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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 94 D O C S h o r e b i r d M i g r a t i o n E d u c a t i o n K i t 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 DOC Shorebird Migration Education Kit 95 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. 96 D O C S h o r e b i r d M i g r a t i o n E d u c a t i o n K i t 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 DOC Shorebird Migration Education Kit 97 ● 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. 98 D O C S h o r e b i r d M i g r a t i o n E d u c a t i o n K i t 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. DOC Shorebird Migration Education Kit 99 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 100 D O C S h o r e b i r d M i g r a t i o n E d u c a t i o n K i t 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. DOC Shorebird Migration Education Kit 101 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. 102 D O C S h o r e b i r d M i g r a t i o n E d u c a t i o n K i t 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 DOC Shorebird Migration Education Kit 103 Miranda bivalves 104 D O C S h o r e b i r d M i g r a t i o n E d u c a t i o n K i t Miranda univalves DOC Shorebird Migration Education Kit 105