line of shells of
different sizes (in size order) to emphasise the fact that big shells grow
from little ones but that shape and surface sculpture does not vary with age
● It
can be instructive to include some shells that have interesting faults,
such as bore holes made by predatory whelks or tunnels made by worms
that live in the walls of thick shells
Storage
Mounted shell sets are easily damaged unless they are protected. A4
mounting boards are recommended because they can be stored safely
in the cartons which bulk supplies of A4 photocopy paper are packed.
Arrange for your office staff to save you a couple with their lids.
Cut two corners off the boards so they can easily be lifted out. Also cut and
staple simple spacers so that stacked boards in each box do not rest on
the shells below.
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Miranda univalves
Miranda bivalves
Arabic volute
Alcithoe arabica
Battleaxe shell
Myadora striata
Ostrich foot
Struthiolaria vermis
Cockle
Austrovenus stutchburyi
Speckled whelk
Cominella adspersa
Trough shell
Cyclomactra ovata
Spotted whelk
Cominella maculosa
Pipi
Paphies australis
Mud whelk
Cominella glandiformis
Wedge shell
Macomona liliana
Trophon
Xymene ambiguous
Horse mussel
Atrina pectinata zelandica
Olive shell
Amalda australis
Scallop
Pecten novaezelandiae
Horn shell
Zeacumantus lutulentus
Pacific oyster
Crassostrea gigas
Turret shell
Maoricolpus roseus
Dredge oyster
Tiostrea chilensis lutaria
Auger shell
Pervicacia tristis
Golden saddle oyster
Anomia trigonopsis
Ricegrain whelk
Marginella pygmaea
Nut shell
Nucula hartvigiana
Saucer limpet
Sigapatella novaezelandiae
Green-lipped mussel
Perna canaliculus
Harbour saucer limpet
Zegalerus tenuis
Asian mud mussel
Musculista senhousia
Smooth slipper limpet
Crepidula monoxyla
Black estuary mussel
Xenostrobus securis
Harbour topshell
Diloma subrostrata
Mud snail
Amphibola crenata
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From left: wedge shell, pipi,
trough shell, cockle, nutshell
For identification illustrations of
the shells in this list see pages
104 (bivalves) and 105 (univalves).
Sea shell identification
Shell shape and surface sculpture
The science curriculum requires students to develop skills for identifying
different organisms. Even at Level 2 there is a requirement for students
to be able to ‘use differences and similarities of external characteristics
to distinguish broad groups of living things’. By level 4 there is an
expectation that students can ‘investigate and classify closely related
living things on the basis of easily observable features’.
Marine molluscs (snails and bivalves) are an important food source
for some migrant birds. At Miranda they are the easiest marine life to
sample, assess and store for post-visit studies as millions of their shells
are washed up along the top of the shore. The variety along the Miranda
shoreline is smaller than on some other beaches so small collections for
class study are not too difficult to sort into species.
Sorting shells can initially be difficult for students because some
different species may appear superficially similar. Confusion can also
be caused by slight differences between individuals of the same species
– just as there are in people. However, there are always several key
characteristic features which allow students to distinguish between
similar species. Using each of the distinguishing categories below will
help students make confident identifications.
Unreliable features
Size, colour and colour patterns
Size
Most species have a typical maximum size range but all species start life
on the shore as minute individuals smaller than a pinhead. Consequently
small specimens may just be very young examples of a species which is
often more familiar as a much larger animal.
Colour
Shell colour is mostly concentrated in the periostracum; a horny layer
covering the shell. In some shellfish, like the green-lipped mussel, it is a
substantial and obvious outer skin but in others, like the cockle, it is very
thin and scarcely noticeable. The colour and patterns of the periostracum
often vary with age and they may also vary according to the colour
pigments in the food eaten. Therefore animals of the same age and size
taken from different habitats may have slightly different colours.
The shells of long dead shellfish may absorb stains from the substrate
if they are buried in sand or mud. Shallowly buried shells often pick
up orange brown pigments of rust-like deposits in sand. Shells deeply
buried in black anaerobic mud or sand (blackened by the waste products
of bacteria) will absorb those black pigments which are mostly sulphides
of iron and other metals.
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Patterns
The presence of colour bands, spots, lines and blotches are often
characteristic of a species but the precise patterns can be very variable.
The dawn shell, Tawera spissa, a common bivalve of fairly open sandy
beaches, is decorated with radiating bands and/or zigzag lines. Either
may be prominent, faint or absent and the patterns on individual shells
are said to be as distinct as the finger prints of people.
Features to look for:
1
●
General shape
Is it round, oval, tear-drop or semi-rectangular?
● Are
there sharp angles (where in related species there
might be smooth curves)?
Mudwhelk
● Is
the shell symmetrical about a line drawn down from
the umbo (= beak)?
● Does
the shell have an obvious ridge, shoulder or flange?
● In
bivalves, are the two shells equal (exact mirror images)
like a pipi or unequal like a wedge shell?
2
Surface sculpture
●
Is the shell smooth or rough to the touch?
●
Does it have ribs or ridges and are they fine or coarse?
● Do
these ribs or ridges fan out from the umbo or do they run round the
shell following the shape of the margin (concentric) or does the shell
have both radial and concentric surface sculpture?
●
Are ridges straight, smooth, sharp, rounded or with frilled extensions?
3
Shell dimensions
● The
ratio of shell length to width (measured with callipers) is usually a
useful identification character but it is not always precisely constant.
Ratios may differ, for example, in populations of pipi from
geographically separated harbours.
● The
ratio of shell length or width to shell depth (how plump or thin the
animal is) is also useful though in some, like the cockle, it can vary with
growth rate or age.
● Shell
thickness is also characteristic. When comparing two species
of a similar size it is usually easy to see and or feel the difference in
thickness. It is not always easy to measure because shells become
thicker as they age and are usually thinner towards the growing edge.
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Estuary flea mussel
4
Internal sculpture
● On
some shells the outer surface may be worn smooth, obscuring fine
lines and ridges and sometimes two similar species may be difficult to
distinguish from external features alone. The internal sculpture and the
details of a bivalve’s hinge tend to be locked together precisely and are
a reliable characteristic for distinguishing species.
● The
most obvious feature inside a bivalve shell is the attachment scars
of the shell closing muscles (adductors), usually one at each end of the
shell. Frequently the attachment scar of the mantle (the retractable
cloak that lines the inside of each shell valve) is also clear. It usually
has a concave depression (the pallial sinus) at the posterior end where
the siphons lie. Deep burrowing bivalves with long siphons have a deep
pallial sinus and shallow burrowing bivalves only have a shallow sinus.
The shape and position of the adductor scars and the mantle sinus are
dependable identification characters.
Cockle, nutshell, stalk-eyed mudcrab
Estuary flea mussle
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Miranda bivalves
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Miranda univalves
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