Lepidurus Arcticus

Egg laying strategies in the Arctic tadpole shrimp (Lepidurus arcticus).

Þorgerður Þorleifsdóttir

Líf- og Umhverfisvísindadeild Háskóli Íslands 2018 i

Egg laying strategies in the Arctic tadpole shrimp (Lepidurus arcticus).

2018 Bachelor of Science Student: Þorgerður Þorleifsdóttir Supervisors: Hrefna Sigurjónsdóttir and Hilmar J. Malmquist

Egg laying strategies in the Arctic tadpole shrimp (Lepidurus arcticus). Varpaðferðir Skötuorms 10 eininga ritgerð sem er hluti af Baccalaureus Scientiarum gráðu í Líffræði

Líf- og umhverfisvísindadeild Verkfræði- og náttúruvísindasvið Háskóli Íslands Askja – Sturlugata 7 101, Reykjavík

Sími: 525 4000

Skráningarupplýsingar: Þorgerður Þorleifsdóttir, 2018, Egg laying strategies in the Arctic tadpole shrimp (Lepidururs arcticus), BS ritgerð, Líf- og Umhvefisvísindadeild, Háskóli Íslands, 25 bls.

ISBN XX

Prentun: Háskólaprent Reykjavík, 30. Janúar 2018

ii Útdráttur

Lítið er vitað um lífsögu skötuormsins (Lepidurus arcticus). Í þessari ritgerð koma fram ný gögn um varphegðun tegundarinnar. Þessum dýrum var safnað í Veiðivötnum sumarið 1996 og þau flutt til Reykjavíkur þar sem rannsókn var framkvæmd á varphegðun dýranna. Tilraunir voru settar upp þar sem einstökum dýrum var haldið í dósum með vatni og mosagrein til að verpa á, og var dýrunum gefin fæða einu sinni á dag. Mosagreinar voru teknar daglega, þær merktar með dagsetningu og númeri dýrsins sem verpti og egg á greinunum talin og mat lagt á varpstærð (fjölda eggja í varpi). Höfundur þessarar ritgerðar tók saman niðurstöður þessarar tilraunar árið 2017 og eru þær birtar hérna. Samanburður á stærð dýranna (skjaldarstærð, carapace) og fjölda eggja sýndi að skýr jákvæð tengsl voru þar á milli þar sem stærri dýr framleiddu fleiri egg. Sams konar tengsl fundust á milli stærðar og hversu oft dýrin verpa. Rannsóknin sýndi að tíðni varps (mælt sem tími milli varps) var breytileg meðal einstaklinga, en algengast var að dýrin verptu með u.þ.b. tveggja sólahringa millibili (annan hvern dag). Meðalfjöldi eggja sem hver einstaklingur verpti var 17,9 og það var marktæk jákvæð fylgni milli stærðar dýrs og fjölda eggja sem var orpið. Einnig var jákvætt samband milli fjölda eggja í hverju varpi og stærðar dýra og flest dýr (64%) sem verptu gerðu það oftar en einu sinni á rannsóknartímanum sem stóð yfir í 2 vikur. Niðurstöðurnar benda til þess að hjá þessum dýrum hafi þróast ákveðin varphegðun (strategy) sem er breytileg á milli einstaklinga en frekari rannsókna er þörf til að sjá hvaða breytur það eru sem hafa mest áhrif.

iii Abstract

Little is known about the life history of the Arctic tadpole shrimp (Lepidurus arcticus). This dissertation presents some new data about the egg laying strategies of the species, provided by experiments. The animals were collected in Veiðivötn area, S-Iceland, in the summer of 1996 and transported to Reykjavík where the experiments were conducted. Individual animals were kept in containers, fed once a day, and provided with a moss branch to lay their eggs on. Each day the moss branches were collected, marked with date and identity number and the branches examined for number of eggs and clutches. In 2017 the author of this dissertation worked on the samples described above and analysed the data. Comparison of the shield size (carapace) of L. arcticus and the number of eggs laid confirmed that more eggs are laid by bigger animals, as well as they lay more clutches of eggs. Also, the results show that the oviposition strategy of L. arcticus has some characteristic features, but also revealed some phenotypic plasticity. For instance, most individuals laid eggs every other day, while others did so every day or with a still longer interval. The average number of eggs per individual was 17,9 and there was a highly significant positive relationship between body size and total number of eggs laid. The relationship between both number of clutches and number of eggs per clutch with shield size was also positive and most individuals laid eggs more than once (64%). Further research is clearly needed to answer questions about the detailed reproductive strategy of L. arcticus.

iv Preface

In the years 1994‒1996 an ecological and behavioural field survey was carried out by Þorleifur Eiríksson (project manager), Hilmar J. Malmquist and Hrefna Sigurjónsdóttir on the Arctic tadpole shrimp (Lepidurus arcticus) in the Veiðivötn area, S-Iceland (Eiríksson et al. 1996). The project was based at The Natural History Museum of Kópavogur and for three years funded by The Icelandic Centre for Research (Rannís). At first, research emphasis was aimed primarily at in situ ecology and feeding behaviour of the species, but later on the focus was extended and directed more at reproduction biology (breeding and egg laying), including experimental set up on egg laying and egg size variation (Árnason et al. 1940, Eiríksson et al. 1996, Ingimarsson 1997). The research presented here and conducted by the author of the dissertation is based on material and data described above and previously collected by others but left unprocessed. The emphasis in this dissertation is to examine reproductive biology of the animals by determining clutch size (number of eggs laid by individuals), how many and how often eggs are laid and if there is any correlation between fertility, number of clutches and eggs per clutch, and size of the animals. My contribution is based on the analyses of earlier records on behaviour and measurements of 137 individual Arctic tadpole shrimps, collected in 1994, including measurement of their size (length), size of egg clutches (number of eggs laid in a single egg laying episode), and how often eggs (egg clutches) were laid over a given time. A research report for the original, overall project has been written for Rannís and another research report on egg production (Ingimarsson 1997). Also, two conference posters with abstracts have been published (Eiríksson et al. 1998, 1999) and a general introduction of the project has been published in the newspaper Morgunblaðið (Eiríksson et al. 1996).

Table of Contents

Útdráttur ...... iii Abstract ...... iv Preface ...... v Table of Contents ...... vii Acknowledgements ...... ix 1 Introduction ...... 1 2 The Arctic tadpole shrimp ...... 2 2.1 Evolutionary history ...... 2 2.2 Morphology...... 3 2.3 Habitat and global distribution ...... 3 2.4 Ecological significance...... 3 2.5 Behaviour ...... 3 2.6 Life history ...... 4 3 LOCATION, MATERIAL AND METHODS...... 5 3.1 Earlier research and data gathering (not done by author) ...... 5 3.2 Research and data collection done by author ...... 6 4 RESULTS ...... 7 4.1 Size distribution ...... 7 4.2 Is body size correlated with the number of eggs laid? ...... 9 4.3 How is the frequency of clutches and clutch size in the sample of 137 individuals distributed? ...... 10 4.4 Does body size affect the number and size of clutches the individuals lays? ...... 11 4.5 How often and how fast does L. arcticus lay its eggs? ...... 12 4.6 Does body size affect the frequency of egg laying? ...... 13 5 Discussions and Conclusions ...... 14 References ...... 15

vii Figures

Figure 1: Lepidurus arcticus seen from above (Photo: Finnur Inigmarsson). 2

Figure 2: Denotation of length measurements on Lepidurus arcticus in the study. 7

Figure 3: Frequency distribution of body length (mm) in 114 L. arcticus. 8

Figure 4: Frequency distribution of shield length (mm) in 114 L. arcticus. 8

Figure 5: Relationship of shield length and body length of L. arcticus. (r: 0,799, F = 399,433, F-crit= 3,88, p< 0,001, df. = 226) Also shown is the linear regression equation of body length (y) on shield length (x) which explains 60% of the variance and is highly significant) 9

Figure 6: Relationship between number of eggs laid and shield size in L. arcticus. Also shown is linear regression equation of total number of eggs per individual (y) on shield length (x). (r = 0,5567, p< 0,001, F= 20,23, F-crit=3,88, df. = 226). 9

Figure 7b: Frequency distribution of clutch size by L. arcticus. 10

Figure 8: Total number of clutches per individual in reference to shield length (mm) of L.arcticus (r = 0,3470, (p < 0,001, F= 153,908, F-crit= 3, df. = 220). 11

Figure 9: Average number of eggs per clutch in L. arcticus. as a dependent variable on shield length. Also shown is the linear regression equation. (r = 0,6098, p< 0,001, F=812,65, F-crit= 3,88. df. = 220). 11

Figure 10: Frequency of egg laying by L. arcticus. 0: none egg laid; 1: eggs(s) laid once; 3: egg(s) laid three times, etc. 12

Figure 11: Frequency of average number of days in between egg laying by L. arcticus. 13

Figure 12: Average egg laying frequency in days (Y- axis) against size (X- axis). (r = 0,071, NS) 13

viii Acknowledgements

Special thanks to the supervisors, Hrefna Sigurjónsdóttir and Hilmar J. Malmquist for diligent work, without which assistance and guidance this dissertation would not have been possible. Thanks to Þorleifur Eiríksson, as well as Hrefna and Hilmar for providing access to samples and unpublished data from the L. arcticus research in Veiðivötn area. Thanks to Þorleifur Eiríksson for assistance and guidance in methods and made the samples available and showed how to use them in the lab of RORUM. Thanks to the staff of RORUM, especially Þorleifur Eiríksson, for assistance, guidance and information. Thanks to the Icelandic Museum of Natural History for research facilities.

ix 1 Introduction

Lepidurus arcticus is Iceland’s biggest freshwater crustacean and the only species in the country that belongs to the order Notostraca. Little research has been done on the ecology and behaviour of L. arcticus in Iceland but it is supposed to be an important food to both fish and birds during the autumn months (Einarsson 1979, Eiríksson et al. 1996, Baldursson 2014). Research on the species in other countries (see below) shows that it feeds on small invertebrates, bacteria, detritus and even plants. Behavioural observations are limited (Lakka 2013). The mode of reproduction is variable. It’s distribution in Iceland seems mostly bound to the highlands and the north (Aðalsteinsson 1975, Hallgrímsson 1990, Eiríksson et al. 1996, Guðbergsson 1997, Baldursson 2014).

The aim of this study is to get some insight into the oviposition strategies of the species, i.e. how many and how often L. arcticus lays its eggs and if that is affected by size or time between egg laying. It has been shown elsewhere (see below) that bigger individuals produce more eggs than smaller once. Here we check if that also applies to the Icelandic population. Observations during the experiment that were carried out (see preface) showed. that L. arcticus can lay eggs more than once a day but did not reveal how many times they lay eggs over the course of the egg laying period (one month in the late summer, Eiríksson et al. 1996).

Egg laying frequency in L. arcticus has not been studied before nor its relation to body size.

The following research questions are hereby put forward:

1. How is the size distribution in the sample?

2. Is body size correlated with the number of eggs laid?

3. How is the frequency of clutches and clutch size in the sample of 137 individuals distributed?

4. Does body size affect the number and size of clutches the individuals lays?

5. How often and how fast does L. arcticus lay its eggs?

6. Does size affect the frequency of egg laying?

Here follows a short, general introduction to the Artic tadpole shrimp emphasizing evolutionary history, ecology, behaviour and life history.

2 The Arctic tadpole shrimp 2.1 Evolutionary history

Notostraca, commonly known as tadpole shrimps (Klausen 2012), is a group of branchiopod crustaceans that has existed since at least the late Devonian period (419.2‒358.9 million years (Mya)) (Korn et al. 2013). Among Notostraca there is one family, Triopsidae, and two genera, Triops and Lepidurus. The animals in the family are often considered living fossils, having not significantly changed in morphology since the Triassic period (25‒199 Mya) (Einarsson 1979, Brendonck et al. 2008, Lakka 2013).

While the general morphology of the species has remained remarkably stable over time, species within both genera, especially those belonging to Lepidurus, are characterized by quite extensive phenotypic variation. Example of this phenotypic variation are differences in armature of spines on the exoskeleton, bodily proportions, the armature of carina and sulcus, the endities of the first thoracic appendage, the number of apodous segments and the size and shape of the dorsal organ. This, along with unusual sexual reproduction, has made species definition rather difficult (Rogers 2001, Hessen et. al 2004, Stenderup et al. 2006, Brendock et al. 2007). Also of significance is the fact that most Notostraca can have populations either with male and females or with only females. Some solitary female populations in a few species have shown hermaphroditism but others not, even reproducing by parthenogenesis. Other species only reproduce sexually (Longhurst 1955).

Of the ten Lepidurus species known most show relatively little morphological differentiation, but the Arctic tadpole shrimp, Lepidurus arcticus (Pallas, 1793), is distinguished by its relatively short supra-anal plate and endites (Longhurst 1955).

Figure 1: Lepidurus arcticus seen from above (Photo: Finnur Inigmarsson).

2.2 Morphology

Like other tadpole shrimps L. arcticus has a broad, flat shell (carapace) that conceals most of the head and thorax and some of the abdomen which it is clearly seen behind the shield (Fig. 1). The abdomen is long, segmented and has numerous pairs of legs. The telson has two long, thin caudal rami (Linder 1952, Hallgrímsson 1990). The species has two compound eyes located on top of the head. It has three pairs of antenna, two significantly reduced (Linder 1952). Fully grown individuals reach about 2‒5 cm in total length and the species is the largest crustacean found in fresh waters in Iceland (Hallgrímsson 1990, Eiríksson et al. 1996, Baldursson 2014). 2.3 Habitat and global distribution

The species inhabits freshwater lakes, ponds, streams and reservoirs in the Arctic, including the Aleutian Islands, Alaska, Labrador, Greenland, Iceland, Bear Island, Spitzbergen, Scandinavia and Siberia, but is also found in the Southern hemisphere (Reed 1962, Longhurst 1955, Carlton 2007, Klausen 2012, Lakka 2013,). In Iceland, it is found all around the island, but is most frequently observed in the highlands (Ólafsson 1988, Eiríksson et al. 1996, Brendonck et al. 2008, Baldursson 2014). However, an in-depth assessment of its distribution remains to be done. The species inhabits both shallow ponds and deep lakes, down to ca. 14 m depth as observed in one lake (Eiríksson et al. 1998). The bottom substrate of the habitats the tadpole-shrimps thrive in is quite variable, ranging from soft sediment to hard lava rock (Einarsson 1979, Eiríksson et al. 1996,). 2.4 Ecological significance

High densities of L. arcticus appear to be associated with low predation pressure since the species is usually absent or in very low densities in lakes with abundance of fish, i.e. Arctic charr (Salvelinus alpinus), brown trout (Salmo trutta) and three-spined stickleback (Gasterosteus aculeatus), along with various ducks and shore birds, who are all known to prey on L. arcticus (Aðalsteinsson 1975, Guðbergsson and Antonsson 1997, Borgstrøm 1975, Fjellheim et al. 2007, Lakka 2013, Baldursson 2014, Jóhannsson and Jónsson 2015). Apparently, the Arctic tadpole shrimp is mostly a predator and scavenger, feeding primarily on other invertebrates such as zooplankton and annelids (Eiríksson et al. 1996). Some analysis suggest that it may also feed on detritus, bacteria and plants (Klausen 2012, Lakka 2013). The strong built, serrated jaws of the species may imply that predation is more important than other forms of feeding habits (Einarsson 1979, Eiríksson et al. 1996, , Hallgrímsson 1990, Christoffersen 2001). 2.5 Behaviour

In general, little is known of L. arcticus behaviour. The basic traits of the species’ behaviour and relations with the environment, e.g. feeding behaviour, have though been studied in Iceland (Garðarsson and Einarsson, 1991, Eiríksson et al. 1996). Most foreign research focuses on morphological variation and genetics of the genus Lepidurus (Longhurst 1955, Borgstøm and Larsson 1974, Rogers 2001, Hessen et al. 2004, Stenderup et al. 2006, Brendonck et al. 2007, Wojtasok and Bryłka−Wołk 2010, Korn et al. 2013). The Arctic tadpole shrimp is primarily a benthic, asocial animal. Individuals crawl and swim in short

3 bursts on and close to the lake/pond bottom, spending most of their time, however, lying still on the bottom. They have also been recorded to swim upwards in the water column to the surface and sink slowly again in a spiral-like track (Lakka 2013). Similar behaviour has been seen in other Notostraca. If it detects food in the sediment it will crawl over the bottom and whirl the sediment up by its appendages and engulf the food. If encountered by another Arctic tadpole shrimp, it may defend its food and attack the intruder (Hallgrímsson 1995). It lays its eggs on moss branches, lava rocks and other available material, primarily in the littoral zone though eggs have been found in deeper parts of lakes (Eriksson et al. 1996).

The way L. arcticus reproduces is not fully clear. It a has been described as hermaphroditic (Sassanan 1997) as well as parthenogenetic (Hessen et al. 2004). Males have been identified but the reasons why males remain absent in some populations and not others are still unclear (Wojtasik and Bryłka−Wolk 2010). The hermaphrodites can either self-fertilize or mate with males but they cannot mate with other hermaphrodites (Longhurst 1955). When males mate the sperm leaves the males body through simple pores on the testis lobes. The eggs are released by the other partner and then held in a cup-like brood pouch on the 11th thoracic appendages (legs). The eggs in the brood pouch are fertilized internally as the partner swims though the sperm cloud. The eggs are retained only for a short time before being laid (Sassanan 1997, Lakka 2013). Similar hermaphroditism has been found in at least two other species – Triops cancriformis and T. longicaudatus (Longhurst 1955). 2.6 Life history

In Iceland L. arcticus has an annual life circle, where most of the eggs are laid in the littoral zone in the autumn. The eggs live throughout the winter while the adults die in late autumn. The eggs may tolerate some drying and freezing but not for an extended time (Eiríksson et al. 1996). The eggs hatch into free-swimming larva in spring, around May or June when lake bottom sediments defrost. The eggs develop quickly as they attain in a matter of hours an adult phenotypic appearance without going through metamorphosis. The data shows that growth is fast with shedding about once a week which is supported by other earlier research by Lakka (2013) in Finland (Eiríksson et al. 1996).

Egg production starts around midsummer once the adult tadpole-shrimp reaches a certain size but L. arcticus stops growing at the start of egg-laying (Lakka 2013). The adults rub against moss branches or rocky substrate and release and attach few eggs at each time in one or more clutches (Eiríksson et al. 1996).

Lakka (2013) showed that bigger size is correlated with more eggs being laid. As individuals get bigger, the more eggs there are in the brood pouches but egg size is constant (Ingimarsson 1997). From this it could be deducted that it could be beneficial for L. arcticus to grow to full potential size and then focus on egg production. In the ideal environment, with low predation and no food scarcity, reaching maximum possible size before egg laying should yield the best outcome for the animal (highest fitness) since potential fitness relates directly with number of viable offspring. What that size is depends on many parameters, both physical and biological. No attempt has been made to answer such a question for this species.

3 LOCATION, MATERIAL AND METHODS 3.1 Earlier research and data gathering (not done by author)

In the years 1994‒1996 an ecological and behavioural field survey was carried out by Þorleifur Eiríksson (project manager), Hilmar J. Malmquist and Hrefna Sigurjónsdóttir on L. arcticus in the Veiðivötn area, S-Iceland (Eiríksson et al. 1996). The project was based at The Natural History Museum of Kópavogur. The Veiðivötn area lies within the neovolcanic zone and emerged in its current form in the year 1477 after a volcanic eruption in the area (Aðalsteinsson 1987). L. arcticus is found throughout the area in lakes and tarns of varying depth and size, which form a complex system of spring fed freshwater bodies, both isolated and connected to running waters. Many lakes have brown trout, Arctic charr and three-spined stickleback. For the above reasons, the area offers both a variety of freshwater environments and Arctic tadpole shrimps in abundance and is ideal to study biology of the species, including life history variation and reproductive strategies.

Map of the lakes where samples were collected. The research presented here and conducted by the author of the dissertation is based on material and data previously collected in the project described above, but left unprocessed. For study on reproductive biology a total of 137 life animals were collected in 1995 from two different bodies of water in Veiðivötn, Litla-Breiðavatn and Skálanestjörn, and later used in my study. Litla-Breiðavatn is relatively large, about 9,54 ha, whereas Skálanestjörn is a small pond on the edge of the lake Skálavatn, 1,96 ha (see map). The difference between these two freshwaters is considerable regarding total biomass and the nature of the

5 ecosystem, such as substrate, predation and food availability. Depth of bodies of waters was not measured, partially since most collection was done on the banks of the lakes where the Arctic tadpole lays its eggs in submerged moss branches. Litla-Breiðavatn is considerably deeper and contains predators (Arctic charr and sticklebacks) while Skálanestjörn is shallower and has a sandier substrate. Samples were collected in August and September when production was at its peak. Specimens where collected, kept in cool water, transported to Reykjavík and reared in a lab. Specimens were kept outside during rearing in an attempt to keep water temperature similar to temperature in the lakes they were collected in.

To minimize the external influence on egg production, such as food availability and proper egg laying substrate, the animals were placed in separate containers, each with their own moss branches as a substrate for egg-laying and fed commercial hobby-fish feed once a day. The branches were exchanged every day and identified with date and the animal under question. Branches where stored in glycerine-isopropanol mixture. The branches where collected for as long as the animal was alive and laying eggs, in most cases for about two weeks. 3.2 Research and data collection done by author

The 137 animals used in the present study were stored by Þorleifur Eiríksson in the Natural History Institute of Westfjords in Bolungarvík (Náttúrustofa Vestfjarða) until it was transported to Reykjavík in autumn 2016. My contribution is based on the examination of these 137 individual Arctic tadpole shrimps, including measurement of their length, total number of eggs laid per individual, size of egg clutches (number of eggs laid in a single egg laying episode), and how often eggs (egg clutches) were laid over a given time.

The moss branches were examined for eggs under stereoscope. The eggs were counted on each branch. If 3‒5 eggs were close together on the same branch or on two adjacent branches it was concluded that they were from single or the same egg laying episode and belonged to the same clutch. Thus, clutch size is the number of eggs laid in one laying episode. If eggs were further away on the moss branches it was concluded that they were from separate clutches. Some eggs were detached and these were dismissed from the study.

The term egg laying frequency was defined as the time (no of days) between egg laying episodes. To find egg laying frequency for each individual the average number of days between egg laying (days where eggs were found on moss branches) was calculated. In one case this number was rounded upwards. This number was then used to find the average time between egg laying.

Average number of eggs laid was found by adding all eggs and divide by number of animals. Number of eggs laid over the research time was calculated for each individual and then compared to its shield-length. Number of days with egg laying was counted as each day- sample were eggs were found on the moss branches.

Egg laying recurrence was defined as the average number of days between egg laying. If egg laying occurred every day, recurrence was marked as zero. If one day elapsed between egg laying it was marked as one.

Both body-length and shield-length was measured and after confirming that body-length and shield-length is correlated shield-length was used for convenience of measuring (Fig. 5).

Length was measured from the tip of the head to the end of the shell for the shield length and tip of head to end of the supra anal plate for body length (Fig. 2). Total length with telson setae was not measured as the setae can be bent, twisted and broken and thus hard to measure exactly. Of the 137 individuals 114 were measured for size since a few had a severe case of Red Carapace Disease (Lakka 2013) that made accurate measuring impossible. Information on clutches was not available for 3 individuals. Thus the sample size is 114 or 111 when correlation between number of eggs, number of clutches and clutch size and body size is investigated. (Fig. 3 and 4).

Figure 2: Denotation of length measurements on Lepidurus arcticus in the study.

4 RESULTS 4.1 Size distribution

Figure 3 shows the frequency distribution of shield length. Smallest shield-length measure was 5,0 mm and the biggest was 14,4 mm. Average shield-length was 9,5 mm with standard deviation 2,2 (Fig. 4).

Figure 4 shows the frequency distribution of body length. Smallest body-length measure was 9,7 mm and the biggest was 23,4 mm. Standard deviation was 2,9. Average body-length was 15,9 mm, with standard deviation 2,9.

6 Numberindividuals of

0 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 body length (mm)

Figure 3: Frequency distribution of body length (mm) in 114 L. arcticus.

10 Numberindividuals of

0 3 4 5 6 7 8 9 10 11 12 13 14 15 shield length (mm)

Figure 4: Frequency distribution of shield length (mm) in 114 L. arcticus.

16 y = 1.0673x + 5.7755

14 R² = 0.6071 Bodylength (mm) 12

8 4 6 8 10 12 14 16 Shield length (mm)

In Figure 6 the total number of eggs laid per individual over the research time is shown as a dependent variable on shield length. The linear regression model shows that the relationship is highly significant (r=0,5567, p <0,001, F=20,23, F-crit=3,88, df.=226). Average number of eggs per individual was 17,9 and the standard deviation was 19,82).

80 y = 5.5768x - 34.787 70 R² = 0.3099 60

10 Totalnumber eggs of laidper individual 0 4 6 8 10 12 14 16 Shield length

4.3 How is the frequency of clutches and clutch size in the sample of 137 individuals distributed?

Figure 7a shows the distribution of the number of clutches in the 137 individuals measured. Majority of individuals lay one, two or three clutches. Figure 7b shows the frequency distribution of clutch size. Most frequent were singular eggs but clutches with two eggs were second most common. When no eggs were found it was marked as zero. Average for figure 7b was 3,9, the median was 2,0 and the mode was 1,0. Standard deviation was 4,26.

Numberindividuals of 10

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Number of clutches

Figure 7a: Frequency distribution of number of clutches per individual L. arcticus.

25 size

20 clutch 15

10 Frequencyof 5

0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 number of eggs per clutch

Figure 7b: Frequency distribution of clutch size by L. arcticus.

4.4 Does body size affect the number and size of clutches the individuals lays?

Figure 8 shows the total number of clutches laid per each individual in relation to its shield length. The relationship is significant. Figure 9 shows that the relationship between number of eggs per clutch is also positively related to size.

20 y = 0.7297x - 3.0037 R² = 0.1204 15

10 Totalnumber clutches of 5

0 4 6 8 10 12 14 16 Shield length (mm)

Figure 8: Total number of clutches per individual in reference to shield length (mm) of L.arcticus (r = 0,3470, (p < 0,001, F= 153,908, F-crit= 3, df. = 220).

8 y = 0.5015x - 2.3825 7 R² = 0.3719

Avergare Avergare numbereggs of per clutch 1

0 4 6 8 10 12 14 16 Shield length

4.5 How often and how fast does L. arcticus lay its eggs?

Here results on egg laying behaviour is shown. First, how often the individual lays eggs based on number of days (Fig. 10) and secondly how frequently they do it (Fig. 11).

20 Numberindividuals of

0 0 1 2 3 4 5 6 7 8 9 15 17 Number of days with egg laying

Figure 10: Frequency of egg laying by L. arcticus. 0: none egg laid; 1: eggs(s) laid once; 3: egg(s) laid three times, etc. In Figure 10 it can be seen that many animals laid eggs only once and some never. Out of 137 animals, 22 (16,1 %) never laid eggs, 51 (37,2%) did so once and 64 (46,7%) laid eggs more than once. The highest number of days with egg laying was 17 times (Min = 1 individual). Of the 64 that laid eggs more than once 7 laid more than once in one day but not again after that.

Figure 11 shows how fast the 57 individuals that laid eggs more than once oviposited their eggs. Thirteen individuals laid eggs every day, 42 did so every other day, one with 2 days interval and one with four days interval.

15 Numberindividuals of 10

0 0 1 2 3 4 Days between egg laying

Figure 11: Frequency of average number of days in between egg laying by L. arcticus.

4.6 Does body size affect the frequency of egg laying?

In Figure 12 the average egg laying frequency (days between egg laying) for the 57 individuals that laid eggs more than once is shown in relation to shield size. No significant correlation was found.

2.5

1.5

1 Average Average laying egg frequency

0.5 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 Shield length (mm)

Figure 12: Average egg laying frequency in days (Y- axis) against size (X- axis). (r = 0,071, NS)

5 Discussions and Conclusions

The Arctic tadpole shrimp laid on average 17,9 eggs over the season. The maximum number of eggs was 80. Body size clearly influences fitness as seen by the statistically significant, positive relationship between shield length (which was used as an indicator of body size) and number of eggs (Fig. 6).

Individuals vary in how many clutches of eggs they lay (Fig. 7a). The most common number is one and the second most common is two. Of interest is the observed great variance with one animal producing up to 17 clutches over a two-week period. A large variation is also found in number of eggs per clutch, the highest number being 22. (Fig. 7b). Size was found to correlate significantly with both number of clutches and number of eggs in a clutch (Fig. 8 and 9)

Regarding the question about the frequency of egg laying it is clear that the strategies vary. Size did not seem to have any effect on the frequency of egg laying (Fig. 12). The most common strategy was to lay eggs once but individuals can spread the oviposition up to 17 days (Fig.10). The time between separate egg laying for those that spread their oviposition over more than one day is in most cases one day between egg laying (Fig. 11). Eggs laid were most often two at a time but could reach up to 11 eggs. Earlier research showed that L. arcticus could lay eggs more than once per day.

To summarize and answer the research questions put forward earlier it is clear that increased body size means more eggs, more clutches and larger clutches. The strategy to lay two eggs in a single clutch with one day in between is the most common strategy but at the same time individuals vary quite a lot. Since the larger individuals lay both more clutches and also more eggs it can be argued that their strategy is to lay eggs over longer time and in more clutches which is likely an anti- predation adaptation. Thus, being large is likely to give higher fitness.

Populations of L. arcticus have been studied for egg production regarding size before in Finland but never in Iceland and the results were similar (Lakka 2013). That bigger individuals lay more and larger clutches has not been shown previously.

Because of the experimental setup there are of course many outside factors that are not considered. Scarcity of food and competition as well as predation are all factors that can encourage L. arcticus to start laying eggs before full size is reached. Further research is also needed on the influence of different environments such as lake size, substrate and biomass on life history and strategy of the species. Further research is needed to answer questions if L. arcticus shows a difference between populations, if individuals always empty their brood pouch and if there is strategy behind when they choose to lay eggs. Egg laying strategies could thus very likely be a good indicator of other life history features including behavioural strategies.

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