G206 Tadpoles

March 2009 March 2009 G206 Tadpoles Contents

1.0 Introduction...... 1 1.1 Native species covered in this publication...... 2 1.2 The foreign species that are sometimes available ...... 3 2.0 Life cycles...... 4 2.1 Life cycle of a tail-less amphibian...... 4 2.2 Life cycle of tailed amphibian ...... 4 3.0 Sources of spawn ...... 5 4.0 Keeping tadpoles indoors ...... 6 5.0 Food and feeding...... 9 5.1 Frogs and toads ...... 9 5.2 Newts and other tailed amphibians ...... 9 6.0 Breeding amphibians to produce tadpoles...... 11 7.0 Health and disease ...... 12 8.0 Studying tadpoles ...... 13 8.1 Keeping a diary of development...... 13 8.2 Making observations quantitative...... 15 8.3 Investigating feeding ...... 16 8.4 Investigating factors that influence growth ...... 17 8.5 Investigating aspects of behaviour...... 18 Appendix 1 Suppliers of equipment and materials ...... 20 Appendix 2 Books and other teaching materials that include frogs and tadpoles ...... 22

© CLEAPSS 2009 Strictly Confidential Circulate to members and associates only

As with all CLEAPSS materials, members and associates are free to copy all or part of this guide for use in their own establishments. CLEAPSS gratefully acknowledges the advice of Dr John Baker, Widespread Amphibian and Reptile Project, The Herpetological Conservation Trust, 655A Christchurch Road, Boscombe, Bournemouth, Dorset, BH14AP. Tel: 01986 872016 / 07884 441521. www.herpconstrust.org.uk & www.arguk.org. The ILEA Centre for Life Studies originally published this guide in 1994 as a title in the Guidelines series. CLEAPSS acquired the copyright of all the CLS publications following its closure.

CLEAPSS Brunel Science Park Kingston Lane Uxbridge UB8 3PQ Tel: 01895 251496 Fax: 01895 814372 E-mail: [email protected] Web Site: www.cleapss.org.uk Guide G206 Tadpoles

1.0 Introduction

Studying spawn each spring is a common activity, particularly in primary schools. Many of the tadpoles that develop from the spawn can die because the animals are not kept in the most appropriate conditions. Section 14 of the CLEAPSS Laboratory Handbook, CLEAPSS guide L124 Aquaria in primary schools: electrical safety and L181 Cold water aquaria give general information about setting up aquaria. This publication discusses only tadpoles kept in such aquaria and has more extensive advice. Hopefully this advice will help to increase the chances of rearing tadpoles successfully. Larval amphibians. Tadpoles are the larval life stage of amphibians. They change into the juvenile stage through a process called metamorphosis. Juveniles look like smaller versions of the adults. Juvenile amphibians can be difficult to rear in classrooms, and this guide covers only the tadpole stage. Spawning in spring. In the UK all native species of amphibians spawn in the spring. In general, tadpoles grow throughout the spring and early summer, turning into juveniles in the summer. Tadpole growth rates vary according to conditions, so that under some circumstances tadpoles develop slowly – in extreme cases not developing completely until the following spring. In a minority of ponds tadpoles can be found throughout the year. This is known as ‘overwintering’. Conservation. It is essential to consider the conservation of the wild population of amphibians and their habitats, and the associated legislation. Although all native amphibians do have some legal protection, it is legally permissible to collect and maintain four native species – although it is illegal to sell them without a licence. Great crested / warty newts (Triturus cristatus) and natterjack toads (Epidalea calamita) are strictly protected and cannot be taken from the wild (Wildlife and Countryside Act 1981). Natterjack toads are a rare species limited to relatively few, usually protected, sites, so you are unlikely to come across them by chance. Great crested newts are widespread so greater caution is required when dealing with newts. (See overleaf for help in identifying species.) Small amounts. When obtaining spawn, ask for or collect only the amount that you can rear with the resources available, e.g. a clump of frog spawn the size of a tennis ball contains as many as 350 eggs. Three hundred and fifty tadpoles need about 100 litres (22 gallons) of water. Returning young adults to the wild. Small, just-metamorphosed amphibians are difficult to rear in captivity. It is best to release the young of native species as soon as possible after metamorphosis at the collection site if possible. (See the end of section 3 for more details.)

1 1.1 Native species covered in this publication Common frog (Rana temporaria)

Common toad (Bufo bufo)

Tadpoles of these two species are virtually indistinguishable to the eye.

Smooth / common newt (Lissotriton [formerly Triturus] vulgaris) and Palmate newt (Lissotriton [formerly Triturus] helveticus)

Frog and toad tadpoles are easier to maintain than newts and feed by grazing on a range of organic materials. Newt tadpoles feed on small living prey. These differences in feeding mean that frog and toad tadpoles are much easier to feed in captivity which makes them more suitable to study in the classroom.

2 1.2 The foreign species that are sometimes available African clawed toad (Xenopus laevis)

Mexican axolotl (Ambystoma mexicanum)

Axolotls are unusual amphibians that breed in the tadpole stage. The tadpoles grow to adult size eventually reaching approximately 30 cm long in a process called neotony.

European fire salamander (Salamandra salamandra)

3 2.0 Life cycles 2.1 Life cycle of a tail-less amphibian I Once external gills have been replaceed by an internal set, the tadpole begins to feed, mainly on plant material. II The gut shortens and the tadpole becomes progressively more carnivorous as its hind limbs grow. III When the left front leg appears through the spiricle then the tadpole stops feeding. IIII The tadpole meta- morphoses into the adult. On land the adult requires small live food.

2.2 Life cycle of tailed amphibian I The tadpole starts to feed before the front limbs appear. II As the rear legs develop, larger food is taken. III The tadpole stops feeding at metamorphosis, shown by the disappearance of the gills and tail fins.

4 3.0 Sources of spawn

From ponds. Frog or toad spawn can be collected from the wild, with the pond owner’s permission. For practical purposes, the source pond should be near to your school and readily accessible. Spawn taken from a teacher’s garden pond or from a pond within the school grounds is ideal. Take only a small amount of spawn and be prepared to return some tadpoles to the pond should you end up with more than you can deal with. A small ball of frog spawn produces many more tadpoles than you can care for. Sometimes spawn clumps get broken up in the pond. A small fragment of spawn is easier to deal with than a whole clump. Toad spawn is harder to find than frog spawn because it is laid in strings. A 10 cm length of toad spawn provides more than enough tadpoles. Make sure you collect viable spawn. Not all spawn is fertilised and it can also be attacked by fungus. Healthy spawn contains dark coloured eggs or developing tadpoles. Spawn that contains pale eggs or shows signs of fungus (furry white growth) should be avoided. Care of spawn. A clump of frog spawn can be hatched in a small plastic aquarium. The aquarium only needs to contain shallow water, around10 to 15 cm. Frog spawn can pollute aquarium water so it is important to do a partial water change (approximately half of the water) every few days or every day if the water appears particularly polluted. A small portion taken from a string of toad spawn can be hatched in quite a small volume of water, e.g. in a plastic margarine container. Transfer spawn. After hatching, it is best to transfer tadpoles to another container. A pipette can be made from plastic tube and a rubber teat. You only need about a dozen or so tadpoles in a small aquarium, e.g. approximately 30 x 20 x 20 cm. It is a good idea to set up several aquaria to rear the tadpoles because this increases the chances of success. Surplus hatchlings or un-hatched spawn should be returned to the original pond.

Foreign species. Clawed toad tadpoles are sold by some suppliers and are available almost all year round. Clawed toad, salamander and axolotl tadpoles may also be obtained from individuals who keep and breed these species. The British Herpetological Society may be able to suggest possible contacts. Salamander and axolotl tadpoles are usually available in late spring and early summer. If the adults are available then it may be possible to obtain tadpoles by breeding from captive stock. Section 6 on breeding gives an outline of how this can be done with reference to a few easy examples.

5 4.0 Keeping tadpoles indoors

Rearing tadpoles indoors can be easy - when you know how. It is important to avoid overcrowding, which can lead to polluted conditions with, understandably, a high death rate. A small aquarium measuring 30 x 20 x 20 cm should be large enough to house 10 to 20 common frog or toad tadpoles. Rearing tadpoles. Tadpoles can be reared in a variety of different types of container. Glass or plastic aquaria are suitable but other watertight containers also work. It is possible to grow a single frog / toad tadpoles in a plastic pint glass. In such small containers it is harder to maintain a stable aquatic environment so conditions need to be monitored carefully and include frequent water changes. Larger containers usually require less ongoing maintenance. Large well-established aquaria can create conditions that require no maintenance throughout the whole tadpole stage. If you have experience of setting up coldwater aquaria then this is a good option for rearing tadpoles. A simple system for rearing common frog or toad tadpoles is to keep them in an aquarium with no gravel. Pond weed can be added for aesthetic purposes and to help condition the water. However, weed is not essential except as an option to help amphibians leave the water at metamorphosis. The advantage of this simple system is that it is easy to clean. Partial water changes can be done as necessary. Dirty water should be removed by siphoning. To make a siphon: fill a length of plastic / rubber tubing with water, place your finger over one end and place the other in the aquarium. Hold the end sealed by your finger over a bucket and below the level of the open end in the tank. When you take your finger off the end of the tubing, water flows from the aquarium into the bucket. You can pick up detritus from the bottom of the aquarium by directing the upper end of the tubing and control the water flow with your finger over the lower end. You should replace 30 to 50% of the water in a partial change. Water directly from the tap is fine for tadpoles but leaving it to stand for 24 hours minimises any adverse effects from chlorine in the tap water. Including a layer of aquarium gravel helps to maintain a natural balance in an aquarium because detritus is held in the gravel and broken down by bacteria. The disadvantage with this is that if the aquarium becomes polluted then it is harder to clean. Filtration. A sub-gravel filter powered by either an air-lift or submerged water pump can be added to the aquarium. Filtration should remove suspended matter to a place where it can be degraded or discarded and also break down any dissolved poisons. For small set-ups this can be done using a bed of zeolite with a particle size between 3 mm and 7 mm or, less preferably, gravel. Such relatively complex aquaria set-ups are not essential, but they do give scope for experienced aquarists to establish natural-looking environments for rearing tadpoles. Established aquarium set-ups are also helpful should you attempt to rear newt tadpoles because they allow small prey to survive.

6 Xenopus tadpoles. When keeping species of Xenopus tadpoles it is important to switch off the filtration for about an hour when feeding. These tadpoles are filter-feeders and so to avoid starvation they must be allowed to consume the food before the aquarium’s filter is switched on. Once an aquarium with a filtration system has been running for about a week, 25% of the water should be removed every two weeks and replaced with fresh, conditioned water. Water temperature. Tadpole growth is strongly affected by temperature. Room temperature (18 °C to 20 °C) is perfectly adequate for rearing frog and toad tadpoles and is normally higher than the temperature in a pond. Healthy tadpoles should grow faster indoors than in a pond, primarily because of this, and these tadpoles can be compared with those in the source pond. There is no need for an aquarium heater. Metamorphosis and leaving the water. Once tadpoles approach metamorphosis there are three important points to consider.

• Young amphibians can climb vertical aquarium walls and escape. • If they cannot escape the water at this point they may drown. • Metamorphosed amphibians are difficult to feed in captivity.

As tadpoles become ready to leave the water it is important to ensure that their rearing container has an escape-proof lid, with adequate ventilation. Some plastic aquaria have snap-on lids which are ideal. An aquarium should be well stocked with pond weed to help the emerging tadpoles escape from the water. If some of the plants lie just under the water surface then the young amphibians can move onto them and push through to the air above. Once the animals are fully air-breathing they need to leave the water via gradually sloping beaches in the form of protruding rocks or a gravel slope. If necessary, the metamorphosing tadpoles can be transferred to another aquarium with only a few centimetres of water and a thin layer of zeolite or gravel. This should be sloped so that at one end of the tank it breaks the water surface to form a beach. Metamorphosis in aquatic species. Fully aquatic species such as clawed toads (Xenopus species) and axolotls (Ambystoma mexicanum) need never leave the water at any stage of their life cycle. Axolotls are unusual in that they normally do not metamorphose but continue to grow until they are breeding size. All these aquatic species can be left in a normal aquarium. However, small specimens of any kind should not be left in the aquarium with adults as they will almost certainly be eaten! Only transfer them into the adults’ tank when they are no longer a potential meal. Feeding. If adults are to be kept for any length of time before release then suitable live food such as aphids, hatchling crickets, fruit flies, etc, must be provided. Releasing native species. Once young native frogs, toads or newts have completed the tadpole stage they spend their juvenile lives on land. Rearing these young amphibians is not easy so native species should be released as soon as possible after metamorphosis. There are two issues to consider when releasing amphibians back into the wild. • The possibility of the spread of amphibian disease. • The potential to release unwelcome water plants with the amphibians. Release native species in wet vegetation at the edge of the original spawning site. Releasing at the original pond minimises any risk of spreading disease from one area to another. If released in the late evening then these vulnerable animals will have time to hide overnight.

7 Pond weeds commonly used in coldwater aquaria can cause problems in the wild. If you stocked your tadpole aquarium with weed, then make sure that none of this is released with the amphibians (unless the weed came from the release pond). Do NOT release foreign species into the wild! These animals must remain in captivity as there are strict laws controlling the release of alien species. Materials. Almost all of the materials needed to keep and rear tadpoles can be bought from local aquarists. (Addresses and telephone numbers can be found in Yellow Pages or Thomson Local Directory.) Some of the larger items can be bought more cheaply through general educational suppliers. Suppliers. Many of the commercial companies trading as ‘Aquarists’, and the more specific manufacturers and suppliers advertise in various magazines for amateur aquarists, e.g. The Aquarist & Pondkeeper and Practical Fishkeeping (both monthly publications are available from local newsagents). Mains electrical items, i.e. filtration pumps, must be bought according to the requirements of your employer (usually the local authority). All mains equipment including heaters, thermostats, air and water pumps and lighting hoods must be installed and used safely. Refer to section 1 of guide L124 Aquaria in primary schools: electrical safety and section 14.3 of the CLEAPSS Laboratory Handbook. Appendix 1 lists a range of products and trade names.

8 5.0 Food and feeding

5.1 Frogs and toads When frog tadpoles hatch they still have some yolk remaining from the egg to nourish them for the first part of their lives. However, by the time they reach the internal gill stage they do require feeding. Fish food. Initially, most frog tadpoles are plant-feeders but as they grow they become progressively more carnivorous. This change normally starts by the time the rear legs appear as buds at the position where the body joins the tail on the lower surface. Most tadpoles eat by scraping the surface of the food so they require a solid piece of food from which to feed. This is not a problem because they can be fed at all stages on pellet or flake fish food. These products come in various formulations and should be given to accommodate the changing preferences of the growing tadpoles, starting with those made from plant products and later switching to formulations prepared for carnivorous fish. Some tadpoles show a preference for feeding at the water surface and for these the floating pellet food can be used. Most modern formulated diets tend not to decompose too quickly. Take care not to overfeed and overload the filter. For clawed toad tadpoles (Xenopus species) the formulations used for fish fry, as either liquid or powder preparations, are very successful. Fresh food. This should be offered daily in such quantities that any one piece is eaten in a couple of hours. Filter-feeders such as Xenopus may need feeding two or three times a day with amounts that are cleared by the tadpoles in less than an hour. If food is uneaten after this period then it should be removed by switching the filtration back on. Older tadpoles should preferably be fed on carnivorous fish food formulations. However, their diet may be varied occasionally by feeding them small pieces of chopped meat such as liver or heart. If these are not eaten then they must be removed before they begin to decompose. Live food in the form of growing plants can remain in the aquarium indefinitely. Even filamentous algae, if unsightly, are a natural and nutritious food. Algae also help to balance the environment by cycling nutrients and by photosynthesising which adds oxygen to the water. 5.2 Newts and other tailed amphibians Newt tadpoles need live food from the time they are independent of their yolk. Supplying live food on a sustained and regular basis usually means that several cultures of the food animals need to be kept. When feeding live food, take care to ensure that it is eaten soon after being offered or else removed from the tank. This prevents pollution from any food animal that has died and started to decompose.

9 Some food animals that are not eaten immediately, such as blood worms, become a nuisance by attacking the tadpoles and, in any case, the movement that attracts the tadpoles is soon ignored and fails to stimulate feeding. The table below shows some of the different live foods that may be fed to newt tadpoles of different sizes.

Tadpole stage Tadpole size Suitable foods (average) Just hatched to 5 to 7 mm Infusoria front leg stage Micro-worms Hatchling brine shrimps Front leg to 7 to 13 mm Vinegar eels rear leg stage Small brine shrimps Small water fleas Grindal worms White worms - chopped Tubifex - chopped Rear leg stage Over 13 mm Medium to adult brine shrimps to metamorphosis Water fleas White worms Tubifex; Blood worms Gnat larvae (Details of where to obtain the live foods in the table are given in Appendix 1.)

Fresh dead food. For convenience some of these food items can be purchased frozen or freeze-dried. Feeding dead food or even suitably small pieces of meat or fish is possible, as just the smell of the food can be sufficient to stimulate feeding.

If the tadpoles show interest but do not actually snap at the food then they may sometimes be fooled into accepting it if it is waved about in front of their mouths. If it is not accepted immediately then remove it.

10 6.0 Breeding amphibians to produce tadpoles

It is best to breed foreign species yourself. Failing this, several people currently breed a wide variety of species in captivity. Surplus tadpoles are sometimes available, especially when there has been a large spawning. Make sure you have the resources to rear these often demanding species and ensure that you have been successful with easier to rear species before contemplating keeping the spawn or tadpoles of exotic species. Natural spawning. If you are attempting breeding then there are several techniques you can try. The easiest is to set up a suitable vivarium containing adults and let them spawn naturally. However, there are comparatively few species that spawn under such conditions. Species that do spawn successfully under these conditions are painted frogs (Discoglossus pictus), oriental fire- bellied toads (Bombing orientalis), European fire salamanders (Salamandra salamandra) and Mexican axolotls (Ambystoma mexicanum). The British Herpetological Society has details of breeding these and other amphibians. Breeding other species. For other species of amphibian, breeding is often complicated and by no means predictable. There has been research into using hormone-induced breeding but the procedures are still very experimental. Some information is available from the British Herpetological Society. Finally, remember the absolute prohibition on releasing foreign species into the wild. All the offspring of non-native species must be found good homes in captivity.

11 7.0 Health and disease

Good husbandry. Tadpoles can suffer from infections caused by viruses, bacteria and fungi. It is difficult to treat disease in tadpoles so prevention, through keeping tadpoles healthy and more resistant, is the only feasible option. Tips on keeping tadpoles healthy. • Do not overcrowd. • Keep the water clean. • Remove any dead tadpoles immediately. Overcrowding. Amphibians naturally produce large numbers of eggs. In ponds, high mortality due to predation ensures that overcrowding is a rare occurrence. Given the lack of natural predators indoors, it is easy to end up with more tadpoles than you can care for properly. It is important to resist the temptation to look after all of them. Ready access to the original pond makes it easy to release excess tadpoles. Clean water. There is an element of luck in setting up aquaria for hatching frog / toad spawn or rearing tadpoles. Sometimes the water reaches a ‘natural balance’ and keeps reasonably clean. Other times it can become unpleasant and smelly. Partial water changes or installing a filtration system gives a better chance of success. If you opt for a simple set up with no gravel then the frequency of water changes can be judged by using your eyes and nose! Uneaten food should be removed frequently from aquaria without gravel. Algae growing on surfaces in an aquarium are not a cause for concern as they provide food for tadpoles. Remove dead tadpoles. Even with careful maintenance, the occasional tadpole may die. At high stocking densities, a dead tadpole may be rapidly eaten by other tadpoles, especially in the later stages of development. However, it is best to remove any dead tadpoles immediately. If several tadpoles die then it may be an indication of disease. There is little you can do to diagnose or treat most diseases in tadpoles. If the tadpoles can be separated into several different aquaria or rearing tanks then it may help to save some of them from infection.

12 8.0 Studying tadpoles

The main focus of interest in keeping tadpoles is the life cycle of amphibians. This can provide opportunities for scientific observations and investigations, some of which can be made quantitative, if appropriate. 8.1 Keeping a diary of development This is an excellent way of encouraging students to make careful daily observations and keep good records of the changes that they see - both essential aspects of scientific studies. The results of the students' observations can be displayed in a variety of ways as a pictorial record of how the tadpoles develop. If the spawn from different amphibians is available then interesting development comparisons can be made. Frog spawn forms clumps of individual eggs in spheres of jelly; toad spawn consists of strings of jelly with the eggs arranged regularly along them. Frog tadpoles develop with a brownish / gold speckled coloration on their bodies, while toad tadpoles are completely black. The speed with which tadpoles develop varies with the species but is most affected by the temperature of the water. In a cold spring it might be 2 to 3 weeks before there is much external evidence of development beginning and further growth is very slow. Therefore, when spawn is brought into school it is difficult to say exactly how old it is. The first stages of egg development will already have taken place by the time the spawn is collected. An approximate indication of the growth that might be expected after various periods of time is shown below for common frog tadpoles. Remember that your tadpoles may grow slower or faster, depending on temperature. Some aspects of development may be different if you have tadpoles of another species of frog, toad or newt. The female lays her eggs and the male sheds his sperm over them. Some, but not all, are fertilised. The outer jelly layer swells with water and the eggs float in a mass at the surface. After two to three hours the fertilised egg cell splits in two, then four, eight and so on. A close-up view of an egg under a magnifier after about one day’s growth shows that the surface of the egg inside the jelly is rather lumpy. The fertilised egg cell has divided many times to form a ball of small cells. The upper layers of the egg are a dark brown / black colour. The lower layers are more yellow - this is the yolk store for the developing tadpoles.

The developing tadpole The upper layers of cells start to grow downwards and cover the lower yellow yolk cells. If the eggs in your spawn are a dark colour all over then they must be at least one day old. After the end of the second day, the tadpoles’ nervous system starts to develop and the ball of cells becomes a bit like a ‘dumb-bell’ shape as the head and tail start to form.

13 Days 2 - 4 The tadpoles grow from about 4 mm to 10 mm long in this time. They start to move inside the jelly, their heart begins beating and their head and tail become more obvious. The tadpoles have a ‘comma’ shape at first but later are more elongated. The tadpoles wriggle free of their balls of jelly by about the 10th day. They have no mouth at this stage and usually hang in clusters attached to the jelly or nearby pond weed using small suckers. They can only feed on the remains of the yolk inside their bodies. The external gills are now visible but not very well formed.

For much of this time, the tadpoles will still just be hanging around on weed and jelly. The external gills grow so that the tadpoles can extract more oxygen from the water, to enable them to be more active.

By the end of two weeks, the mouth opens. The tadpoles can now rasp away with their teeth at algae growing on the surface of pond weed. The tadpoles now develop a distinct division into body and tail. The external gills start to disappear as internal gills form. A flap of skin called the operculum starts to grow backwards from the front of the head and covers the remains of the gills. Now the tadpoles breathe by taking in water through the mouth. This passes over the internal gills where oxygen is extracted and out through a set of gill slits that are underneath the operculum. The water finally leaves through a hole in the operculum on the left side - called the spiracle.

During this time the operculum grows completely over the body. The tadpoles grow considerably in size, initially without any changes in the form of the body. They generally feed on various types of plant material. Then the back legs start to develop - first as small buds at the rear of the body and later as perfectly formed limbs, although they are not yet used for propulsion.

The hind legs continue to grow, as do the lungs. A sure sign of this are the tadpoles coming to the surface to gulp in air. The tadpoles’ diet also gradually changes with more meat or dead animals being eaten. (Any tadpoles that die are quickly finished off by their fellows - students should be warned about this.) Towards the end of this period the front legs develop. The left leg is usually the first to appear and grows out through the spiracle in the operculum. The right leg is at first a bulge under the body but then breaks through the operculum.

14 The tadpoles, now nearly young frogs, crawl out of the water and the tail stump finally disappears. This entire change from tadpole to frog is called metamorphosis. The young frogs stay in damp vegetation and eat as many insects as possible to grow quickly before hibernating for the winter.

There are many detailed observations that students can make. They should be encouraged to be as careful ‘tadpole detectives’ as possible, both looking at the changes in the tadpoles’ bodies and also the way in which they behave. Using some form of magnifier to look at the tadpoles is always rewarding. If appropriate, students can make plasticine models of the major changes they see in the tadpoles’ life cycle. 8.2 Making observations quantitative

Aspects of number and measurement can be incorporated into working with tadpoles. Success rate. Students can count the number of eggs in the spawn and relate this to the number that hatch (several of the eggs may not have been fertilised by any sperm) and later to the numbers that finally become adults. This reveals why amphibians need to produce so many eggs, although keeping tadpoles in a protected environment indoors can give an overestimate of the number of adults that normally develop. In nature there are many factors that can considerably reduce the number of surviving adults. A cold spell in spring may freeze and kill the eggs or they may be attacked by fungus. Many animals prey on the tadpoles or the young adults as they crawl out of the water. Growth. Making precise measurements of tadpole growth is a valuable experience and can be tackled in a variety of ways. The figures obtained can be used to plot a graph of growth against time. Students can be set the theoretical problem How could you measure the length of a tadpole? without initially being given any help. (Taking tadpoles out of the water to measure them is obviously not allowed!) Measuring length. One possible technique is to draw up a small tadpole into a clear glass or plastic tube. This can be made by fitting a rubber bulb on the end of any available transparent tube or tubing as shown.

Tadpoles can be measured by placing the tube alongside a ruler. This method is best for young tadpoles. As they grow, a wider tube can be used but this might not be so easy to obtain. An alternative technique is to place a tadpole in water inside a large ‘bug box’. This has a magnifying lid and a millimetre grid on its base. A cheaper technique is simply to use a hand lens with a glass dish on top of a piece of graph paper. The problem with this method is that the tadpole's movements are not restricted and so it may be difficult to get it to stay still for long enough. Students will have to learn to be patient! There is often a great deal of variation in how long individual tadpoles grow. Taking average growth increase measurements for all the tadpoles gives a more reliable picture of growth rate. Students might also be asked if measuring length is the best way of recording growth. At the later stages of growth, students’ own graphs will quickly show the limitations of concentrating on length as the criterion for growth.

15 Weighing tadpoles. Using the increasing mass of the tadpole is a better way of monitoring growth but it’s tricky to measure. How could students do this? (It is unlikely that measuring equipment sufficiently sensitive to weigh individual tadpoles is available in primary schools, although this will be less of a problem in secondary schools. It may be necessary to weigh several tadpoles together and then calculate average figures.) Students can be given the problem of how to weigh tadpoles in water; as with length measurements, weighing them ‘dry’ is obviously not allowed. One technique might be to transfer the tadpoles with a net to a plastic bag containing a known amount of water, hang this from a spring balance and then subtract the mass of the water. Swimming speed. This is another problem you can pose your students. A stopwatch is essential. It is not easy to time the movements of tadpoles but students should be given the opportunity of devising possible methods for themselves. One way might be to place a tank of water on a large sheet of paper with two marks drawn on it a known distance apart. Individual tadpoles can then be added to the tank and their movements timed between the two marks. How might this method be inaccurate? Alternatively, a wide plastic tube of the type used for transporting posters could be filled with water and tadpoles timed as they swim along it. Students should be encouraged to see the need to make several readings with one tadpole and then take an average. Several tadpoles should be timed. If a reasonably accurate method of timing can be used then ask your students ‘What makes one tadpole swim faster than another?’ Is speed related to the length of the tadpole, its mass, age, or what? There is plenty of scope here for investigations. Breathing. At the later stage of tadpole development, when the lungs begin to form, the tadpoles periodically come to the surface to breathe. How long can they stay submerged before needing a refill of air? How is this time period related to the tadpole’s activity or its size? 8.3 Investigating feeding The dual feeding habit of tadpoles, firstly herbivorous (plant-eating) and later carnivorous (meat-eating), provides an obvious area for investigation. When the tadpoles hatch out of the jelly they often remain attached to it and many students mistakenly deduce that they are feeding on the jelly. At this stage the mouth is not even open. Encourage careful observations by asking students what exactly the tadpoles are doing at this time. Observing the tadpoles with a magnifier shows no signs of feeding. Students can identify the time at which the mouth opens by observing when the tadpoles start to feed on plant material placed in the water. Again, encourage close-up observations under magnifiers to see how the tadpoles feed. Do the tadpoles eat the pond weed itself or are they feeding on something growing on the surface of the weed? To get a good view of tadpoles feeding, you might try this. Grind up various foods, e.g. fish food pellets or flake, or pellets used for rabbit or guinea pig food and mix this with gelatine powder. Add hot water so that the gelatine dissolves and allow the mixture to cool. Before the gelatine sets, spread the mixture onto the inside vertical surfaces of a small glass tank, dish or beaker, lying on its side. This produces a film of jelly and food that sticks onto the glass. Fill the container with water, add one or more tadpoles and hopefully you can watch them feeding off the food on the sides of the dish or tank. A hand lens helps to show the tadpole rasping away at the food. Alternatively, you can attach a flat plastic Fresnel lens to the side of the tank that acts as a magnifier. These are available in various sizes from a number of suppliers. Food preferences. There is plenty of scope to study which types of plant food tadpoles prefer to eat. They can be provided with pond weed and a variety of flake or pellet fish foods as described earlier in section 5. Do not provide too much food as it may remain uneaten and foul the water. Also do not provide the tadpoles with too many choices at once.

16 Do the tadpoles prefer to feed off very solid pieces of food? Do they prefer to eat at the bottom of the container or at the surface of the water on floating food? Do tadpoles prefer ‘natural’ foods such as pond weed to ready-made fish food? Are early-stage tadpoles exclusively vegetarian? You can test this easily by supplying them with very small amounts of meat such as liver, as well as plant food. The liver can be hung in the water on lengths of cotton thread. Carefully observe whether the meat is nibbled. (Do not leave the meat in with the tadpoles. If it is uneaten then it decays and pollutes the water.) Changing diet. The stage when tadpoles become more dependent on animal food can be identified by observing any changes in feeding when the tadpoles are provided with a variety of plant and animal fish food. When the hind limbs develop there is an increasing move towards eating ‘meat’. Are there any preferences in the type of animal food they eat, e.g. chopped liver, heart or animal-based fish food? Time spent feeding. Plant food is generally less nutritious than animal food, so early tadpoles at the vegetarian stage need to feed more to obtain all the nutrients they require. The tadpoles do not have to feed so intensively when they become carnivorous. Students can be encouraged to compare the amount of feeding activity in early and late tadpoles. (If they look very closely at the tadpoles’ bodies, they may even be able to see that the length of the intestine gets shorter as the tadpoles begin to eat more and more animal food.) Feeding stops when tadpoles begin to metamorphose into the adult stage and students should be alerted to look out for this as the first sign of the changes to come. Detecting food. Do tadpoles use their eyes to locate food, or do they move towards it because they can smell or taste chemicals from the food that are released into the water? It’s easy to pose questions like this but not so easy to test them! Let your students suggest how they would go about investigating their tadpoles’ feeding behaviour. If their ideas have faults then encourage them to see where there may be problems. In the dark. It is quite difficult to disentangle the effects of the stimuli of light and chemicals in the water. You could keep tadpoles in the dark and see if food is eaten readily but if it is then the tadpoles may simply have swum into the food by accident and started to feed. It’s also sometimes not easy to tell if a piece of food has been partially eaten. If the food remains uneaten then it might simply mean that the tadpoles aren’t hungry at the time! A more profitable approach is to enclose food inside small muslin or nylon stocking ‘bags’, preferably with the food crushed up to help release the chemicals. The bags are then suspended on cotton threads in the water. Are tadpoles attracted to the bags? They could still be attracted by the sight of the bags so repeat the tests with the tadpoles in the dark and at intervals turn on the light to see whether the tadpoles have collected near the food. 8.4 Investigating factors that influence growth Tadpoles are poikilothermic (cold-blooded) animals which means that they are at the same temperature as their surroundings. If the weather is warm then cold-blooded tadpoles have warm blood! Since all body reactions slow down with a drop in temperature, the rate at which the tadpoles develop is largely determined by the temperature of the water. This is why it is not possible to be precise about the timing of the various stages of development. Tadpole eggs are black on top and the tadpoles are also black or very dark brown. This is thought to help in absorbing heat and so speed up development. Is it true that black structures absorb more heat than others? There is an opportunity here for an investigation. For example, place thermometers in two similar-sized bottles filled with the same amount of water. One bottle is painted black. Shine a desk light or other heat source on both from the same distance. Which bottle warms up fastest?

17 The effects of temperature. How temperature affects the life cycle can easily be studied by keeping tadpoles at various temperatures. Some could be kept outdoors (and a minimum / maximum thermometer used to measure average temperatures) and others kept at a cool temperature indoors. A further batch is kept at normal room temperature and some more tadpoles in water warmed slightly with a heater / thermostat. As in all investigations in which one condition is being studied, it is important to ensure that all other factors are the same in each batch of tadpoles. This means that there should be the same number of eggs or tadpoles in each group and the same amount of food. The size of the containers for each batch of tadpoles must be the same, as must the volume of water in each. Water should be topped up daily using water that has been allowed to stand for 24 hours to remove the chlorine. If tadpoles are kept outdoors then it is necessary to cover the container with a mesh to prevent birds preying on them. The tadpoles’ growth in each batch should be monitored and comparisons made of their size and the dates when they reach important growth stages. Overcrowding. If tadpoles are overcrowded then they grow more slowly and are smaller. The frogs that are formed are also smaller. The effects of crowding can be studied by keeping varying numbers of tadpoles in similar sized containers and giving them the same environmental conditions, including food. Growth is then measured over the subsequent weeks. (It is important not to crowd the tadpoles too much or their health may be seriously affected.) Ask your students how overcrowding can have these effects on growth. An immediate response might be that the individual tadpoles don’t get as much food and so grow less well. This idea can be tested by increasing the food supply to the more crowded tadpoles. Research with tadpoles has shown that they can release a chemical into the water that has an inhibitory effect on other tadpoles and, in sufficient concentrations, slows down these tadpoles’ growth. Perhaps this could be the reason for the tadpoles' reduced growth when crowded? Students can test this idea by keeping tadpoles crowded together for some time and then taking samples of their water and adding it to tadpoles that are not crowded. Is there any effect on growth? With these tests, it is important to have a ‘control’ tank of similarly uncrowded tadpoles to which plain water is added and comparisons then made with the treated tadpoles. Why is this necessary? 8.5 Investigating aspects of behaviour Feeding behaviour has already been discussed but there are other aspects of behaviour that can also be studied. Reacting to danger. If you walk by the side of a pond containing a lot of tadpoles then they will usually swim away as you come closer to the edge of the water. Are they detecting your presence by shadows falling on the pond, by picking up the vibrations caused by walking or can they see you? This behaviour can easily be tested indoors by watching the response of tadpoles in a tank of water. Look closely at the tadpoles, cast a shadow over them and cause vibrations in the water by gently knocking on the tabletop or the sides of the tank. How do the tadpoles react? If they respond to vibrations then try testing how sensitive they are. Camouflage Tadpoles are in danger of being eaten by predators, so you might expect that they will try to make themselves as invisible as possible. So do they? Several factors might be involved. Do tadpoles try to camouflage themselves so that they merge into the background? Try testing this by making up a tank with a light-coloured substrate, e.g. silver sand in one half and a much darker background in the other, e.g. soil or gravel. Place a piece of card on top of the sand / soil and pour water onto the card to avoid disturbing the substrate when you fill the tank. Remove the card once the tank has been filled. Shine a light evenly onto the tank from above and place the tadpoles into the water. Record where the tadpoles are in the tank at regular intervals. (You might expect more to be on the darker side.) Are there any other factors that are different between the two ends of the tank that could influence the tadpoles?

18 Frogs and toads can quickly change their colour if they are put into a light- or dark-coloured container. You could investigate this when your tadpoles turn into adults. Can tadpoles change colour too? It seems to be an advantage if they can do this when they move from the light to dark areas of the pond. Ask your students to investigate. Response to light. Do you expect tadpoles to swim towards or away from the light? Ask your students to suggest what tadpoles might do in a pond. Swimming away from the light seems sensible so that the tadpoles are less likely to be seen by predators such as birds. Plants grow in the light so swimming towards the light might bring the tadpoles closer to their food. Which response is ‘correct’? The only way to find out is by testing the tadpoles’ reactions in the classroom or laboratory! Get your students to design a ‘fair test’. One method could be to shine a light down onto one half of a tank of tadpoles while the other half is covered on all sides with card. Then count the number of tadpoles on each side every minute until there is no change. What other factors might be important in the tests? Could the tadpoles be responding to anything else other than light? Is it better to test one tadpole at a time or lots together? How many counts should be made? An extension to the study of tadpoles’ response to light is to place various objects into their tank and look to see if tadpoles try to hide under them. Choose anything suitable, e.g. a half a flowerpot on its side. Don’t use pond weed. The tadpoles may disappear under it only because they are eating it! Do the tadpoles only hide when ‘danger’ threatens? Does the colour of the background in the tank make any difference?

19 Appendix 1 Suppliers of equipment and materials

Note that in addition to these suppliers you local aquarist is worth considering as are a number of other aquarists which offer supplies by mail order and can be found via the internet. AQUATICS DIRECT HOGG LABORATORY SUPPLIES 6/3 Newlands Avenue See: Scientific & Chemical Supplies Brackla Industrial Estate Bridgend MONKFIELD NUTRITION LTD CF31 2DA Church Farm Barn Tel: 01656 663427 Wendy Email: [email protected] Near Royston Web site: www.aquatics-direct.co.uk Hertfordshire SG8 0HJ BLADES BIOLOGICAL Tel: 01223 208261 Cowden Fax: 01223 208424 Edenbridge Email: [email protected] Kent We site: www.monkfieldnutrition.co.uk TN8 7DX Tel: 01342 850242 PEREGRINE LIVEFOODS LTD Fax: 01342 850924 Rolls Farm Barns Hastingwood Road GRIFFIN EDUCATION Magdalen Laver Bishop Meadow Road Ongar, Essex Loughborough CM5 0EN Leicestershire Tel: 0800 919 631 LE11 5RG Fax: 0845 450 7401 Tel: 01509 233344 Email: [email protected] Fax: 01509 231893 Web site: www.peregrine-livefoods.co.uk Email: [email protected] Web site: www.griffineducation.co.uk SCIENTIFIC & CHEMICAL SUPPLIES Carlton House PHILIP HARRIS Livingstone Road Hyde Buildings Bilston Ashton Road West Midlands Hyde WV14 0QZ Cheshire Tel: 01902 402402 SK14 4SH Fax: 01902 402343 Tel: 0845 120 4520 Fax: 0800 138 8881 Email: [email protected] Web site: www.philipharris.co.uk

20 TECHNOLOGY TEACHING SYSTEMS TIMSTAR LABORATORY SUPPLIES (TTS Group) Timstar House Park Lane Business Park Marshfield Bank Kirkby-in-Ashfield Middlewich Road Nottinghamshire Crewe,Cheshire NG17 9LE CW2 8UY Tel: 0800 318 686 Tel: 01270 250459 Fax: 01623 447999 Fax: 01270 250601 Email: [email protected] Email: [email protected] Web site: www.tts-shopping.com Web site: www.timstar.co.uk

21 Appendix 2 Books and other teaching materials that include frogs and tadpoles

Family of frogs. ISBN 0713632178 Metamorphoses: Egg, Tadpoles, Frog. A & C BLACK ISBN 9781846430321 36 Soho Square CHILD’S PLAY (International) Ltd London Ashworth Road W1D 3QY Bridgemead Tel: 020 7758 0200 Swindon Fax: 020 7758 0222 Wiltshire Email: [email protected] SN5 7YD Web site: www.acblack.com Tel: 01793 616286 Fax: 01793 512795 Banta Bioviewer & Tadpole to Frog Bioset Z31 Email: [email protected] BANTA Ltd Web site: www.childs-play.com Unit 9, Burnt Oak Business Park Back Lane Frog life cycle & Frog life cycle puzzle Waldron GALT EDUCATION East Sussex Johnsonbrook Road TN21 0NL Hyde Tel: 01435 810200 Cheshire Fax: 01435 810210 SK14 4QT Email: [email protected] Tel: 0845 120 3005 Web site: www.bantabiology.co.uk Fax: 0800 056 0314 Email: [email protected] Frogs and Toads. ISBN 0905483383 Website: www.galt-educational.co.uk BOOK SYSTEMS PLUS (WHITTETT BOOKS) Amphibians. DVD / VHS 09671 BSP House VIEWTECH FILM & VIDEO Station Road 7 - 8 Falcons Gate Linton Northavon Business Centre Cambridge Dean Road CB1 6NW Yate Tel: 01223 894870 Bristol Fax: 01223 894871 BS37 5NH Email: [email protected] Tel: 01454 858055 Web site: www.booksystemsplus.com Fax: 01454 858056 Email: [email protected] Web site: www.viewtech.co.uk

22 Keeping minibeasts. SC1222 Wall chart. Frog story. WG4339 Wall frieze. WILDGOOSE EDUCATION The Old Toy Factory Jackson Street Coalville Leicestershire LE67 3NR Tel: 01530 518568 Fax: 01530 811900 Email: [email protected] Web site: www.wildgoose.ac

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